rotating equipment chapter 3 pumps

Technical Training Programme Rotating Equipment

TriStar T.S M - RE (Rev. 3) May 2004 Page 1 of 125

CHAPTER 3

PUMPS



CHAPTER 3

PUMPS

Objectives:

At the end of this chapter the trainee will be able to:

Understand pump types, components, application, performance,auxiliaries, operation and maintenance.



CHAPTER 3

PUMPS

CONTENTS Page Number

SECTION 3.1

The Function of the Pumps

SECTION 3.2

Classification of Pumps

3.2.1 Positive displacement pumps 93.2.1.1 Reciprocating positive displacement pump .. 103.2.1.2 Rotary positive displacement pumps 13

3.2.2 Dynamic pumps 153.2.2.1 Pump theory .. 153.2.2.2 Centrifugal pumps . 15

SECTION 3.3

Centrifugal Pump Components

3.3.1 Centrifugal pump components .. 17

3.3.2 Function of the components 18

3.3.3 Stuffing boxes & mechanical seal .. 203.3.3.1 Packed stuffing box 213.3.3.2 How does it work 223.3.3.3 Lantern rings .. 233.3.3.4 Arrangements of the lantern ring to meet

specific services . 233.3.3.5 Packing selection 243.3.3.6 Conventional packing draw backs . 25

3.3.4 Mechanical seals 263.3.4.1 Mechanical seals overview . 263.3.4.2 Mechanical seals construction 263.3.4.3 Sealing points for mechanical seal 283.3.4.4 How does it work .. 283.3.4.5 Advantages of mechanical seals 293.3.4.6 Comparison between conventional packing

and mechanical seals . 30



SECTION 3.4

Classification of Centrifugal Pumps

SECTION 3.5

Pump Performance Curves, Pump Power and Efficiency

3.5.1 Factors affecting pump performance . 423.5.2 Effects of specific gravity . 423.5.3 Effect of viscosity . 433.5.4 Specific speed 433.5.5 Typical characteristic curves for a centrifugal pump 443.5.6 Pump power .. 48

3.5.6.1 Definition .. 483.5.6.2 Pump power . 483.5.6.3 Pump power and efficiency .. 49

SECTION 3.6

Pump Operation

3.6.1 Safety 503.6.2 Priming . 503.6.3 Starting . 503.6.4 Running 513.6.5 Stopping .. 513.6.6 Operation against closed discharge . 51

SECTION 3.7

Operating Difficulties

3.7.1 General .. 52

SECTION 3.8

Cavitations

3.8.1 What is the cavitation? 543.8.2 The main reasons of cavitation .. 543.8.3 What is the effect of cavitation on the pump . 563.8.4 Symptoms of cavitation . 56



SECTION 3.9

Pump Auxiliaries

3.9.1 Pump drive 573.9.2 Couplings . 583.9.3 Strainers 58

SECTION 3.10

Pump Maintenance

3.10.1 Safety .. 593.10.2 Lubrication . 593.10.3 Gland packing . 603.10.4 Mechanical seal .. 613.10.5 Coupling . 613.10.6 Overhauling overhung pump shaft . 61

3.10.6.1 General instructions . 613.10.6.2 Dismantling . 623.10.6.3 Inspection of components 663.10.6.4 Assembly . 68

3.10.7 Maintenance of centrifugal pump(in between bearings pumps) ... 713.10.7.1 Dismantling the pump . 713.10.7.2 Inspection 723.10.7.3 Reassembling the pump .. 74

SECTION 3.11

Reciprocating Pumps

3.11.1 How it works 763.11.2 Reasons for using reciprocating pumps .. 763.11.3 Disadvantages of reciprocating pumps 763.11.4 Pump classification .. 773.11.5 Liquid end components 80

3.11.5.1 The liquid cylinder . 803.11.5.2 Pumping element .. 803.11.5.3 Stuffing boxes 833.11.5.4 Valves 91

3.11.6 Drive End Components 94



3.11.7 Flow Characteristics . 953.11.8 Power Pump Drive Systems .. 983.11.9 System design .. 98

3.11.9.1 The suction vessel should 983.11.9.2 The suction piping should 993.11.9.3 The discharge piping should 99

3.11.10 Remedies for low NPSHA 1013.11.11 Unloading the pump .. 1013.11.12 Slurry applications 103

3.11.12.1 Stuffing box area (packing) . 1033.11.12.2 Pump valves . 1043.11.12.3 Plunger or piston rod

(in case of piston pump) .. 104

3.11.13 Reciprocating Pump Maintenance 1043.11.13.1 Liquid end components maintenance . 1043.11.13.2 Drive end components maintenance .. 106

SECTION 3.12

Pulsation Dampeners

3.12.1 The function of pressure pulsation dampener 1083.12.2 Installation . 108

3.12.2.1 Mounting . 1083.12.2.2 Precharging . 109

3.12.3 General precharging instructions .. 1093.12.4 Maintenance 110

3.12.4.1 Precharge .. 1103.12.4.2 Troubleshooting 1113.12.4.3 Diaphragm removal . 1113.12.4.4 Diaphragm installation . 112

SECTION 3.13



SECTION 3.1

FUNCTION OF PUMPS

A wide variety of pumps are used in petroleum industry. A pump is used toincrease the total energy content of a liquid in the form of pressure increase.Pumps transfer liquids, for example, between vessels. They are the fluid moversof liquids.

The pumps are used to perform one of the following jobs:

1- Move liquids from low level to high level (figure 3.1)2- Move liquids from low pressure location to high pressure location

(figure 3.2)3- To increase the flow rate of a liquid (figure 3.3)

Figure 3.1 Figure 3.2Move liquid from low level to high level Move liquid from low pressure

location to high pressure location



Figure 3.3To increase the flow rate of liquid



SECTION 3.2

CLASSIFICATION OF PUMPS

The most common classification of pumps is based on the way energy is addedto the liquid and pump geometry. They are classified into two main categories:

Positive Displacement Pumps Dynamic Pumps

3.2.1 Positive Displacement Pumps

Energy is added to the liquid by the application of force that moves the liquidfrom the low-pressure side (suction) to the high-pressure side (discharge).

See figure 3.4

Figure 3.4 Positive displacement pump

They are classified as follows:

Positive displacement Pumps

Reciprocating Reciprocating Diaphragm 1- Gear pumpPiston Pump Plunger Pump Pump 2- Lobe pump

3- Screw pump

Reciprocating Pumps Rotary Pumps



3.2.1.1 Reciprocating Positive Displacement Pumps

Reciprocating positive displacement pumps include three designs:

1- Reciprocating piston pump (figure 3.5 and figure 3.6)2- Reciprocating plunger pump (figure 3.7)3- Diaphragm pump (figure 3.8)

Figure 3.5 Reciprocating piston pump (single acting)

Figure 3.6 Double acting reciprocating piston pumps



Figure 3.7 Plunger pump

Fig. 3.8 Diaphragm pump



Figure 3.9 Suction and discharge strokes of diaphragm pump



3.2.1.2 Rotary Positive Displacement Pumps

Single rotorScrew pumps, sliding vane pumps, etc.

Multiple rotorGear pumps (figure 3.10, 3.11), lobe pumps (figure 3.12) and screwpumps (figure 3.13).

Figure 3.10 External gear pump

Figure 3.11 Internal gear pump



Figure 3.12 Lobe pump

Figure 3.13 Screw pump



3.2.2 Dynamic Pumps

The liquid velocity is increased inside the pump to a value higher than thedischarge velocity. Velocity reduction within or after the pump is converted topressure.

3.2.2.1 Pump Theory

The rotating impeller imparts a centrifugal force and kinetic energy in the formof velocity to the liquid.

