pump primciples

PUMPPRINCIPLESMANUAL

GLOBALTRAININGEDUCATIONANDDEVELOPMENTPROGRAM

CHESTERTON®

PUMP

PRINCIPLES

MANUAL

Tableof

Contents

1

Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1Course Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3Course Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Chapter 1 – PumpsPump Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Pump Classifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8ANSI Pump Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Chapter 2 – Pump PartsOverhung Impeller Centrifugal Pump Configuration . . . . . . . . . . . . . . . . . . . . . . . .17Overhung Impeller Centrifugal Pump Wet End Parts and Function . . . . . . . . . . . . . .18Overhung Impeller Centrifugal Pump Power End Parts and Function . . . . . . . . . . . .28Overhung Impeller Centrifugal Pump Driver Parts and Function . . . . . . . . . . . . . . . .35Overhung Impeller Centrifugal Pump Support Structure Parts and Function . . . . . . .39Split Case Centrifugal Pump Part Differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

Chapter 3 – Pump TermsPressure Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45Fluid Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48Pressure/Head Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52Temperature Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

Chapter 4 – Pump OperationFluid Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59Fluid Energy Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62

Chapter 5 – Pump CurvesPump Performance Curve Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65Pump Performance Curve Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68Element Effects – Various Operating Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . .73

Chapter 6 – A Simple Pumping SystemElements of a Pumping System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83Total Suction Head (System) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89Total Discharge Head (System) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92Total Head (System) Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95Net Positive Suction Head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99Pump’s Operating Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102

Chapter 7 – Typical Pump FailuresRadial Loading and Shaft Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107Bearing Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111Classic Cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117

IndexA to Z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125

Table of Contents

PUMP

PRINCIPLES

MANUAL

Disclaimer

2

DisclaimerThis publication is designed to provide a compilation of technical data and information from various sources. This data and information is presented with the understanding thatA.W. Chesterton® Company is not herein engaging in the design of machinery or systems nor in the rendering of technical services or advise.

A.W. Chesterton Company makes no representationsor warranties with respect to the completeness oraccuracy of the data or information contained herein.In addition, A.W. Chesterton Company does notassume any liability for any losses or damages resulting from the use or application of the data or information contained herein.

PUMP

PRICIPLES

MANUAL

CourseIntroduction

CourseObjective

3

Course Introduction

Course Objective

This course is designed to educate you with

the appropriate knowledge of pumping principles.

A thorough comprehension of underlying principles

is required for you to effectively carry out

your day to day business. Greater system utilization

and production availability of pumping systems

is growing in demand. Profits are realized by

increasing the reliability of plant pump systems.

The reduction of maintenance and operating

expenses will add to the financial bottom line.

Upon completion of this course you will be

able to identify pump classifications, describe the

purpose of pump parts, define various pump terms,

explain pump operation, describe the components

of a pump curve, describe the elements and

effects of a simple pumping system, and describe

typical pump failures and their causes.

CHAPTER 1

PUMPS

4

CHAPTER 1

PUMPS

Introduction

Objective

PumpDefinition

What is

a pump?

5

Chapter 1 Pumps IntroductionPumps are the second most common machine in use today. They are exceeded in numbers only by the electric motor. To study pumps, a thorough understanding of their various types and functions is essential.This chapter will define what a pump is, how pump types are classified, how these types function and their main differences. The ANSI industrialpump standard will also be discussed.

Because pumps are used extensivelythroughout industry, it is essential to know what a pump is and how it is used. This lesson will provide a definition of a pump and a description of how and why pumps are used. This definition and description of purpose are essential as we begin our study of pump principles.

What is a pump?The pump can be thought of as the earliest form of a machine which substituted natural energy for humanmuscular effort. This definition is unfortunately not comprehensive enough for most of us. To that end, a pump is defined as a mechanical device that rotates or reciprocates tomove fluid from one place to another.

Most pumps usually are made up of many individual components, each serving a specific function. However, some pumps, like waterwheels and the Archimedeanscrew, can be very simplistic in their mechanical composition. These simple, early pump designs have limited parts and are still in use in some areas of the world today.

Because pumps are probably the second most common machine in use, exceeded in numbers only by the electric motor, they can come in an almost endless variety of shapes, sizes and types.

Various pump designs are shown in Figures 2 – 6

ObjectiveUpon completion of this chapter you will be able to define what a pump is and its purpose. You will also be able to describe how pumps function, pump type classifications, and describe a pump built according to the ANSI pump standard.

Figure 1 Ancient Archimedean screw

Pump Definition

CHAPTER 1

PUMPS

6

Figure 3 Typical centrifugal pumps in an industrial plant

Figure 6 Typical positive displacement pump cutaway

Figure 5 Typical centrifugal (split case) pump cutaway

Figure 2 Modern waterwheel

Figure 4Modern Archimedean screw

Stuffing Box

•

Stuffing Box

Suction Eyes

Bearings

•

• •

•

Bearings

•

Closed Impeller

•

PumpDefinition

What is

a pump?

CHAPTER 1

PUMPS

PumpDefinition

What is

the purpose

of a pump?

7Figure 7 A typical pumping system

What is the purpose of a pump?A pump is designed to transfer fluid from one point to another. Pumps transfer fluid from low pressure areas to higher pressure areas, low elevations to higher elevations, and from local locations to distant locations.

Examples of low pressure areas are atmospheric tanks, tanks under vacuum, bodies of water or any fluid storage area where the surface pressure is equal to or less than atmospheric pressure. High pressureareas are pressurized tanks, boilers or any fluid storage area where the surface pressure is greater than atmospheric pressure.

To move a fluid against gravity requires a pump to add energy to it in order to achieve this higher elevation. Without this energy transfer the fluid will not be raised to its new, higher level. In rare instances pumps are sometimes used to move fluid down in elevation to achieve a specific flow or transfer rate.

To move fluid through a channel or conduit, such as a pipe, requires a pump to add energy to overcome the losses due to friction between the fluid and the pipe walls. Without this energy transfer the fluid will not move down the pipe.

CHAPTER 1

PUMPS

8

Pumps are classified as either Kinetic or Positive Displacementpumps. This lesson will describe the different ways in which these pumps add energy to the pumped fluid to generate movement. It will also discuss some of the mechanical configurations typically seen and how they operate. It is very important to understand the differences between these two classifications as we continue our study of pump principles.

How Pumps Are ClassifiedBecause pump designs are so numerous, they can be classified in any number of ways. Some examples of how pumps can be put into different classes are as follows:• By the applications the

pumps serve.• By the materials from which

the pumps are constructed.• By the liquids the pumps

can handle.• By the pump orientation.

All of the above classes are limited and usually overlap each other.Therefore, a more basic classification system is needed. If pumps are categorized by the way in which energy is added to the liquid, there is no overlap and the classification is related only to the pump itself. To that end, pumps are generally classified as either Kinetic or Positive Displacement pumps.

Figure 8 Pump classifications

Pump Classifications

REGENERATIVETURBINE

SPECIALEFFECT RECIPROCATINGCENTRIFUGAL ROTARY

KINETIC PUMPSPOSITIVE

DISPLACEMENTPUMPS

PUMPCLASSIFICATIONS


How Pumps

Are

Classified

CHAPTER 1

PUMPS


Kinetic Pumps

Centrifugal

Pumps

9

Figure 9Centrifugal pump pumping to 100 feet

Figure 10Centrifugal pump classifications

Kinetic Pumps Kinetic pumps add energy continuously.This energy increases the fluid velocitywithin the pump to levels above thoseoccurring at the discharge. When thehigh-speed fluid reaches the discharge it has to slow down to equal the lowerfluid velocity found there. The resulting velocity reduction within or beyond the pump produces a pressure increase.This increase in pressure moves thepumped fluid if it exceeds the resistancefound in the system. If the pump pressure does not exceed the systemresistance, the fluid does not move.

Kinetic pumps are further classified as Centrifugal, Regenerative Turbine, and Special Effect.

Centrifugal Pumps The most common form of Kineticpump, by far, is the Centrifugal pump.This type of pump is a machine that uses the dynamic principle of accelerating fluid, through centrifugalactivity, and converting the kinetic energy into pressure.

Centrifugal pumps will only pump, or build pressure, to a designed level.When this level is reached, the fluid no longer moves and all the kinetic energy is converted to heat. This heatcan cause the fluid to vaporize or build pressure within the pump, sometimes exceeding its design limit.Caution should be used when operating a Centrifugal pump at low or zero flows.

The Centrifugal pump is the pump typehighlighted for discussion throughout this book. Centrifugal pumps are foundin the majority of pumping services, both industrial and residential. They are most commonly typed by their mechanical configuration and can be classified as follows: • Overhung Impeller • Impeller between Bearings

Overhung Impeller Centrifugal pumps can be close-coupled, separately-coupled, or sealless.

Impeller Between Bearings pumps (split case) are separately-coupled pumps that can be single-stage or multi -stage. The pumps can also be radially (vertically)or axially (horizontally) split.

We will primarily focus on the Overhung Impeller Centrifugal Pump in this book.

IMPELLERBETWEEN

BEARING PUMPS

OVERHUNGIMPELLER

PUMPS

CENTRIFUGALPUMPS

100 Feet

CHAPTER 1

PUMPS


Positive

Displacement

Pumps

Rotary Pumps

10

Positive Displacement PumpsPositive Displacement (PD) pumps addenergy periodically. A force is applied to the movable boundaries of anenclosed volume of fluid. This results in a pressure increase to move the fluid through valves or ports into the discharge line.

PD pumps push or pull liquid from one point to another using various mechanical configurations. A positive displacement pump will continue to pump fluid until relieved. It is only limited by the pressure limitation of the pump materials and system. This characteristic means that the pressure will continue to build until alleviated. Relief can come in a controlled form at relief valves, rupture disks or other pressure relief devices. It can also come in an uncontrolled form at a flange gasket, mechanical seal, etc. possibly causing damage or injury.

PD pumps are classified as Rotary or Reciprocating. They are often used to pump thick, viscous fluids (i.e., #6 Fuel Oil) or where a finiteamount of fluid is required in a given time (i.e., metering pumps).

Rotary PumpsA Rotary pump is a positive displacement pump consisting of a chamber(s) containing gears, lobes, cams, vanes, screws or similar elements driven by a rotating shaft. Close running clearances enable the pump to generate the proper hydraulics. As these clearances increase, due to wearing of parts, the pump’s efficiency drops dramatically.

The pump discharge lines must remain open during operation or damage to the pump and/or system can occur. Relief valves are recommended to prevent pressure from building beyond the pressure limitation of the system. Flow rates are determined by the Revolutions Per Minute (RPM) of the pump drive shaft.

The most common types of rotarypumps are gear, lobe, vane, and screw. Other types of rotary pumps include piston, flexible member and circumferential piston.

Figure 11Rotary pump classifications

GEAR PUMPS LOBE PUMPS VANE PUMPS SCREW PUMPS

ROTARYPUMPS

CHAPTER 1

PUMPS


Rotary Pumps

11

Figure 14Vane pump

Figure 13Lobe pump

Figure 15Screw pump

Figure 12Gear pump

CHAPTER 1

PUMPS


Reciprocating

Pumps

Power Pump

12

Power PumpA power pump is a pump driven by power from an outside source applied to the crankshaft of the pump. It is a constant-speed,constant-torque, constant-capacity reciprocating machine. The power pump is further divided as either piston or plunger. The pump’s capacity fluctuates with the number of plungers or pistons.

Figure 18 Power plunger pump

Figure 17Power piston pump

Reciprocating PumpsA Reciprocating pump is a positive displacement pump consisting of a liquid end and a drive end. The liquidend consists of a device to displace a fixed volume of fluid for each stroke of the drive end. Suction and discharge flow is usually determined by the position of check valves. Like rotary pumps, it is critical that

discharge lines remain open and relief valves are used during operation to prevent pressure from building beyondthe system limitations. Reciprocatingpumps are divided into three generaltypes: power, controlled volume andsteam.

Figure 16Typical reciprocating pump

Pump Discharge

CrankshaftLiquid End Drive End

Pump Suction

•

•

•

Controlled Volume PumpA controlled volume pump is a pump used to accurately displace a predetermined volume of fluid in a specified period of time. It is also called a “metering”, “proportioning”, or “chemical injection”pump. These pumps also are driven by power from an outside source applied to the pump mechanism. The controlled volume pump is further divided as either piston, plunger or diaphragm.

CHAPTER 1

PUMPS


Controlled

Volume Pump

Steam Pump

13

Figure 20Reciprocating double diaphragm pump (air operated)

Figure 19 Controlled volume diaphragm pump

Steam PumpA steam pump is a reciprocating steamengine and a liquid end built together as a unit. Steam is usually used as thedriving medium, but compressed air

or natural gas can also be used. No outside power source is necessary.Steam pumps are further divided aseither piston or plunger.

Figure 21Reciprocating steam pump

Steam Side Liquid Side

Discharge Valve Assembly

Suction ValveAssembly

Plunger

Diaphragm

Hydraulic Oil

Flow

Liquid End Head

•

••

•

•

•

CHAPTER 1

PUMPS

ANSI PumpStandard

What is

the

ANSI Pump

Standard?

14

Final approval by the B73 StandardsCommittee and the American NationalStandards Institute (ANSI) came in 1977.This approved standard was designatedANSI B73.1 – 1974.

There have been two (2) major revisions to this standard since the original publication. They came in 1984 and in 1991. The latest pump standard is ASME B73.1M – 1991. The American Society of MechanicalEngineers (ASME) submitted this standard that was later approved by the ANSI and the B73 StandardsCommittee. If the pump is horizontal the specification is B73.1 and if it is vertical the specification is B73.2.

Key1. Casing 2. Impeller 3. Gasket 4. Seal Chamber Cover5. Seal6. Gland7. Adapter8. Seal, Inboard

Bearing Cover9. Inboard Bearing

10. Frame11. Pump Shaft 12. Outboard Bearing 13. Outboard

Bearing Cover14. Bearing Locknut15. Seal, Outboard

Bearing Cover16. Coupling Key

Figure 22 Overhung Impeller – Separately Coupled – Single Stage – ANSI Pump

As we have seen, pumps can be classifiedin various ways. A popular American centrifugal pump standard is the ANSIpump standard. This lesson will definethe ANSI pump standard, its history and its purpose. Because many pump companies manufacture pumps using this standard, it is important to understand its strengths and weaknesses.

What is the ANSI Pump Standard?In 1955 the Chemical Process Industry(CPI) and the Standards Committee on Centrifugal Pumps for ChemicalIndustry Use, B73, started working on a centrifugal pump standard to meet their industry needs. After many yearsand many discussions, the first pumpspecification was published in 1974.

ANSI Pump Standard

• • ••

•

•

•

•

•

•

•• ••

••

1

2 3 54 6 7 8 9 10 12 1314 15 1611

CHAPTER 1

PUMPS

ANSI PumpStandard

What is

the purpose

of the

ANSI Pump

Standard?

Review

Questions

1 through 5

15

An ANSI pump can be defined as an overhung impeller, end suction, single stage, centerline discharge, centrifugal pump. ANSI pumps can beboth horizontal and vertical designs. This type of pump is so designated as an ANSI pump because it fits the dimensional and operational criteria and requirements of the ASME B73 – 1991 pump specification.Some important dimensional criteriainclude the following:• Centerline height• Footprint• Overall length• Pump sizes (Suction x Discharge

x Nominal Impeller Diameter)

Operational criteria gives approximateflow and head recommendations for given pump sizes.

What is the purpose of the ANSI Pump Standard?The ANSI pump is covered by a standard that requires all such pump types to be dimensionally interchangeable and to have certaindesign features to facilitate installationand maintenance. This standard requires that all ANSI pumps, regardless of the pump manufacturer, be interchangeable with respect to mounting dimensions, size and location of suction and discharge nozzles, input shafts, baseplates, and foundation bolt holes, etc. A complete description of this standard is found under ANSI/ASME B73.

An ANSI dimensional pump is not guaranteed to be less expensive or more reliable than a non-ANSI pump.Being ANSI dimensional means nothingmore than being interchangeable withpumps of similar ANSI-defined sizes from different pump manufacturers. Therefore, the ANSI pump standard provides a good minimum set of requirements when purchasing new pumps.

Review Questions1. Pumps are mechanical

devices that

a. only reciprocate to move fluid.b. rotate or reciprocate

to move fluid.c. come in one design type.d. are not very common in industry.

2. The purpose of a pump is to transfer fluid from one point to another such as high pressure areas to low pressure areas.a. True.b. False.

3. If pumps are classified only by the applications they serve, this would capture all pumps and have no overlap of styles.a. True.b. False.

4. What criteria does the ANSI pump fit?a. Dimensional & Operational

criteria of ANSI/ASME B73.b. Operational & Engineering

criteria of ANSI/ASME B73.c. Engineering & Manufacturing

criteria of ANSI/ASME B73.d. Dimensional & Manufacturing

criteria of ANSI/ASME B73.

5. The ANSI pump standard for horizontal and vertical centrifugal pumps

a. requires these pumps to be dimensionally interchangeable.

b. is described under the ASME B73 specification.

c. Both 1 and 2.d. None of the above.

Answers – Located on the Inside Back Cover

CHAPTER 2

PUMP

PARTS

16

CHAPTER 2

PUMP

PARTS

Introduction

Objective

OverhungImpeller

CentrifugalPump

Configuration

What are

the four

sections of

an Overhung

Impeller

Centrifugal

Pump

Configuration?

17

Chapter 2 Pump Parts IntroductionA thorough knowledge of how a pumpis built begins our pump discussion. This chapter will identify and describe the main sections of an OverhungImpeller Centrifugal Pump and the various parts found in each section. It is essential to understand the purpose and function of each sectionand the parts contained therein.

Our study of pump principles will focus on centrifugal pumps. This lesson will define the four basic sections found on the Overhung Impeller Centrifugal Pump.

What are the four sections of an OverhungImpeller Centrifugal Pump Configuration?The four sections of an Overhung Impeller Centrifugal Pump consist of the following:• Wet End• Power End• Driver• Baseplate and Foundation

ObjectiveUpon completion of this chapter you will be able to describe the sectionsand parts of an Overhung ImpellerCentrifugal Pump. You will also be able to identify the differences betweenthis pump and an Impeller BetweenBearings Centrifugal Pump.

Figure 1Typical Overhung Impeller Centrifugal Pump Configuration

Overhung Impeller Centrifugal Pump Configuration

Wet-End Power-End

Baseplate and Foundation

Driver

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump

Wet EndParts andFunction

What are

the parts in

the wet end

section of

an Overhung

Impeller

Centrifugal

pump?

18Figure 2Wet End Components

The wet end is the first section we will discuss. This lesson will identifyand describe the parts found in the wet end section of an OverhungImpeller Centrifugal Pump. The purpose of each part will also be defined.

What are the parts in the wet end section of an Overhung Impeller Centrifugal pump?The parts of the wet end section consist of the following:A -Volute (Casing)B - ImpellerC -Wear RingD - Stuffing Box (Seal Chamber)E - Sealing Device

(Packing or Mechanical Seal)F - Backcover (Backplate, Head,

or Stuffing box cover)G -Cooling JacketH - Frontcover (Suction cover)I - Lantern Ring

Overhung Impeller Centrifugal Pump Wet End Parts and Function

H

A

D

B

C

F

I E

G-Not shown

•

•

•

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

Volute?

19Figure 3 External view of volute looking into suction side (Note: Impeller is visible.)

Figure 4 Internal view of volute showing impeller rotation and fluid flow

What is the function of the Volute?The volute, or casing as it is sometimesreferred to, is a spiral-shaped component surrounding the impeller.This section provides a path to acceptand discharge the fluid being pumped.On most Overhung Impeller Centrifugalpumps, each casing has an inlet (suction) side and an outlet (discharge)side. The fluid to be pumped is fedunder pressure to the inlet side of the pump. The casing collects this fluid discharged by the impeller and converts its velocity energy to pressureenergy. The fluid then leaves the casingthrough the outlet at a higher pressure.

A centrifugal pump volute increases in area from its initial point until it encompasses the full 360° around the impeller and then flares out to the final discharge opening. The wall dividing the initial section and the discharge nozzle portions of the casing is called the tongue of the volute or the “cutwater.” The cutwater on manycentrifugal pumps (especially horizontal end-suction) is usually positioned atapproximately the 0° (top) position.

Volute

Cutwater

•

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

Impeller?

20

Figure 5Internal view of impeller inside casing

Figure 6 Impeller Shapes Chart as defined by hydraulic geometry

What is the function of the Impeller?The impeller is the bladed memberattached to the pump shaft (rotatingassembly) and imparts the principle force to the fluid being pumped. The rotation of the impeller is what adds velocity energy to the pumpedfluid. This velocity energy is later transformed to a pressure energy inside the casing near the outlet or discharge side.

Impeller shapes can be classified as either radial flow (straight-vane), Francis-vane, mixed flow, or axial flow.These shapes are defined by thehydraulic geometry of the fluid as it flows through the impeller. One can see from the Impeller ShapesChart (in Figure 6) below that there is actually a continuous change from the radial flow impeller, which developspressure primarily by the action of centrifugal force, to the axial flowimpeller, which develops most of itspressure by the propelling or liftingaction of the vanes on the liquid.

Another method of classifying impellers is by mechanical design. As such, impellers defined in this manner can be designed as open, semi-open, or closed. Both impellershapes and mechanical designs are discussed in detail later in this lesson.

Impellers are usually manufactured using one of two casting processes.“Sand casting” uses a sand mold intowhich molten metal is poured andallowed to cool. Sand casted impellers

are the most common and least expensive to produce. However, theyvery often leave a rough surface on the wetted areas of the impeller. Thiscan lead to lower pump efficiencies.

“Investment casting” uses a mold made from an extremely viscous (and expensive) waxy-like material.Molten metal is poured into this mold.As the metal cools, the mold melts leaving a smooth metal surface.Impellers made this way are usually more expensive to produce than sandcasted impellers. However, because they leave a much smoother surface on the wetted areas of the impeller,higher pump efficiencies are oftenachieved.

Impeller

•

Hub Hub Hub Hub HubVanesVanesVanes

ImpellerShrouds

Impeller Shrouds

HubAxis ofRotation

Vanes Vanes

Axial-Flow AreaMixed-Flow AreaFrancis -Vane AreaRadial-Vane Area

••

••• •

••• •

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


Radial flow(straight-vane)

impeller

Mixed flow

impeller

21

Figure 7Radial flow impeller showing flow patterns

Figure 9Mixed flow impeller showing flow patterns

Figure 8 Radial flow impeller

Figure 10 Mixed flow impeller

Radial flow (straight-vane) impellerOn a radial flow impeller the vane surfaces are generated by straight lines parallel to the axis of rotation. On most pumps the axis of rotation is the pump shaft. Flow strictly follows a line perpendicular to this axis of rotation. The liquid enters the impeller at the hub and flows radially to the periphery. In other words, the liquid enters the impeller and makes a 90° turn and runs parallel to the vanes until it exits the impeller at the vane tips.

Mixed flow impellerIn a mixed flow impeller the vane surfaces have both an axial and radial component. Flow follows this mix of components with axial and radial movement. Flow enters the pump axially and discharges in an axial and radial direction. A mixed flow impeller with more radial than axial flow is sometimes called a Francis-vane impeller.

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


Axial flow

impeller

22

Figure 11Axial flow impeller showing flow pattern

Figure 13 Axial flow pump

Figure 12 Axial flow impeller

Axial flow impellerIn an axial flow impeller the vane surfaces are perpendicular to the axis of rotation. On most pumps the axis of rotation is the pump shaft. Flow strictly parallels this axis of rotation. The liquid enters the pump inlet axially and discharges nearly axially. This means the flow enters the impeller and keeps on going straight through, parallel to the shaft.Pumps that use these types of impellers are sometimes called propeller pumps.

