pipe flow fundamentals rev 0

PIPE FLOW FUNDAMENTALS COURSE

Gayungsari Timur 5 Blok MGH No. 923 – 24 Nopember 2013

by Wendi Junaedi

What You Should Know

Course only for two days !!

You may not become a superman nor even goku in 2 days....

Introduction

What is piping system ?

A Piping system consist of tanks, pumps, valves, and components connected together by pipelines to deliver a fluid at a spesific flow rate and/or pressure in order to perform work or make a product. The piping system may also contain a variety of instrumentation and controls to regulate the processes that are occuring within the boundaries of the piping system

What is piping system ?

Introduction

Value of a Clear Picture of a Piping System

To see the piping system clearly, the system boundaries must be defined, including where the system begins and ends, what device are installed in the system, and how all the devices in the system are configured.

Introduction

A clear picture of system operation Understanding normal operation (flow, pressure, level,

temperature, etc) Understanding why and how they changed at different operating

condition Understanding the function and expected of hydraulic performance Understanding the processes are occuring inside the piping, and

how the processes measured and controlled.

Introduction

A clear picture for troubleshooting Not only provides a better understanding of normal condition Helps to identify abnormal condition

A clear picture for energy consumption and cost Transporting fluid requires energy Energy loss occurs due to friction, noise, vibration, inefficient in the

motor and pump, head loss in the components such as piping, valves, fitting, etc

Surely, energy costs money

Introduction

Understanding Total System

Understand type of piping system Single path open system Branching system Single path closed loop system Multi loop closed system

Understand hydraulic performance

Understand piping system curve vs pump curve

Understand total energy graph

Understand abnormal condition

Terminology, Unit, and Physical Laws

Fluid Properties Any characteristic of a system is called a property.

Familiar: pressure P, temperature T, volume V, and mass m.Less familiar: viscosity, thermal conductivity, modulus of

elasticity, thermal expansion coefficient, vapor pressure, surface tension.

Intensive properties are independent of the mass of the system. Examples: temperature, pressure, and density.

Extensive properties are those whose value depends on the size of the system. Examples: Total mass, total volume, and total momentum.

Extensive properties per unit mass are called specific properties. Examples include specific volume v = V/m and specific total energy e=E/m.

Terminology, Unit, and Physical Laws

Fluid PropertiesThe properties relevant to fluid flow are summarized below:Density:

This is the mass per unit volume of the fluid and is generally measured in kg/m3. Another commonly used term is specific gravity. This is in fact a relative density, comparing the density of a fluid at a given temperature to that of water at the same temperature.

Terminology, Unit, and Physical LawsFluid PropertiesViscosity: This describes the ease with which a fluid flows. A substance like treacle has a high viscosity, while water has a much lower value. Gases, such as air, have a still lower viscosity. The viscosity of a fluid can be described in two ways.

• Absolute (or dynamic) viscosity: This is a measure of a fluid's resistance to internal deformation. It is expressed in Pascal seconds (Pa s) or Newton seconds per square meter (Ns/m2). [1Pas = 1 Ns/m2]

• Kinematic viscosity: This is the ratio of the absolute viscosity to the density and is measured in metres squared per second (m2/s).

Terminology, Unit, and Physical LawsFluid PropertiesReynolds Number:

• Critical Reynolds number (Recr) for flow in a round pipe

Re < 2300 laminar2300 ≤ Re ≤ 4000 transitional Re > 4000 turbulent

• Note that these values are approximate.

• For a given application, Recr depends upon

– Pipe roughness– Vibrations– Upstream fluctuations, disturbances

(valves, elbows, etc. that may disturb the flow)

Terminology, Unit, and Physical LawsFluid PropertiesLaminar vs Turbulent

Terminology, Unit, and Physical LawsPressure Loss in Pipe

Whenever fluid flows in a pipe there will be some loss of pressure due to several factors:

a) Friction: This is affected by the roughness of the inside surface of the pipe, the pipe diameter, and the physical properties of the fluid.

b) Changes in size and shape or direction of flow

c) Obstructions: For normal, cylindrical straight pipes the major cause of pressure loss will be friction. Pressure loss in a fitting or valve is greater than in a straight pipe. When fluid flows in a straight pipe the flow pattern will be the same through out the pipe. In a valve or fitting changes in the flow pattern due to factors (b) and (c) will cause extra pressure drops.