They are classified as follows:

3.2.2.2 Centrifugal Pumps3.2.2.3 Special Pumps

3.2.2.2 Centrifugal Pumps

How it Work?

Figure 3.14 shows the impeller and pump casing of centrifugal pump. Let ussee how it works?

Figure 3.14 Centrifugal pump

1. Liquid flows through the pump inlet and into the eye of the impeller.

2. The impeller whirls the liquid around in a circle. The liquid is forcedfrom the center to the outside of the impeller.Centrifugal force pushes the liquid outward from the eye.



3. Liquid enters the pump casing when it leaves the outer edge of theimpeller.When the liquid enters the casing, speed decreases, as the speed of theliquid decreases, its pressure increases.

4. As centrifugal force moves the liquid away from the impeller eye, a low-pressure area (zone) is formed in the suction eye. This low pressure areain the suction eye causes liquid to flow into the suction eye.

A typical centrifugal pumps is show in figure 3.15.

Figure 3.15 Typical centrifugal pump



SECTION 3.3

CENTRIFUGAL PUMP COMPONENTS

3.3.1 Centrifugal Pump Components

Pump consist of rotating components (rotor) and stationary components.Figure 3.16 show centrifugal pump components.

Figure 3.16 Horizontal single stage centrifugal pump



The following table shows the correct name for each item.

Table 3.1 Recommended names of centrifugal pump parts

3.3.2 The Function of Pump Components

1- Impeller

An impeller is the part which imparts energy to the liquid being pumped.Energy is added to the liquid as it moves through the rotating vanes of theimpeller (figure 3.17)



Figure 3.17 The impeller

2- Shaft

The impeller is firmly attached to the shaft and rotates with it. The shaftperforms two jobs:

Carry the impeller (s) and all other rotating parts and keep them in theircorrect position with respect to the pump casing.

Transmit the required driving power to rotate the impeller (s)

3- Shaft Sleeve

To protect the shaft from wear in stuffing box area. As spacer between different impellers in multi-stage pump.

4- Coupling

Transmits the required power to drive the pump shaft and all other rotatingparts.

5- Wear Rings

One wear ring is fixed to the impeller and rotate with it (impeller wear ring).One wear ring is fixed to the pump casing and does not rotates (case wear ring).

These two wear rings together work to minimize the internal leakage inside thepump.



6- Pump Casing

It contains all rotating parts (shaft, impeller, impeller wear ring) etc.Pump casing directs the liquid which leaves the impeller to the discharge nozzle(pump discharge).

7- Stuffing Box

It is a cylindrical cavity where the shaft passes into the casing. The packingmaterial presses around the shaft in this cylindrical cavity to minimize theleakage of liquid to outside the pump.A mechanical seal may be used instead of packing.

8- Bearings

Its function is to carry the pump rotor and keep it in its correct position withrespect to the casing.

3.3.3 Stuffing Boxes

Any pump converts the energy of a prime mover, such as an electric motor, intovelocity or pressure energy of the liquid being pumped.

In a centrifugal pump, the product enters the suction of the pump at the center ofthe rotating impeller. See Figure 3.18

Figure 3.18 Fluid flow in a centrifugal pump



As the impeller vanes rotate, they transmit motion to the incoming product,which then leaves the impeller, collects in the pump casing, and leaves thepump under the pressure through the pump discharge. Discharge pressure willforce some product down behind the impeller to the drive shaft, where itattempts to escape along the rotating shaft. Pump manufacturers use variousdesign techniques to reduce the pressure of the product trying to escape. Suchtechniques include:

1. The addition of balance holes through the impeller to permit most of thepressure which acting behind the impeller to escape into the suction sideof the impeller. (figure 3.19)

2. The addition of small pumping vanes on the back side of the impeller.(figure 3.20).

However, as there is no way to eliminate this pressure completely, sealingdevices are necessary to limit the escape of the product to the atmosphere. Suchsealing devices are typically either compression packing or mechanical seal.

Figure 3.19 Back wear ring and Figure 3.20 Back vanesbalancing holes

3.3.3.1 Packed Stuffing Box

Stuffing boxes have the primary function of protecting the pump againstleakage at the point where the shaft passes out through the pump casing.



If the pump handles a suction lift and the pressure interior stuffing box end isbelow atmospheric, the stuffing box function is to prevent air leakage into thepump.

If this pressure is above atmospheric, the function is to minimize liquid leakageout the pump.

3.3.3.2 How Does it Work?

1- Early attempts to control the leakage of the product around rotating shaftsconsisted of merely restricting the clearance between the shaft and thewall of the pump casing by packing a soft, resilient material around theshaft within an extension of the pump back head called a stuffing box.

2- Figure 3.21 Shows a typical stuffing box sealed with square rings ofcompression packing.

3- The compression packing rings, which must be carefully installed in aclean stuffing box, are held in place by a gland.

4- As the gland bolt nuts are tightened, pressure applied to the gland istransmitted to the compression packing, forcing it against the shaft orshaft sleeve and effecting a seal. Because this pressure is not evenlydistributed throughout the packing, most of the sealing and consequentlymost the wear occurs in the first few rings adjacent to the gland. (Figure3.22)

5- Frictional heat, which develops where the compression packing contactsthe rotating shaft or shaft sleeve, is reduced by permitting the product toleak to the atmosphere at a controlled rate. This leakage is essential tocarry away the frictional heat and as lubricant between the shaft (or shaftsleeve) as rotating element and the packing rings as stationary element.

Figure 3.21 Stuffing box with Figure 3.22 Pressure distributioncompression packing



3.3.3.3 Lantern Rings

Figure 3.23 Lantern ring Figure 3.24 Stuffing box with lantern ring

The lantern ring (figure 3.23) is a device made from a rigid material such asbronze, stainless steel, nylon or TFE, and is of open construction to allow freepassage of sealing liquid ( or lubricant). Normally, the sealing liquid (orlubricant) enters the outside of the ring, and flows to fill the space between thepacking rings and the shaft ( or shaft sleeve). The lantern ring usually haspacking rings on either side (figure 3.24)

3.3.3.4 Arrangements of the Lantern Ring to Meet SpecificServices

1- When a pump operates with negative suction head

See Figure 3.25 a:

The inner end of the stuffing box (product side) is under vacuum, and air tendsto leak into the pump. For this type of service, packing is usually separated intotwo sections by a lantern ring (seal cage).

Sealing fluid is introduced under pressure into the space, causing flow ofsealing fluid in both axial directions. This construction is useful to assure liquidfor cooling and lubrication between the packing rings and the shaft or shaftsleeve.



Figure 3.25 Arrangement of lantern ring to meet specific services

2- See Figure 3.25 b

This construction is useful for pumps handling flammable or chemically activeand dangerous liquids since it prevents outflow of the pumped liquid.

3- If the product being pumped is too contaminated with abrasives

See Figure 3.25 c:

Clean liquid flush to lantern ring to prevent dirty liquid to enter the stuffing boxarea. If the abrasives lodged between the packing rings and the shaft (or shaftsleeve) it will act completely like a cutting tool against the shaft or shaft sleeve.

3.3.3.5 Packing Selection

Factors that must be considered in selecting a packing involve:

The fluid's conditions, such as temperature, lubricity and pressure.

All equipment parameters:

Shaft speed.

Shaft size (in stuffing box area).

Stuffing box dimensions.

Continuous or intermittent service.



3.3.3.6 Conventional Packing Draw-Backs

The drawbacks of conventional packing are:

Packing operates on the principle of controlled leakage. They neverattempt to totally prevent fluid from leaking from the equipment. Thisleakage will cause:

a. Waste of product, b. Pollution.

It requires regular adjustment of the gland.