ImpellerVanes •

•

SUCTION

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


Completely

open

impeller

Semi-open

impeller

Closed

impeller

23

Figure 14Completely open impeller

Figure 16Closed impeller

Figure 15Semi-open impeller

Completely open impellerAn open impeller, strictly speaking, consists of nothing but vanes attachedto a central hub. Because of inherentweaknesses in this design, ribs are usually used to strengthen the vanes.

Semi-open impellerA semi-open impeller uses a single shroud, usually at the back of the impeller. This shroud may or may not have pump out vanes or balance holes to modify the pressure behind the impeller.

Closed impellerA closed impeller uses shrouds that totally enclose the impeller from the suction eye to its edges. This design prevents liquid slippage from the discharge to suction by using a running joint between the casing and the impeller. This runningjoint is often called a wear ring.

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

wear ring?

What is

the function

of the

stuffing box?

24Figure 19A stuffing box that is integrally cast with the casing

Figure 18Stuffing box with packing

What is the function of the stuffing box?The stuffing box is a cylindrical opening in the pump casing where the shaft passes through to the impeller. It has the primary function of containing a sealing device that will minimize or eliminate leakage at this point. If a mechanical seal is used as the sealing device, the stuffing box is often called a seal chamber.

Figure 17 Front and back wear rings on a closed impeller

What is the function of the wear ring?Wear rings are sacrificial componentsinstalled on the casing and impeller to inhibit fluid from recirculating back to suction from the discharge. They provide a renewable restrictionbetween a closed impeller and the casing. Wear rings are often installed on both the front and back of theimpeller. When wear rings are installedon the back of the impeller, another set of rings is installed in the backcover.

Lantern Ring Port

Lantern Ring

StuffingBox

Packing Gland

Sealing Device (Packing)

•

•

••

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

sealing device?

25Figure 21 Mechanical seal in a stuffing box

Figure 20 Mechanical packing in a stuffing box

What is the function of the sealing device?A sealing device comes in two forms. It is either mechanical packing or amechanical seal. The sealing device is placed inside the stuffing box to control or eliminate leakage from thepump casing.

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

backcover?

What is

the function

of the pump

stuffing box

jacket?

26

Figure 22 The pump backcover

Figure 23 Pump jacket. Note the restriction bushing in the bottom of the seal chamber

What is the function of the backcover?The backcover is a removable component of the volute, often with the stuffing box attached, that enclosesthe back side of the impeller inOverhung Impeller Centrifugal Pumps.The backcover is sometimes also referred to as a backplate or a head.

What is the function of the pump stuffing box jacket?In some services, high temperatures can complicate the problem of maintaining the sealing device within the stuffing box. Pumps in these services are usually provided with jacketed, liquid-cooled stuffing boxes. A heat transfer medium (usually water)reduces the temperature of the fluidbeing pumped, only in the stuffing box area, and not in the volute. The temperature in the stuffing box is reduced by the flow of water completely surrounding the stuffing box. This improves the service conditions (i.e., reduces temperature) and increases the life of the sealing device.

In rare occasions, the pump stuffing boxjacket is actually used to keep a fluidwarm or even hot. Liquids such as soap,asphalt and various polymers are someexamples of liquids that need a jacketfor heating instead of cooling. Withoutthis external source of heat to keep itwarm, the fluid would solidify or at leastbecome extremely viscous. This greatlyreduces the life of any sealing device.

Pump Jacket

Pump Jacket

Heat Transfer Fluid In

Heat Transfer Fluid Out

•

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

frontcover?

What is

the function

of the

lantern ring?

27

Figure 24The pump frontcover

Figure 25Lantern ring within a packing set

What is the function of the frontcover?The frontcover, or suction cover, is a removable piece used to enclose the suction side of Overhung Impeller Centrifugal Pumps. Typically the inlet nozzle (suction nozzle)is part of this piece.

What is the function of the lantern ring?The function of the lantern ring is to establish a liquid seal around the shaft and to provide a path to inject lubrication, thus reducing the heat generation of the stuffing box packing. This ring can be made from softer metals (like brass), carbon and various polymers (like PTFE). Another common name for lantern ring is seal cage.

The lantern ring is very important in rotating equipment, although it is also used in static equipment like valves. Properly used, this ring provides external lubrication to the packing and a hydrodynamic seal if the injection liquid is at a higher pressure than the fluid in the stuffing box. This increases the life of the packing. When not used properly, the lantern ring does nothing to enhance the life of the packing and could actually decrease packing life.

Stuffing Box Area

Lantern Ring (Seal Cage)Gland

Stud-Boltsand Nuts

Gland

Packing

Throat

•

•

••

•

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump

Power EndParts andFunction

What are

the parts

in the

power end

section

of an

Overhung

Impeller

Centrifugal

pump?

28

Overhung Impeller Centrifugal Pump Power end Parts and Function

The power end is the second section we will discuss. This lesson will identify and describe each part found in the power end section of an Overhung Impeller Centrifugal Pump. A definition of the purpose of each part will also be given.

What are the parts in the power end section of an Overhung ImpellerCentrifugal pump?The parts in the power end section consist of the following:A -Bearing housingB - Frame adapterC - ShaftD -BearingsE -Bearing protectionF -Oil sump areaG -Oil sight glassH - Power-end cooling jacketI -BreatherJ - Snap ringK -DeflectorL -Oil flingerM- Thrust bearing cartridge

Figure 26Power end components

GA C

KB

DD

L I

H F

J E

E M

•••••

•

•

•

•

•

•

•

•

••

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

bearing

housing?

What is

the function

of the

frame adapter?

What is

the function

of the shaft?

29

What is the function of the bearing housing?The bearing housing provides a body in which the bearings are mounted. This housing, technically, encompassesmany of the other components foundwithin the power end. The items listed below are often considered part of thebearing housing when it is discussed.However, each item is defined separately later in the course. • Oil sump• Breather• Bearing protection • Snap ring• Oil sight glass • Deflector• Power end cooling jacket• Oil flinger• Thrust bearing cartridge

Figure 27 Bearing housing (highlighted without other components)

Figure 28 Frame adapter cast integrally with the bearing housing

Figure 29Pump shaft

What is the function of the frame adapter?The frame adapter is a machined component used to permit assembly of the power end section to the wet end section of the centrifugalpump. The frame adapter can be a separate bolted-on piece or can be cast as part of the bearing housing.

What is the function of the shaft?The shaft is part of the rotating assembly used to transmit energy(power) from the driver (motor) to the fluid through rotation of the impeller. The shaft is the main component of the rotating assembly and is supported by the bearings.

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

bearings?

What is

the function

of the

thrust bearing

cartridge?

What is

the function

of the

snap ring?

30

What is the function of the thrust bearing cartridge?The thrust bearing cartridge provides an adjustable platform for the thrustbearing assembly. This cartridge allows for easier installation of the bearings and for performing impeller adjustments. It is becoming a standarditem found on many Overhung Impeller Centrifugal Pumps.

Figure 31Thrust bearing cartridge

Figure 32Snap ring

What is the function of the snap ring?The snap ring maintains the positioningof the thrust bearings to prohibit anyaxial movement of the bearing inside the bearing housing or thrust bearingcartridge. The snap ring is installed into a groove that is machined into the housing or cartridge. Sometimes the groove is machined in the outer race of the thrust bearing.

A drawback with using a snap ring in this location is that the groove can become worn and enlarged allowing excessive axial movement. This movement can cause problems with the thrust bearing and the mechanical seal leading toward premature failure.

Figure 30Pump thrust and radial bearings

What is the function of the bearings?The bearings function to keep the shaft or rotor in correct alignment with the stationary parts under theaction of radial or transverse loads.

Bearings that provide radial positioning are referred to as radial or line bearings. These bearings maintain the shaft alignment withrespect to movement up, down, and/or sideways. Radial bearings arealmost always located closest to theimpeller. This is because the majority of the radial loads occur inside the casing acting on the impeller.

Bearings that locate the rotor (shaft)axially are called thrust bearings. These bearings maintain the shaft alignment with respect to movementback and forth. In most applications the thrust bearings act as thrust andradial bearings. Thrust bearings are usually located closest to the coupling.

Radial Bearing

Thrust Bearing

•

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

bearing

protection?

31

Figure 33Lip seal installed on bearing housing

Figure 34 Examples of lip seals

What is the function of the bearing protection?Bearing protection functions to keep oil (lubricants) in the oil sump area and contaminants out.Contaminants include water, dirt, and any other abrasive material that can damage the bearings. Bearing protection is located on each end of the bearing housing. The three typical forms of bearing protection are lip seals, labyrinth seals, and face seals. Without this protection, the bearings would fail very quickly.

Lip seals are the most common and least expensive form of bearing protection. They have been in use since the early 1900’s. These seals use a “lip”, usually made from a rubber- like material, that rides on the shaft. The shaft actually rotates inside the lip.Unfortunately, their life expectancy is extremely short when used on pumps. Often lip seals installed onpumps fail in three to six months (2000 – 4000 hours) when used continuously.

Examples of Special Purpose Shells

Environment

GarterSpring

SealingElementPrimary

Sealing Lip

Bearing SideShell

Examples of External Designs

Open Packing Spring-Lock Spring-Cover

•

•

•

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

bearing

protection?

32

Labyrinth seals have also been around for a long time. However, their use on pumps has only evolvedextensively in the last 10 to 15 years.These seals are more expensive than lip seals, but they have virtually an infinite life. These seals are basically made up of two parts, a rotor and a stator. Under normal operation, they do not contact each other. The two parts acting together act like a series of turns to minimize contaminant entry.1. Stationary element:

Nickel plated steel2. Mounting ring: Secure mounting

without equipment alterations3. Positive O-rings: Non-slip,

will not fret or damage shaft4. Oil drain: Helps retain lubricant

in bearing housing5. Hooded design: Prevents direct

penetration of dripping fluids6. Labyrinth: Traps liquid

contaminants and directs to gravity drain

7. Rotary element: 316 stainless steel for corrosion resistance, acts as slinger when rotating

8. Gravity drain: Allows liquid trapped by labyrinth to safely exit

Figure 35 Typical labyrinth seal

Figure 36Typical face seal

Face seals are not very common. These seals have only recently beenintroduced to the marketplace. They act similarly to a mechanical seal. They cost about the same aslabyrinth seals, but have a finite life.However, this life can exceed 10 years when used correctly.A - Rotary faceB - Positive drive O-ringC - Seal housingD - RetainerE - Stationary faceF - SpringG - Secondary O-ringH - Excluder ring

52 •1 • •

6•

7•8•

4 •

3••

•

C•D•E•

G•B •

A • F•

H•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

deflector?

What is

the function

of the

oil sump area?

What is

the function

of the oil

sight glass?

33

Figure 37Typical deflector installed on radial bearing side of bearing housing

Figure 38 “Bulls-eye” oil sight glass

What is the function of the deflector?The deflector is a device located on the wet end side of the power end. The purpose of the deflector is to keep fluid from traveling down the shaft and entering through the bearing protection component, typically a lip seal. The deflector is supposed to “deflect” the fluid away from this bearing protectiondevice. Often, this device provides little additional protection for the bearings because it fails to deflect anything.

What is the function of the oil sump area?The oil sump area is where the oil is contained to provide lubrication and cooling for the bearings. Sometimes grease is used instead of oil for bearing lubrication. In this case the sump area is usually very small.

What is the function of the oil sight glass?The oil sight glass is used to indicate the level and condition of the oil contained in the oil sump area. The device comes in many forms. Two of the most common found on pumps are “Bulls-eye” and tube sight glasses.

A device that is often confused with the sight glass is the“constant- level”or bulb type oilers found on many pumps. These devices are designed to provide additional lubrication to the oil sump area in the event of an oil leak through the bearing protection, usually a lip seal. They provide no viewing of the level or condition of the oil found inside the pump.

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPump


What is

the function

of the

breather?

What is

the function

of the

oil flinger?

What is

the function

of the

power end

cooling jacket?

34

What is the function of the breather?As the shaft rotates in the enclosed oil sump area, the air heats up and expands. The breather acts as a small vent allowing the air from the enclosed oil sump area to escape. Air also re-enters the oil sump area when the pump stops due to its cooling.

Figure 39Typical oil sump breather

Figure 41Power end cooling jacket

Figure 40 Oil flinger

What is the function of the oil flinger?The oil flinger is a device attached to the shaft that is located inside the oil sump area. The oil flinger distributes the oil throughout the bearing housing to assist in the lubrication of the bearings.

What is the function of the power end cooling jacket?The power end cooling jacket is a separate chamber cast into the bearing housing used to control the temperature of the oil in the oil sump area. This chamber is often located on the bottom of the housing, but could also be cast circumferentially around it.

The chamber is fed with a heat transfer medium, usually water, to cool the oil. Oil will get hot for many reasons. The rotation of the shaft and bearings add heat to the oil. However, the most common reason a cooling jacket is used is because the pumped fluid is at a high temperature. The fluid adds additional heat to the oil. As oil heats up its lubrication qualities break-down resulting in premature bearing failure.

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalPumpDriver

Parts andFunction

What are

the parts in

the driver

section of

an Overhung

Impeller

Centrifugal

pump?

What is

the function

of the motor?

35

Figure 42 A typical “squirrel -cage” induction motor

Overhung Impeller Centrifugal Pump Driver Parts and Function

The pump driver is the third section we will discuss. This lesson will identifyand describe the parts found in the driver section of an Overhung ImpellerCentrifugal Pump. A definition of thepurpose of each part will also be given.

What are the parts in the driver section of an Overhung ImpellerCentrifugal pump?The parts in the driver section can consist of the following:• Motor (Driver)• Coupling• Motor adapter• Belts• Gears

The driver section need not contain all of the items listed above. As a minimum,a driver (usually a motor) is required. The coupling, belts and gears are power transmission devices that may or may not be required with the pump.

What is the function of the motor?Energy is required to move the fluid.Many types of drivers are used to provide this energy. The motor is, by far,the most common device used to drivepumps. It is an electrical component that provides the input energy (power)being transferred through the powerend to the fluid being pumped.Motors come in a varying array of types,sizes, enclosures and insulation classes.However, the motor that is most oftenused is the “squirrel-cage” inductionmotor.

CHAPTER 2

PUMP

PARTS

OverhungImpeller


Parts andFunction

What is

the function

of the

coupling?

36

What is the function of the coupling?A coupling is a power transmission device that is used to connect the motor (driver)shaft to the power end shaft of the pump. The primary purpose of a coupling is to transmit rotary motion and torque from the motor to the pump. Couplings often are required to perform other secondary functions as well. These other functions include accommodating misalignment between shafts, transmitting axial thrust loads from one machine to another, permitting adjustment of shafts to compensate for wear and maintaining precise alignment between connected shafts.

Many times pumps use couplingsinstalled with a spacer. A spacer coupling allows the pump to be disassembled without moving piping, the pump casing or motor. A typical installation showing a spacer coupling is shown below.

Figure 43A coupling

Figure 44 A typical pump disassembly using a spacer coupling

Spacer

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller


Parts andFunction

What is

the function

of the motor

adapter?

What is

the function

of the belts?

37

Figure 45 A motor adapter

Figure 46 Typical belt drive

What is the function of the motor adapter?The motor (driver) adapter is a machined component used to assemblethe motor to the power end section of the pump. Its primary purpose is to allow easier pump and motor alignment, maintain this alignment, and compensate for thermal growth.This adapter is required for most vertical pump applications and is now becoming more in demand for horizontal applications.

What is the function of the belts?The belts are power transmission devices that are usually used to change the speed (higher or lower)of the driver. They are connected to the motor and pump power endshafts by the use of a sheave on each shaft. Sometimes belts are used, not to change the driver speed, but to supply power and torque from a location not directly behind the pump (i.e.,above or to the side of the pump).

Belts come in many shapes, sizes andtypes depending on the application.Often belts are used in multiples for larger pump and motor applications.

Figure 47Typical belts

CHAPTER 2

PUMP

PARTS

OverhungImpeller


Parts andFunction

What is

the function

of the

gears?

38

What is the function of the gears?Gears are power transmission devicesoften used to change the speed of the driver. Most times the gears, and gearboxes, are used to reduce the speed to an applicable level.Sometimes, however, gears are

employed to increase the speedof the pump because it is required to run at a speed higher than the driver.Many types of gears are used withpumps. Some gear types and typicalgear drives are shown below.

Figure 49High-speed parallel -shaft gear drive

Figure 50Cross-section of a spiral-bevel vertical pump drive

Figure 48 Various gear types

SPUR HELICAL

STRAIGHT TOOTH BEVEL

ZEROL WORM GEAR SETS

SPIRAL BEVEL

DOUBLE HELICAL

CONTINUOUS HERRINGBONE

HYPOID

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalSupport

StructureParts andFunction

What are

the parts

in the support

structure of

an Overhung

Impeller

Centrifugal

pump?

39Figure 51Pump support structure showing foundation, baseplate, grout (in grout hole), shims (under motor feet) and jack-bolts

Overhung Impeller Centrifugal Pump Support Structure Parts and Function

The pump support structure is the fourth, and final, section we will discuss. This lesson will identify and describe each component found in the pump support structure of anOverhung Impeller Centrifugal pump. A definition of the purpose of each part will also be given.

What are the parts in the support structure of an Overhung ImpellerCentrifugal pump?The parts in the support structure consist of the following:• Baseplate (Bedplate)• Foundation• Grouting• Jack-bolts• Shims

Not all of the above parts are required for pump installation. Pumps are usually mounted on the baseplate which, in turn, ismounted securely to the foundation. These two parts, along with the shims for alignment, are most often the minimum structure setup used. However, good installation and maintenance practices grout the baseplates and provide jack-bolts for alignment.

Baseplate Grouting

Foundation

ShimsJack-bolts

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalSupport


What is

the function

of the

baseplate?

What is

the function

of the

foundation?

What is

the function

of the

grouting?

40

What is the function of the baseplate?The baseplate is the structure used to furnish mounting surfaces for the pump and motor feet so they can be rigidly attached to the foundation. Baseplates are usually made of metal, but sometimes composite non-metallic materials are used for corrosion resistance. Metal baseplates can be cast in almost any shape or can be made of heavy-gauge channel. All baseplates should be milled flat and machined to avoid any pump alignment problems.

What is the function of the foundation?The foundation is the structure to which the pump, motor and baseplate are mounted. It is usuallymade of reinforced concrete. Often times the bolts used to secure the baseplate are installed before the concrete is poured. This ensures that the baseplate, pump and motor will not move.

A foundation’s mass should be at least 3 to 5 times the total mass of the pump, motor, and baseplate to be effective. When the foundation is large enough, it provides a permanent rigid support to the full area of the baseplate and absorbs any strains or shocks. A pump baseplate supported by “stilts” or “legs” is not very secure and will over time cause more operational and maintenance problems than if it had a proper foundation.

What is the function of the grouting?The purpose of grouting is to prevent lateral shifting of the baseplate, to increase its mass and thereby reduce vibration, and to fill in irregularities in the foundation.Grout is a cement-like substance that solidifies the connection between the baseplate and the foundation. It should be of a consistency to flow freely under the baseplate filling in the cavity completely. Grout can be made of a variety of materials. At the very least, it should be a corrosion resistant, non-shrinking, non-expanding, epoxy cement.

Figure 52Cross-section of foundation and baseplate showing proper grouting procedures

Dam LugWasherPipe Sleeve

Allow 3/4”

to1-1/2” for Grout

FinishedGrouting

•

BaseplateLevelingWedges orShims leftin place

Top of Foundation left Rough – Clean and Wet down

•

•

•

•

•• •

•

CHAPTER 2

PUMP

PARTS

OverhungImpeller

CentrifugalSupport


What is

the function

of the

jack-bolts?

What is

the function

of the

shims?

41

What is the function of the jack-bolts?Jack-bolts are specially designed bolts mounted on the baseplate near the motor feet. They are used to properly position and maintain themotor (driver) laterally in the correctalignment position. These bolts are often not installed during the initial pump installation, but can be retrofitted later if necessary.

Jack-bolts can easily and smoothly move a motor the required smallamount for proper alignment. They eliminate the need to use sledge hammers and bars for moving heavy, large motors.

What is the function of the shims?Shims are thin pieces of metal (usually stainless steel) placed under the feet of the motor. They are used to properly position and maintain motor (driver) height in the correct alignment position. Shims can come as sheets that are cut as needed or as pre-cut sets. The pre-cut sets come in various sizes depending on the bolt and motor feet size. Examples of sizes and sets are shown in Figures 54 and 55.

Figure 54Typical kit of pre-cut shims

Figure 53Close-up view of jack-bolts

Figure 55 Typical sizes for pre-cuts shims

D

C

B

A.015

Slot SizesA = 5/8”B = 3/4”

C = 1-1/4”D = 1-5/8”

CHAPTER 2

PUMP

PARTS

Split caseCentrifugalPump PartDifferences

Overhung

Impeller and

Impeller

Between

Bearings

Centrifugal

Pump

differences?

42

Split Case Centrifugal Pump Part Differences

Although we have focused mainly on the Overhung ImpellerCentrifugal pump, let’s look at another common kinetic pump that is found in industry. This other pump is an Impeller Between Bearings pump or sometimes called Split Case pump. It operates similarly to the Overhung Impeller pump, but has some distinct differences. This lesson will describe the main differences between the OverhungImpeller and Impeller Between Bearings Centrifugal Pumps.

Overhung Impeller andImpeller Between BearingsCentrifugal Pump differencesThe functions of the Overhung Impeller and Impeller Between Bearings Centrifugal pumps are primarily the same. Most of the differences are a result of the mechanical design and configuration of each pump type.

Overhung Impeller Centrifugal pumps have the impeller located at one end of the pump shaft. This configuration consists of one stuffing box area where the shaft enters the volute and one set of bearings that support the impeller from the opposite end of the shaft. This design was previously discussed in this chapter.

Impeller Between Bearings (Split Case) Centrifugal pumps have the impeller located in the center of the pump and supported by bearings on each side. These pumps can be split axially (horizontally) or radially (vertically). With this design, there are two stuffing box areas where the shaft enters the volute. There is no separate power end area as found in the Overhung Impeller design because the bearings are on both sides of the casing.

Figure 56Impeller between bearings (split case) centrifugal pump

Stuffing Box

•

Stuffing Box

Suction Eyes

Bearings

•

• •

•

Bearings

•

Closed Impeller

•

CHAPTER 2

PUMP

PARTS

Split caseCentrifugalPump PartDifferences

Overhung

Impeller and

Impeller

Between

Bearings

Centrifugal

Pump

differences?

Review

Questions

1 through 5

43

Figure 57 Multiple-stage pump with three stages (impellers)

The split case design offers a largeamount of flexibility when more than one impeller is required. Because of this fact, most multiple-stage (multiple impeller) pumps are designed and built using the ImpellerBetween Bearings configuration.

Another common name for ImpellerBetween Bearings Centrifugal pumps is“Double Suction” pumps. This name is often used because theimpeller is a closed impeller with twosuction eyes, one on each side, hencethe name Double Suction pump.

Review Questions1. Which is not part of the four

sections in an Overhung Impeller Centrifugal Pump configuration?a. Coupling.b. Wet end.c. Baseplate/Foundation.d. Driver.

2. What part is not found in the wet end section of an Overhung Impeller Centrifugal pump?a. Casing.b. Deflector.c. Impeller.d. Lantern ring.