Pressure drops can be measured in a number of ways. The SI unit of pressure is the Pascal. However pressure is often measured in bar.

This is illustrated by the D’Arcy equation:

Before the pipe losses can be established, the friction factor must be calculated. The friction factor will be dependant on the pipe size, inner roughness of the pipe, flow velocity and fluid viscosity. Theflow condition, whether ‘Turbulent’ or not, will determine the method used to calculate the friction factor.

Moody Chart can be used to estimate friction factor. Roughness of pipe is required for friction factor estimation. The chart shows the relationship between Reynolds number and pipe friction. Calculationof friction factors is dependant on the type of flow that will be encountered. For Re numbers <2320 the fluid flow is laminar, when Re number is >= 2320 the fluid flow is turbulent.

The following table gives typical values of absolute roughness of pipes, k. The relative roughness k/d can be calculated from k and inside diameter of pipe.

Calculate pressure drop for a pipe of 4” diameter. carrying water flow of 50 m3/h through a distance of 100 meters. The pipe material is Cast Iron

Terminology, Unit, and Physical LawsPressure Loss in Components in Piping System

Minor head loss in pipe systems can be expressed as:

Valves

Valves isolate, switch and control fluid flow in a piping system. Valves can be operated manually with levers and gear operators or remotely with electric, pneumatic, electro-pneumatic, and electro-hydraulic powered actuators. Manually operated valves are typically used where operation is infrequent and/or a power source is not available. Powered actuators allow valves to be operated automatically by a control system and remotely with push button stations. Valve automation brings significant advantages to a plant in the areas of process quality, efficiency, safety, and productivity.

ValvesGate Valves

Best Suited Control: Quick OpeningRecommended Uses:

Fully open/closed, non-throttling Infrequent operationMinimal fluid trapping in line

Advantages: High capacityTight shut off, Low cost, Little resistance to flow

Disadvantages:Poor controlCavitate at low pressure dropsCannot be used for throttling

Applications: Oil, Gas, Air, Slurries, Heavy liquids, Steam, Non-condensing gases, and Corrosive liquids

ValvesGlobe Valves

Best Suited Control: Linear and Equal percentageRecommended use-

Throtteling services/flow regulationFrequent operation

Advantages: Efficient throttlingAccurate flow control valvesAvailable in multiple ports

Disadvantages:High pressure dropMore expensive than other

Applications: Liquids, vapors, gases, corrosive substances, slurries

ValvesBall ValvesBest suited control – Quick opening linear.Recommended uses –

Fully open/closed limited throttlingHigher temperature fluids

Advantages – Low costHigh capacityLow leakage & maintenanceTight sealing with low torque

Disadvantages – Poor throttling characteristicsProne to cavitation

Applications – Most Liquids, high temperatures, slurries

ValvesButterfly Valves

Best Suited Control: Linear, Equal percentageRecommended Uses:

Fully open/closed or throttling servicesFrequent operationMinimal fluid trapping in line

Advantages:Low cost and maint.High capacityGood flow controlLow pressure drop

Disadvantages –High torque required to controlProne to cavitation at lower flows

Applications: Liquids, gases, slurries, liquids with suspended solids

ValvesCavitation on Valves

Pump & Pumping System

• 20% of world’s electrical energy demand• 25-50% of energy usage in some industries• Used for

• Domestic, commercial, industrial and agricultural services

• Municipal water and wastewater services

What are Pumping Systems

Pump & Pumping SystemWhat are Pumping Systems

Objective of pumping system

(US DOE, 2001)

• Transfer liquid from source to destination

• Circulate liquid around a system

Pump & Pumping SystemWhat are Pumping Systems

• Main pump components• Pumps• Prime movers: electric motors, diesel engines, air

system• Piping to carry fluid• Valves to control flow in system• Other fittings, control, instrumentation

• End-use equipment• Heat exchangers, tanks, hydraulic machines

33© UNEP 2006

• Head• Resistance of the system• Two types: static and friction

• Static head• Difference in height between

source and destination• Independent of flow

Pumping System Characteristics

destination

source

Statichead

• Static head consists of• Static suction head (hS): lifting liquid relative to

pump center line• Static discharge head (hD) vertical distance

between centerline and liquid surface in destination tank

• Friction head• Resistance to flow in pipe and fittings• Depends on size, pipes, pipe fittings, flow

rate, nature of liquid• Proportional to square of flow rate• Closed loop system

only has friction head(no static head) Friction

In most cases:Total head = Static head + friction head

System head

Static head

Friction head

Systemcurve

Systemhead

Static head

Frictionhead

Systemcurve

Pump performance curve• Relationship between head and flow

• Flow increase• System resistance increases• Head increases• Flow decreases to zero