Pressure limits: Packing not suitable selection for high pressure workingconditions like water injection pumps.

Power consumption: The packing consume more power. Packingrubbing on a shaft (or shaft sleeve) similar to driving an automobile withthe handbrake engaged. This relatively high power consumption willincrease the running cost.

Maintenance cost: Most of the time, the shaft ( or shaft sleeve) should bechanged due to damage. The rubbing between the packing rings and theshaft will cause score marks and rough surface on the shaft in the stuffingbox area. That means extra maintenance cost and more downtime. Besidethis, most bearing failure is caused by contamination rather thanoverloading. The easiest way to contaminate a bearing is from theleakage coming through the packing.

Speed limits: Packing have limited speed, if you try to use it in speedshigher than its limits, the failure will happen.

The argument for packing usually centers around four statements:

1. You don't have to take the pump apart to change packing.2. In an emergency, you can always add a ring of packing.3. Packing is cheaper.4. Packing is less complicated.

Let's look at each of these statements if it is true:



Statement 1:

You do have to take the pump apart to change sleeves and bearings. Shaftsleeve replacement is a normal part of repacking a pump. The fact of the matteris that you will have to dismantle a packed pump more than a sealed pump.

Statement 2:

If you need reliability, use a mechanical seal with an auxiliary packing gland.

Statements 3:

Packing is cheaper if you consider the packing alone. Bicycles are also cheaperthan automobiles.

Statement 4:

Packing is less complicated only to an inexperienced man. If you have ever triedto teach an apprentice how to inspect a stuffing box and shaft, cut packing,install it so as to align the lantern ring, tamp it in place, and adjust it properly soas to keep leakage to a minimum and not generate excessive heat (you have todo it by feel), then you know just how complicated packing really is.

3.3.4 Mechanical Seals

3.3.4.1 Mechanical Seals Overview

The mechanical seals was developed to overcome the disadvantages ofcompression packing. Leakage can be reduced to a level meeting theenvironmental standards.

3.3.4.2 Mechanical Seals Construction

All mechanical seals are constructed of four basic sets of parts. As shown infigure 3.26, these are:

1. A set of seal faces which are called sometimes primary sealing device.One that rotates (rotating face) and one that is stationary (stationary face).

2. A set of secondary seals known as secondary sealing device or gasketssuch as 0-rings, wedges, U-cups and V-rings.



3. Spring (s).

4. Mechanical seal hardware including seal flange (gland ring), shaft sleeve,etc.

Figure 3.26 A simple mechanical seal



3.3.4.3 Sealing Points for Mechanical Seal

There are four main sealing points, (see figure 3.26)

1. The primary seal is at the seal face, point A.

The primary seal is achieved by two very flat, lapped faces which create adifficult leakage path perpendicular to the shaft. Rubbing contact betweenthese two flat mating surfaces minimizes leakage.

As in all mechanical seals, one face is held stationary in a housing(stationary face), and the other face is fixed to, and rotates with the shaft(rotating face). These two faces are made from two dissimilar materials,one of them is softer than the other. For example carbon graphite (assoft face), the other is usually hard material (as tungsten carbide as hardface). The seal faces are made from two different materials in order tohelp prevent adhesion of the two faces.

2. The leakage path at point B (between the floating seal face and the shaftor shaft sleeve) is blocked by floating seal face gasket (either 0-ring, U-cup, a V-ring, or a wedge).

3. Leakage path at point C (between the seal flange and stuffing box face) isblocked by seal flange gasket which could be 0-ring or any other shape ofstatic gaskets.

4. Leakage path at point D (between the seal flange and the stationary face)is blocked by stationary face gasket or seat gasket.

3.3.4.4 How Does it Work?

1. The two flat seal faces are pushed together by axial force from the closingmechanism (spring or metal bellows) and by product pressure in thestuffing box cavity.

2. When the seal is in operation, the two seal faces are lubricated by thesame product inside the stuffing box. It is known that, for the seal to workefficiently, it is necessary for a stable fluid film to exist between the sealfaces. In the majority of cases this film is a liquid. The function of thisliquid film between the seal faces is for cooling (carry away the frictionalheat) and lubrication. If this film stability is destroyed, excessive weartakes place leading to rapid seal failure.



3. Improperly positioned seals could allow a wide gap between the faces,causing a leak path. The faces could also be squeezed so tightly togetherthat no lubrication is present, causing rapid seal failure.

3.3.4.5 Advantages of Mechanical Seals

Mechanical seals replace packing in stuffing boxes where the liquid must becontained. These seals offer:

1. Reduced friction power loss.

2. Eliminate the wear on the shaft or shaft sleeve in the stuffing box area.

3. Invisible or minimum leakage.

4. Ability to function in relative extremes of shaft deflection and end play.

5. Suitable for high working pressures and high running speeds.



3.3.4.6 Comparison Between Conventional Packing andMechanical Seals

Packing

1. How Does it Work?

Figure 3.27

Packing (figure 3-27) forced into a stuffing box around the shaft. It seals bythrottling the fluid trying to leak between the packing and shaft. Packing wearsthe shaft and increases the power needed to rotate the shaft.

2. Shaft Run-Out

Shaft run-out is one costly enemy of conventional packing. It beats out packing,making sealing problem tough. If the shaft run-out is over 0.003 inch, it'simpossible to seal properly, especially at high speeds.



Mechanical Seal

1. How Does it Work?

Figure 3.28

Mechanical seal (figure 3-28) has two seal faces at right angles to the shaft. Oneseal face is fastened to the shaft and revolves with it, while the other isstationary and is held against the machine casing. The wearing faces that sealhave a small area compared to the area conventional packing seals against.Because of this small area, and the preloaded spring(s) forcing the two facestogether, there's less friction at the seal faces. And of course there is no wear onthe shaft because seal faces take it all, they can relapped or replaced whenneeded.

2. Shaft Run-Out

Mechanical seals can take more shaft run-out without leaking. Reason is thatsealing faces are at right angles to shaft. The elastomeric gaskets and thespring(s) allow for some misalignment between the seal faces which couldhappen due to shaft run-out.



Figure 3.29 Effect of shaft run-out

3. The Effect of Shaft Axial Float on Packing

Figure 3.30 The effect of shaft axial play



End play (shaft axial play) is common with most shafts especially when startingup or shutting down. Such shaft movement does not affect the packing if shafthas no grooves in packing area. But usually shafts or sleeves do groove after ashort while. Then shaft end play disturbs packing, open it up and causesleakage.

The Effect of Shaft Axial Float on Mechanical Seal

Figure 3.31 The effect of shaft axial play on the mechanical seal

Shaft end play (shaft axial play) does not affect the mechanical seal if this endplay within certain limits (about 0.003" for rolling element bearings as a thrustbearing and about 0.015" for slide surface bearing as a thrust bearing). Thespring (s) will keep the seal faces close.

4. Power Consumption

It is relatively high in case of packing (about three times the power consumptionin mechanical seal for the same shaft size and speed).

5. The Required Time for Replacing:

Figure 3.32 Replacing packing and mechanical seal



In case of Packing:

Can be packed in place. It needs adjusting several times after start upuntil it reach the normal running conditions.

Mechanical Seal:

Installed over shaft end. It needs more time for installation. It does notneed any additional adjustment after the installation.

6. Pollution

Is relatively high in case of packing because the packing must leak for coolingand lubrication.

In case of mechanical seal in normal running conditions, less leakage i.e. lesspollution. Double seals are able to stop product leakage 100%.

7. Cost of Product

In case of packing, the cost of product is high- due to high leakage rates.

In case of mechanical seal, the cost of product is low- due to very smallleakage rates.

8. High Pressures and Big Shaft Diameter Services

The packing not suitable sealing device for big shaft diameter or highpressures.