3. Which part listed below is not one the parts in the power end section of an Overhung ImpellerCentrifugal pump?a. Shaft.b. Bearings.c. Backcover.d. Oil sump.

4. Baseplates should be machined and milled flat to avoid pump alignment problems.a. True.b. False.

5. The Overhung Impeller Centrifugal Pump and the Split Case pump

a. are both configured the same.b. are both end suction pumps.c. are both kinetic pumps.d. All of the above.


Impellers

• • •

CHAPTER 3

PUMP

TERMS

44

CHAPTER 3

PUMP

TERMS

Introduction

Objective

PressureTerms

What is

pressure?

What is

atmospheric

pressure?

45

Chapter 3 Pump Terms IntroductionAs with any subject, there are essentialterms that we must understand in order to discuss the subject. Learning about pumps is no different.This chapter will define the proper terminology and nomenclature used in describing pumps and pumping systems and how they interact with one another. Without this knowledge of terms, it is very difficult to discusspump operation and systems.

This lesson discusses the subject of pressure. Pressure is a term that hasmany meanings to different people.Besides defining pressure, we will discussthe other types of pressure terms thatrequire clarification. It is important that we define these various pressureterms and their differences in order to understand pump operation.

What is pressure?Pressure is defined as a force acting over an area. Pressure, when applied to a surface such as a fluid, acts in alldirections equally. Pressure will movethrough the path of least resistance.Pressure is often expressed in pounds per square inch (psi) or bar.

What is atmospheric pressure?Atmospheric pressure is the force exerted on an area by the weight of the atmosphere. Atmospheric pressurewill be different in Denver, Colorado than it is in Miami, Florida. The reason for this is due to the elevation with respect to sea level. Places closer to sea level will have moreof the atmosphere weighing down onthem and hence, more pressure.Atmospheric pressure at sea level is defined as 14.69 pounds per square inch (psi) or 1.013 bar.

ObjectiveUpon completion of this chapter you will be able to define pressure terms, fluid terms and pressure and head conversions. You will also be ableto describe how temperature affectsthese terms and what effect they have on pump operation.

Pressure Terms

CHAPTER 3

PUMP

TERMS

PressureTerms

What is

absolute

pressure?

What is

gauge

pressure?

What is

vacuum?

46Figure 1Absolute and gauge pressure comparisons

What is vacuum?Vacuum is the term used to express any pressure below atmospheric pressure. Vacuum is typically expressed in inches or millimeters (mm) of mercury (Hg).Atmospheric pressure at sea level is 29.92 inches or 760 mm of Hg.Pressure below 0 psig or 0 bar-gauge is considered a vacuum. As the number of inches or mm of Hg rises the pressure is actually falling.

The amount of vacuum that can be generated depends on the total atmospheric pressure. For example, the most vacuum you can have on the earth is 29.92 inches or 760 mm of Hg. The atmospheric pressure on Venus is 1330 psi or 92 bar. You can have a tremendous vacuum on Venus. Conversely, the atmospheric pressure on Mars is 0.14 psi or 0.01 bar. Not much vacuum can be generated there.

What is absolute pressure?Absolute pressure is the true total pressure. Absolute pressure is the sum of gauge pressure and atmospheric pressure. Another way of defining absolute pressure is, the amount by which the measured pressure exceeds a perfect vacuum.Absolute pressure is measured in units of psia or bar abs.

What is gauge pressure?Gauge pressure is the difference between a given fluid pressure and that of the atmosphere. Gauge pressure is measured by a mechanical or electrical device known as a pressure gauge. Another way of stating gauge pressure is, the amount by which the measured pressure is greater than atmospheric pressure. Gauge pressure is measured in units of pounds per square inch – gauge (psig) or bar – gauge (barg)

Absolute Pressure Gauge Pressure

Atmospheric Pressure at Sea Level

0” or 0 mm Hg Vacuum(Normally read on Pressure Gauge)

30” or 760 mm Hg Vacuum(Full or Perfect Vacuum)

0 psia0 bar abs

14.7 psia1.013 bar abs

0 psig0 barg

CHAPTER 3

PUMP

TERMS

PressureTerms

What is

vapor

pressure?

47Figure 2Typical vapor pressure curves of various chemicals

-40 -20 0 20 50 100 150 20010080

60

40

20

108

6

4

2

1.0

0.8

0.6

0.4

0.2

0.1

-50 0 50 100 150 200 250 300 400

What is vapor pressure?Vapor pressure is the pressure at which a liquid will flash into a vapor at a given temperature. Vapor pressure is measured in absolute pressure units, such as psia or bar-absolute. At 60°F (15.5°C), water has a vapor pressure of 0.256 psia (0.018 bar abs). At 212°F (100°C), water’s vapor pressure has increased to 14.69 psia(1.013 bar abs), therefore it boils under atmospheric pressure. Anotherway of defining vapor pressure is the amount of pressure exerted on a liquid to keep it liquid.

TEMPERATURE (°C)

SULPH

UROUS ACID

ETHER

CARBON DISU

LPHIDE

ACETONE

FORM

IC A

CID

MET

HYL A

LCOHOL

ETHYL

ALC

OHOL

TOLU

OL

ACETIC

ACID

TURP

ENTIN

EPH

ENO

L

CARBON

TETR

ACHLORID

E

BENZO

LE

TEMPERATURE (°F)

VAPO

R PR

ESSU

RE (

PSIA

)

CHAPTER 3

PUMP

TERMS

FluidTerms

What is

hydraulics?

What is

density?

48

What is density?Density is sometimes referred to as specific weight. It describes the weight per unit volume of a substance. Water has a density of 8.34 pounds per gallon at 14.69 psia and at 60°F. In the metric system, water’s density is 1.0 kg per liter at 1.013 bar and 15.5°C.

Figure 3 A typical hydraulic pumping system

Fluid Terms

Fluids have certain physical properties to describe themselves. Terms like weight, density and viscosity have different meanings to different people. This lesson will define typical fluid terms used in industry and clarify the differences among them.

What is hydraulics?Hydraulics is the study of fluids at rest or in motion. Fluids include both liquids and gases. When referring to pump hydraulics we are referring only to liquids.Centrifugal pumps cannot pump gases, vapor or liquids with entrained gases very well.

CHAPTER 3

PUMP

TERMS

FluidTerms

What is

specific

gravity?

49Figure 4Illustration of how liquids with different specific gravities and the same column height produce different pressures at the bottom

Specific gravity of a liquid affects pressure relative to the height of the liquid. The power requirementchanges as specific gravity changes.Common sense will tell you that the heavier a fluid is, more energy will be required to pump it.

Specific gravity should not be confused with viscosity. Viscosity is a measure of a liquid’s resistance to flow. Specific gravity and viscosity are both properties of liquids, but there is no correlation between them.

What is specific gravity?Specific gravity is the density ratio of a liquid as compared to water at a given temperature. Water is used as the standard at 14.69 psia (1.013 bar abs) and at 60°F (15.5°C). Its specific gravity is 1.0 at this standard temperature and pressure. Because specific gravity is a ratio of the same property(i.e.,density), it has no units.

If a liquid has a specific gravity ratiogreater than 1.0, it will sink in waterbased on the standard. If a liquid has a specific gravity less than 1.0, it will float in water based on the standard.

Pressure36.7 psi2.5 bar



CausticS.G. 1.5

WaterS.G. 1.0

GasolineS.G. 0.85

100’30 m

SecondsFordCup #4

31 1.00 – 29.0 – 1.00 6200.00 – – – – – –35 2.56 – 32.1 – 1.16 2420.00 – – – – – –40 4.30 – 36.2 5.10 1.31 1440.00 – – – – – –50 7.40 – 44.3 5.83 1.58 838.00 – – – – – –

60 10.30 – 52.3 6.77 1.88 618.00 – – – – – –70 13.10 12.95 60.9 7.60 2.17 483.00 – – – – – –80 15.70 13.70 69.2 8.44 2.45 404.00 – – – – – –90 18.20 14.44 77.6 9.30 2.73 348.00 – – – – – –

100 20.60 15.24 85.6 10.12 3.02 307.00 – – – – – –150 32.10 19.30 128.0 14.48 4.48 195.00 – – – – – –200 43.20 23.50 170.0 18.90 5.92 144.00 40.0 – – – – –250 54.00 28.00 212.0 23.45 7.35 114.00 46.0 – – – – –

300 65.00 32.50 254.0 28.00 8.79 95.00 52.5 15 6.0 3.0 30 30400 87.60 41.90 338.0 37.10 11.70 70.80 66.0 21 7.2 3.2 42 28500 110.00 51.60 423.0 46.20 14.60 56.40 79.0 25 7.8 3.4 50 34600 132.00 61.40 508.0 55.40 17.50 47.00 92.0 30 8.5 3.6 58 40

700 154.00 71.10 592.0 64.60 20.45 40.30 106.0 35 9.0 3.9 67 45800 176.00 81.00 677.0 73.80 23.35 35.20 120.0 39 9.8 4.1 74 50900 198.00 91.00 762.0 83.00 26.30 31.30 135.0 41 10.7 4.3 82 57

1000 220.00 100.70 896.0 92.10 29.20 28.20 149.0 43 11.5 4.5 90 62

1500 330.00 150.00 1270.0 138.20 43.80 18.70 – 65 15.2 6.3 132 902000 440.00 200.00 1690.0 184.20 58.40 14.10 – 86 19.5 7.5 172 1182500 550.00 250.00 2120.0 230.00 73.00 11.30 – 108 24.0 9.0 218 1473000 660.00 300.00 2540.0 276.00 87.60 9.40 – 129 28.5 11.0 258 172

4000 880.00 400.00 3380.0 368.00 117.00 7.05 – 172 37.0 14.0 337 2305000 1100.00 500.00 4230.0 461.00 146.00 5.64 – 215 47.0 18.0 425 2906000 1320.00 600.00 5080.0 553.00 175.00 4.70 – 258 57.0 22.0 520 3507000 1540.00 700.00 5920.0 645.00 204.50 4.03 – 300 67.0 25.0 600 410

8000 1760.00 800.00 6770.0 737.00 233.50 3.52 – 344 76.0 29.0 680 4659000 1980.00 900.00 7620.0 829.00 263.00 3.13 – 387 86.0 32.0 780 520

10000 2200.00 1000.00 8460.0 921.00 292.00 2.82 – 430 96.0 35.0 850 575

15000 3300.00 1500.00 13700.0 – 438.00 2.50 – 650 147.0 53.0 1280 86020000 4400.00 2000.00 18400.0 – 584.00 1.40 – 860 203.0 70.0 1715 1150

SecondsSayboltUniversalssu

SecondsSayboltFurolssf

CHAPTER 3

PUMP

TERMS

FluidTerms

What is

viscosity?

50

SecondsParlinCup #20


DegreesBarbey

SecondsRedwood2(Admiralty)

There are two basic viscosity parameters, dynamic (absolute)viscosity and kinematic viscosity. Dynamic viscosity is usually measured in terms of centipoise (cP)in both English and metric systems. Kinematic viscosity is equal to the dynamic viscosity divided by the fluid’s specific gravity. This is true only when the kinematic viscosity is measured in centistokes (cSt) and the dynamic viscosity is measured in cP. Again, the cSt is used in both the English and metric systems.

Water at a temperature of 68°F (20°C)has a kinematic viscosity of 1cSt. Viscosity is not related to the weight,density or specific gravity of the fluid.

KinematicViscosity (cSt)

=

DynamicViscosity (cP)

SpecificGravity

Figure 5Viscosity comparison table

What is viscosity?Viscosity is the measure of a fluid’s resistance to flow under an applied force at a given temperature. Viscositycan be thought of as the “thickness”of a fluid as it moves due to this force.

The majority of liquids found in industryare considered true or Newtonian fluids.This is because their viscosity remainsconstant at a given temperature andpressure. Examples of Newtonian fluidsare water and mineral oil.

Other liquids, such as molasses, grease, starch and paint act differently from Newtonian liquids. The viscosity of these liquids does notremain constant at a given temperatureand pressure. These liquids exhibit lower viscosity as they are agitated at a constant temperature and are called thixotropic fluids.

Still other liquids, such as clay slurries,candy mixtures and quicksand show an increase in viscosity as they are agitated and are called dilatant fluids.

KinematicViscositycSt

SecondsRedwood1(Standard)

SecondsParlinCup #7


SecondsFordCup #3

DegreesEngler

CHAPTER 3

PUMP

TERMS

FluidTerms

What is

capacity?

What is

head?

51

What is head?Head is a pump term often used to describe the mechanical energy added to the fluid by centrifugal force. It is the quantity used to express the energy content of the liquid per unit weight of the liquid.

Head is a good term to use with centrifugal pumps because they are constant energy devices. This means that for a given pump operating at a certain speed and handling a definite fluid volume, the energy transferred to this fluid (in foot- lbs. per foot or Newtons per meter of fluid) is the same for any fluid regardless of density.

The head generated by a given pump at a certain speed and capacity will remain constant for all fluids, barring any viscosity effects. Therefore, head when applied to centrifugal pumps is commonly expressed in feet (or meters) of liquid. Head can also be used to represent the vertical height in feet (or meters)of a static column of liquid.

There are many other units for kinematic viscosity. The most common of which is the Seconds Saybolt Universal (SSU). For values of 70 cSt or above, the following conversion applies:

SSU = cSt x 4.63

Other units are shown in the table in Figure 5.

Centrifugal pump performance is greatly affected by viscosity. When a fluid’s viscosity is greater than water, (more than 1 cSt.), the pump performance will change. Common sense will tell you that the “thicker” the fluid is, the more energy is required to pump it.

What is capacity?Capacity is defined as the volume of liquid per unit time delivered by the pump. It can also be described as the volumetric flowrate of the fluid being transferred. The most common units for capacity are usually gallons per minute (GPM) or cubic meters per hour. Centrifugal pumps do not offer a large amount of flexibility in capacity variations without affecting the pump efficiency. Capacity is often designated by the letter “Q” in current nomenclature.

When specifying capacity requirements for a centrifugal pump a range of capacities should be stated. Minimum and maximumcapacity limits are very important. These limits ensure proper pump operation, both mechanically and hydraulically.

Figure 6Head

100 Feet

CHAPTER 3

PUMP

TERMS

Pressure/Head

Conversions

How do you

convert

pressure

to head?

52

Figure 7Pressure vs. head relationship using water

Figure 8Different view of the pressure vs. head relationship using water

Weight of water12 in x 12 in = 144 in2

62.4 lbs/144 in2 = 0.433 psi

Weight of water2.31 feet of head of water equals 1 psi

Pressure/Head Conversions

Pressure and head are two pump terms that are related by a constant. This lesson will identify the conversionsneeded to complete head and pressure calculations.

How do you convert pressure to head?To explain the pressure vs. head relationship we will use a very wellknown liquid… water.

In English units:• 1 cubic foot of water

weighs 62.4 lbs.• 1 cubic foot of water

displaces 144 square inches• 62.4 divided by 144 = 0.433• Therefore 1 ft. of water creates

0.433 psi of pressure or 2.31 ft. of water creates 1 psi pressure.

In the metric system:• 1 cubic meter of water

weighs 1000 kg.• 1 cubic meter of water

displaces 10,000 square cm.• 1000 divided by 10,000 = 0.1.• Therefore 1 m of water creates

0.1 kg/cm2 of pressure or 10 m of water creates 1 kg/cm2 pressure.

Pressure can be converted to head by the equations:

0.433 psi

1 psi

1ft

1ft

2.31ftof head

1ft

1ft

1ft

Pressure (psi) x 2.31Specific Gravity

Head (ft.) =

Pressure (kg/cm2) x 10Specific Gravity

Head (m) =

Pressure (bar) x 10.2Specific Gravity

Head (m) =

Illustrations of the above relationshipsare shown graphically.

CHAPTER 3

PUMP

TERMS

Pressure/Head

Conversions

How do you

convert

head to

pressure?

53

Figure 9Pressure vs. head relationship using water

Figure 11Pressure to head conversion equation

Figure 13 Head to pressure conversion equation

Figure 12Pressure to head conversion equation (metric)

Figure 14Head to pressure conversion equation (metric units)

Figure 10 Different view of the pressure vs. head relationship using water (metric units)

Weight of water100 cm x 100 cm = 10,000 cm2

1000 kg/10,000 cm2 = 0.1 kg/cm2

Weight of water10 m of head of water = 1 kg/cm2

How do you convert head to pressure?Using basic algebraic substitutions, head can be converted to pressure by the equations:

Pressure (psi) =

Pressure (kg/cm2) =

bar =Meters x S.G.

10.2

Psi =Feet x S.G.

2.31

1 kg/cm2

1m

1m

10 mof head

0.1 kg/cm2

100 cm

100 cm

100 cm

Head (ft) x Specific Gravity2.31

Head (m) x Specific Gravity10

Pressure (bar) =

Head (m) x Specific Gravity10.2

Feet =PsiS.G.

x 2.31

Meters =bar.S.G.

x 10.2

CHAPTER 3

PUMP

TERMS

TemperatureEffects

How does

temperature

affect

specific

gravity?

54Figure 15 Specific gravity vs. temperature chart for various liquids

Temperature Effects

Temperature is very important when discussing the terms we have learned. It can greatly affect any or all of the physical properties of a fluid. This lesson will describe the effect that temperature has on pump operation.

How does temperature affect specific gravity?A fluid’s specific gravity will vary inversely with temperature. Typically, a fluid’s specific gravity will decrease with increasing temperatures and vice versa.

The chart below shows how temperature affects the specific gravity of various liquids.

CARBON TETRACHLORIDESULPHUROUS ACID

CARBON DISULPHIDE

FORMIC ACID

HEAVY WATER

ACETIC ACID

BENZOLELHYL-METHYL ALCOHOL

PHENOL

ANILINETOLUOL

ACETONE

ETHER

TEMPERATURE (°C)

TEMPERATURE (°F)

SPEC

IFIC

GRA

VIT

Y

CHAPTER 3

PUMP

TERMS

TemperatureEffects

How does

temperature

affect

viscosity?

55Figure 16 Viscosity vs. temperature chart for various liquids. Specific gravities shown are for 60°F (15.6°C)

How does temperature affect viscosity?A fluid’s viscosity will vary inversely with temperature. Typically, a fluid’s viscosity will decrease with increasing temperatures and vice versa.

This explains why most fluids flow faster and easier when they are heated. A chart showing how temperature affects viscosity of various fluids is shown below.

STEAM CYLINDER OIL APPROX. 0.80MOLASSES 1.42

CASTOR OIL 0.96CRUDE OIL 0.925

RAPE OIL 0.93

GAS OIL 0.89

KEROSENE 0.79

20% NaCI 1.16

GASOLINE 0.716

GASOLINE 0.784

GASOLINE 0.68

WATER 1.00

ETHYL ALCOHOL

DOWTHERMA A 1.056

CRUDE OIL 0.855

TEMPERATURE (°C)

TEMPERATURE (°F)

KIN

EMA

TIC

VIS

CO

SITY

(cS

t)

VIS

CO

SITY

(SS

U)

CHAPTER 3

PUMP

TERMS

TemperatureEffects

How does

temperature

affect

vapor pressure?

56Figure 17 Vapor pressure vs. temperature chart for various liquids

How does temperature affect vapor pressure?Every fluid has its own unique vapor pressure curve where the vapor pressure is plotted in relation to temperature. As temperaturesincrease, the vapor pressure of the fluid also increases. This means that as a fluid’s temperature increases, it requires more pressure to keep it from boiling and remain liquid. A chart showing the affects of temperature on the vapor pressure of various liquids is shown below.

TEMPERATURE (°F)

ABS

OLU

TE P

RESS

URE

– P

SIA

GA

UG

E PR

ESSU

RE –

PSI

GVA

CU

UM

– IN

CH

ES O

F M

ERC

URY

CARBON DIOXIDE

ETHANE

MONOCHLOROTRIFLU

OROMETHANE HYDROGEN SULFIDE

PROPYLENE

METHYL CHLORIDE

BUTANE

ETHYL C

HLORIDE

METHYL F

ORMATE

DICHLOROET

HYLENE

CARBON TE

TRACHLO

RIDE

TRIC

HLORO

ETHYL

ENE

WATE

R

ACETONE

CHAPTER 3

PUMP

TERMS

Review

Questions

1 through 11

57

Review Questions1. Pressure acts in one direction only.

a. True.b. False.

2. Density is thea. weight per cubic centimeter

of a substance.b. weight per unit volume

of a substance.c. weight per gallon of

a substance.d. All of the above.

3. Specific gravity of a liquid will not affect pressure.a. True.b. False.

4. A fluid’s viscosity is affected only by temperature.a. True.b. False.

5. Capacity is defined as the velocity of the fluid being transferred.a. True.b. False.

6. Barring any viscosity effects, the head generated by a given pump at a certain speed and capacity will

a. increase as the fluid specific gravity increases.

b. remain constant for all fluids.c. decrease as the fluid density

decreases.d. change with fluid density.

7. Pressure can be converted to head by the following equations:

8. Which equation is correct?

9. Typically, a fluid’s specific gravity will increase with increasing temperatures.a. True.b. False.

10. Typically, a fluid’s viscosity will increase with increasing temperatures.a. True.b. False.

11. As temperatures decrease, the vapor pressure of the fluid increases.a. True.b. False.


Pressure (psi) x SG

2.31a. Head (ft.) =

Pressure (bar) x SG

10.2Head (m) =

Pressure (psi) x 2.31

Specific Gravityb. Head (ft.) =

Pressure (bar) x10.2

Specific GravityHead (m) =

Pressure (psi) x SG

23.1d. Head (ft.) =

Pressure (bar) xSG

1.02Head (m) =

Pressure (psi) x 23.1

Specific Gravityc. Head (ft.) =

Pressure (bar) x1.02

Specific GravityHead (m) =

a. Pressure =

b. Pressure (psi) =

Head (ft) x SG

2.31c. Pressure

(bar) = Head (m) x SG

10.2

Head x SG

Constant

d. All of the above.

CHAPTER 4

PUMP

OPERATION

58

CHAPTER 4

PUMP

OPERATION

Introduction

Objective

Fluid Flow

What is

the pump

suction?

59

Chapter 4 Pump Operation IntroductionA centrifugal pump moves fluid from one point to another, but how? This chapter will look at pump operation by describing how fluid flows through a pump and how a pump changes energy to perform this fluid flow. Understanding how a pump operates is the essential first step to learning pump and system interaction.

We will now begin our study of pump operation. This lesson will define how fluid flows into, through and out of a pump. The various pump wet end areas will also be discussed.

What is the pump suction?The pump suction is the inlet or flange area where the fluid enters the volute (casing). It can be oriented horizontally or vertically, depending on pump configuration.

ObjectiveUpon completion of this chapter you will be able to describe pump operation by identifying the fluid flow and energy conversion that takes place within a pump.

Figure 1The pump suction or inlet side mounted horizontally

Fluid Flow

POINT OF LOWEST PRESSURE WHERE VAPORIZATION STARTS

Figure 3 The pump discharge or outlet side

CHAPTER 4

PUMP

OPERATION

Fluid Flow

What is

suction

pressure?

What is

the pump

discharge?

60 Figure 2Pump pressure profile showing higher pressure outside the pump suction flange and a common gauge location for measuring suction pressure

What is the pump discharge?The pump discharge is the outlet orflange area where the fluid leaves thevolute (casing). The discharge flange is usually oriented up (or vertically), but can also be mounted sideways (or horizontally) if the application requires it.