• Zero flow rate: risk of pump burnout

Performance curve for centrifugal pump

Pump operating point

• Duty point: rate of flow at certain head

• Pump operating point: intersection of pump curve and system curve

Static head

Pump performance curve

System curve

Pump operating point

Pump suction performance (NPSH)• Cavitation or vaporization: bubbles inside pump• If vapor bubbles collapse

• Erosion of vane surfaces• Increased noise and vibration• Choking of impeller passages

• Net Positive Suction Head• NPSH Available: how much pump suction exceeds

liquid vapor pressure• NPSH Required: pump suction needed to avoid

cavitation

Type of Pumps

Centrifugal Pump

Are classified as nonpositive displacement pumps because they do not pump a definite amount of water with each

revolution. Rather, they impart velocity to the water and

convert it to pressure within the pump itself.

Centrifugal Pump

• Pump shaft power (Ps) is actual horsepower delivered to the pump shaft

• Pump output/Hydraulic/Water horsepower (Hp) is the liquid horsepower delivered by the pump

How to Calculate Pump Performance

Hydraulic power (Hp):Hp = Q (m3/s) x Total head, hd - hs (m) x ρ (kg/m3) x g (m/s2) / 1000

Pump shaft power (Ps):Ps = Hydraulic power Hp / pump efficiency ηPump

Pump Efficiency (ηPump): ηPump = Hydraulic Power / Pump Shaft Power

hd - discharge head hs – suction head, ρ - density of the fluid g – acceleration due to gravity

Centrifugal PumpEnergy Efficiency Opportunities

1. Selecting the right pump2. Controlling the flow rate by speed variation3. Pumps in parallel to meet varying demand4. Eliminating flow control valve5. Eliminating by-pass control6. Start/stop control of pump7. Impeller trimming

1. Selecting the Right Pump• Pump performance curve for centrifugal pump• Oversized pump

• Requires flow control (throttle valve or by-pass line)• Provides additional head• System curve shifts to left• Pump efficiency is reduced

• Solutions if pump already purchased• VSDs or two-speed drives• Lower RPM• Smaller or trimmed impeller

2. Controlling Flow: speed variation

Explaining the effect of speed• Affinity laws: relation speed N and

• Small speed reduction (e.g. ½) = large power reduction (e.g. 1/8)

3. Parallel Pumps for Varying Demand

• Multiple pumps: some turned off during low demand• Used when static head is >50% of total head• System curve

does not change• Flow rate lower

than sum ofindividual flow rates

4. Eliminating Flow Control Valve• Closing/opening discharge valve (“throttling”) to

reduce flow

• Head increases: does not reduce power use

• Vibration and corrosion: high maintenance costs and reduced pump lifetime

5. Eliminating By-pass Control

• Pump discharge divided into two flows• One pipeline delivers fluid to

destination• Second pipeline returns fluid

to the source• Energy wastage because part of

fluid pumped around for no reason

6. Start / Stop Control of Pump• Stop the pump when not needed• Example:

• Filling of storage tank• Controllers in tank to start/stop

• Suitable if not done too frequently• Method to lower the maximum demand (pumping at

non-peak hours)

7. Impeller Trimming

• Changing diameter: change in velocity• Considerations

• Cannot be used with varying flows• No trimming >25% of impeller size• Impeller trimming same on all sides• Changing impeller is better option

but more expensive and not always possible

Comparing Energy Efficiency Options

Parameter Change control valve

Trim impeller VFD

Impeller diameter

430 mm 375 mm 430 mm

Pump head 71.7 m 42 m 34.5 mPump efficiency 75.1% 72.1% 77%

Rate of flow 80 m3/hr 80 m3/hr 80 m3/hrPower

consumed23.1 kW 14 kW 11.6 kW

Centrifugal Pump

Pipeline

Generally need to deliver oil or gas at a specified flow rate and pressure Hydraulic design required for preliminary selection of pipeline diameterFluid must be kept above a minimum velocity