The mechanical seal is suitable sealing device for big shaft diametersand / or high running speed.



SECTION 3.4

CLASSIFICATION OF CENTRIFUGAL PUMPS

Centrifugal pumps can be classified with respect to the following parameters:

1- With respect to the impeller design.

2- With respect to the flow of liquid after it leave the impeller.

3- With respect to the pump case design.

4- With respect to the split of the casing.

5- With respect to the number of stages.

6- With respect to the shaft position.

1- With Respect to the Impeller Design

Single suction impeller (figure 3-33) or double suction impeller (figure 3-34)

Figure 3.33 Single Suction Impeller



Figure 3.34 Double suction

2- With Respect to the Flow of Liquid After Leave the Impeller

Radial flow Axial flow mixed flow (figure 3.35)

Axial flow Mixed flow Radial flow

Figure 3.35



3- The Pump Case Design

The pump case could be volute design (figure 3.36) or double volute(figure 3.37) or diffuser type (figure 3.38)

Figure 3.36 Volute design pump case



Figure 3.37 Double volute pump case

Figure 3.38 Diffuser type pump



4- The Split of the Casing

Could be axial split casing (figure 3-39) or radial split casing (figure 3-40)

Figure 3.39 Axial split casing

Figure 3.40 Radial split casing



Fig

ure

3.41



5- Number of Stages

The pump could be single stage (one impeller) figure 3-16 or multi stage(figure 3-41).

6- With Respect to the Shaft Position

The pump could be horizontal shaft (figure 3-41) or vertical shaft (figure 3-42)

Figure 3.42 Vertical pump



SECTION 3.5PUMP PERFORMANCE CURVES

The centrifugal pump is now the most widely used type of pumps in thepetroleum industry. It must be correctly sized, fitted, and installed to operatesatisfactorily. This section is a guide to the selection, application, andlimitations of centrifugal pumps.

3.5.1 Factors Affecting Pump Performance

The following factors does affect the performance of centrifugal pump:

1- Running speed.2- Impeller size (diameter).3- Liquid specific gravity.4- Liquid viscosity.5- NPSH (net positive suction head)

At constant speed the characteristics of centrifugal pumps are:

1- The capacity varies directly as the diameter of the impellers; if thediameter of the impeller is increased 10% the capacity is increased 10%.

2- The head or pressure developed by the pump varies directly as the squareof the diameter of the impeller; if the diameter of the impeller is reducedto 90% of its original diameter, the head developed is reduced to 90%squared, or 81% of the original head.

3- The horsepower required varies as the cube of the impeller diameter, ifthe impeller diameter is reduced to 90% of its original value, thehorsepower required reduced to 90% cubed, or 72.9% of its originalvalue.Under ideal conditions, a change in speed of the pump changes thecapacity, head and horsepower in the same ratio or proportions directly,square and cube as does the same percentage change in impeller size.

3.5.2 Effects of Specific Gravity

Because the discharge pressure and horsepower required to drive the pump are afunction of the specific gravity of the liquid, both are affected in directproportion to changes in specific gravity.



Centrifugal pump can push a heavy liquid and a light liquid to the same height,even if water is piped to the pump instead of lighter oil, it continues to raise theliquid to the same elevation. The pressure on the discharge line would increaseas a result of the higher specific gravity of water. The pump would have toexert more force and would automatically require more horsepower.

3.5.3 Effect of Viscosity

Viscosity is a measure of the friction between the particles in a liquied,molasses has a high viscosity, water a low viscosity. A change in viscosity willchange the capacity, head, efficiency and brake horsepower (input to the pumpshaft) requirements of a pump. These effects are hardly noticeable up to about70 SUS; however, above viscosities of 100 SUS, these effects become quitepronounced, causing a reduction in efficiency and a drop in head at a given flowrate.

SUS (Saybolt Universal Second) is the standard unit of measurement forviscosity of oil in the American petroleum industry. It is the number of secondsrequired for a measured quantity of oil at a constant temperature to run througha small hole in the tube of a standard saybolt viscosimeter. An oil which takes60 second to run through this hole is said to have a saybolt universal secondsviscosity of 60, or 60 SUS.

A change in temperature greatly affects the viscosity of some crude oils; usuallythe higher the temperature, the lower the viscosity.

3.5.4 Specific Speed

Specific speed is a term used to compare the performance of impellers,irrespective of their size. Specific speed (usually designated by the symbol NS)is the speed, in revolutions per minute, at which a geometrically similar impellerwould run if it were or such size as to discharge one gallon per minute againstone foot head.

The specific speed equation is

NS =4

3

2

1

H

QN

Where N = impeller speed, revolutions per minute.Q = capacity. Gallons minuteH = total head in feet per stage

Specific speed should be thought of as an index or type number referring to theperformance or general proportions of an impeller, but not to its actual size orrevolutions per minute.



3.5.5 Typical Characteristic Curves for a Centrifugal Pump

Two methods are commonly used for plotting the characteristic curves of acentrifugal pump.

Figure 3-43 shows the method used in presenting pump performance for asingle speed and a fixed impeller size, these curves result from a test of a pumpat a constant speed and are curves a manufacturer commonly uses to certify theperformance of a pump.

Figure 3-44 shows the method used to express more fully the entire range ofperformance of a pump with maximum and minimum diameters of impellers ata given speed. These curves are commonly used in the selection of a pump fora specified service.

The curves in figure 3.44 are made-up from the average performances of anumber of tests for various diameter impellers that have been ploted in the formshown in figure 3-43.

Figure 3-45 shows a third method of plotting characteristic curves for acentrifugal pump driven at variable speed but with fixed impeller diameter.

Characteristic curves of the pump and the system through which it pumpsshould be plotted on the same chart. These curves will assist in the selection ofthe proper pump for the desired service, and will show the effect of operatingconditions, such as changes in speed, viscosity or system characteristics.

Practically, all performance curves furnished by manufactures are for pumpingwater; if the pump is to handle some other liquid, proper corrections must bemade for viscosity and specific gravity.



Fig

ure

3.43



Fig

ure

3.44



Fig

ure

3.45



3.5.6 Pump Power

There are three factors does affect the power consumed. These factors are:

1- The flow rate (Q).2- The discharge pressure.3- The specific gravity of the liquid.

3.5.6.1 Definitions

Capacity: The pump capacity Q is the volume of liquid per unit time deliveredby the pump.

In English measure it is usually expressed in gallons per minute (GPM) and forlarge pumps, in cubic feet per second (ft3/sec).

In metric measure the units are liters per second (L / Sec.) and cubic meters persecond (M3 / Sec)

Head: The pump head (H) represents the net work done on a unit weight ofliquid in passing from the inlet or suction flange (S) to the discharge flange (d).

Power: There are liquid horsepower and mechanical horse power.

Liquid horsepower = Lhp =960,3

QSH

Q in GPMS specific gravityH in feet

3.5.6.2 Pump Power

B.H.P. = Horsepowereffeciency2450

PsipressurealdifferentiBPH

B.H.P. = Breake horsepowerB.P.H. = Barrels per minuteDifferential pressure = in Psi

In metric system B.H.P. =



3.5.6.3 Pump Power and Efficiency

The power is sometimes called the liquid power or hydraulic power. The inputpower, P (in kW), represents the driving shaft mechanical power and issometimes called brake power if we allow for friction losses.

The pump hydraulic power = .g.Q.HThe pump efficiency = .g.Q.H / P

Where, Q pump capacity in m3 / s, H total pressure head in m, liquiddensity in kg/m3, g is the acceleration due to gravity = 9.81 m/s2.



SECTION 3.6

PUMP OPERATION

3.6.1 Safety

Ensure that all protective guarding is in position and securely fixed.