What is suction pressure?Suction pressure is the actual pressure,positive or negative, at the pump suction connection as measured on a gauge. Pumps do not “suck” fluid as the pump suction name implies. The higher pressure outside the volute in the suction piping moves to the lower pressure area, at the impeller eye, resulting in available fluid for the impeller.

An example of this would be a straw in a drink. As the air is removed from thestraw (creation of a low pressure area),atmospheric pressure pushes the liquidup the straw and into the mouth.

A pump pressure profile showing the suction pressure is shown in Figure 2.

INCR

EASI

NG

PRE

SSU

RE

A

C

D

E

BA

POINTS ALONG LIQUID PATH

ENTRANCELOSS

FRICTION

TURBULENCEFRICTION

ENTRANCELOSS AT

VANE TIPS

INCREASINGPRESSURE

DUE TOIMPELLER

B C D E

Suction pressure

•

•

•

CHAPTER 4

PUMP

OPERATION

Fluid Flow

What is

discharge

pressure?

How does

a fluid pass

through

a pump?

61Figure 5Illustration of fluid velocity profile as it enters the impeller eye and exits the volute

What is discharge pressure?Discharge pressure is the actual pressure at the pump discharge connection as measured on a gauge. This pressure is equal to the pump suction pressure plus the total head (pressure)developed by the pump.

A pump pressure profile highlighting the higher discharge pressure is shown in Figure 4.

Figure 4Pump pressure profile showing common discharge pressure gauge measurement location

How does a fluid pass through a pump?Fluid flows through the pump by first entering the suction flange. After passing through the suction flange,the fluid enters the lowest pressure areain the pump, the impeller eye.

From here the fluid is picked up by the spinning impeller vanes. The fluid passes along the vanes wherevelocity and energy are added to it.

Through centrifugal force, the fluid is thrown to the outside tips of theimpeller, against the volute and towardthe discharge flange. At this point,because the fluid is confined by thevolute, the velocity decreases therebyincreasing the pressure (head).

The fluid velocity is decreasing becausethe volute is shaped in such a way thatthe impeller is not centered inside it. Rather, the impeller is offset from thecenter. This offset causes the impeller to volute clearance to increase from the cutwater to the discharge area. As the clearance increases the velocity decreases and the pressure increases.

The final act inside the pump is for thefluid to move through the inside edge ofthe volute to the discharge flange whereit exits the pump at a higher pressure.

INCR

EASI

NG

PRE

SSU

RE

A


ENTRANCELOSS

FRICTION

TURBULENCEFRICTION

ENTRANCELOSS AT

VANE TIPS

INCREASINGPRESSURE

DUE TOIMPELLER

B C D E

POINT OF LOWEST PRESSURE WHERE VAPORIZATION STARTS

C

D

E

BA

Discharge pressure

High velocityLow pressure

Impellereye

Rotation

Low velocityHigh pressure

•

•

•

•

•

•

CHAPTER 4

PUMP

OPERATION

Fluid EnergyConversion

What is

energy?

What is

velocity energy?

What is

pressure

energy?

How does

the energy

conversion

take place

within

a pump?

62

Fluid Energy Conversion

Now that we know how fluid passesthrough a pump, we must understandhow energy changes state in order tomove fluid. This lesson will describe the types of energy found in a pump and the energy conversions that take place there.

What is energy?Energy exists in many forms. It can be fuel, electricity, water power,wind power or that possessed by a body in motion. Energy exists as basically two forms:• Kinetic Energy –

the energy of motion.• Potential Energy –

the energy of position.

A pump converts the mechanical energy (from a motor) to liquid energy for the purpose of moving liquid from one point to another within a piping system. Liquid energyfrom a pump is measured in terms of head (or pressure). It also exists in two forms:• Pressure energy or Pressure head• Velocity energy or Velocity head

It is this conversion of the velocity energy of a body in motion (the fluidwithin a pump) to pressure energy, that makes the fluid want to move down the pipe even though there are system pressures to be overcome.

What is velocity energy?Velocity energy represents the kinetic energy of a unit weight of liquid moving with a certain velocity.Velocity energy is often referred to as velocity head.

What is pressure energy?Pressure energy or pressure head is defined as the energy required to move a given weight of liquid against a certain pressure. It is this energy, converted from velocity energy, that moves the liquid from the pump down the pipe.

How does the energy conversion take place within a pump?The fluid at the suction flange has a liquid velocity equal to the velocity of the fluid in the pipe. The speed of the impeller takes this energy of the fluid and increases it to the speed of the impeller vane tips. At the point where the fluid begins to leave the impeller vane tips, the fluid begins to slow down.

The fluid, upon reaching the discharge flange, has slowed down to the velocity of the fluid in the discharge pipe. This slowing of the fluid can be thought of as basically hitting a wall. As a result, there is a conversion of velocity energy to pressure energy at the discharge flange. This pressure energy is what is needed to overcome the resistance in the pipe from friction (friction head losses) and changes in fluid level (static head).

CHAPTER 4

PUMP

OPERATION

Review

Questions

1 through 9

63

Review Questions1. The area of the pump

where the liquid enters is called

a. the pump discharge.b. the pump suction.c. the pump impeller eye.d. the pump outlet.

2. Suction pressure is always positive and can never be a vacuum.a. True.b. False.

3. The pump discharge is the outlet or flange area where the fluid leaves the volute (casing).a. True.b. False.

4. Which statement is correct?a. Discharge pressure is equal

to suction pressure plus the pump pressure (head).

b. Discharge pressure minus suction pressure equals the pump pressure (head).

c. Pump pressure (head) plus suction pressure equals the discharge pressure.


5. What part of the pump has the lowest pressure?a. The impeller eye.b. The impeller vanes.c. The suction flange.d. The discharge flange.

6. Kinetic energy is the energy of position.a. True.b. False.

7. Velocity energy represents the kinetic energy of a unit weight of liquid moving with a certain velocity.a. True.b. False.

8. Pressure head is the energy required to move a given weight of liquid against a pressure.a. True.b. False.

9. Which statement is correct?a. The fluid at the suction

flange has a liquid velocity less than velocity of the fluid in the pipe.

b. The speed of the impeller takes the energy of the fluid and increases it to the speedof the impeller vane tips.

c. At the point where the fluid begins to leave the impeller vane tips, the fluid begins to speed up.

d. The fluid, upon reaching the discharge flange, has increased to the velocity of the fluid in the discharge pipe.


CHAPTER 5

PUMP

CURVES

64

CHAPTER 5

PUMP

CURVES

Introduction

Objective

PumpPerformance

CurveDefinition

Single Pump

Performance

Curve

65

Chapter 5 Pump Curves IntroductionWe now know how fluid flows into and out of a pump. We also know how a pump converts energy to make it move. Pumps will performaccording to their pump head-capacityperformance curves. This chapter will define this head-capacity curve. It will also identify its elements, its use and define how operating condition changes affect this curve.

A pump head-capacity performancecurve identifies how a pump will operate in a given system. It is thepump’s fingerprint. This lesson will define the pump head-capacity performance curve, how it is created and why it is used. A pump curve, as it is commonly called, is the basis for identifying pump operation in a system.

Single Pump Performance CurveA pump head-capacity performancecurve, or pump curve, is determined from actual pump performance data in a laboratory. This curve is a plot of a pump’s ability to generate fluid flow (capacity) against a certain head. Every pump, regardless of the manufacturer, has its own unique curve.

ObjectiveUpon completion of this chapter you will be able to define a pump performance curve, describe the curve elements and identify the effect of various operating conditions on these elements.

Pump Performance Curve Definition

We learned earlier that a pump takes mechanical energy from a motor and transforms it to velocity energy at the impeller vanes. The pump casing then changes the velocity energy to a pressure energy at the pump discharge. This pressure energy dissipates as the fluid moves through a system (i.e., pressure drop). The pump’s head-capacity curve defines how much energy is available at a given flow rate, impeller diameter and shaft speed for each pump size. This pump curve is often called a “Single Pump Performance Curve”.

There are numerous other elements for which curves are often generated from this test data in addition to the head-capacity curve. They will be further defined in the next lesson, but are listed below:• Efficiency• Power requirements • Net Positive Suction Head Required

(NPSHR)

CHAPTER 5

PUMP

CURVES

PumpPerformance

CurveDefinition

Single Pump

Performance

Curve

Composite

Pump

Performance

Curve

66

Figure 1A typical single pump performance curve

Composite Pump Performance CurveEach pump is tested at various impeller diameters and speeds.Sometimes the information is plotted and compiled individually.Displaying the information in this matter can take up quite a bit of paper and book space. Many times it is prudent and useful to show the information for all the head-capacity and other element curves on one plot. This minimizes space and consolidates the information onto one page.

When the head-capacity curve is plotted with the other elements for two or more impeller diameters it is commonly called a “Composite Pump PerformanceCurve.”This curve is comprised of data from numerous impeller, efficiency, power requirement and NPSHR curves all on one page for one pump size at a given speed. Most pump manufacturers show their pump data on composite pump curves for clarity and to reduce space.

Head-Capacity Curve

Efficiency Curve

Impeller Diameter

NPSHR Curve

Power Requirement Curve

•

•

•

•

•

CHAPTER 5

PUMP

CURVES

PumpPerformance

CurveDefinition

Composite

Pump

Performance

Curve

67

Figure 2A typical composite pump performance curve

9"

10"

11"

12"

13"200

160

120

80

40

0

20

30

40

50

60

40 80 120 160 200 240 280 320 360 400 440

10

2' 3' 4' 5'

6'

NPSHR

40%

45%

50%

55%

57%

60%

50%

48%

40%

30HP25HP

20HP

15HP

10HP

5HP

(2.0)(1.6)

(1.3)

(1.0)

(.6)

NPSHR

7.5kW11.2kW

14.9kW

18.7kW

22.4kW

3.7kW

Impeller Diameters

NPSHR Curves

Efficiency Curves

XYZ Pump Co. 1.5 x 3 – 13 Speed: 3550 rpm

Power RequirementHead-Capacity Curves

T D HEADMETERS FEET

•

•

••

••

••

•

CHAPTER 5

PUMP

CURVES

PumpPerformance

CurveElements

Capacity

Head

68

ANSI: 1.5 x 3 – 6IPP: 40 x 80 – 150Speed: 3550 rpm

Figure 3Head - Capacity curve

HeadHead is a pump term often used to describe the mechanical energy added to the fluid by centrifugal force. It is the quantity used to express the energy content of the liquid per unit weight of the liquid.

Head is a good term to use with centrifugal pumps because they are constant energy devices. This means that for a given pump operating at a certain speed and handling a definite fluid volume, the energy transferred to this fluid (in foot-lbs. per foot or Newtons per meter of fluid) is the same for any fluid regardless of density.

The head generated by a given pump at a certain speed and capacity will remain constant for all fluids, barring any viscosity effects. Therefore, head when applied to centrifugal pumps is commonly expressed in feet (or meters) of liquid.Head can also be used to represent the vertical height in feet (or meters)of a static column of liquid.

Pump Performance Curve Elements

We defined a pump performance curve and identified its use in the previous lesson. Each curve is composedof various elements. This lesson willdefine the elements found on a pump performance curve and identifytheir individual uses. Knowing how to read and comprehend a pump performance curve with all its elements is the first step in understanding how a pump and system interact.

CapacityCapacity is defined as the volumetricflowrate of the fluid being transferred by the pump. The most common units for capacity are usually gallons per minute (GPM) or cubic meters per hour. Centrifugal pumps do not offer a large amount of flexibility incapacity variations without affecting the pump efficiency.

When specifying capacity requirements for a centrifugal pump a range of capacities should be stated. Minimum and maximum capacity limits are very important. These limits ensure proper pump operation, both mechanically and hydraulically.

Capacity varies inversely proportional with head. This means as capacity increases, head decreases. Capacity is often designated by the letter “Q” on pump curves.

CHAPTER 5

PUMP

CURVES

PumpPerformance

CurveElements

Total

Dynamic

Head

Net Positive

Suction Head

Required(NPSHR)

69

Figure 5Net positive suction head required (NPSHR) curve

Total Dynamic Head Total Dynamic Head (TDH) is the difference between the discharge headand the suction head of the pump. Total Dynamic Head is the amount ofenergy, expressed in feet or meters,added to the fluid by the pump. Total Dynamic Head may be referred to as pump head or pump total head.TDH is typically designated as H.

For example, if a water pump has a discharge head reading of 100 ft. and a suction head reading of 40 ft., the TDH (pump head) developed by the pump is 60 ft.

In the metric system, if a water pump has a discharge head reading of 30 m and a suction head reading of 10 m, the TDH developed by the pump is 20 m.

Net Positive Suction Head Required (NPSHR)As liquid enters the pump, there is a reduction of pressure and subsequent head. This head reduction is a function of the specific pump and is determined by laboratory testing to be stated by the pump manufacturer on a pump curve. Net Positive Suction Head Required (NPSHR)is the measurement of this head reduction to determine the minimum suction head conditionrequired to prevent the liquid from vaporizing in the pump.

Notice on the NPSHR curve below, as the pump capacity increases and head decreases, more NPSHR is required to prevent cavitation from occurring.


Figure 4Head

NPSHR Curve

100 Feet

•

CHAPTER 5

PUMP

CURVES

PumpPerformance

CurveElements

Efficiency

70

EfficiencyEfficiency is power output of a mechanical device, such as a pump, divided by power input to the device. Pump efficiency is the ratio of liquid power (also known as water power) divided by the power input to the pump shaft, (also known as brake power).

where Q is in cubic meters per hour and H is in meters.

where Q is in GPM and H is feet.

Water power, designated as whp (English) and wkW (metric), is the useful work delivered by the pump and is expressed by the following formulas:

Figure 6A typical efficiency curve showing the “ideal capacity“ and BEP

Every centrifugal pump has a capacity at which it works best. This is known as the “Ideal Capacity”. At this Ideal Capacity the pump efficiency is at its maximum. This point of highest efficiency is called the Best Efficiency Point (BEP). When operating a pump it is recommended that the pump be sized to run as close to the BEP as the application allows. Notice from the efficiency curve shown below that the efficiency falls as capacity is changed higher or lower than the Ideal Capacity.


BEPEfficiency Curve

Ideal Capacity

•

•

•

Water Power

Brake Powerx 100

Pump efficiency =

Q x H x Specific Gravity

3960whp =


367wkW =

CHAPTER 5

PUMP

CURVES

PumpPerformance

CurveElements

Power

Requirements

Impeller

Diameter

Pump Speed

71Figure 7A typical power requirement curve

Power RequirementsA pump will require a certain amount of power to move the liquid. By performing some simple algebraic substitutions to the efficiency equations shown earlier, the pump power requirement is calculated below.

English units:

bhp = whp

Pump efficiency

Metric units:

When entering the pump efficiency the decimal equivalent should be used.An efficiency of 45% should be entered into the equation as 0.45.


Impeller DiameterThe size of the impeller impacts Total Dynamic Head, Capacity, NPSHR,Efficiency, and Power Requirements.

A pump designated as being a 1.5 x 3 – 10 (USCS)or 40 x 80 – 250 (SI) indicates that the largest impeller that can be installed in a pump is 10 inches or 250 mm in diameter. It is important to always physically check the impeller diameter when reading a pump curve. The actual size can and may be smaller than the pump curve dimensions. Estimating the impeller size from the pump nameplate often gets one into trouble because it may not be accurate.

Impeller sizes are determined from the actual system requirements and may not be shown on the composite pump performance curve. Interpolation is then needed to determine the correct diameter.

Pump SpeedPump speed is the rotational speed of the pump as determined by the motor (driver) and any other power transmission device such as the coupling, gearbox, or belts. The pump speed can also be changedelectrically by varying the power input frequency from the normal 60 Hz or 50 Hz to some other value. Pump speed is typically designated as revolutions per minute (rpm).

Pump speed directly impacts capacity,head and power requirements. As the pump speed increases all three of these parameters also increase, but at different rates. Knowing the speed of the pump is critical in identifying and reading the correct pump performance curve.

Power Requirement Curve

•

bkw =wkw

Pump efficiency

bkw =Q (m3/hr) x H (m) x S.G.

Pump efficiency x 367

bhp = Q (gpm) x H (ft.) x S.G.


CHAPTER 5

PUMP

CURVES

PumpPerformance

CurveElements

Pump Size

What is the

relationship

between

Total

Dynamic

Head and

Capacity?

72

Pump SizePump size is determined by the pumping requirements. The minimum sizing requirements include the capacity to be pumped against a specified head, the fluid specific gravity, the fluid viscosity, and the pump NPSHR. This requires an analysis of the entire pumping system. Pump size is typically shown as 1.5 x 3 – 10 (USCS) or 40 x 80 – 250 (SI).

The 3 or 80 indicates the size of the suction nozzle measured in inches and millimeters, respectively. The suction nozzle will always be larger than or equal to the discharge nozzle. The 1.5 or 40 indicates the size of the discharge nozzle. The discharge nozzle will be smaller than or sometimes equal to the suction nozzle.

The 10 or 250 represents the largest possible impeller diameter for this pump size. System requirements may deem it necessary to cut down the impeller.Always physically check the actualimpeller diameter when reading the pump performance curve for an existing application.

What is the relationshipbetween Total Dynamic Head and Capacity?Total Dynamic Head varies inversely with Capacity. This means that as the head increases, the capacity willdecrease and as head decreases, the capacity will increase.

An example of this phenomenon is the flow of water out of a gardenhose. Unimpeded, the water has the largest capacity, but the lowest head (pressure). This is evidenced by a stream of water that does not extend out from the hose very far.However, when you put your thumb over the end of the hose, the capacitygoes down, but the pressure goes up. Now the stream will extend much further from the hose and you can put water farther away from you.

A typical head-capacity curve shownbelow reflects this discussion.


Figure 8Illustration of TDH vs. Capacity relationship

Highest TDH & Smallest Capacity

Head Decreases as Capacity Increases

Lowest TDH & Highest Capacity

•

•

•

CHAPTER 5

PUMP

CURVES

Element Effects;Various

OperatingConditions

What effect

does viscosity

have on a

pump curve?

73Figure 9Performance correction chart for viscous fluids

Element Effects; Various Operating Conditions

Pump curves are developed through laboratory testing with water. As a pump transfers different fluids,pump operation and the pump curve are affected by the varying physical properties of these fluids, especially if they differ greatly from water.

What effect does viscosity have on a pump curve?

This lesson will describe the effect of fluid properties on pump operation. It will also define various operating conditions when describing pump operation on a pump curve.

100

80

60

100

80

60

40

20

152030406080

100150200300400600

40 60 80 100

150200

300400

600800100015002000

30004000

6000800010000

15000

1 2 4 6 8 10 15 20 40 60 80 100

0.6 x QNW

0.8 x QNW

1.0 x QNW

1.2 x QNW

HC

QC

OC

10 15 20 32 43 65 88

132176220

330440

660880

1320176022003300

600400300200150100806040

CENTISTOKES

VISCOSITY-SSUCAPACITY IN 100 GPM

HEA

D IN

FEE

T ( P

ER S

TAG

E)C

APA

CIT

Y A

ND

EFF

ICIE

NC

Y

CO

RREC

TIO

N F

AC

TORS

HEA

D

CHAPTER 5

PUMP

CURVES


OperatingConditions

What effect

does viscosity

have on a

pump curve?

74

Viscosity is a measurement of a fluid’s resistance to flow at a given temperature. As viscosity increases, a viscosity correction to the pump’s head/capacity curve will be required.Pump curves are developed in laboratory testing using water that has a viscosity of 1cSt (Centistoke).Viscosity greater than 1cSt results inlower head/capacity outputs even if both are using the same size impeller. As a result of this lower head and capacity output, the impeller and sometimes the pump size need to be increased.

A viscosity performance correction chart is shown in Figure 9 on the previous page. It is used only withNewtonian liquids as non-Newtonian liquids may produce widely varyingresults. Re-rated head, efficiency andpower requirements are taken offthis chart and plotted on a pump

performance curve. Notice, in Figure 10,how the curves for a higher viscosity liquid (1000 SSU) are re-rated to a different value than the water curves.The new head and efficiency curves arelower than water. Whereas, the newpower curve is higher than water.

Figure 10A sample set of re-rated pump performance curves. The solid lines represent water viscosity and the dashed lines represent the re-rated 1000 SSU viscosity curves.

CAPACITY – GPM

WATER

WATER

HEAD

EFFICIENCY

HEA

D IN

FEE

T

( %)

EFFI

CIE

NC

YBH

P

CHAPTER 5

PUMP

CURVES


OperatingConditions

What effect

does specific

gravity and

viscosity

have on the

power

requirement

curve?

What is

the Best

Efficiency

Point of

a pump?

75

What effect does specific gravity and viscosity have on the power requirement curve?As we have defined, specific gravity is the density ratio of a liquid as compared to water at a given temperature. It was also shown previously that specific gravity is part of the equation to calculate power consumption.

Pump Power Requirement =

From this equation we can see that as the specific gravity increases (the fluid becomes heavier), the pumppower requirements also increase.

Also shown previously, viscosity is themeasurement of a fluid’s resistance to flow at a given temperature. We learned in the preceding section that as a fluid becomes thicker (viscosity increases), the pump power requirements also increase.

Knowledge of both of these fluid physical properties is required when sizing a pump and motor.

What is the Best Efficiency Point of a pump?We learned earlier that efficiency is power output, of a mechanical device such as a pump, divided by power input to the device. Pump efficiency is the ratio of liquid power (also known as water power) divided by the power input to the pump shaft (also known as brake power).

Water power, designated as whp(English) and wkW (metric), is the useful work delivered by the pump and is expressed by the following formulas:

where Q is in GPM and H is feet.

where Q is in cubic meters per hour and H is in meters. Efficiency for bothequations is expressed as a decimal or percent value.

Every centrifugal pump has a capacity at which it works best. This is known as the “Ideal Capacity.”At this Ideal Capacity the pump efficiency is at its maximum. This point of highest efficiency is called the Best Efficiency Point (BEP). When operating a pump it is recommended that the pump be sized to run as close to the BEP as the application allows.

As a pump impeller size is cut, its efficiency curve and therefore its BEP will change. When reading composite pump performance curves, interpolation of efficiency values is often required.


Figure 11BEP and “Ideal Capacity”

BEPEfficiency Curve

Ideal Capacity

•

•

Capacity x Head x Specific Gravity

Pump Efficiency x Constant

Water Power

Brake Power

Pump efficiency = x 100


3960whp =


367wkW =

•

CHAPTER 5

PUMP

CURVES


OperatingConditions

What effect

on efficiency

does pump

operation

have when

operating

away from

the pump

Best

Efficiency

Point?

What effect

does changing

capacity have

on Net

Positive

Suction Head

Required?

76

What effect on efficiency does pump operation have when operating away from the pump Best Efficiency Point?Pumps run best when operating at their Best Efficiency Point. At this point, the hydraulic forces within the pump are balanced. This point also indicates the pump output is the best it can be in relation to the pump power input.

Pump operation away from the Best Efficiency Point causes the hydraulic forces within the casing to become unbalanced. These unbalanced forces can cause the pump shaft to bend or deflect. This condition, if severe enough, could impact pump reliability. Further information about these forces will be discussed later in the manual. The power input in relation to the pump output changes resulting in lower efficiencies.


Figure 12Efficiency curve illustrating decreasing efficiencies as the capacity moves away from the ideal capacity and BEP

Figure 13Illustration showing NPSHR increasing as capacity increases

What effect does changing capacity have on Net Positive Suction Head Required?We learned earlier that Net Positive Suction Head Required (NPSHR) is the minimum hydraulic suction head condition required to prevent cavitation in the pump. This minimum condition is determined by laboratory testing and is stated by the pump manufacturer.

