Lecture 5 - Pump Selection

8/21/2019 Lecture 5 - Pump Selection

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Pump selection and hydraulicPump selection and hydraulicPump selection and hydraulicPump selection and hydraulic

designdesigndesigndesign

Copyright@Dominic Foo H82PLD - Plant Design Pump - 2

Lecture outlineLecture outlineLecture outlineLecture outline

Different types of pumps

Application of Bernoulli equation

in pumping system

Pump characteristic & system

curves

Flow control & affinity law

Series vs. parallel operation


IntroductionIntroductionIntroductionIntroduction Purpose of pumps:

To transport

To supply energy in the form of pressure

Major types of pumps:


Positive displacement pumpsPositive displacement pumpsPositive displacement pumpsPositive displacement pumps

Reciprocating diaphragm pump

Piston pump Plunger pump


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Positive displacement pumpsPositive displacement pumpsPositive displacement pumpsPositive displacement pumps

Internal gear pumpExternal gear pump

Screw pump


Positive displacement pumpsPositive displacement pumpsPositive displacement pumpsPositive displacement pumps Contains inlet & outlet valves

During liquid suction, the camber is filled with liquid, withinlet valve open & outlet valve closed; during discharge,inlet valve closed & outlet valve opened.

Valves opening & closing cause fluctuating flowrate &discharge pressure reduced by multiple cylinders inparallel.

Deliveryra

te

Time Deliveryrate

Time

Single cylinder Multiple cylinder

(Coulson & Richardson, 1998)


Centrifugal pumpsCentrifugal pumpsCentrifugal pumpsCentrifugal pumps Most widely used type in the chemical &

petroleum industries.

Handle liquids with wide ranging properties &

suspensions with high solid content (e.g. cement). May be constructed from a wide range of corrosion

resistant materials.

Fluid is fed to the centre of a rotating impeller &thrown outward by centrifugal action.

Due to high speed rotation, the liquid acquires a

high kinetic energy.(Coulson & Richardson, 1998)


Centrifugal pumpsCentrifugal pumpsCentrifugal pumpsCentrifugal pumpsImpeller

(Seider et al., 2003)


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Advantages of centrifugal pumpsAdvantages of centrifugal pumpsAdvantages of centrifugal pumpsAdvantages of centrifugal pumps

Simple in constructionmade in a wide range ofmaterials

Completely absent of valves

Operates at high speed couple directly toelectric motor

Steady delivery

Lower maintenance cost than other type

No damage if delivery is blocked (in short period)

Smaller than other pumps of equal capacity

Handle liquid with high proportions of suspendedsolid



DisadvantagesDisadvantagesDisadvantagesDisadvantages Single pump does not develop high pressure

Multiple-stage pumps develop greater heads butvery expensive & cannot be made by corrosive-resistant material

High efficiency only to a limited range ofconditions

Not self-priming

If no valve installed, liquid may return to suctiontank once pump stops

Viscous liquid cannot be handled efficiently(Coulson & Richardson, 1998)


Centrifugal pump selectionCentrifugal pump selectionCentrifugal pump selectionCentrifugal pump selection

104

103

102

1010 102 103 104 105

Totalhead,m

Flowrate, m3/h

Single stage

1750 rpm

Single stage

3500 rpm

High speedsingle* or

multi*

Reciprocating

Multi-stage

* Single stage > 1750 rpm;multi-stage 1750rpm

(Sinnott, 2005)


Some comparisonSome comparisonSome comparisonSome comparison

Totalhead,m

Flowrate, m3/h

Centrifugal

Positivedisplacement

Efficiency

Flowrate, m3/h

Centrifugal

Positivedisplacement

(www.pumpschool.com)


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Factors influence pump selectionFactors influence pump selectionFactors influence pump selectionFactors influence pump selection Quantity of liquid:

Affect the size of pump Determine the use of parallel pumps

Head against the liquid to be pumped Pressure difference Vertical height of the downstream & upstream reservoirs Friction losses in the delivery line

Nature of liquid Liquid viscosity determines the friction losses & power Corrosive nature determine the material of construction Pump clearances must be large handling liquid with suspensions

Nature of power supply high speed centrifugal or rotary pump ispreferred with the use of electric motor/internal combustion engine

Intermittent use corrosive troubles more likely than continuousoperation

It may be advantageous to select a cheap pump & pay higher maintenance coststhan to install expensive/high efficiency pump.



More resources on pumpsMore resources on pumpsMore resources on pumpsMore resources on pumps

INTERNET


Why pump is needed?Why pump is needed?Why pump is needed?Why pump is needed?

Energy

Pressuresensor

(a) Supply pressure (b) Supply height

Energy

Change inelevation

(Wood, 1995)


Why pump is needed?Why pump is needed?Why pump is needed?Why pump is needed?