3.6.2 Priming

The pump must not be run dray at any time. The pump casing and inlet linemust be completely filled with liquid before the pump will operate.

3.6.3 Starting

In general, the following procedure should followed:

1- When fitted ensure that flushing and / or cooling liquid supplies areturned on.

2- Close the outlet valve (on the discharge line of the pump).

3- Be sure that the suction valve (on the suction line of the pump) is open.

4- Prime the pump

5- Start the motor and immediately check outlet pressure.

If the gauge does not register positive pressure, stop the motor and checkfor air leakage or any other possible cause.

6- If the pressure is satisfactory, slowly open outlet valve. Do not operatethe pump with valve closed for more than a few minutes.



3.6.4 Running

1- Packed gland should leak, and leakage should take place soon after thestuffing box is pressurized. Until steady leakage takes place, the pumpmay overheat. If this happens, the pump should be stopped and allowedto cool and when re-started, leakage should take place.

In general, gland nuts should not be slackened but when hot liquids arebeing pumped, this may become necessary. After the pump has beenrunning for ten minutes with steady leakage, tighten the gland nuts.Continue to tighten gland nuts until leakage is reduced to an acceptablelevel. When adjustment is completed there should be drip leakage fromthe gland, ensuring that overheating does not take place.

2- With a mechanical seal, no adjustment is necessary and any slight initialleakage will disappear when the seal is run in.

3.6.5 Stopping

Close outlet valve and switch off motor.

3.6.6 Operation Against Closed Discharge

Closing the discharge valve on an operating centrifugal pump reduces the flowrate even though the pressure is a maximum. However, the power consumed by

the pumps is not zero, it is about2

1 the rated power because of friction of parts

and the churning of the enclosed liquid. This condition requires that precautionbe taken.

No centrifugal pump should be operated against a closed discharge valve. Itmay be necessary to hold the valve closed or nearly closed for few second whenstarting the unit. Power absorbed in rapidly churning the liquid results in adangerous temperature increase. This condition is usually provided for bymeans of a small by pass with a check valve around the discharge valve.Another protective measure is the installation of by pass with valve to thepump sump or the suction side of the pump.



SECTION 3.7

OPERATING DIFFICULTIES

3.7.1 General

This section gives information on fault diagnosis and possible remedies tooperating difficulties. The matrix in next page details a list of ten possiblesymptoms to which the possible cause or causes can be ascertained by readingoff composite the black rectangles.



Figure 3.46 Fault diagnosis matrix



SECTION 3.8

CAVITATION

3.8.1 What is the Cavitation?

The formation and subsequent collapse of vapor-filled cavities in a liquid due todynamic action are called cavitation. The cavities may be bubbles, vapor-filledpockets, or a combination of both. Cavitation happens if the local pressurebecomes equal to or below the vapor pressure of the liquid at this temperature,and the cavities must encounter a region of pressure higher than a vaporpressure in order to collapse.

Dissolved gases are often liberated shortly before vaporization begins.This maybe an indication of impending cavitation, but true cavitation requiresvaporization of the liquid. Bubbles which collapse on a solid boundary maycause severe mechanical damage. All known materials can be damaged byexposure to bubble collapse for a sufficiently long time. This is properly calledcavitation erosion or pitting.

3.8.2 The Main Reasons of Cavitation

Centrifugal pumps begin to cavitate when the suction head (NPSH) isinsufficient to maintain pressures above the vapor pressure throughout the flowpassages. This could happen due to one or more of the following reasons:

1. The unfavorable inlet flow conditions, believed to have been thecause of the cavitation, were at least partly due to elbows in theapproach piping. Elbows in the suction side of the pump willcause additional pressure drop beside the pressure drop due to theflow of liquid inside the suction line (frictional pressure drop).Modification to the approach piping and the pump inlet passagesreduced the cavitation.

2. In between/bearing pump double-suction impeller, cavitation canhappen in this type of pumps, in addition to the reason #1, due to abend (elbow) in suction line in the horizontal level figure 3.47



This bend (elbow) may cause uneven distribution of flow to theimpeller. The liquid will flow toward the outside of the elbow andresult in an uneven flow distribution into the two inlets (suctioneyes) of the double suction impeller. One side of the impeller willget enough liquid and there is no enough liquid on the other side,(figure 3-47). In such cases, there is a great probability ofcavitation to happen in one suction eye of the impeller which doesnot get enough liquid.

There is another thing that will happen prior to cavitation, onestuffing box-on the side where there is no enough liquid - willsuffer from pressure drop, which can change completely thehydraulic forces on the seal faces.

When such elbow cannot be avoided, it should be in a verticalposition if possible. Where it is necessary for some reason to use ahorizontal elbow, it should be a long radius elbow and there shouldbe a minimum of two diameters of straight pipe between the elbowand the pump suction as shown in figure B.

Figure 3.47 Horizontal elbow in suction line



3. Blockage in the suction side also can cause cavitation. If thesuction valve is not fully opened or strainers on the suction side ispartially blockaged additional pressure drop will occur. This maylead to cavitation.

4. Another reason can cause cavitation in deep-well pumps. Deep-well pumps are usually provided with a check valve close to thedischarge. However, this valve cannot prevent the liquid elevatedin the column from flowing back into the well and creating avacuum between the liquid level and the check valve. When thispump is started again, vaporization due to this vacuum may lead tocavitation shocks.

3.8.3 What is the Effect of Cavitation on the Pump?

1. One of the effects of Cavitation is almost always to cause the pump to rununevenly with strong radial and axial vibrations. If the axial vibration dueto cavitation increased and becomes as strong axial vibration, this cancause rapid breakdown of the. sealing faces of the mechanical seal.

2. The large pressure pulsation in the stuffing box can have a determintaleffect on the performance and life of a mechanical seal. If the pressure inthe stuffing box reduced to certain limit, in this case there is no enoughhydraulic pressure to create a liquid film between the seal faces and thenthe seal running dry.

3. The bubbles which collapse when reaching a zone of higher pressure mayexert enormous local stresses on the surfaces on which they collapse,causing damage.

3.8.4 Symptoms of Cavitation

1- Simmering noises and cracking are heared.2- Severe axial vibration in the pump shaft.3- Unsteady state of flow of liquid. Liquid is mixed with air and gas

bubbles.



SECTION 3.9PUMP AUXILIARIES

3.9.1 Pump Drive

Electrical Motors

Motors are the major drivers to supply energy to rotating pumps. The motorpower should be greater than the pump input power in order to allow for frictionand other types of losses.

Steam Turbines

The availability of steam may suggest a turbine drive. A steam turbine drive isusually chosen where exhaust or high-pressure steam is available.

Gas Turbines

A gas turbine drive may be used to power a rotating pump. Gas turbines areusually available in the large size power.

Gas Engines

A gas engine drive may be used when gas is available at a low cost. Gas enginesare used to provide power to drive reciprocating or centrifugal pumps.

Diesel engine

This drive is similar to the gas engine drive. The difference is the fuel used. Thecompression ratio of a diesel engine is greater than that of a gas engine.



3.9.2 Couplings

On most centrifugal pumps, couplings (Figure 3.48) join the shafts of thedrivers and pumps. Alignment of shafts is very important for good and smoothoperation of the pump. Misalignment will cause the shaft and other pump partsto vibrate. Serious vibration can generate enough stresses to break the shaft andcoupling. Also, vibration can cause bearings to wear, internal parts to rub sothey become unbalanced, etc. All of these conditions require maintenance andresult in equipment downtime.

3.9.3 Strainers

The primary function of a strainer is to protect the equipment. Normallystrainers are placed in the line at the inlet to pumps, control valves or any otherequipment that should be protected against damage. The strainer is selected forthe design capacity of the system at the point where it is to be inserted in theline.