As pump capacity increases and head decreases, more NPSHR is required to prevent cavitation. This is shown in Figure 13.


BEPEfficiency Curve

Ideal Capacity

NPSHR Curve

•

•

•

•

CHAPTER 5

PUMP

CURVES


OperatingConditions

What is

a Shut-off

or Dead-head

condition?

What is a

Non-overloading

or Run-out

condition?

77

Figure 14An illustration of a shut-off or dead-head condition on the head-capacity curve

Figure 15An illustration of a non-overloading condition on the head-capacity curve

What is a Shut-off or Dead-head condition?A Shut-off or Dead-head conditionoccurs when the pump is pumping at zero capacity and is at its highestdesign head. This condition can be caused by shutting the pump discharge valve completely, a blockage in the discharge pipe, pumping against a full filter, or excessive static discharge head (i.e., trying to pump up a hill that is too high).

Pumping at a dead-head condition for an extended period of time can lead to premature pump failure andreduced reliability. Also, because the pump is not moving any fluid, all or most of the energy from the motor is transferred into heat. This heat can raise the temperature of the liquid very rapidly and possiblycause catastrophic failure of pump components (i.e., the pump casing can explode). A pump head-capacitycurve illustrating a dead-head condition is shown in Figure 14.


What is a Non-overloading or Run-out condition?When a pump is pumping at its greatest design capacity it is called a Non-overloading or Run-out condition. This maximum capacity is determined by hydraulic testing from the pump manufacturer. Beyond this maximum capacity the pump operates erratically. Examples of non-overloading are a large reduction in head requirement for the pump due to a system change or a pipe burst at the pump discharge.

A pump that runs continuously at this Non-overloading condition is probably sized incorrectly. This operating point is very far from the pump’s ideal capacity and should be resized to operate more efficiently. A pump head-capacity curve illustrating a non-overloading condition is shown in Figure 15.


Shut-off or Dead-head Non-overloading or Run-out

•

•

CHAPTER 5

PUMP

CURVES


OperatingConditions

What is

the effect of

operating at

Shut-off

on power

consumption?

What is

the effect of

operating at

Non-overloading

on power

consumption?

78

What is the effect of operating at Shut-off on power consumption?Usually, when a pump is operating in a Shut-off or Dead-head condition its power requirements are at their minimum. This is true for pumps with a typical non-overloading powercurve. Overhung Impeller CentrifugalPumps with radial or Francis-vaneimpellers fall into this category. Axial flow impellers, however, are just the opposite and have power curves that have the maximum powerrequirement at dead-head.


Figure 16A shut- off or dead-head power curve found on radialor Francis-vane overhung impeller centrifugal pumps

Figure 18A non-overloading power curve found on radial or Francis-vane overhung impeller centrifugal pumps

Figure 19A non-overloading power curve found on axial flow overhung impeller centrifugal pumps

Figure 17A shut- off or dead-head power curve found on axial flow overhung impeller centrifugal pumps

What is the effect of operating at Non-overloadingon power consumption?Usually, when a pump is operating in a Non-overloading or Run-out condition its power requirements are usually at their maximum. This is true for pumps that have a non-overloading power curve.Overhung Impeller Centrifugal Pumps with Radial and Francis-vane impellers fall into this category. Axial flow pumps, however, are just the opposite of this and the powerrequirements are at their minimum.


Lowest Power Requirement

Lowest Power RequirementHighest Power Requirement

Highest Power Requirement

•

•

•

•

CHAPTER 5

PUMP

CURVES


OperatingConditions

What are

the effects

and limitations

of Impeller

trimming?

79

What are the effects and limitations of Impeller trimming?To reduce cost, pump volutes are usually designed to accept several different impeller diameters. The ability to reduce the impeller diameter also allows a variety of operating requirements to be met.

Trimming or reducing impeller diameter will result in lower pump head, capacity and power requirement.Impeller trimming or reduction is rarely more than 10 to 20% of the original full diameter. The recommended maximum impeller reduction is usually around 30% from maximum, however this determination is case by case and varies with pump manufacturer.Composite pump performance curves often show a minimum impeller diameter as the smallest size it could be trimmed.

CHAPTER 5

PUMP

CURVES

Review

Questions

1 through 10

80

6. NPSHR is the same for all pumps of the same size.a. True.b. False.

7. As pump capacity moves away from its ideal capacity the efficiency

a. increases.b. decreases.c. remains the same.d. changes depending

on the pump.

8. A pump’s power requirement can be calculated by which equation?a. Capacity times head

times specific gravity times a constant divided by pump efficiency.

b. Capacity times head times specific gravity divided by a constant and pump efficiency.

c. Capacity times head divided by specific gravity, a constant and pump efficiency.

d. None of the above.

9. Which of the following is the rotational speed of the pump? a. Capacity.b. Net Positive Suction Head

Required (NPSHR).c. Pump Speed.d. Efficiency.

10. A pump size is 25 x 50 – 150. The 25 is the The 50 is the The 150 is the a. discharge, suction,

maximum impeller diameter.b. shaft diameter, stuffing box ID,

pump speed (divided by 100).c. suction, discharge,

maximum impeller diameter.d. suction, discharge,

pump power requirement.

Review Questions1. A pump head-capacity

performance curve is

a. the same for all manufacturers of a given pump size.

b. determined by calculations using a pump curve standard.

c. determined from testing actual pump performance data in a laboratory.


2. What is a Composite Pump Performance Curve?a. A pump curve for non-metallic

(composite) pumps.b. A pump curve for pumps

from different manufacturers.c. A pump curve for many

impeller diameters of the same size pump.

d. A pump curve for many pump sizes with the same size impeller.

3. Efficiency of the pump is not affected by changes in the pump capacity.a. True.b. False.

4. The generated by a given pump at a certain speed and capacity

for all fluids, barring any viscosity effects. a. pressure, will remain constant.b. head, will remain constant.c. head, will change.d. None of the above.

5. Which of the following is the difference between the discharge head and the suction head of the pump? a. Total Dynamic Head. b. Power Requirements.c. Head.d. Net Positive Suction Head

Required (NPSHR).

CHAPTER 5

PUMP

CURVES

Review

Questions

11 through 21

81

11. As capacity increases TDH decreases. a. True.b. False.

12. As a fluid’s viscosity increases, the

a. motor size may need to be increased.

b. impeller size may need to be increased.

c. pump size may need to be increased.


13. In most cases pump power requirements will decrease as fluid temperature increases.a. True.b. False.

14. The BEP is the same for impeller sizes in a given pump. a. True.b. False.

15. As a pump operates away from the BEP, the efficiency increases.a. True.b. False.

16. NPSHR does not change as its capacity changes. a. True.b. False.

17. Running the pump at shut-off can cause which one of the following to occur? a. Premature bearing failure.b. The pump can explode.c. The mechanical seal can fail.d. All of the above.

18. A pump running at non-overloading

a. is probably sized incorrectly and should be resized.

b. is using the most power on the head-capacity curve.

c. can occur if the pipe burst near the pump discharge.


19. Which statement is correct?a. When an axial flow pump

is operating in a dead-head condition its power requirements are usually at their maximum.

b. When a radial vane pump is operating in a dead-head condition its power requirements are usually at their minimum.

c. Power requirements will vary as capacity changes.


20. When a pump is operating in a Non-overloading condition its power requirements are typically the same as found at the BEP. a. True.b. False.

21. A common limit for maximumimpeller reduction is closest to

of largest impeller diameter. a. 5%.b. 10%.c. 20%.d. 30%.


CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

82

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Introduction

Objective

Elementsof a

PumpingSystem

What are

the

elements of

a pumping

system

83

Chapter 6 A Simple Pumping System IntroductionPumps are never installed without piping. They are always a part of a pumping system. If not properlydesigned, the system can cause thepump to fail, or at least, cause it to perform poorly. This chapter will look at the pump as a part of a simple pumping system. It will identify the system elements and how these elements affect the pump. It is veryimportant to understand these systemelements. One must realize that theseelements are what determines how apump operates, and not the reverse.

When we think of pumping systems, we typically think of just the mechanical and fluid segments. However, there is much more to a pumping system than meets the eye. The lesson will discuss the various elements found in a pumping systemthat we commonly think of and a few that are not normally considered.

What are the elements of a pumping system?A pump operates as part of a system.One must realize that the system dictates what the pump does, not theother way around. Understanding theoperation of a pump means – identifying and understanding the various elements in a pumping systemand how they affect pump operation.Understanding the pumping system is critical, because every element in the system can potentially require the pump to operate in ways it was not designed and sacrifice reliability.

ObjectiveUpon completion of this chapter you will be able to describe the elements of a simple pumping systemand their affect on pump operation. You will also be able to define total suction head, total discharge head, total head, net positive suction head and the pump operating point on a pump performance curve.

Elements of a Pumping System

Although we typically think of the pumping system as mechanical components, the pumping system is more than just a pump and some piping. A typical system consists of the following:• Pump• Motor• Sealing Device• Coupling• Process Fluid• Tanks/Piping/Valves• Foundation and Support• Instrumentation• Operators• Mechanics• Procedures• Practices• Purchasing• Management• Vendor/Suppliers• Contractors

What is Static Head?Static head is defined as the distancemeasured vertically above or below an arbitrarily selected horizontal datum level. In a simple pumping system the horizontal datum level is usually the pump centerline ( ).

In a pumping system, this head represents the energy required to raise the liquid from the pump centerline to the point in the pipe that the liquid needs to be raised. This energy will vary with specific gravity as the pressure energy needed to raise the liquid will increase with increasing specific gravity.

Static head consists of two parts: Suction static head and Discharge static head. The difference betweenthese two is called total static head or sometimes just static head.

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Elementsof a

PumpingSystem

What is

Static Head?

84

Figure 1A simple pumping system

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Elementsof a

PumpingSystem

What is

Static Head?

85

Figure 2Illustration of static head for a flooded suction

Figure 3Illustration of static head for a suction lift

SUCTIONSTATIC

LIFT

SUCTIONSTATICHEAD

TOTALSTATICHEAD

DISCHARGESTATICHEAD

TOTALSTATICHEAD

DISCHARGESTATICHEAD

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Elementsof a

PumpingSystem

What is

Velocity Head?

What is

Friction Head?

86

What is Velocity Head?Velocity head is the kinetic energy per unit weight of a flowing liquid.Velocity head is a change in energy form. A liquid that is not flowing willresult in a determined static head pressure. When the liquid begins to flow the static head pressure will drop.Energy has not been lost with this drop in static head pressure, rather it has been converted into velocity head.

Velocity head represents the kinetic or velocity energy present in a movingliquid. Velocity head is equivalent to the vertical distance the mass of liquidwould have to fall to acquire the velocity (in a perfect vacuum).

Another way of defining velocity head is the head necessary to accelerate thefluid pumped. It is usually insignificantand can be ignored in most high headsystems. It is, however, significant in low head systems and should be calculated. Velocity head is expressed by the following equation:

hv = Velocity Head =

Where: V = Velocity of Liquid in Pipeg = Gravitation Constant

(32.2 ft/sec2

or 9.81 m/sec2)

V2

2g

Figure 4Velocity head calculation

What is Friction Head?Friction head is the energy required per unit weight of liquid pumped to overcome friction losses that occur as the liquid flows through the pipingsystem. All moving liquids in a piping system generate a resistance to this flow as they move down the pipe. This resistance varies with pipe size,roughness, pipe length, the capacity and the type and quantity of valves and fittings used.

Friction loss is the calculation (actually estimation) of this friction head resistance at a given capacity. This loss will increase with increasingcapacity. For the liquid to move at the desired capacity, the friction lossesneed to be overcome by the pump.

There are many methods for estimating friction loss. Many involve the use of extensive equations andtables. A common standard method for calculating friction loss in pipes is demonstrated in the EngineeringData Book published by the Hydraulic Institute. Pipe is listed in various diameters of asphalt-dipped cast iron, steel, or cast iron. Flow rates are stated in charts andinclude velocity, velocity head and friction loss. When the pipe diameter and flow rate are known, the pipe friction loss will be listed in the chart. As with pump curves, all rates are determined using the viscosity of water (1Cst). Any change in viscosity will result in added frictional loss.

0.0111 5 0.217 0.000732 0.0112 0.227 0.000800 0.01280.0223 10 0.434 0.00293 0.0372 0.454 0.00320 0.04350.0334 15 0.651 0.00659 0.0762 0.681 0.00720 0.09000.0446 20 0.868 0.0117 0.126 0.908 0.0128 0.15100.0557 25 1.085 0.0183 0.189 1.13 0.0200 0.2280

0.0668 30 1.30 0.0263 0.262 1.36 0.0288 0.3200.0780 35 1.52 0.0359 0.347 1.59 0.0392 0.4270.0891 40 1.74 0.0468 0.443 1.82 0.0512 0.5490.100 45 1.95 0.0593 0.547 2.04 0.0648 0.6830.111 50 2.17 0.0732 0.662 2.27 0.0800 0.830

0.123 55 2.39 0.0885 0.789 2.50 0.0968 0.9930.134 60 2.60 0.105 0.924 2.72 0.115 1.1700.145 65 2.82 0.124 1.07 2.95 0.135 1.360.156 70 3.04 0.143 1.22 3.18 0.157 1.560.167 75 3.25 0.165 1.39 3.40 0.180 1.78

0.178 80 3.47 0.187 1.57 3.63 0.205 2.020.189 85 3.69 0.211 1.76 3.86 0.231 2.280.201 90 3.91 0.237 1.96 4.08 0.259 2.550.212 95 4.12 0.264 2.17 4.31 0.289 2.820.223 100 4.34 0.2927 2.39 4.54 0.320 3.10

0.245 110 4.77 0.354 2.86 4.99 0.387 3.730.267 120 5.21 0.421 3.37 5.45 0.461 4.400.290 130 5.64 0.495 3.92 5.90 0.541 5.130.312 140 6.08 0.574 4.51 6.35 0.627 5.930.334 150 6.51 0.659 5.14 6.81 0.720 6.80

0.356 160 6.94 0.749 5.81 7.26 0.820 7.710.379 170 7.38 0.846 6.53 7.72 0.925 8.700.401 180 7.81 0.948 7.28 8.17 1.04 9.730.423 190 8.25 1.06 8.07 8.62 1.16 10.800.446 200 8.68 1.17 8.90 9.08 1.28 11.9

0.490 220 9.55 1.42 10.7 9.98 1.55 14.30.535 240 10.4 1.69 12.6 10.9 1.84 17.00.579 260 11.3 1.98 14.7 11.8 2.16 19.80.624 280 12.2 2.29 16.9 12.7 2.51 22.80.668 300 13.0 2.63 19.2 13.6 2.88 26.1

0.713 320 13.9 3.00 22.0 14.5 3.28 29.70.758 340 14.8 3.38 24.8 15.4 3.70 33.60.802 360 15.6 3.79 27.7 16.3 4.15 37.80.847 380 16.5 4.23 30.7 17.2 4.62 42.20.891 400 17.4 4.68 33.9 18.2 5.12 46.8

0.936 420 18.2 5.16 37.3 19.1 5.65 51.50.980 440 19.1 5.67 40.9 20.0 6.20 56.41.025 460 20.0 6.19 44.6 20.9 6.77 61.51.069 480 20.8 6.74 48.5 21.8 7.38 66.81.114 500 21.7 7.32 52.5 22.7 8.00 72.3

1.225 550 23.9 8.85 63.2 25.0 9.68 871.337 600 26.0 10.5 74.8 27.2 11.5 1021.448 650 28.2 12.4 87.5 29.5 13.5 1211.560 700 30.4 14.3 101 31.8 15.7 1421.671 750 32.5 16.5 116 34.0 18.0 162

1.782 800 34.7 18.7 131 36.3 20.5 1841.894 850 36.9 21.1 148 38.6 23.1 2072.005 900 39.1 23.7 165 40.8 25.9 2322.117 950 41.2 26.4 184 43.1 28.9 2582.228 1000 43.4 29.27 204 45.4 32.0 285

3 InchNOMINAL CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Elementsof a

PumpingSystem

What is

Friction Head?

87

STEEL SCHEDULE 40I.D. = 3.068 Inches e/D = 0.000587

Figure 5Typical pipe friction loss table. Note the columns for velocity (V), velocity head (V2/2g), and friction loss (hf)

DISCHARGE

ASPHALT DIPPED CAST IRONI.D. = 3.00 INCHES e/D = 0.00160

CFS GPM

hffeet per 100 feet of Pipe

hffeet per 100 feet of Pipe

Vft/sec

Vft/sec

V2/2gfeet

V2/2gfeet

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Elementsof a

PumpingSystem

What is

Friction Head?

88 Figure 6Typical resistance coefficient (K-factor) chart for various valves and fittings.

Determining the friction loss for valves and fittings is similar to that of pipe. The velocity head must be known. Additionally there is a K factor that is must be included. The K factor is the resistance coefficient for valves and fittings. Values of the K factor will vary with various valves, fittings and diameters.

As with pipe friction, the Engineering Data Bookpublished by the Hydraulic Institute provides the standard for this determination. Friction losses incurred in passing through auxiliary equipmentsuch as strainers, filters, or heat exchangers should be obtained from the equipment manufacturer.

GLOBE VALVE

GATE VALVE

ANGLE VALVE

SWING CHECKVALVE

BASKETSTRAINER

FOOT VALVE

BALL VALVE FULL PORT FULL OPEN

BALL VALVE 25%REDUCED PORT FULL OPEN

BUTTERFLY VALVE FULLOPEN

COUPLINGS AND UNIONS

SCREWED

SCREWED

SCREWED

SCREWED

FLANGED

FLANGED

FLANGED

FLANGED

REDUCINGBUSHINGAND COUPLING

SUDDEN ENLARGEMENT V1 – V22

2g

V12

2g

If A2 = ∞ so that V2 = 0

h =

h =

Feet of Fluid

Feet of Fluid

V22

2gh = K

V2

2g

V2

V2

V1

V1

hf = K

Used as reducerK = 0.05 – 2.0Used as increaser loss is up to 40% more than that caused by a sudden enlargement

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Elementsof a

PumpingSystem

What is

Pressure Head?

What is

vapor pressure?

TotalSuction Head

(System)

What is

Suction

Static Head?

89

What is Pressure Head?Pressure head exists when the pumping system begins or terminates in a tank under pressure other thanatmospheric. It is the pressure acting on the surface of the liquid in the tank.This pressure can be positive (aboveatmospheric) or negative (vacuum).

In the case of positive pressure, the pressure is converted into feet ormeters of head using the equationslearned earlier. If the pressure is a vacuum, it can be converted using the following equations:

What is vapor pressure?Vapor pressure is the pressure at which a liquid will flash into a vapor at a given temperature. Vapor pressure is measured in absolute pressure units, such as psia or bar-absolute. At 60° F (15.5° C), water has a vaporpressure of 0.256 psia (0.018 bar abs.).At 212° F (100° C), water’s vapor pressure has increased to 14.69 psia(1.013 bar abs.), therefore it boils under atmospheric pressure. Anotherway of defining vapor pressure is the amount of pressure exerted on a liquid to keep it liquid.

Total Suction Head (System)

To properly size a pump, we need to know how much energy (head)the pump has to provide in oder to move the fluid at the desired capacity.Knowing how much energy (head)is available on the suction side of thepump is the first step in determining the correct pump size required for any system. This lesson will define the terms necessary to calculate the energy (head) that exists on the pump inlet side.

What is Suction Static Head?Suction Static Head is defined as the distance measured vertically above or below the height of the liquid surface in the suction tank relative to the centerline of the pump.

If the liquid surface is above the centerline, the suction static head will be positive. This condition is referred to as a suction head condition or commonly just a flooded suction.

If the liquid surface is below the centerline, the suction static head will be negative. This condition is referred to as a suction lift condition.

Figure 7Typical suction lift application

inches of Hg x 1.135 Vacuum (ft.) =

Specific Gravity

mm of Hg x 0.0136 Vacuum (m) =

Specific Gravity

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

TotalSuction Head

(System)

What is

Suction

Static Head?

90

Figure 8Suction static head for a flooded suction

Figure 9Suction static head for a suction lift condition

SUCTIONSTATICHEAD

SUCTIONSTATIC

LIFT

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

TotalSuction Head

(System)

What is

Suction

Velocity Head?

What is

Suction

Pressure Head?

What is

Suction

Friction Head?

What is

Total Suction

Head (System)?

91

What is Suction Velocity Head?Suction velocity head is the kinetic energy per unit weight of a flowing liquid in the suction piping.

Suction velocity head is expressed by the following equation:

Figure 10Suction velocity head calculation

hvs = Suction Velocity Head =

Where: V = Velocity in the Suction Pipeg = Gravitation Constant

(32.2 ft/sec2

or 9.81 m/sec2)

V2

2g

What is Suction Pressure Head?Suction Pressure Head exists because the suction tank is under a pressure other than atmospheric. It is the pressure acting on the surface of the liquid in the suction tank. This pressure can be positive (aboveatmospheric) or negative (vacuum).


Figure 11Total system suction head equation

hs = Total System Suction Headhs = hss – hfs + hvs ± hps

Where:hss = Suction Static Headhfs = Suction Friction Head hvs = Suction Velocity Head hps = Suction Pressure Head

What is Suction Friction Head?Suction Friction Head uses the Suction Velocity Head to determine the actual friction lossesthrough piping, valves, fittings, contractions and enlargements, and entrance and exit losses. A method for calculating friction loss using the Hydraulic InstituteEngineering Data Book was learned earlier.

What is Total Suction Head (System)?We now can determine the total system head requirement for the suction piping. This is called Total System Suction Head. This is also sometimes called Total Dynamic Suction Head. The equation to calculate this head requirement is shown below.

Suction static head is positive when there is a flooded suction and negative when there is a suction lift.

Pressure head is zero if the tank is atmospheric. It is added when above zero gauge pressure and subtracted when under vacuum.

Velocity head theoretically is part of the System Suction Head equation. In practical application, it is rarely considered as its value is minimal.


Specific Gravity


Specific Gravity

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

TotalDischarge

Head(System)

What is

Discharge

Static Head?

92Figure 12Discharge static head as measured to the level of the liquid surface in the discharge tank

Total Discharge Head (System)

To properly size a pump, we need to know how much energy (head) the pump has to provide in oder to move the fluid at the desired capacity. Knowing how much energy (head) is required on the discharge side of the pump is the second step in determining the correct pump size for any system. This lesson will define the terms necessary to calculate the energy (head) that exists on the pump discharge side.

What is DischargeStatic Head?Discharge Static Head is defined as the distance measured vertically above or below the height of the liquid surface in the discharge tank relative to the centerline of the pump.

If the liquid surface is below the centerline, the discharge static head will be negative.

If the liquid surface is above the centerline, the discharge static head will be positive.

In the event of an open end pipe above the liquid surface in the discharge tank, the centerline of the open end is then considered the highest liquid surface in the discharge system.

DISCHARGESTATICHEAD

TANK LIQUID SURFACE LEVEL

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

TotalDischarge

Head(System)

What is

Discharge

Velocity Head?

What is

Discharge

Friction Head?

93

Figure 13Discharge static head as measured from the centerline of open pipe above the discharge tank

What is Discharge Velocity Head?Discharge Velocity Head is the kineticenergy per unit weight of a flowing liquid in the discharge piping.

Discharge Velocity Head is expressed by the following equation:

Figure 14Discharge velocity head calculation

hvd = Discharge Velocity Head =

Where: V = Velocity in the Discharge Pipeg = Gravitation Constant

(32.2 ft/sec2

or 9.81 m/sec2)

V2

2g

What is Discharge Friction Head?Discharge Friction Head uses the Discharge Velocity Head to determine the actual friction lossesthrough piping, valves, fittings, contractions and enlargements, and entrance and exit losses. A method for calculating friction loss using the Hydraulic InstituteEngineering Data Book was learned earlier.