(c) Supply velocity

Energy

Fluidwith

velocity

(d) Overcome friction

Obstructionsin the line

Energy

(Wood, 1995)


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Why pump is needed?Why pump is needed?Why pump is needed?Why pump is needed? All these pressure needs can be summed to yield

the total pressure requirements:

It is convenient to express this total needs in unitof pressure that would be independent of theliquid density.

Can be done by dividing pressure by gravity force

acting on a mass contained in a unit volume offluid:

+

+

+

=

loss

Friction

difference

Velocity

difference

Height

difference

PressureneedsTotal

head""calleddistanceofunitg

Pressure=

(Wood, 1995)


Application of Bernoulli Eq.Application of Bernoulli Eq.Application of Bernoulli Eq.Application of Bernoulli Eq.

Include energy losses into Bernoulli Equation:

where HLD & HLS = head loss due to piping, fittings & other equipmentin discharge (D) & suction (S).

When level changes are slow, i.e. vL & vD 0:

LD

2

DD

DPLS

2

SS

S

22H

g

vZ

g

PHH

g

vZ

g

P+++=+++

PS

PD

HP

ZS

ZD

( ) ( )LSLDSDSD

P HHZZg

PPH +++

=

Case 1


Application of Bernoulli Eq.Application of Bernoulli Eq.Application of Bernoulli Eq.Application of Bernoulli Eq.

PS

PD

HP

ZS

ZD

( ) ( )LSLDSDSD

P HHZZ

g

PPH ++++

=

Change of sign

Case 2: Source below pump level


Nett positive suction headNett positive suction headNett positive suction headNett positive suction head Flow conditions at the pump suction are of special

importance & care must be taken if cavitation is to beavoided.

Cavitation: formation of vapour bubbles (flashing of liquid)as pressure drops below the vapour pressure at flowing

temperature. During cavitation, vapour bubbles collapse pressure builds,

lead to severe damage at impeller & pump performancedrops.

To ensure the pressure cannot drop below the vapourpressure, pump manufacturers specify the total suctionhead must exceed the head equivalent, i.e. called the nettpositive suction head (NPSH).

NPSH available > NPSH specified by pump manufacturer

TPP

Hg

vZ

Plowingpressure@fvapour;g2gavailableNPSH

VPVPLS

2

SSS =++=


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Hydraulic & actual powerHydraulic & actual powerHydraulic & actual powerHydraulic & actual power Hydraulic power input of a pump:

Power = m Hpg = Q Hpgwhere m = liquid mass flowrate, Q = volumetricflowrate

Actual power input:

where = efficiency

inputActualpowerActual =


Characteristic curvesCharacteristic curvesCharacteristic curvesCharacteristic curves

Efficiency reaches itsmaximum & then falls

Head falls slowlyinitially & falls offrapidly

Head,m

Flowrate, m3/h

Head

Efficiency

Power



Characteristic curvesCharacteristic curvesCharacteristic curvesCharacteristic curves effect ofeffect ofeffect ofeffect ofrotation raterotation raterotation raterotation rate



Characteristic curvesCharacteristic curvesCharacteristic curvesCharacteristic curves effect ofeffect ofeffect ofeffect ofimpeller diameterimpeller diameterimpeller diameterimpeller diameter



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System curveSystem curveSystem curveSystem curve

Pump is not shown as we are concerning about head inputby the pump.

2 components in the pressure head to be supplied by pump

in a piping system: static head & dynamic loss Bernoulli equation can be interpreted as follow:

PS

PD

ZS

ZD

( ) ( )LSLDSDSD

P HHZZg

PPH +++

=

Static head Dynamic head loss


System curveSystem curveSystem curveSystem curve Static head:

Independent of flowrate

Can be calculated immediately for any pumping service Dynamic head loss:

Dependent on flowrate (increase as flowrate increases)

At no flow, term is zero (HLS + HLD = 0)

System curve: a plot of total liquid head vs. liquid flowrate

Head

Flowrate

Static head

Dynamichead loss

(Wood, 1995)


Suction head & system curveSuction head & system curveSuction head & system curveSuction head & system curve

Head

Flowrate

Static head

Dynamichead loss

ZS

ZD

ZS ZD

ZS+ZS

Head

Flowrate

Dynamichead loss



Interaction of pump & system curvesInteraction of pump & system curvesInteraction of pump & system curvesInteraction of pump & system curves

Given the pumpcharacteristics & systemcurve, if we superimpose

one on the other, theoperating point (Q1, H1) isobtained.

However, the flowrateachieved is not alwayswhat we need flowratecontrol needed.

Head, m

Flowrate, m3/h

Pumphead

System

H1

Q1 = design

flowrate

Operatingpoint

(Wood, 1995)


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Flowrate control for pumpingFlowrate control for pumpingFlowrate control for pumpingFlowrate control for pumping Main methods of flow control

Throttling of pump discharge, with pump runs at constant speed

Varying pump speed suitable for larger pump (e.g. steam turbine) Throttling of pump discharge

Throttling (globe) valve is added to pump discharge.