Figure 3.48 Couplings



SECTION 3.10

MAINTENANCE

3.10.1 Safety

The following safety precautions should be observed before commencement ofmaintenance.

1. Do not attempt any maintenance on the pump whilst it is in operation.

2. Before carrying out dismantling or maintenance, isolate the power supplyto the pump driving unit and/or automatic starting devices.

3. Ensure inlet and outlet isolating valves are closed and holding.

4. Drain pump casing to a safe area. Wear the correct protective clothing tosuit the pumped liquid when removing drain plug.

3.10.2 Lubrication

1- Pump and Motor Bearings Grease Lubrication

Machines are supplied with bearings pre-packed with grease and ready to putinto service. For re-lubrication, a lithium based grease such as Shell Alvania R3is recommended. As a guide to quantity of grease, the actual bearing should befilled and then one third of the housing.

Note that Excessive Grease Can Cause Overheating

Units without grease nipples. Clean out and recharge housings with fresh greaseat 6000 hours operation or 3 years whichever is earlier.Some motors have "sealed for life" bearings which cannot be re-lubricated.

2- Pump Bearings Oil Lubrication

Oil lubricated units are supplied without oil. To fill the reservoir either removethe breather or hinge back the constant level oiler. When the oil level can beseen in the bend of the oiler, cease adding oil and fill the oil bottle. Hingeforward to allow oil to run into reservoir. Repeat as necessary until level in oilbottle remains constant. Top up bottle as necessary during operation.



When a pump has been initially operated with a particular grade of oil, thebearing temperature should be checked. You must use the oil which is specifiedby the manufacturer.

The oil should be changed every six months when the pump is operating for 8hours per day. When conditions are more severe, such as hot service, damp orcorrosive atmosphere or continuous service, the oil should be changed morefrequently.

3.10.3 Gland Packing (When Fitted)

1- New gland packing has to be run-in and it is normal practice to start thepump with the stuffing box gland relatively loose i.e., gland nuts onlyfinger tight.There should ALWAYS be a slight leakage of liquid from the gland tokeep an efficient seal and to lubricate and cool the packing. Should thegland need re-packing;, it should be noted that some pumps have a splitgland which can be completely removed to make packing a simple task.Other pumps have a pressed stainless steel gland which allows space forre-packing.

Figure 3.49 Stuffing box with lantern ring



2- stuffing boxes are equipped with a lantern ring (Figure 3.49) which canbe used to feed water or a compatible fluid to the packing. This is useful,when the inlet pressure is less than atmospheric, as air leakage into thepump is prevented. Also, when an independent fluid source is used, it ispossible to flush the packing of any grit or solids handled by the pump.

3- The gland should be inspected at frequent intervals to check thatoperation is correct.

3.10.4 Mechanical Seals

A mechanical seal, whatever type, when correctly selected for type of liquid andapplication, should give a long period of service without any attention. Noadjustment is necessary and any slight initial leakage will be eliminated whenthe seal is run-in.

The seal and any auxiliary flushing should be examined frequently for correctoperation.

3.10.5 Coupling

The coupling should be examined at frequent intervals to ensure that correctalignment is maintained and that the driving elements are not worn.

3.10.6 Overhaul

3.10.6.1 General Instructions

1- The frequency of a complete overhaul depends upon the hours ofoperation of the pump, the severity of service and the care the pumpreceives during operation. DO NOT open the pump for inspection unlessthere is evidence of trouble inside the pump or in the bearings.

2- Should dismantling prove necessary, great care must be taken. For ease ofre-assembly, lay out all parts in the order in which they are removed.

3- Protect all machined faces against metal to metal contact and corrosion.Do not remove the bearings unless they are to be replaced.



3.10.6.2 Dismantling

The sequence of steps to strip down the pump depends mainly on the type of thepump. The pump could be overhang pump or in between bearings pump. Forthis reason this section divided into two parts. Each one covers one of thesetwo designes.

One important thing should taken into consideration:

The manufacturers instructions should be followed when dismantling andassembling the pump.

For this pump (overhang) please check figure 3-50 and spare parts list on thenext two pages.

1- The pump is of the "back pull-out" type which enables the pump casingto remain secured to the baseplate and pipework when dismantled. Thespacer coupling enables the motor to remain secured to the baseplate andconsequently no further alignment should be necessary after re-assembly.Care must be exercised during dismantling, to prevent damage to internalcomponents.



Figure 3.50



Pump Parts List



The dismantling procedure should be carried out in the following order:

1. Close inlet and outlet valves, and drain liquid from pump.

2. Disconnect auxiliary piping (cooling, flushing, etc).

3. Drain oil from the bearing housing, and remove the correct level oiler

4. Remove the coupling spacer.

5. Remove the bearing housing support foot from the baseplate and removethe nuts which secure the integral frame/bearing housing and adapter tothe pump casing.

NOTE:On larger units do not remove the support foot. Fit a bolt (hand tight) intothe tapped hole in the frame to act as a support.

6. Remove the rotating element together with the housing as one unit forfurther dismantling.

7. Prise open impeller nut lockwasher and remove impeller, nut (right-handthread). If an inlet inducer is fitted this should be removed first (right-hand thread).

8. Pull off impeller.

9. For pumps fitted with gland packing, remove the gland nuts securing thegland, and remove the gland from the studs. Unscrew the stuffing boxcover/bearing bracket bolts (where fitted), before removing stuffing boxcover.

Remove gland packing, lantern ring and packing seating ring.

Remove shaft sleeve.

10.For those pumps fitted with mechanical seals, the manufacturer'sinstructions should be followed when dismantling and assembling. Forstandard mechanical seals the following procedures give generalguidance:



Remove the nuts securing the seal plate (seal flange) to the casing and slidethe seal flange away.

Unscrew the stuffing box cover/bearing adapter bolts (where fitted) andremove the stuffing box cover. The inboard seat on a double seal will comeaway in the stuffing box cover.

Mark the position of the seal drive collar to the shaft. Loosen the drivescrews in the seal drive collar and remove the rotating element of the seals,from the shaft sleeve. For double seals, smooth any marks on the sleeve madeby inboard seal before removing outboard seal.

Remove stationary seat, from the seal plate. This should only be done if thestationary face or its seating ring are being replaced.

Remove shaft sleeve.

Dismantling Bearing Frame (Bearing Housing)

This operation should only be carried out when bearings are being replaced:

1. Pull off the pump half coupling and remove coupling key.

2. Remove liquid thrower and both bearing covers. Note that metal labyrinththrowers are secured by socket head setscrews.

3. Press shaft, with bearings, out of bearing housing, removal direction istowards the coupling end. In some designs the outer race and roller ofpump end bearing will stay in the housing. These can now be removed.

4. Remove bearing locknut (and lockwasher where fitted) and drive offbearings.

5. Check the condition of the oil thrower which is fitted between thebearings when bearing cooling feature is supplied.

3.10.6.3 Inspection of Components

When the pump has been dismantled the components should be examined tofined out the parts which should be replaced.



1. Shaft

Examine the shaft carefully. Its condition should be checked where the impeller,shaft sleeve and bearings fit. The shaft may become damaged by corrosion orpitting, caused by leakage along the shaft at the impeller end of the shaft sleeve.

Check the shaft keyway for distortion. Excessive thermal stresses orcorrosion may loosen the impeller on the shaft and subject the keyway toexcessive shock. Replace a shaft that is bent or.

Check the shaft for possible runout.

2. Bearings

Extreme care is to be taken when removing the bearings as they may bedamaged to such an extent that they are no longer usable.

Always check the bearings immediately after removal for anyimperfections, or for any play between the races.