DISCHARGESTATICHEAD

OPEN PIPECENTERLINE

LEVEL

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

TotalDischarge

Head(System)

What is

Discharge

Pressure Head?

What is

Total

Discharge

Head (System)?

94

What is Total Discharge Head (System)?We now can determine the total system head requirement for the discharge piping. This is called Total System Discharge Head. This is also sometimes called Total Dynamic Discharge Head. The equation to calculate this head requirement is shown below.

Discharge Static Head is positive when above the centerline of the pump, and negative when below the centerline of the pump.

Pressure head is zero if the tank is atmospheric. It is added when above zero gauge pressure and subtractedwhen under vacuum.

Velocity Head theoretically is part of the System Discharge Head equation. In practical application, it is rarely considered as its value is minimal.

hd = Total System Discharge Headhd = hsd + hfd + hvd ± hpd

Where: hsd = Discharge Static Head hfd = Discharge Friction Headhvd = Discharge Velocity Head hpd = Discharge Pressure Head

Figure 15Total system discharge head equation

What is Discharge Pressure Head?Discharge Pressure Head exists because the discharge tank is under a pressure other than atmospheric. It is the pressure acting on the surface of the liquid in the discharge tank. This pressure can be positive (aboveatmospheric) or negative (vacuum).



Specific Gravity


Specific Gravity

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Total Head(System)

Relationships

Total Head (System)

95

Total Head (System) Relationships

We just determined how much energy(i.e., head) is required on both the suction and discharge sides of the pump.To properly size a pump, it is essential to know how much total energy (i.e., total head) the pump has to providein order to move the fluid at the desiredcapacity. This lesson will calculate thistotal head requirement and identify it as Total System Head.

Total Head (System)Total System Head is the differencebetween Total Discharge System Headand Total Suction System Head. This system head is the amount of energy required by the pump to providethe desired capacity. When the pump is producing this desired capacity it also is producing head equal to the TotalSystem Head. This head produced by the pump is called Total Dynamic Head(TDH).

Let’s explain this a little further…

The system requires a certain amount of head to move the fluid at a givencapacity. We just defined this as TotalSystem Head. In order for this desiredflow to happen, however, the pump has to produce an amount of TDH equal to this Total System Head. If the pump can do this, the fluid will move at the desired capacity. If pump cannot produce enough TDH, it will not move the fluid at the capacitydesired. It will move the fluid at somecapacity, just not the desired capacity.

What can be seen here is that the system dictates what the pump can do and not the other way around. As the Total System Head changes, so does the TDH requirement of thepump. If more capacity is desired, we know the Total System Head willincrease; therefore more TDH from the pump is needed to give us this new capacity.

We learned in the Pump Curves chapterthat head and capacity vary inversely. So if we need more capacity and head as stated above, what do we have to do to get it? We cannot just open the valve more and hope that works. This is because the pump curve tells usthat as a pump produces more flow, itproduces less head. So what do we do?We have to change something within the pump. Increasing the impeller diameter is probably the most common change made to a pump. Other options include increasing thespeed or changing to a larger pump.

The equation in Figure 16 shows how to calculate Total System Head from the previous two lessons. As we just discussed this Total System Head equals the pump Total Dynamic Head for producing the capacity required. To solve this equation all terms must be in the same units. In other words, the head values must be either all gauge units or all absolute units,for either English or metric.

H = hd – hs

Where:H = Total System Head hd = Total System Discharge Headhs = Total System Suction Head

After substitution: H = (hsd – hss) + (hfd + hfs)

+ (hvd – hvs) + (hpd – hps)

Where:hsd = Discharge Static Head hss = Suction Static Head hfd = Discharge Friction Head hfs = Suction Friction Head hvd = Discharge Velocity Head hvs = Suction Velocity Head hpd = Discharge Pressure Head hps = Suction Pressure Head

Figure 16Total system head calculation

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Total Head(System)

Relationships

Total Head (System)

Total Head (System)

Relationships

96

TDH = (hsd – hss) + (hfd + hfs)+ (hvd – hvs) + (hpd – hps)

Increases Decreases


Total Head (System)RelationshipsIt is important to understand that every factor involved in determining Total System Head affects the pump TDH (Total Dynamic Head) requirement. This is true whether on the suction or discharge side of the system headequation and involves static, friction, and pressure head. This section willexplain each factor and its effect on the pump TDH requirement.

The relationship of Total Head(System) on pump operation whenthe suction tank level is raisedWhen the suction tank level is raised, the total suction system head isincreased. From this the total systemhead decreases. This means the pumpTDH requirement also decreases.

Decreases Increases

The relationship of Total Head(System) on pump operation whenthe suction tank level is loweredWhen the suction tank level is lowered,the total suction system head isdecreased. From this the total systemhead increases. This means the pumpTDH requirements will increase.

Figure 17Total system head calculations cancelling velocity head and pressure head

H = (hsd – hss) + (hfd – hfs)

+ (hvd – hvs) + (hpd – hps)

H = (hsd – hss) + (hfd + hfs)

Many times both the suction and discharge piping are the same size and the suction and discharge tanks are atmospheric. When this happensboth the velocity head and pressure head terms cancel and you are left with this equation:

0 0

Figure 18Total dynamic head when the velocity head and pressure head are cancelled out

Figure 19Total dynamic head


TDH = (hsd – hss) + (hfd + hfs)

Because Total System Head equals Total Dynamic Head, we substitute TDH for H and arrive at the following:

Whenever the velocity and pressureheads differ, this TDH calculation is the amount of Head (i.e., energy)the pump needs to produce in order to provide the desired capacity through the system.



CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Total Head(System)

Relationships

Total Head (System)

Relationships

97

Decreases

IncreasesIncreases

Decreases

The relationship of Total Head(System) on pump operation with increased friction loss on the pump suctionWhen the friction losses increase on the suction, the total suction systemhead is decreased. From this the total system head is increased. This means the pump TDH requirements increase.

The relationship of Total Head(System) on pump operation when the discharge tank level is lowered and the discharge pipeexits above the liquid surfaceWhen the discharge tank level is lowered and the discharge pipe exits above its liquid surface, the total discharge system head is unchanged.Therefore, the total system head isunchanged. This means the pump TDH requirements will be unchanged.

The relationship of Total Head(System) on pump operation with decreased friction loss on the pump suctionWhen the friction losses decrease on the suction, the total suction systemhead is increased. From this the total system head decreases. This means the pump TDH requirement decreases.



Decreases Decreases

The relationship of Total Head(System) on pump operation whenthe discharge tank level is raisedWhen the discharge tank level is raised,the total discharge system head isincreased. From this the total systemhead is increased. This means the pump TDH requirement increases.

Increases Increases

The relationship of Total Head(System) on pump operation whenthe discharge tank level is loweredWhen the discharge tank level is lowered, the total discharge system head is decreased. From this the total system head is decreased. This means the pump TDH requirements will decrease.


Decreases

The relationship of Total Head(System) on pump operation with decreased friction loss on the pump dischargeWhen the friction losses decrease on the discharge, the total discharge system head is decreased. From this thetotal system head decreases. This meansthe pump TDH requirement decreases.

Decreases




Decreases

Increases



CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Total Head(System)

Relationships

Total Head (System)

Relationships

98

The relationship of Total Head(System) on pump operation with increased pressure head on the pump dischargeWhen the pressure head increases on the discharge, the total discharge systemhead is increased. From this the total system head increases. This means thepump TDH requirement increases.

Increases

Increases

The relationship of Total HeadSystem on pump operation with decreased pressure head on the pump dischargeWhen the pressure head decreases on the discharge, the total discharge system head is decreased. From this the total system head decreases. This means the pump TDH requirement decreases.

Decreases

Decreases

The relationship of Total Head(System) on pump operation with increased friction loss on the pump dischargeWhen the friction losses increase on the discharge, the total discharge system head is increased. From this the total system head increases. This means the pump TDH requirement increases.

Increases Increases

The relationship of Total Head(System) on pump operation with increased pressure head on the pump suctionWhen the pressure head increases on the suction, the total suction systemhead is increased. From this the total system head decreases. This means the pump TDH requirement decreases.

Decreases

The relationship of Total Head(System) on pump operation with decreased pressure head on the pump suctionWhen the pressure head decreases on the pump suction, the total suction system head is decreased. From this thetotal system head increases. This meansthe pump TDH requirement increases.

Increases

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Net PositiveSuction

Head

Net Positive

Suction Head (NPSH)

Net Positive

Suction Head

Required (NPSHR)

99

Net Positive Suction Head

We call the proximity of the liquid to its vapor pressure its“available” NPSH and the pressure reduction inside the pump,the “required” NPSH. As long as the available NPSH is greater than the required NPSH the pump will not cavitate.

Net Positive Suction Head Required (NPSHR)As liquid enters the pump, there is a reduction of pressure and subsequent head. This head reduction is a function of the specific pump and is determined by laboratory testing to be stated by the pump manufacturer on a pump curve. Net Positive Suction Head Required (NPSHR)is the measurement of this head reduction to determine the minimum suction head conditionrequired to prevent the liquid from vaporizing in the pump.

As shown in Figure 20 below, the pressure drops to a minimum level at the impeller eye before the liquid is acted upon by the impellervanes. It is this minimum value that is determined by testing and given the name Net Positive Suction Head Required (NPSHR).

Net Positive Suction Head (NPSH)is one of the most critical yet least understood pump hydraulic characteristics. Understanding the concept of NPSH is crucial to proper pump operation. This lesson will define NPSH and explain the differences between NPSH Available and NPSH Required.

Net Positive Suction Head (NPSH)Net Positive Suction Head (NPSH)is defined as the minimum hydraulic head condition in which a pump can meet its head and capacity requirements without the liquid vaporizing inside the pump. Vaporization of the liquid causes cavitation. This cavitation reduces a pump’s performance and may damage the pump.

Remember that any liquid can vaporize at any temperature if the pressure acting on the liquid is low enough. If the pressure inside the pump suction approaches the liquid’s vapor pressure cavitation could result. Comparing the pressure at the pump suction to the liquid’s vapor pressure is what we are doing when we check the NPSH conditions.

Figure 20Pump pressure profile showing point of lowest pressure (NPSHR).

INCR

EASI

NG

PRE

SSU

RE

A


ENTRANCELOSS

FRICTION

TURBULENCEFRICTION

ENTRANCELOSS AT

VANE TIPS

INCREASINGPRESSURE

DUE TOIMPELLER

POINT OF LOWEST PRESSURE WHERE VAPORIZATION CAN OCCUR (NPSHR)

B C D E

C

DE

BA

•

•

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Net PositiveSuction

Head

Net Positive

Suction Head

Available (NPSHA)

Pump System

Factors

affecting

Net Positive

Suction Head

Available (NPSHA)

100

Pump System Factors affecting Net Positive SuctionHead Available (NPSHA)We learned in the last section how to calculate NPSHA from two different equations. From these equations we can see various system factors that affect NPSHA.

The most important factor is probably the temperature of the liquid. This will determine the liquidvapor pressure. As the temperature and the vapor pressure increase, the NPSHA decreases.

Suction piping can have a large affect on the NPSHA, especially if it is very long or has many fittings. The more friction loss the suction piping has, the less NPSHA.

The static head is also important, especially for a suction lift application. As the suction lift increases (i.e., liquid level is falling farther below pump centerline) the NPSHA decreases.

A common, yet often overlooked, situation is where the pump takes suction from a vessel under vacuum. As the vacuum increases (i.e., surface pressure decreases)the NPSHA decreases.

Net Positive Suction HeadAvailable (NPSHA)Net Positive Suction Head Available(NPSHA) is the difference between the total suction system head and the fluid vapor pressure at the suction flange in absolute terms. NPSHA depends on the system layout and must always be greater than NPSHR to ensure proper pump operation and eliminate cavitation.

Knowing the liquid vapor pressure at the pumping temperature, we can calculate NPSHA from the following equations:

NPSHA = hpsa + hss – hfs – hvp

Where: NPSHA = Net Positive Suction Head

Availablehpsa = Suction Surface Pressure

converted to feet or meters of Head Absolute

hss = Suction Static Headhfs = Suction Friction Head

hvp = Vapor Pressure of theLiquid at the Pumping Temperature converted to feet or meters of Head Absolute

Figure 21NPSHA calculation using the friction loss estimation

NPSHA = ha + hgs + hvs – hvp

Where: NPSHA = Net Positive Suction Head

Availableha = Atmosphere Pressure

converted to feet or meters of Head Absolute

hgs = Suction Pressure Gaugeconverted to feet ormeters of Head

hvs = Suction Velocity Headhvp = Vapor Pressure of the

Liquid at the Pumping Temperature converted to feet or meters of Head Absolute

Figure 22NPSHA calculation using a suction pressure gauge

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Net PositiveSuction

Head

The effect

of changing

Capacity on

Net Positive

Suction Head

Required (NPSHR)

The

relationship

of NPSHA

to NPSHR

for proper

pump

operation

101

The effect of changing Capacity on Net PositiveSuction Head Required(NPSHR)We have learned that NPSHR is a function of the pump only and not the system. What happens when a pump increases its capacity? From the pump performance curves we can see that as capacityincreases and head decreases, pump NPSHR increases.

This is why when a pump is cavitating, closing the discharge valve a little bit will reduce or sometimes eliminate the cavitation. Closing the valve decreases the capacity and thereby decreases theNPSHR until the NPSHA is greater.


Figure 23A typical NPSHR curve showing that it increases with increasing capacity

The relationship of NPSHA to NPSHR for proper pump operationAs long as NPSHA is greater than the NPSHR, no vaporization will occur and the pump will perform as expected. However, if the opposite is true, cavitation results. When cavitation occurs, vapor bubbles form due to a pressure lower than the liquid’s vapor pressure. These bubbles move along the impeller vanes. As they reach the higher pressure areas of the vane tip regions, they are crushed, or implode, resulting in noise, vibration and impeller damage.

The standard tests for NPSHR tell us that even if NPSHA equals NPSHR there still is still a mild incipient cavitation occurring. Therefore, we need a little safety margin. A good margin to use is:

NPSHA > NPSHR +3 ft. (1m)

This margin can vary with pump type, impeller type, and fluid being pumped. However for most Overhung Impeller Centrifugal Pumps the 3 ft (1m) safety margin is usually satisfactory.

NPSHR CURVE

•

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Pump’sOperating

Point

How do you

convert

pressure to

head?

Determine

a pump’s

Operating

Point

from suction

and

discharge

gauges

102

Pump’s Operating Point

Determining a pump’s operating point is the best way to troubleshoot pumpproblems. Knowing how to determinethe pump capacity, head, NPSHR andpower requirement allows us to see if the pump is doing what it is supposed to be doing. This lesson will describe how to calculate pump TDH from pressure gauges. It will also enable you to identify the pump capacity, NPSHRand power requirement at its operatingpoint using a pump performance curve.

How do you convert pressure to head?If you remember nothing else from this book you need to remember how to convert pressure to head. This conversion is required when determining a pump’s operating point. To review, pressure can be converted to head by the equations:

Pressure (psi) x 2.31 Head (ft.) =

Pressure (bar) x 10.2

Head (m) =

Head (m) =

Pressure (kg/cm2) x 10

Specific Gravity

Specific Gravity

Specific Gravity

In order to perform this task you will need to know or obtain the following information:• Pump Size • Pump Speed• Pump Impeller Diameter• Pump Performance Curve• Fluid Specific Gravity• Suction and Discharge Pressure

Gauge Readings • Suction and Discharge Pressure Gauge

Distance from Pump Centerline (if any)

Let’s use the pump curve in Figure 24 as an example.• Pump Size: 3 x 4 – 13• Speed: 3550 rpm• Impeller Diameter: 11 inches• Specific Gravity: 1.2• Suction Gauge Pressure: 30 psig• Suction Gauge Height

from Shaft Centerline: 1 ft.• Discharge Gauge Pressure: 290 psig • Discharge Gauge Height

from Shaft Centerline: 3 ft.

First, calculate the differential pressure:290 psig – 30 psig = 260 psig

Second, calculate the differential head using the pressure to head conversion equation:260 psig x 2.31/1.2 = 500.5 ft.

This differential head is the amount of head the pump is producing at thecapacity we are trying to determine.Therefore this head is equal to the pump TDH for all intents and purposes.

Since this pump TDH is usually calculated from Suction and Dischargegauge pressure at the pump centerline,we need to subtract the Suction gaugeheight from the Discharge gauge height.This will be added to the above calculation to determine the actual point of operation.3 ft. – 1 ft. = 2 ft.500.5 ft. + 2 ft. = 502.5 ft. (153.2 m)

Determine a pump’s Operating Point from suctionand discharge gaugesDetermining a pump’s operating point is the best way to troubleshoot pumpproblems. Knowing the pump capacity,head, NPSHR and power requirementallows us to determine if the pump isdoing what it is supposed to be doing. To do this we have to install pressuregauges on both the suction and discharge. A pressure gauge on the discharge only is not acceptable and will not provide all the information we need.

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Pump’sOperating

Point

Determine

a pump’s

Operating

Point

from suction

and

discharge

gauges

103

Using the appropriate pump curve, identify the impeller size on it. Draw a line from the calculated head to intersect with the impeller curve. Draw a second line straight down until it intersects the capacity axis. The head calculated along with the estimated capacity is the pump’s operating point. From this point we can determine NPSHR and the power requirement. We can also ascertain if the pump is operating where it’s supposed to or if it is not performing as expected.

From the curve we see the estimatedcapacity is 640 GPM (145 m3/hr). The pump efficiency is approximately69%. From this we calculate the power requirement to be:

NPSHR at the operating point is shown to be about 15 ft (4.6 m).

BHP =

Capacity (GPM) x Head (ft)x Specific Gravity

= 141.2 HP (105.4 kW)

0.69 x 3960 =

640 x 502.5 x 1.2


Figure 24Illustration of determination of pump operating point knowing pressure gauge readings and using the proper pump performance curve

Impeller Size 11”

Pump Operating Point

NPSHR = 15’ (4.6 m) Pump Eff. = 69%

•

•

XYZ Pump Co. 3 x 4 – 13 Speed: 3550 rpm

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Review

Questions

1 through 9

104

6. Vapor pressure

a. is the pressure at which a liquid will flash into a vapor at a given temperature.

b. is measured in psia.c. will increase with increasing

temperature.d. All of the above.

7. Suction Static Head is negative if there is a suction lift condition.a. True.b. False.

8. Calculate the Total System Suction Head with the following parameters:• The liquid has a

specific gravity of 1.2• The tank is pressurized to

14.5 psig or 1 barg.• The velocity head is 3 ft (0.9 m).• The friction head is estimated

to be 6 ft (1.8 m).• The tank level is 22 ft (6.7 m)

above the pump centerline.a. 59 ft (18 m).b. 47 ft (14.3 m).c. 33.5 ft (10.2 m).d. 40 ft (12.2 m).

9. Discharge Static Head is the distance of the liquid surface in the discharge tank

relative to the centerline of the pump. a. or the centerline

of an open pipe above the liquid surface.

b. or the suction tank.c. and the suction tank.d. None of the above.

Review Questions1. The pumping system dictates

how a pump operates.a. True.b. False.

2. In a simple pumping system, static head is defined as the distance measured vertically above or below the pump centerline.a. True.b. False.

3. Velocity head is

a. the kinetic energy per unit weight of a flowing liquid.

b. the amount of head necessary to accelerate the fluid pumped.

c. equal to the vertical distance the mass of liquid would have to fall to acquire the velocity (in a perfect vacuum).


4. Friction head

a. increases with increasing pump TDH.

b. depends on the pump size.c. increases with increasing capacity.d. is the energy required to move

the fluid from a low elevation to a high elevation.

5. Pressure Head is the pressure in a tank that is anything other than atmospheric pressure.a. True. b. False.

CHAPTER 6

A SIMPLE

PUMPING

SYSTEM

Review

Questions

10 through 16

105

13. Calculate the NPSHA:• Specific Gravity is 1.2.• Suction Pressure is 5 psig

(0.35 barg).• Velocity Head is 2 ft (0.65 m).• Vapor Pressure is 3.5 psia

(0.24 bar-abs).a. 39.5 ft (12.0 m).b. 33.2 ft (10.01 m).c. 26.8 ft (8.2 m).d. 29.7 ft (9.1 m).

14. As suction tank level increases, NPSHA decreases.

a. True.b. False.

15. As a pump head output increases, the NPSHR

a. increases.b. decreases.c. is not affected.d. Need more information.

16. Determine the pump operating point, NPSHR and power requirement using the pump curve above for the 3 x 4 – 13 pump running at 3550 RPM and the following parameters.

• Specific Gravity is 1.5.• 10” impeller.• Suction Pressure is 10 psig.• Discharge Pressure is 205 psig.a. Capacity = 1000 GPM,

Head = 400 ft, NPSHR = 22 ft, Power = 154 HP.

b. Capacity = 600 GPM, Head = 430 ft, NPSHR = 12 ft,Power = 102 HP.

c. Capacity = 1100 GPM, Head = 300 ft, NPSHR = 32 ft, Power = 179 HP.

d. Capacity = 900 GPM, Head = 350 ft, NPSHR = 26 ft, Power = 143 HP.


10. Calculate the Total System Discharge Head with the following parameters:

• The liquid has a specific gravity of 1.5.

• The tank is not pressurized.• The velocity head is 2 ft (0.6 m).• The friction head is estimated

to be 33 ft (10 m).• The open pipe discharges into

the tank and is 143 ft (43.6 m)above the pump centerline.

a. 112 ft (34.1 m).b. 143 ft (43.6 m).c. 178 ft (54.2 m).d. 130 ft (39.6 m).

11. The pressure reduction inside the pump is sometimes called

a. “Available” NPSH.b. “Required” NPSH.c. Vapor Pressure Margin.d. Friction Loss.

12. Calculate the pump TDH for the following system:

• The specific gravity of the fluid is 0.9.

• Both tanks are atmospheric.• The velocity head for the

suction and discharge is 1 ft (0.3 m) and 4 ft (1.2 m), respectively.

• The suction has a 7 ft (2.1 m)suction lift while the discharge pipe maximum level is 65 ft (19.8 m).

• The friction loss on the suction is 3 ft (0.9 m) and the discharge is 42 ft (12.8 m).

a. 106 ft (32.3 m).b. 114 ft (34.7 m).c. 120 ft (36.5 m).d. 122 ft (37.1 m).

CHAPTER 7

TYPICAL

PUMP

FAILURES

106

CHAPTER 7

TYPICAL

PUMP

FAILURES

Introduction

Objective

RadialLoading

and ShaftDeflection

Hydraulic

Force

Unbalanced

and

Radial Loads

The

relationship

of pump

operation

away from

the Best

Efficiency

Point on

Radial Loads

107

Chapter 7 Typical Pump Failures IntroductionIf the pump and system are not properly designed for each other, significant problems can result. This chapter will look at some typicalpump and pumping system issues. These problems are found throughout all industries and, to some degree, on all centrifugal pumps. By learningwhat causes these problems and how to correct them, pump reliability can be maximized.

Inside a pump there is pressure. Pressure is constantly working on the surface areas found in the pump. One of these areas is the impeller. Forces acting on the impeller can bequite large. Luckily, many of these forces balance each other. Sometimesthey do not. This lesson will explore the causes of force imbalance and the subsequent radial loads and pumpshaft deflection that can occur.