• Minimise surging • Prevent build up of solids

Fluid flow must be below a maximum velocity• Prevent erosion • Optimise pumping requirements

Hydraulic Design

PipelineHydraulic Design

Hydrocarbons for transport may beLiquid (incompressible: straightforward to analyse) Gas (compressible & properties vary along pipe:

more challenging to analyse) Multi-phase (e.g. gas & condensate)

(highly complex)

Pipeline

For liquid lines:Max velocity 4 m/secMin velocity 1 m/sec

For gas lines: Max velocity 18-25 m/secMin velocity 4-5 m/sec

Trade off between - CAPEX (Large pipe diameter) and

- OPEX (Lower pumping costs)

Fluid Velocities

Pipeline

Pressure drop in liquid pipelines is principally due to

• Change in elevation (described by change in hydraulic head, or Pressure = gh )

• Friction loss

Pressure Drop

The remainder of the section on hydraulic design will be concerned with liquid pipelines

Pipeline

There are two equations that may be used for calculating the friction loss

Darcy-Weisbach

Fanning

Friction Loss Calculation

2L DARCYL Vh fD g

æ öæ ö= ç ÷ç ÷è ø è ø2

2L FANNINGL Vh fD g

æ öæ ö= ç ÷ç ÷è ø è ø

Oil pipelines

Gas pipelines

So, fDARCY = 4fFANNING

Pipeline

Friction Loss Calculation

For Laminar Flow

For Turbulent Flow use the Moody Chart Depends on pipe relative roughness

64ReDARCYf = For Re < 2300

For Re > 4000

Best PracticeCompressed Air System

Best PracticeCompressed Air Leakage

Leaks can be a significant source of wasted energy in an industrial compressed air system and may be costing you much more than you think. Audits typically find that leaks can be responsible for between 20-50% of a compressor’s output making them the largest single waste of energy. In addition to being a source of wasted energy, leaks can also contribute to other operating losses:

• Leaks cause a drop in system pressure. This can decrease the efficiency of air tools and adversely affect production

• Leaks can force the equipment to cycle more frequently, shortening the life of almost all system equipment (including the compressor package itself)

• Leaks can increase running time that can lead to additional maintenance requirements and increased unscheduled downtime

• Leaks can lead to adding unnecessary compressor capacity

Best PracticeCompressed Air Leakage

Best PracticeSteam Distribution

There are numerous graphs, tables and slide rules available for relating steam pipe sizes to flow rates and pressure drops.

To begin the process of determining required pipe size, it is usual to assume a velocity of flow. For saturated steam from a boiler, 20 - 30 m/s is accepted general practice for short pipe runs. For major lengths of distribution pipe work, pressure drop becomes the major consideration and velocities may be slightly less. With dry steam, velocities of 40 metres/sec can be contemplated -but remember that many steam meters suffer wear and tear under such conditions. There is also a risk of noise from pipes.

Pipe Selection

Best PracticeSteam Distribution

Recommended Thickness of Insulation (inches) for Mineral Wool

Best PracticeWater Distribution

As a rule of thumb, the following velocities are used in design of piping and pumping systems for water transport:

If you want to pump 14.5 m3/h of water for a cooling application where pipe length is 100 metres, the following table shows why you should be choosing a 3” pipe instead of a 2” pipe.

If a 2” pipe were used, the power consumption would have been more than double compared to the 3” pipe. It should be noted that for smaller pipelines, lower design velocities are recommended. For a 12” pipe, the velocity can be 2.6 m/s without any or notable energy penalty, but for a 2” to 6” line this can be very lossy.

Recommended water flow velocity on suction side of pump

Capacity problems, cavitation and high power consumption in a pump, is often the result of the conditions on the suction side. In general - a rule of thumb - is to keep the suction fluid flow speed below the following values:

References

1. Piping System Fundamental, The Complete Guide to Gaining a Clear Picture of Your Piping System, 2012 Engineered Software, inc

2. Best Practice Manual, Fluid Piping System, 2006, Devki Energy Consultancy Pvt. Ltd.

3. Pump Handbook, 2004 Grundfos Industry4. Valve Sizing & Selection, Ranjeet Kumar5. Pumps & Pumping System, 2006, www.energyefficiencyasia.org6. Pumps for Process Industry, Ranjeet Kumar7. Critical Pump Selection, Webinar8. Repair Engineering

Simulation & Modelling

pipe flow fundamentals rev 0

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