It is recommended that new bearings are installed, because very oftendamage caused by removal cannot detected until the pump is put backinto service.

3. Impeller

After removal, the impeller should be checked for corrosion, blocked waterwaysand worn spots. Impellers should be statically balanced after any machiningwork is carried out.

4. Joints

It is recommended that new joints (gaskets, O-rings) are installed after the pumphas been dismantled. The joints should be of the same material and thickness asthe original joint, so that they will compress to the same thickness.

5. Stuffing Box

The packing rings should be replaced by new set with the same size, samematerial.



3.10.6.4 Assembly

General

1. To assemble the pump, reverse the dismantling procedure previouslydescribed. Consult the exploded-view illustrations and take note of thefollowing procedures which detail special considerations of assemblywhich must be observed.

2. Ensure all gaskets are assembled correctly. Clean the inside of theintegral frame/bearing housing and adapter and the bores for thebearings.

3. The inner races of the bearings are an interference fit on the shaft. Theseshould be fitted by either heating or by using hydraulic press.

NOTE

The method described in the previous paragraph is preferred. Heat the bearingsin an oil bath or electric oven to uniform temperature and mount it quickly onthe shaft.

If the alternate method (using of hydraulic press) is used, apply the force usingan arbor press (hydraulic press), in forcing the bearing onto the shaft, ensurethat the race is never misaligned. The inner race should be checked with afeeler gauge to ensure it is right up against the shaft shoulder.

Heat the bearing to expand it so that it can easily be placed in positionand allowed to shrink to grip the shaft. To avoid damage to bearing shieldand grease, care is to be taken that the temperature is not raised above100 deg C.

Force the bearing onto the shaft using equipment that can provide asteady, even load. Care is to be taken to avoid damage to the bearing andthe shaft.

4. On grease lubricated pumps, pack the bearings with grease and pack thebearing cover cavity approximately one third full with grease.



5. When fitting labyrinth thrower deflector the groove in the pump endbearing cover should be filled with grease and the labyrinth throwerpositioned to give a clearance of 0.5 to 0.65 mm in front of the bearingcover. Screws for the drive end bearing cover should be tighteneduniformly.

6. When refitting the shaft sleeve, ensure the joint or "0"-ring is correctlyfitted.

7. The stuffing box should be packed with good quality packing, suitablefor the liquid being handled.

It is important to fit the lantern ring and the packing" seating ring in theircorrect positions to ensure that the lantern ring is situated in line with thegland seal connections. The packing scarf joints should be staggered by90-180 degrees from the previous ring.

8. When replacing a mechanical seal, extreme cleanliness is required. Thetwo sealing faces of the seal and the surface of the sleeve must be freefrom scratches and other damage.

9. Carefully press the stationary seat into the seal flange ensuring that theseat sealing ring is not deformed, that where an anti rotation pin is fittedcorrect engagement with slot is achieved, and that the face is square tohousing.

10. The rotating element (rotating face) should be carefully mounted onto theshaft sleeve ensuring that the sealing ring is not damaged.

11. Position the seal into the same position, remembering to check settingdimension that it originally occupied and tighten the drive screws in theseal drive collar.

When refitting the seal plate (seal flange), check that the seal iscompressed by the action of moving the seal plate into position. It shouldnot be over compressed and locked up solid.

Renewable Rings (Wearing rings)

When fitted, these should only be removed from the casing and the impellerwhen they are to be replaced. To decide if these rings should be replaced or not,you should check the wear ring clearance.



Wear Ring Clearance

Check the conditions of case wear ring, any signs for corrosion, wear dueto rubbing. Check if it is fixed in its position or it become loose.

Check the conditions of the impeller wear ring, any sings for corrosion,wear due to rubbing with case wear ring. Check its fixation to theimpeller.

Check the running clearance (wear ring clearance). Compare theobserved values with the manufacturer given value.If the clearance is greater than the specified value, the wear rings (one ofthem or both) should be replaced.

They should be prised out using levers behind them. Alternatively, carefuldrilling followed by chiseling to split the ring. Will facilitate removal.

Replacement rings are to be pressed into position (the pump casing wear ringand impeller wear ring). Ensure entry is square to the recess.

Replacement of Oil Lip Seal

1. Oil lip seals, like mechanical seals, are not totally leak free devices. Inmechanical seals, leakage is usually in the form of vapour and is notvisible. Oil from a bearing housing does not evaporate and can bevisually unpleasant in a clean pump room.

2. Very careful fitting practice for oil lip seals is therefore essential,otherwise oil leakage will be unacceptable in any environment.Particular attention must be paid to protecting the seal from keyways, byusing shimming or tape and careful handling of the shaft to avoid evenfine longitudinal scratches.

3. Perfectly assembled seals can have a leakage rate from almost zero tovery small amount. This is the equivalent of approximately 2 drops perhour. At this leakage rate, the constant level oiler would need filling onlyevery 6 months.

4. Refit coupling hub, which should be heated for fitting. It should not beknocked onto the shaft causing loading and damage to the bearings.

5. Rotate pump by hand to ensure there is no binding.

6. Refit pump to baseplate and check coupling alignment.

7. Replace all safety guards.



3.10.7 Maintenance of Centrifugal Pump (in Between BearingsPump)

The required steps in this case will be as follow

The steps which are given here are general guide lines. Before starting to docomplete overhaul for this type of pumps read the manufacturers instruction.

The manufacturers instruction should be followed when dismantling,inspection and assembling.

3.10.7.1 Dismantling the Pump

In overhauling a pump of this types, it is unnecessary to disconnect the suctionor discharge piping, or to move the pump, unless it is necessary to remove thecase from the base plate. Necessary steps to dismantle the pump are:(Refer to figure 3-51 on next page)

1. Open the case drain to drain off any liquid could be inside the pump case.

2. Disconnect the air vent line.

3. Disconnect the mechanical seal piping system.

4. Put match marks on all mating parts to b sure that no mistakes couldhappen during reassemble.

5. Open the motor to pump coupling and remove the spacer (or the spool)if spacer type coupling is used.

6. Remove caps and top halves of bearings shells from inboard and outboardbearings.

7. Remove cap screws holding glands to case.

8. Remove nuts from studs holding the upper and lower halves of the casetogether.

9. Use the jacking bolts to crack off the joint between upper and lowerhalves of the pump casing.

10.Lift off top half of the case, making certain that the shaft is not lifted, thismay occur if the case wear rings are extremely tight in their groves inthe top half.



11.Lift out the rotor of the pump including shaft, impeller (s), mechanicalseals and ball thrust bearing.

12.Procedure from this point depends almost entirely on the nature of thetrouble and the extent of repairs or replacements.

3.10.7.2 Inspection

Washing and cleaning is essential before start inspection of the parts.

Remove the old gaskets (between upper and lower halves of the pumpcasing) and use oil stone for cleaning these surfaces and to make it flat.

1- Rotor Inspection

Check the straightness of the shaft on lathe machine check any signs ofcorrosion or wear shaft sleeves: Corrosion or wear due to rubbing.

Impellers: Any sings of corrosion,

2- Impeller Wear Rings

Signs of corrosion, wear due to rubbing, wear ring clearance.

3- Thrust Bearings

4- Throat Bushings

5- Case Wear Rings

Any signs of corrosion, wear due to rubbing check the grooves of wearing rings,throat bushing.

6- Pump Casing



Figure 3-51 Typical 5 stage case



3.10.7.3 Reassembling the Pump

Before reassembly of the unit, all parts which have a machined fit such aswear rings, throat bushings, sleeves and bearings should be check separate forfitting clearance with the meeting part (or component). These clearances mustbe within the specified values.

The pump is reassembly in reverse order to disassembly

1- All gaskets and O rings must be replaced, use the same thicknessesand materials.