Hydraulic Force Unbalance and Radial Loads When a pump operates at its BEP and is pumping its ideal capacity, the hydraulic forces within the volute are virtually balanced. This hydraulic balance of radial loads has a positiveeffect on pump, bearing and sealingdevice reliability. Unfortunately the pump does not always run at the BEP. At all other flows the pressure is not uniform around the impeller andhydraulic force unbalance is the result.

Radial loads are the effect of these unbalanced hydraulic forces. They can act upon a pump impeller and shaft. Radial loads move the pumpshaft in various directions causing undue strain and premature failure of sealing devices and bearings.

ObjectiveUpon completion of this chapter you will be able to describe three typical pump problems and ways to correct them. You will be able todefine and remedy causes of pump shaft deflection, bearing contamination failure and classic (vaporization) cavitation.

Radial Loading and Shaft Deflection

The relationship of pump operation away from the Best Efficiency Point on Radial LoadsPumps should be sized to operate at their BEP if possible. At a pump’s BEP, the hydraulic forces within the volute are balanced. This balance stabilizes the pump shaft and impeller increasingpump life and reliability.

As system changes cause the pump TDH requirements to move, the pumpwill no longer operate at its BEP. This movement away from the BEP causes the hydraulic forces within the volute to become unbalanced. As the forces become unbalanced,unequal radial loads try to move thepump impeller and shaft in various directions. These radial loads can be quite large. In some cases they can exceed 5000 lbs. (2300 kg.). This results in shaft bending or deflection and excessive sealing device, bearing, and other pumpmechanical failures.

CHAPTER 7

TYPICAL

PUMP

FAILURES

RadialLoading

and ShaftDeflection

The effects

of running at

Shut-off

on

Radial Loads

The effects

of running at

Non-overloading

on

Radial Loads

108

Figure 1Direction of unbalanced radial forces when throttling the pump discharge (left of the BEP),240° from pump cutwater

Figure 2Direction of unbalanced radial forces when the pump operates at excessive capacity (right of the BEP), 60° from pump cutwater

The effects of running at Shut-off on Radial LoadsPumps should be sized to operate at their BEP. As the pump TDH requirements move toward a shut-offcondition, the design point moves left of the BEP on the pump head/capacity curve. This movement of the design point to the left of BEP isoften referred as moving up, or back, on the pump curve.

This movement on the pump curve also represents an unbalanced radial load acting on the shaft at a pointapproximately 240 degrees from the cutwater counter-clockwise viewed facing suction flange. When examiningthe stuffing box area, backcover or casing, damage may be seen at approximately 60 degrees viewed facing suction flange.

The effects of running at Non-overloading on Radial LoadsPumps are designed to operate at their BEP. As the pump TDH requirements move toward a run-outcondition, the design point moves to the right of the BEP on the pumphead/capacity curve. This movement of the design point to the right of the BEP is often referred as moving down or forward on the pump curve.

This movement down the pump curve represents an unbalanced radial load effect acting toward the shaft at a point approximately 60 degrees from the cutwater counter-clockwise viewed facing suction flange. Just the opposite of what happens during throttling the discharge. When examining the stuffing box area, backcover or casing, damage may be seen at approximately 240 degrees viewed facing suction flange.

The radial forces increase greatly as the pump operating point moves back on the curve. They increase to amaximum when the pump operates at shut-off. At this point the forces are at their greatest and are causing the most damage to the pump, bearings and sealing device.

Throttleddischarge240°

Too muchcapacity60°

CHAPTER 7

TYPICAL

PUMP

FAILURES

RadialLoading

and ShaftDeflection

The effects

of running at

Non-overloading

on

Radial Loads

Shaft

Slenderness

Ratio

Shaft

Deflection

109Figure 3Pump shaft slenderness ratio measurement

The radial loads also increase, but not nearly as much as what happens at shut-off. However, running a pump at the non-overloading or run-out condition is still not good for the pump, bearings, or sealing device. It also means the pump is requiring the most power on the pump curve. The system should be checked to see if the pump can be modified to run closer to the BEP.

Shaft Slenderness RatioThe Shaft Slenderness Ratio is a ratio of shaft length to shaft diameter. It is used to determine how reliable a pump may be for a given set of operating parameters. Simply take the shaft length from the centerline of the radial bearing to the centerline of the impeller and cube it. Then take the shaft diameter (not sleeve diameter)and raise it to the fourth power. Divide the first number by the second to calculate the ratio.

The lower the ratio, the stronger the shaft and the greater the resistance to unbalanced radial loads.This means less shaft deflection.Obviously, the higher the ratio, the weaker the shaft and the less the resistance to unbalanced loads. This means more shaft deflection and possible pump failures.

Shaft DeflectionShaft deflection is the result of unbalanced radial loads. The amount of shaft deflection or bending depends on the amount of unbalance radial forces and a pump’s slenderness ratio. The higher the forces or the larger the slenderness ratio, the more shaft deflection will occur. Shaft deflection results in sealing device, bearing, and other pump mechanical failures.

Centerline ImpellerL

L

d

d

Centerline Impeller

SHAFTWITH

SLEEVE

L3

d4SOLIDSHAFT

CHAPTER 7

TYPICAL

PUMP

FAILURES

RadialLoading

and ShaftDeflection

Shaft

Deflection

The effect

of Shaft

Deflection

on Sealing

Device Failure

Troubleshooting

Rubbed Parts

110 Figure 5Chart identifying causes and symptoms of various pump maladies, including shaft deflection

Figure 4Effective pump operational zones as a function of capacity

Zone L3/d4 in–1 L3/d4 mm–1

A Sealless or Mag PumpsHigh L3/d4 Pumps (–10% to +10%) >80 >3,2

B Medium L3/d4 Pumps (–20% to +15%) 60 to 80 2,4 to 3,2

C Low L3/d4 Pumps (–40% to +20%) 26 to 60 1,0 to 2,4

D LD Pumps (–50% to +25%) < 26 <1,0

The Pump Design Should Reflect:• L3/d4 stiffness ratio less than 60 in–1 or 2,4 mm–1

• L3/d4 stiffness ratio between the bearings less than 15 in–1 or 0,6 mm–1

• Operation with sufficient NPSH to avoid cavitation

NOTE:These parameters are relevant to typical constant velocity, single volute pumps

The charts in Figure 4 below show how the radial forces change depending on the pump operating point and how the slenderness ratio limits a pump’s operating window of reliability. The pump operating window is a function of the BEP. The smaller the slenderness ratio (stronger pump)the larger the operating window.This chart is strictly a guideline which represents the operational zones ofpower ends with various stiffness ratioscombined with a specific wet end. Values shown are not representative of all wet end sizes nor indicative ofsuccessful operation at all speeds.

The effect of Shaft Deflection on Sealing Device FailureUnderstanding shaft deflection, and its effects, is crucial in increasingsealing device reliability and life. Shaft deflection causes packing, mechanical seals, and bearings to fail prematurely. When removing the sealing device or repairing a pump, it is recommended that a thoroughinspection of both the sealing device and pump be performed to identify failures attributed to shaft deflection.

The chart in Figure 5 shows how to identify worn pump parts (including the mechanical seal)to determine the cause of failure.

Cause Symptom onStationary Unit

Symptom onRotating Unit

Shaft Deflection

Pipe Strain

Bent Shaft

Bearing Failure

ImpellerUnbalanced

Shaft, PumpNot Concentric

Wrong ClearancesExcessive Expansions

All Around

All Around

All Around

All Around

All Around

All Around

All Around

One Section

One Section

One Section

One Section

One Section

One Section

One Section

Troubleshooting Rubbed Parts

PUMP CURVE

50 40 20 10 0%– + 10 15 20 25

BEP

FLOW (% OF BEP)

FLOW

RAD

IAL

LOA

D

BEP

HEA

D

CHAPTER 7

TYPICAL

PUMP

FAILURES

BearingFailure

The

function of

Bearing

Protection

Lip Seals

111

Bearing Failure

Bearing failure is one of the most common pump failures found in industry. Assuming the bearing is properly sized and installed, the most common cause of bearing failure is lubrication contamination. This lesson will identify ways to reduce lubrication contamination using various methods of bearing protection.

The function of Bearing ProtectionBearing protection functions to keep oil (lubricants) in the oil sump area and contaminants out. Contaminants include: water, dirt, and any other abrasive material that can damage the bearings. Bearing protection is located on each end of the power end. The three typical forms of bearing protection are lip seals, labyrinth seals, and gear box seals.

Lip SealsLip seals are positive contact rubbing seals that are located on the pump power end. Lip seals contact the rotating pump shaft. These seals are designed to allow a very small amount of leakage. The leakage acts as a lubricant for the lip seal contact area.

Lip seals have some very limiting drawbacks. The majority of these seals have a small finite life span. This averages out to be 2- 4 months on most pumps. Another problem with lip seals is that they fret or groove the shaft over time. This fretting allows oil leakage out and contaminants into the pump, not to mention the damage it does to the shaft.

Figure 6Typical lip seals

Examples of Special Purpose Shells

Environment

GarterSpring

SealingElementPrimary

Sealing Lip

Bearing Side

Shell

Examples of External Designs

Open Packing Spring-Lock Spring-Cover

•

•

•

•

CHAPTER 7

TYPICAL

PUMP

FAILURES

BearingFailure

Labyrinth

Seals

112Figure 7Typical labyrinth seal

Labyrinth SealsLabyrinth seals are another type of bearing protection. They are non-contacting seals. This means that they have parts that do not contact each other and so they do not wear.

1. Stationary Element: Nickel plated steel

2. Mounting Ring: Secure mounting without equipment alterations

3. Positive O-rings: Non-slip, will not fret or damage shaft

4. Oil drain: Helps retain lubricant in bearing housing

5. Hooded Design: Prevents direct penetrationof dripping fluids

6. Labyrinth: Traps liquid contaminantsand directs to gravity drain

7. Rotary Element: 316 stainless steel for corrosionresistance, acts as slinger when rotating

8. Gravity drain: Allows liquid trapped by labyrinth to safely exit

Labyrinth seals take many forms but most are constructed with a series of grooves and teeth to provide protection against intrusion of contaminants. These grooves are often located in the rotor of the labyrinth seal, but can also be found in the stator. On the rotor the grooves rotate and expel contaminants by centrifugal force into the stator that does not rotate. This contamination then flows by gravity out the drain groove located at the bottom of the stator and away from the oil and bearings.

Labyrinth seals have a minimum of three possible leak paths that have to be eliminated. The first path is along the shaft. This path is shut down by at least one, sometimes more, static o-rings. Static o-rings do not damage the shaft like lip seals because they rotate with the shaft and are not moving relative to it. The second path is around the stator. This can be eliminated by a gasket or o-ring. The third path is, of course, between the rotor and stator. This is the path in which the labyrinth seal gets its name. As stated above, the grooves form a tortuous path in which contamination is reduced and often eliminated.

Labyrinth seals typically last a lifetime and eliminate shaft damage as there is no rubber lip running against the shaft as incorporated with lip seal designs. These seals are becoming very popular in industry. Their advantages over lip seals are that they do not damage the shaft and they provide a much better form of protection at virtually unlimited life.

52 •1 • •

6•

7•8•

4 •

3••

•

CHAPTER 7

TYPICAL

PUMP

FAILURES

BearingFailure

Gear Box

Seals

113Figure 8Typical gear box seal

Gear Box SealsGear box seals are another type of bearing protection. This seal operates very similar to a mechanical seal. There is a stationary element installed in the gear box bore and a rotary element that rotates with the pump shaft.

The rotary and the stationary each contain a seal ring. These seal rings are lapped extremely flat and contact each other to seal in lubricants and seal out contaminants. The face flatness also eliminates any air exchange between the air inside the bearing housing and the surrounding atmosphere. Because these seals operate like amechanical seal they can be installed in vertical applications to contain the oil and eliminate contamination. They can also be used where the equipment may become immersed or completely flooded. A typical gear box seal is shown in Figure 8.A - Rotary faceB - Positive drive O-ringC - Seal housingD - RetainerE - Stationary faceF - SpringG - Secondary O-ringH - Excluder ring

C•D•E•

G•B •

A • F•

H•

CHAPTER 7

TYPICAL

PUMP

FAILURES

BearingFailure

The effect of

contamination

on

Bearing Life

The

limitations of

Lip Seals

on

Bearing Life

114

Figure 9Bearing life reduction vs. water contamination

Figure 10Lip seal life for two common lip seal manufacturers

The effect of contamination on Bearing LifeLubrication contamination is the cause of nine out of ten premature bearing failures. The calculated life of bearings by design in an average pump is over 20 years. Actual life is often less than 3 years. The most common contaminant found to cause bearing failure is water.

Research conducted by Mobil Oil and Lubrication Engineering found an alarming problem with water contamination of bearing oil. The following chart shows water content measured against the corresponding percentage of bearing life reduction with a given load. It can be concluded that a minimalamount of water contamination greatly reduces bearing life. Because water contamination can not be seen by the naked eye untilapproximately 0.1% concentration, the oil may contain water and this fact is not known until it is too late.

The limitations of Lip Seals on Bearing LifeLip seals are the most commonly used form of bearing protection. These seals usually use a lip made from rubber to provide a seal. They are installed in the bearing bore of the pump power end on each side.

Because lip seals contact the pump shaft, the rubber lip wears as the shaft rotates. Eventually, the lip wears enough to cause a leak path at the point of contact between the seal and the shaft allowing excessive leakage of lubricant. This leak path also allows contaminants to enter the pump power end and reduce bearing life.

The charts below show the life of two typical lip seal manufacturers both graphically and in tabular format. Note that there are 8760 hours in one year. This means that nearly 100% of the lip seals from both manufacturers will have failed in 57 days and 125 days, respectively for pumps that run 24 hours a day, 7 days a week.Water

Content(%)

Bearing Life Reduction

(%)

0.002 480.014 543.0 786.0 83

National

Hours To Failure

(%Failure) Median

Rank

CR Waveseal®

HoursTo Failure

182 6 .70 144188 16 .32 576275 25 .94 1343300 35 .57 1736556 45 .19 1763648 54 .81 1973772 64 .43 1979902 74 .06 2946

1264 83 .68 29821366 93 .30 3003

CHAPTER 7

TYPICAL

PUMP

FAILURES

BearingFailure

The

limitations of

Lip Seals

on

Bearing Life

The

limitations of

Labyrinth

Seals

115

Figure 11Lip seal life for two common lip seal manufacturers

Figure 12Illustration of a fretted shaft caused by a lip seal

Figure 13A typical labyrinth seal

Another limitation with lip seals is that they fret or groove the shaft overtime. This fretting damages the shaft.When worn lip seals are replaced with new seals, they will not seal properly on a damaged shaft. The shaft often has to be repaired or replaced. The illustration below shows a shaft damaged by a lip seal.

The limitations of Labyrinth SealsLabyrinth seals are becoming more popular to use as a form of bearing protection. There are several manufacturers of labyrinth seals.

Labyrinth seals are two-piece, non-contacting seals that are installed in the bearing bore of the pump power end on each side. Labyrinth seals do not contact the pump shaft at the sealing point so they do not fret the pump shaft like lip seals. Labyrinth seals form a tortuous path that makes it difficult for lubricants to exit or contaminants to enter the pump power end. Since labyrinth seals do not wear out, they will virtually last a lifetime.

One limitation to labyrinth seals is the fact that they are not positive sealing devices. This means that pressure on either side of the seal can, if pushed through the tortuous path, result in lubricants exiting and/or contaminants entering the pump power end. They also cannot be used in verticalapplications, as oil would drain out.

Fretted Shaft

PERC

ENT

FAIL

URE

TIME HOURS TO FAILURE

■ CR (Mean Life 1844 hours)● National (Mean Life 645 hours)

•

CHAPTER 7

TYPICAL

PUMP

FAILURES

BearingFailure

The

limitations of

Gear Box

Seals

The effect of

Bearing Failure

on pump

operation

116

The limitations of Gear Box SealsA Gear box seal is the best seal type to use as a form of bearing protection. There are several manufacturers of gear box seals. This seal operates very similar to a mechanical seal. There is a stationary element installed in the pump bearing bore and a rotary element that rotates with the pump shaft. Because gear box seals do not contact the pump shaft at the primary sealing point, they do not fret the shaft like lip seals.

Gear box seals are designed to operate under positive pressure in the oil sump area making it impossible for lubricants or air to exit or contaminants or air to enter the pump power end. They can be installed vertically to eliminate the oil leakage that occurs with lip seals. The main limitation to gear box seals is that they can wear out over time. However, since the lubricating media of this seal is oil, their life is still quite long.

The effect of Bearing Failure on pump operationBearings play a critical role in pump operation. The purpose of the bearing is to allow the shaft to rotate with virtually no friction. The lower the friction, the lower the horsepower requirements.

Bearings also function to hold the rotating element in position relative to the stationary parts of the pump. This includes radially, as well as axially, so rubbing will not occur. This means that they must be able to absorb any forces transmitted to them from the impeller, motor, etc.

Being able to absorb these forces means greater reliability in the sealing device chosen for the pump. Any movement of the shaft can cause sealing device failure and other mechanical damage.

Choosing a pump with a good shaft slenderness ratio, the appropriate bearing type and the appropriate bearing protection will help to ensure pump reliability and life. Bearing protection that is effective in eliminating leakage and lubrication contamination will ensure high pump reliability.

Figure 14A typical gear box seal

CHAPTER 7

TYPICAL

PUMP

FAILURES

ClassicCavitation

Net

Positive

Suction Head

Required (NPSHR)

117Figure 15Pump pressure profile showing point of lowest pressure (NPSHR) inside a pump casing

Cavitation means many things to many people. It also can come in a variety of forms. The most common type of cavitation is VaporizationCavitation or “Classic Cavitation.”This lesson will describe the causes of Classic Cavitation and how it can be reduced or eliminated.

Net Positive Suction Head Required (NPSHR)As liquid enters the pump, there is a reduction of pressure and subsequent head. This head reduction is a function of the specific pump and is determined by laboratory testing to be stated by thepump manufacturer on a pump curve.Net Positive Suction Head Required(NPSHR) is the measurement of this head reduction to determine the minimum suction head conditionrequired to prevent the liquid from vaporizing in the pump.

As shown below, the pressure drops to a minimum level at the impeller eye before the liquid is acted upon by the impeller vanes. It is this minimumvalue that is determined by testing and given the name Net Positive Suction Head Required (NPSHR).

Classic Cavitation

C

DE

BA

POINT OF LOWEST PRESSURE WHERE VAPORIZATION CAN OCCUR (NPSHR)

INCR

EASI

NG

PRE

SSU

RE

A


ENTRANCELOSS

FRICTION

TURBULENCEFRICTION

ENTRANCELOSS AT

VANE TIPS

INCREASINGPRESSURE

DUE TOIMPELLER

B C D E

•

•

CHAPTER 7

TYPICAL

PUMP

FAILURES

ClassicCavitation

Net

Positive

Suction Head

Available (NPSHA)

Vaporization

or “Classic”

Cavitation

118

Net Positive Suction Head Available (NPSHA)Net Positive Suction Head Available(NPSHA) is the difference between thetotal suction system head and the fluidvapor pressure at the suction flange inabsolute terms. NPSHA depends on the system layout and must always begreater than NPSHR to ensure properpump operation and eliminate cavitation.

Knowing the liquid vapor pressure at the pumping temperature, we can calculate NPSHA from the following equations:

NPSHA = hpsa + hss – hfs – hvp

Where: NPSHA = Net Positive Suction

Head Availablehpsa = Suction Surface Pressure

converted to feet ormeters of Head Absolute

hss = Suction Static Head hfs = Suction Friction Head

hvp = Vapor Pressure of theLiquid at the PumpingTemperature convertedto feet or meters of Head Absolute

NPSHA = ha + hgs + hvs – hvp

Where: NPSHA = Net Positive Suction

Head Availableha = Atmospheric Pressure

converted to feet ormeters of Head Absolute

hgs = Suction Pressure Gauge converted to feet ofmeters of Head

hvs = Suction Velocity Head hvp = Vapor Pressure of the

Liquid at the PumpingTemperature convertedto feet or meters of Head Absolute

Figure 16NPSHA calculation using the friction loss estimation

Figure 17NPSHA calculation using a suction pressure gauge

Vaporization or “Classic” CavitationCavitation has a variety of meanings to different people. The most precise definition of cavitation is the formation and subsequent collapse of vapor-filled pockets in a liquid. It can also be called VaporizationCavitation or “Classic” Cavitation.

Vaporization cavitation occurs when the pressure inside the pump drops below the vapor pressure of a given liquid. We learned in the previous discussion that at theimpeller eye the pressure is at its minimum. As this pressure drops and eventually reaches this minimum,vaporization can and does occur within the liquid. This vaporizing of the liquid forms bubbles, starting at the eye, that travel along the trailing edge of the impeller vanes to the higher pressure areas at the vane tips where they implode. This implosion sounds like rocks or marbles in the volute of the pump. As these bubbles implode on theimpeller, they can severely damage it by removing metal.

CHAPTER 7

TYPICAL

PUMP

FAILURES

ClassicCavitation

The

relationship

of NPSH to

Vaporization

Cavitation

119

The relationship of NPSH to Vaporization CavitationWe already know that at the impeller eye the pump pressure is at its lowest point. It is this minimum pressure value that we gave the name Net Positive Suction Head Required (NPSHR). This value is a function of the pump design and manufacturer and is determined by testing.

We also defined Net Positive Suction Head Available (NPSHA)as the difference between the total suction system head and the fluid vapor pressure at the suction flange in absolute terms. This value is a function of the system and is determined from our knowledge of the system configuration.

We defined vaporization cavitation as the formation of vapor bubbles when the liquid pressure in the pump falls below the vapor pressure of the liquid. This can also be defined as the formation of vapor bubbles when total system head, minus the pressure drop of the pump, is less than the liquid vapor pressure.

In other words, vaporization cavitation occurs when NPSHA (the system) falls below NPSHR (the pump requirement). Conversely, vaporization cavitation can be eliminated or reduced when we raise the NPSHA or lower the NPSHR. The following statement is a good place to start for NPSH issues.

NPSHA > NPSHR + 3 ft. (1m)

The difference between NPSHR andNPSHA depends on the pump size,design, speed, etc. Sometimes it’s more than 3 ft. (1m) and sometimes it’s less.

To raise the NPSHA we can do the following:• Raise the liquid level

in the suction tank• Elevate the suction tank• Lower the pump• Reduce the suction piping

friction losses• Pressurize the suction tank• Lower the liquid temperature• Install a booster pump

in the suction

To lower the NPSHR we can do the following:• Use a double suction

(split-case) pump• Use a larger pump at

a lower speed• Increase the impeller eye

diameter• Install an inducer on the

impeller• Use several smaller pumps• Reduce the pump capacity

It is very important to be able to change either the system or the pump if vaporization cavitation is a problem. If nothing can be done, it is imperative to use a pump that is built rugged enough to handle the cavitation.

CHAPTER 7

TYPICAL

PUMP

FAILURES

ClassicCavitation

The effect of

Vaporization

Cavitation

on Pump

Hydraulic

Operation

120Figure 18Illustration of head-capacity drop due to varying suction conditions

The effect of VaporizationCavitation on Pump Hydraulic OperationVaporization cavitation will definitelyaffect pump hydraulic operation. When bubbles form in the liquid theytake up space and reduce the head and capacity that can be produced by the pump. More vaporization means more bubbles which also means more head and capacity loss.