2- For mechanical seals (if it is used), the seal faces must be re-lapped andall gaskets should be replaced. Assemble the mechanical seal on thepump shaft.

3- Re-install the bearing brackets (bearing housing), inboard and outboardsides, install the duel pins for the bearing housing. Re-install the lowerhalves of the bearing shell on both sides.

4- Assemble the rotor components. Check the spaces between differentimpellers.

5- Lower the rotor in the lower half of the pump, wear rings must fitcorrect in its position wear ring lock pins should fit correctly in itspositions.

6- Added some oil between the shaft and the bearings and rotate the shaftslowly. Check if there is any rubbing between the rotor and stationaryparts. Make the required corrections.

7- Put the gasket between lower and upper halves of the pump casing.

8- Lower the top half of the pump casing. Be sure that it fit properly in itsplaces. Use guide pins. Before seating the top half completely, insertthe duel pins. Tight all nuts hand tight. Apply the required fightingtorque as sequence given by the manufacture.

9- Rotate the shaft slowly to check any rubbing could happen. Checkthe shaft axial play. Make the required correction if it is needed.



10- Added the lubricating oil in the bearing housing. Connect all pipingeither for pump venting or mechanical seal system.

11- Check the shaft alignment between the motor and the pump. Make therequired corrections.

12- Re-assemble the coupling. Check the rotation of the system.



SECTION 3.11

RECIPROCATING PUMPS

3.11.1 How it Works

A reciprocating pump is a positive displacement machine, i.e, it traps a fixedamount (fixed volume) of liquid at near suction conditions, compresses it todischarge pressure, and pushes it to discharge nozzle.

In a reciprocating pump, this is accomplished by the reciprocating motion of apiston, plunger or diaphragm.

3.11.2 Reasons for Using Reciprocating Pumps

The justification for selecting a reciprocating pump instead of a centrifugalpumps must be:

1- The total cost of reciprocating pumps is less than centrifugal pumps (totalcost including: Initial cost + maintenance cost + power consumption).

2- High efficiency (84% up to 94%).

3- Suitable for high pressures (10,000 Psig).

4- Suitable for abrasive and / or viscous slurries.

5- The reciprocating pump capacity is a function of speed.

3.11.3 Disadvantages of Reciprocating Pumps

1- Pulsating flow. Because of the pulsation, special consideration must begiven to system design.

2- In most applications, the maintenance cost is greater than that incentrifugal pump.

3- The direct acting pump has a low thermal efficiency when driven by agas such as steam. Big improvement happen if a flywheel and the pumpis driven by electric motor.



3.11.4 Pump Classification

Reciprocating pumps are usually classified by their features:

Drive end, i.e. power (driven by electric motor) or direct acting(figure 3-55)

Orientation of centerline of the pumping element i.e. horizontal orvertical.

Number of discharge strokes per cycle of the crank shaft (drive rod) i.e.single acting or double acting.

Configuration of the pumping element, i.e. piston, plunger or diaphragm.

Number of drive rods, i.e, simples, duplex or multiplex (also it refer tothe number of cylinders)

Number of cylinders Term1 Simplex2 Duplex3 Triplex4 Quadruplex5 Quintuplex6 Sextuplex7 Septuplex8 Nonuplex



Figure 3-53 illustrates this classification in chart form

Figure 3-53 Classification of reciprocating pumps



Figure 3-54 shows two examples of power reciprocating pumps use an electricmotor for the drive.

Figure 3-54 Power pumps use an electric motor(single acting horizontal pump)

Figure 3-55 Shows direct acting pump (on the next page)

Figure 3-55 Power pump use an electric motor



The size of a power pump is normally designated by listing first the diameter ofthe plunger (or the piston), and second the length of the stroke. For example apump designated as 2 x 3 has a plunger diameter 2 inches and a stroke length of3 inches.

For a direct acting pump, the same convention is followed, except that thediameter of the drive piston precedes the liquid end element diameter.

For example, a pump designated 6 x 4 x 6 has a drive piston diameter of 6inches, a liquid piston diameter of 4 inches and a stroke length of 6 inches.

3.11.5 Liquid End Components

The liquid end is that portion of the pump that does the pumping. Elementscommon to all reciprocating pump liquid ends are:

1- The liquid cylinder.2- Pumping element.3- Stuffing boxes and4- Valves

3.11.5.1 The Liquid Cylinder

The liquid cylinder is the major pressure retaining part of the liquid end, andforms the major portion of the pumping chamber. It usually contains or supportall other liquid end components.

A piston pump is normally equipped with a replaceable liner (sleeve) thatabsorbs the wear from the piston rings.

Because a plunger contacts only stuffing box components, plunger pumps donot require liners.

3.11.5.2 Pumping Element

All reciprocating pumps contain one or more pumping elements (pistons,plungers or diaphragms) that reciprocate into and out of pumping chambers toproduce the pumping action.

A piston (figure 3.56) is a cylindrical disk, mounted on a rod, and usuallycontains some type of sealing rings (piston rings). Could be two rings or more.



The number of piston rings and its dimensions, configuration and materialsdepends on the working conditions (fluid, pressure and temperature) and thecylinder size.

The piston rings must be able to move freely in the piston ring grooves and thepiston head assembly (piston head and the piston rings) must be able to moveinside the cylinder with enough tighting pressure against the cylinder wall.

The function of the piston rings is to stop the leakage between the piston headand the cylinder wall (or cylinder liner) during the pumping stroke (dischargestroke)



Fig

ure

3-56

Pis

ton

head

, pis

ton

rod

and

pist

on r

ings



A plunger (figure 3-57) is a smooth rod and, in its normal configuration, canonly be single acting.

Figure 3-57 Plunger (in vertical plunger pump)

A plunger must seal only in the stuffing box, and touches only the packing andpossibly stuffing box bushings.

3.11.5.3 Stuffing Boxes

Sealing between the pumping chamber and atmosphere is accomplished in astuffing box (or packing box). The stuffing box contains rings of packing thatconform to and seal against stuffing box I.D. and the piston rod (or the plungerin case of plunger pumps).

If a lubricant, sealing liquid or flushing liquid is injected into the center of thepacking, a lantern ring or seal cage is required. The lantern ring provides anannular space between the packing rings so that the injected fluid can freelyflow to the rod surface.



Stuffing box designs

1- Standard non lubricated stuffing box

The following figure (figure 3-58) shows this design

Figure 3-58 A Standard non lubricated stuffing box

Figure 3-58 B Pressure gradient across the packing



Application

For cool water and fluids with comparable lubricity. Total packing length must be less than plunger stroke length to properly

wet the last ring of packing with pumpage.

2- Standard lubricated stuffing box

Figure 3-59 Standard lubricated stuffing box

Application

Most of lubricant migrates into pumpage.

Packing may be square, chevron.

Suitable for non lubricated liquids (less lubricity).

Suitable for dirty liquids contaminated liquids.



3- Alternative lubricated stuffing box

Figure 3-60 Alternative lubricated stuffing box

Application

Puts lubricant under last ring where it is needed most.

Allows use of low pressure and drip type lubricators.

Very little lubricant migrates into pumpage.

Packing may be square, chevron.

4- Standard box used to bleed off pumpage

Figure 3-61 Standard box used to bleed off pumpage



Application

High friction causes excess heat.

Short life of packing and plungers.

Poor application improper use of standard box.5- Modified gland follower to allow bleed off

Less friction and lower temperature than unit in figure 3.61Longer life of packing and plungers.Secondary packing cannot be adjusted to compensate for wea

rotating equipment chapter 3 pumps

Documents

pump power

pump classification

pump types

pump auxiliaries3

pump drive

pump operation3

pump maintenance3

pump theory