This resultant fall-off of both head and capacity varies. The head-capacitypump performance curve may only partially breakdown with cavitation or it may breakdown much more. A typical grouping of head-capacitycurves as the NPSHA falls is shownbelow. Note how the head-capacitycurve falls sharply as the NPSHA fails to exceed the NPSHR for a typical pump.

This type of graphical representation of the head-capacity drop is very difficult to determine in existing systems without very expensive equipment. The most important item to remember is that when a pump is cavitating due to vaporization, the existing printed head-capacity curve from the manufacturer will not be accurate.

Normal Head –Capacity CurvewhereNPSHA > NPSHR

NPSH

A =

30’ (9 m)

NPSH

A =

20’ (6 m)

NPSH

A =

10’ (3 m)

NPSH

R = 25’ (7.5 m

)

NPSH

R = 15’ (4.5 m

)

NPSH

R = 5’ (1.5 m

)

CHAPTER 7

TYPICAL

PUMP

FAILURES

ClassicCavitation

The effect of

Classic or

Vaporization

Cavitation

on Pump

Mechanical

Operation

121

Figure 20Illustration of vaporization cavitation damage to trailing edge of impeller near the eye

Figure 19Illustration of a vapor bubble imploding on impeller surface

The effect of Classic or Vaporization Cavitation on Pump Mechanical OperationVaporization cavitation will also affect pump mechanical operation. The result of this cavitation is sometimes severe mechanical damage. This damage usually occurs on the impeller, but it is sometimes found on the volute. The extent of this damage depends on the severity of the cavitation and the hardness of the impeller and volute materials.

When bubbles of vapor implode on a metal surface they do so in such a way as to form a toroidal shape as shown in Figure 19. This shape creates pressures in excess of 150,000 psi (10,000 bar). No known materials can withstand this type of punishment. The damage will begin near the impeller eye and continue along the trailing edge of the impeller vanes as shown in Figure 20.

The collapse of these bubbles creates extreme noise and vibration. This vibration can, and often does, cause other mechanical damage to the bearings or mechanical seal.Reduced pump reliability is the ultimate result.

Rotation

Increasing Pressure

Impeller Surface

VaporBubble

Cavitation damage fromlow NPSHA

•

•

CHAPTER 7

TYPICAL

PUMP

FAILURES

Review

Questions

1 through 8

122

4. Which statement is incorrect?a. As the pump TDH

requirements move toward a non-overloading condition, the design point moves to the right of the BEP on the pump head/capacity curve.

b. Movement of the design point to the right of the BEP is often referred to as moving down the pump curve.

c. When a pump is running near non-overloading an unbalanced radial load is acting on the shaft at a point approximately 240 degrees from the cutwater.

d. At the BEP, the pump hydraulic loads are balanced.

5. As the Shaft Slenderness Ratio increases, the pump reliability decreases. a. True.b. False.

6. More shaft deflection will result from pumps with large slenderness ratios. a. True.b. False.

7. A rub mark all the way around the shaft and a spot on the backcover is a symptom of

a. a bent shaft.b. shaft deflection.c. bearing failure.d. impeller unbalance.

8. Which of the following are positive contact rubbing seals that are located on the pump power end? a. Labyrinth seals.b. Lip seals.c. Gear box seals.d. All of the above.

Review Questions1. Which statement is incorrect?

a. When a pump operates at its Best Efficiency Point (BEP), the hydraulic forces within the volute are unbalanced.

b. Hydraulic balance of radial loads has a positive effect on pump reliability and life.

c. Radial loads move the pump shaft in various directions causing undue strain and premature failure of sealing devices and bearings.

d. Radial loads are the effect of unbalanced hydraulic forces that act upon a pump impeller and shaft.

2. Which statement is correct?a. At a pump’s BEP,

the hydraulic forces within the volute are unbalanced.

b. Pumps should be designed to operate at their BEP.

c. As the system changes causing the pump TDH requirements to move, the pump will continue to operate at its BEP.

d. As the flow changes, a pump’s operating point remains the same.

3. As the pump TDH requirements move toward shut-off, the design point moves right of the BEP on the pump head/capacity curve.a. True.b. False.

CHAPTER 7

TYPICAL

PUMP

FAILURES

Review

Questions

9 through 17

123

14. Gear box seals will seal oil in vertical applications.a. True.b.False.

15. Vaporization cavitation is defined as air bubbles that get trapped in the liquid causing noise and impeller damage.a. True.b.False.

16. NPSHA should be greater than NPSHR to avoid vaporization cavitation.a. True.b.False.

17. The effect of vaporization cavitation on the pump head-capacity curve is

a. difficult to determine for a pump installed in the field.

b.a drop in head and capacity.c. usually overlooked when

trying to figure out pump operating point.

d.All of the above.


9. Which of the following are non-contacting seals?a. Lip seals.b.Labyrinth seals.c. Gear box seals.d.None of the above.

10. In a gear box seal, the o-ring on the rotary

a. rotates with the shaft.b.will not fret or damage

the shaft.c. seals in the lubrication. d.All of the above.

11. As little as 0.002% water in the oil can cause bearing life to decrease by

a. about one-half.b.10%.c. about one-quarter.d.0.1%.

12. Which statement is incorrect?a. Lip seal life is usually

less than one year.b.Lip seals cause shaft

damage over time.c. Lip seals never allow the

bearing lubrication to become contaminated.

d.Lip seals will usually leak when installed on a damaged shaft.

13. One limitation to labyrinth seals is the fact that they are not positive sealing devices.a. True.b.False.

PUMP

PRINCIPLES

MANUAL

INDEX

124

PUMP

PRINCIPLES

MANUAL

INDEX

Index

A through F

125

Index Pump Principles Manual AAbsolute pressure.

See Pressure, absolute . . . . . . . . . . . .Absolute viscosity.

See Dynamic viscosity. . . . . . . . . . . . . .ANSI B73 Pump Standard

origin of . . . . . . . . . . . . . . . . . . . . .14purpose of . . . . . . . . . . . . . . . . . . 15

Archimedean Screw . . . . . . . . . . . .5, 6ASME B73.1M Pump Standard . . . . .14Atmospheric pressure.

See Pressure, atmospheric . . . . . . . . . .

BBackcover . . . . . . . . . . . . . . . . . . . . .26Baseplate . . . . . . . . . . . . . . . . . .39- 40Bearing failure . . . . . . . . . . . . . . . . . 111

effect on pump operation . . . . . . 116Bearing housing . . . . . . . . . . . . . . . .29Bearing protection . . . . . . . . . . . . . . .31

function of . . . . . . . . . . . . . . . . .111Bearings . . . . . . . . . . . . . . . . . . . . . .30

radial . . . . . . . . . . . . . . . . . . . . . .30thrust . . . . . . . . . . . . . . . . . . . . . .30

Bedplate. See Baseplate. . . . . . . . . . . . . .Belts . . . . . . . . . . . . . . . . . . . . . . . . .37BEP. See Best efficiency point . . . . . . . . .Best efficiency point . . . . . . . . . . 70, 75

operating away from . . .76, 107-108Breather . . . . . . . . . . . . . . . . . . . . . .34

CCapacity . . . . . . . . . . . . . . . .51, 68, 72

ideal . . . . . . . . . . . . . . . . . . . . . . .70relation with total dynamic head . .72

Casing. See Volute. . . . . . . . . . . . . . . . .Cavitation . . . . . . . . . . . .101, 117, 119

effect on pump hydraulic operation . . . . . . . . . . . . . . . . .120

effect on pump mechanical operation . . . . . . . . . . . . . . . . .121

relationship with Net Positive Suction Head . . . . . . . . . . . . . . .119

C-frame motor adapter. See Motor adapter . . . . . . . . . . . . . . .

Classic cavitationSee Cavitation . . . . . . . . . . . . . . . . . .

CCooling jacket

See Jacket, stuffing box . . . . . . . . . . .Couplings . . . . . . . . . . . . . . . . . . . . .36

spacer . . . . . . . . . . . . . . . . . . . . . .36

DDead-head

See Shut-off . . . . . . . . . . . . . . . . . . .Deflector . . . . . . . . . . . . . . . . . . . . . .33Density . . . . . . . . . . . . . . . . . . . . . . .48D-frame motor adapter

See Motor adapter . . . . . . . . . . . . . . .Dilatant fluids . . . . . . . . . . . . . . . . . .50Discharge friction head

See Head, discharge friction . . . . . . . .Discharge pressure head

See Head, discharge pressure . . . . . . .Discharge pressure

See Pressure, discharge . . . . . . . . . . . .Discharge static head

See Head, discharge static . . . . . . . . .Discharge velocity head

See Head, discharge velocity . . . . . . . .Discharge

See Pump, discharge . . . . . . . . . . . . .Double suction pump

See Pump, impeller between bearings centrifugal pump . . . . . . . .

Dynamic viscosity . . . . . . . . . . . . . . .50

EEfficiency . . . . . . . . . . . . . . . . . . . . .70

pump . . . . . . . . . . . . . . . . . . . . . .70Energy . . . . . . . . . . . . . . . . . . . . . . .62

conversion inside pump . . . . . . . . . 62kinetic . . . . . . . . . . . . . . . . . . . . . .62potential . . . . . . . . . . . . . . . . . . . .62pressure . . . . . . . . . . . . . . . . . . . .62

FFace seals . . . . . . . . . . . . . . . . . . . . .32Face seals.

See also Gear box seals . . . . . . . . . . . .Flooded suction . . . . . . . . . . . . . .85, 90Foundation . . . . . . . . . . . . . . . . . . . .40Frame adapter . . . . . . . . . . . . . . . . .29

PUMP

PRINCIPLES

MANUAL

INDEX

Index

F through M

126

FFriction head

See Head, friction . . . . . . . . . . . . . . . .Friction loss . . . . . . . . . . . . . . . . . . . .86

K factor chart . . . . . . . . . . . . . . . .88pipe chart . . . . . . . . . . . . . . . . . .87valves and fittings . . . . . . . . . . . .88

Frontcover . . . . . . . . . . . . . . . . . . . .27

GGauge pressure

See Pressure, gauge . . . . . . . . . . . . . .Gear box seals . . . . . . . . . . . . . . . . .113

limitations of . . . . . . . . . . . . . . . .116Gears . . . . . . . . . . . . . . . . . . . . . . . .38Grout . . . . . . . . . . . . . . . . . . . . . . . .40

HHead . . . . . . . . . . . . . . . . . . . . .51, 68

converting to pressure . . . . . . . . . .53discharge friction . . . . . . . . . . . . . .93discharge pressure . . . . . . . . . . . . .94discharge static . . . . . . . . . . . . . . . .92open end pipe above liquid surface . . . . . . . . . . . . . . . .93

discharge velocity . . . . . . . . . . . . .93friction . . . . . . . . . . . . . . . . . . . . .86pressure . . . . . . . . . . . . . . . . .62, 89static . . . . . . . . . . . . . . . . . . . . . . .84suction static . . . . . . . . . . . . . .89, 90suction friction . . . . . . . . . . . . . . .91suction pressure . . . . . . . . . . . . . .91suction velocity . . . . . . . . . . . . . . .91total dynamic . . . . . . . . . .69, 95, 96total system . . . . . . . . . . . . . . .95, 96effect when discharge friction loss changes . . . . . . . . . .97

effect when discharge pressure head changes . . . . . . . .98

effect when discharge tank level changes . . . . . . . . . . . . . . .97

effect when suction friction loss changes . . . . . . . . . . . . . . .97

effect when suction pressure head changes . . . . . . . . . . . . . .98

effect when suction tank level changes . . . . . . . . . . . . . . .96

relationship with total dynamic head . . . . . . . . . . .95, 96

when velocity head and pressure head cancel . . . . . . . . .96

Htotal system discharge . . . . . . . . .94total system suction . . . . . . . . . . .91velocity . . . . . . . . . . . . . . . . .62, 86

Hydraulic forces . . . . . . . . . . . . .76, 107Hydraulics . . . . . . . . . . . . . . . . . . . . .48

IIdeal capacity . . . . . . . . . . . . . . . . . .75Impeller . . . . . . . . . . . . . . . . . . . . . .20

axial flow . . . . . . . . . . . . . . . . . . .22closed . . . . . . . . . . . . . . . . . . . . . .23completely open . . . . . . . . . . . . . .23diameter . . . . . . . . . . . . . . . . . . . .71mixed flow . . . . . . . . . . . . . . . . . .21radial flow . . . . . . . . . . . . . . . . . . .21semi-open . . . . . . . . . . . . . . . . . .23shapes of . . . . . . . . . . . . . . . . . . .20trimming . . . . . . . . . . . . . . . . . . . .79

Impeller between bearings centrifugal pump See Pump, impeller between bearings centrifugal pump.

JJack-bolts . . . . . . . . . . . . . . . . . . . . .41Jacket

power end . . . . . . . . . . . . . . . . . .34stuffing box . . . . . . . . . . . . . . . . . .26

KKinematic viscosity . . . . . . . . . . . . . .50

LLabyrinth seals . . . . . . . . . . . . .32, 112

limitations of . . . . . . . . . . . . . . . .115Lantern ring . . . . . . . . . . . . . . . . . . .27Lip seals . . . . . . . . . . . . . . . . . .31, 111

limitations of . . . . . . . . . . . . . . . .114Lubrication contamination

effect on bearing life . . . . . . . . . .114

MMaximum capacity

See Non-overloading . . . . . . . . . . . . .Motor adapter . . . . . . . . . . . . . . . . .37Motor jack-bolts

See Jack-bolts . . . . . . . . . . . . . . . . . .Motors . . . . . . . . . . . . . . . . . . . . . . .35

PUMP

PRINCIPLES

MANUAL

INDEX

Index

N through P

127

NNet Positive Suction Head

definition of . . . . . . . . . . . . . . . . .99Net Positive Suction Head

Available . . . . . . . . . . . . . . . . . . .118calculation using friction loss estimation . . . . . . . . . . . . .100, 118

calculation using suction pressure gauge . . . . . . . . .100, 118

definition of . . . . . . . . . . . .100, 118effect of changing capacity . . . . .101liquid temperature effects . . . . . .100methods to increase . . . . . . . . . .120relationship with Net Positive Suction Head Available . . . . . . .101

static head effects . . . . . . . . . . . .101suction piping effects . . . . . . . . . .100

Net Positive Suction Head Required . . . . . . . . . . . . . . . .69, 117definition of . . . . . . . . . . . . .99, 117effect of changing capacity . . . . . .76methods to decrease . . . . . . . . . .120

Newtonian fluids . . . . . . . . . . . . . . . .50Non-overloading . . . . . . . . . . . . . . .77

effect on pump power requirement . . . . . . . . . . . . . . . .78

NPSHA See Net Positive Suction Head Available . . . . . . . . . . . . . . . . . . . . .

NPSHR See Net Positive Suction Head Required . . . . . . . . . . . . . . . . . . . . . .

OOil flinger . . . . . . . . . . . . . . . . . . . . .34Oil sight glass . . . . . . . . . . . . . . . . . .33Oil sump . . . . . . . . . . . . . . . . . . . . . .33Operating window

See Pump operating zones . . . . . . . . .PPower

brake . . . . . . . . . . . . . . . . . . . . . .71water . . . . . . . . . . . . . . . . . . . . . .70

Pressure . . . . . . . . . . . . . . . . . . . . . .45absolute . . . . . . . . . . . . . . . . . . . .46atmospheric . . . . . . . . . . . . . . . . .45converting to head . . . . .52-53, 102discharge . . . . . . . . . . . . . . . . . . .61gauge . . . . . . . . . . . . . . . . . . . . . .46suction . . . . . . . . . . . . . . . . . . . . .60vapor . . . . . . . . . . . . . . . . . . .47, 89temperature affects . . . . . . . . . . .56

PPressure head

See Head, pressure . . . . . . . . . . . . . . .Pump

centrifugal . . . . . . . . . . . . . . . . . . . .9classifications of . . . . . . . . . . . . . . .8definition of . . . . . . . . . . . . . . . . . .5discharge . . . . . . . . . . . . . . . . . . .60fluid flow through . . . . . . . . . . . . .61fluid velocity change . . . . . . . .61-62impeller between bearings centrifugal pump . . . . . . . . . . . . . .9

differences with overhung impeller centrifugal pump . . . . . .42

kinetic . . . . . . . . . . . . . . . . . . . . . . .9multi -stage . . . . . . . . . . . . . . . .6, 43operating point determination . . . . . . . . . .102-103

operation . . . . . . . . . . . . . . . . . . .59overhung impeller centrifugal pump . . . . . . . . . . . . . . . . . . . . . .9driver parts . . . . . . . . . . . . .35-38power end parts . . . . . . . . .28-34sections of . . . . . . . . . . . . . .17-41support structure parts . . . . .39-41wet end parts . . . . . . . . . . .18-27

positive displacement . . . . . . . . .6, 10reciprocating . . . . . . . . . . . . . . . .12controlled volume . . . . . . . . . . .13power . . . . . . . . . . . . . . . . . . . .12steam . . . . . . . . . . . . . . . . . . . .13

rotary . . . . . . . . . . . . . . . . . . . . .10gear . . . . . . . . . . . . . . . . . . . . .11lobe . . . . . . . . . . . . . . . . . . . . .11screw . . . . . . . . . . . . . . . . . . . .11vane . . . . . . . . . . . . . . . . . . . . .11

power requirement . . . . . . . . . . . .71viscosity and specific gravity effects . . . . . . . . . . . . . . . . . . . . .75

purpose of . . . . . . . . . . . . . . . . . . .7size . . . . . . . . . . . . . . . . . . . . . . . .72speed . . . . . . . . . . . . . . . . . . . . . .71suction . . . . . . . . . . . . . . . . . . . . .59

Pump curves See Pump performance curves . . . . . .

Pump operating zones . . . . . . . . . .110Pump performance curves . . . . . .65-79

composite . . . . . . . . . . . . . . . .66-67single . . . . . . . . . . . . . . . . . . . . . .65viscosity effects . . . . . . . . . . . . 73-74

Pump system . . . . . . . . . . . . . . . . . .83elements of . . . . . . . . . . . . . . . . . .83

PUMP

PRINCIPLES

MANUAL

INDEX

Index

Q through Z

128

QQ. See also Capacity . . . . . . . . . . . . . . . .

RRadial bearings

See Bearings, radial . . . . . . . . . . . . . .Radial loads . . . . . . . . . . . . . . . . . .107

effects of running at non-overloading . . . . . . . .108-109

effects of running at shut-off . . . . . . . . . . . . . . . . . .108

Rubbed partstroubleshooting of . . . . . . . . . . . .110

Run-out See Non-overloading . . . . . . . . . . . . .

SSealing device . . . . . . . . . . . . . . . . . .25

mechanical packing . . . . . . . . . . . .25mechanical seal . . . . . . . . . . . . . . .25

Shaft . . . . . . . . . . . . . . . . . . . . . . . .29Shaft deflection . . . . . . . . . . . . . . . .109

effect on sealing devices . . . . . . .110Shaft slenderness ratio . . . . . . . . . .109Sheaves . . . . . . . . . . . . . . . . . . . . . .37Shims . . . . . . . . . . . . . . . . . . . . . . . .41Shut-off . . . . . . . . . . . . . . . . . . . . . .77

effect on pump power requirement . . . . . . . . . . . . . . . .78

effects of . . . . . . . . . . . . . . . . . . . .77Slinger

See Deflector . . . . . . . . . . . . . . . . . . .Snap ring . . . . . . . . . . . . . . . . . . . . .30Specific gravity . . . . . . . . . . . . . . . . .49

pressure effects . . . . . . . . . . . . . . .49temperature effects . . . . . . . . . . . .54

Split case pump See Pump, impeller between bearings centrifugal pump . . . . . . . . .

Static head See Head, static . . . . . . . . . . . . . . . . .

Stiffness ratio See Shaft slenderness ration . . . . . . . .

Stuffing box . . . . . . . . . . . . . . . . . . .24Suction friction head

See Head, suction friction . . . . . . . . . .Suction lift . . . . . . . . . . . . . . . . .85, 90Suction pressure head

See Head, suction pressure . . . . . . . . .

SSuction pressure

See Pressure, suction . . . . . . . . . . . . .Suction static head

See Head, suction static . . . . . . . . . . .Suction velocity head

See Head, suction velocity . . . . . . . . . .Suction

See Pump, suction . . . . . . . . . . . . . . .System

See Pump system . . . . . . . . . . . . . . . .

TTDH

See Head, total dynamic . . . . . . . . . . .Thixotropic fluids . . . . . . . . . . . . . . . .50Thrust bearing cartridge . . . . . . . . . .30Thrust bearings

See Bearings, thrust . . . . . . . . . . . . . .Total dynamic head

relation with capacity . . . . . . . . . . .72Total dynamic head

See Head, total dynamic . . . . . . . . . . .Total system discharge head

See Head, total system discharge . . . .Total system head

See Head, Total System . . . . . . . . . . . .Total system suction head

See Head, total system suction . . . . . .

VVacuum . . . . . . . . . . . . . . . . . . . . . .46

convert to head . . . . . . . . . . . . . . .91Vapor pressure

See Pressure, vapor . . . . . . . . . . . . . . .Vaporization cavitation

See Cavitation . . . . . . . . . . . . . . . . . .Velocity head

See Head, velocity . . . . . . . . . . . . . . .Viscosity . . . . . . . . . . . . . . . . . . . . . .50

temperature effects . . . . . . . . . . . 55unit comparison . . . . . . . . . . . . . .50

Volute . . . . . . . . . . . . . . . . . . . . . . . .19

WWaterwheel . . . . . . . . . . . . . . . . . .5-6Wear rings . . . . . . . . . . . . . . . . . . . .24

PUMP

PRINCIPLES

MANUAL

Answersto the

Review Questions

Chapter 1 – Pumps1. b, 2. b, 3. b, 4. a, 5. c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Chapter 2 – Pump Parts1. a, 2. b, 3. c, 4. a, 5. c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

Chapter 3 – Pump Terms1. b, 2. d, 3. b, 4. b, 5. b, 6. b, 7. b, 8. d, 9. b, 10. b, 11. b . . . . . . . . . . . . . . . . .57

Chapter 4 – Pump Operation1. b, 2. b, 3. a, 4. d, 5. a, 6. b, 7. a, 8. a, 9. b . . . . . . . . . . . . . . . . . . . . . . . . . . . .63

Chapter 5 – Pump Curves1. c, 2. c, 3. b, 4. b, 5. a, 6. b, 7. b, 8. b, 9. c, 10. a, . . . . . . . . . . . . . . . . . . . . . .8011. a, 12. d, 13. a, 14. b, 15. b, 16. b, 17. d, 18. d, 19. d, 20. b, 21. d . . . . . . . .81

Chapter 6 – A Simple Pumping System1. a, 2. a, 3. d, 4. c, 5. a, 6. d, 7. a, 8. b, 9. a, . . . . . . . . . . . . . . . . . . . . . . . . . .10410. c, 11. c, 12. b, 13. b, 14. b, 15. b, 16. c . . . . . . . . . . . . . . . . . . . . . . . . . . . .105

Chapter 7 – Typical Pump Failures1. a, 2. b, 3. b, 4. c, 5. a, 6. a, 7. b, 8. b, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1229. b, 10. d, 11. a, 12. c, 13. a, 14. a, 15. b, 16. a, 17. d . . . . . . . . . . . . . . . . . . .123

Review Question Answers

A.W.CHESTERTON CO.Middlesex Industrial Park, 225 Fallon RoadStoneham, Massachusetts 02180-9101 USATelephone: 781-438-7000Fax: 781-438-8971Web Address: www.chesterton.com© A.W.CHESTERTON CO., 2000. All rights reserved.® Registered trademark owned and licensed by

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