As valve progressively closed, operating point moves up the head vs.flowrate curve.

PD

HP

PS

Throttlingvalve

Q

Pumphead

Valve fullyopened

HValve closed Valve

closed


Flowrate control for pumpingFlowrate control for pumpingFlowrate control for pumpingFlowrate control for pumping

Varying pump speed:

More economic technique

with regards to power

consumption.

Flowrate is increased Q1

Q2Q3 by increasing pump

speedQ

N3

H

N2N1

System

Q1 Q2 Q3


Affinity lawsAffinity lawsAffinity lawsAffinity laws

Q

H

N2N1

System

Q1 Q2

H1

H2

=

1

212N

NQQ

2

1

212

=

N

NHH

Power

P1

P2

=

1

212N

NQQ

3

1

212

=

N

NPP

Mapping for powerMapping for rotation

Summary:

Q1 Q2Q

5323 ;)(; DNPNDHNDQ


Example 1Example 1Example 1Example 1 System curve & head vs.

flowrate data for a pumpoperating at 2800 rpm aregiven.

Questions: If varying pump speed is

used for flow control, whatspeed is needed to producea flowrate of 114 m3/h?

Make a rough estimation ofpower saving as comparedto pump discharge

throttling on 2800 rpm.[Ans: 27%]

36.137.438.7H(m)

1208040Q (m3/h)

35.430.526.420H(m)

140110800Q (m3/h)

System curve:

Pump curve:


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134

Example 1Example 1Example 1Example 1

0

10

20

30

40

15050 100Flowrate, m3/h

H,m


Series/parallel operationSeries/parallel operationSeries/parallel operationSeries/parallel operation

Head, m

Flowrate, m3/h

Single pumph

2h

h

Doublepump

Head, m

Flowrate, m3/h

Single pump

Qh

Doublepump

Q

2Q

Series operationParallel operation


Example 2Example 2Example 2Example 2 2 identical pumps (with

similar characteristic curve)

are run in parallel.

Questions:

Determine the flowrate

through the system when

both pump are operating.

[Ans: 59.5 m3/h]

What happens if one pump

trips & the other continues

running?

[Ans: 41.5 m3/h]

30.040.050.055.5H(m)

47.536.519.00Q (m3/h)

45.035.529.525.0H(m)

6040200Q (m3/h)

System curve:

Pump curve:


41.5 59.5

Q Q

2Q


0

10

20

30

40

50

60

50 10010 20 30 40 60 70 80 90Flowrate, m3/h

H,


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Example 3Example 3Example 3Example 3 Liquid of density 900 kg/m3 is to be pumped between 2 vessels with

elevations relative to the pump inlet. The liquid depth in each vesselmay vary & the pump is expected to deliver 80 m3/h under allconditions.

The pressure in the vapour space above the liquid level in the receivingvessel operates within a defined range whilst the pressure in the suctionvessel is held constant.

Total head losses due to piping & other fittings (including a fully openglobe valve) have been calculated at 25 m for a flowrate 100 m3/h.

HP

5 m1 m

4 m

2 bar

Globevalve 15 m

1 m

4 m

Max liquid level

Min liquid level

Pressure range

3.854 4.119 bar


Example 3Example 3Example 3Example 31. Estimate & plot the system curve, including open globe

valve, for the most severe pumping operation.

2. A pump having the head vs. flowrate characteristic asshown in table. What would be the resulting flowrate if theglobe valve remains fully open? [Ans: 98 m3/h]

3. If the globe valve is partially closed, make a rough sketch

on your plot of what you think the system curve will looklike to produce a flowrate of 80 m3/h.

4. What is the very largest flowrate that could be achieved,and under what conditions? [Ans: 112 m3/h]

65.2

30

60.162.063.666.0H(m)

130100700Q (m3/h)



0

20

40

60

10020 40 60 80Flowrate, m3/h

H,

120 140 160

80

100


Solution for Example 3Solution for Example 3Solution for Example 3Solution for Example 31. Most severe case = largest P & Z

( ) ( )LSLDSDSD

P HHZZg

PPH +++

=

HP

5 m1 m

2 bar Globevalve 20 m

Max liquid level

4.119 bar

Data available Data unavailable(correlation is

needed)


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Solution for Example 3Solution for Example 3Solution for Example 3Solution for Example 3 Additional note for estimating system curve:

From Affinity law:

( ) ( )LSLDSD

SDP HHZZ

g

PPH +++

=

Static head

Dynamic head loss

Head

Flowrate

( ) ( )1LSLD

2

1

2

2LSLD HHQ

QHH +

=+

2

2

1

2

1

=

Q

Q

H

H

25 m

100 m3/h


Solution for Example 3Solution for Example 3Solution for Example 3Solution for Example 3

2. Plot pump characteristiccurve

3. When Q = 80 m3/h,system curve movessteeper.

Q

H

System

H


Solution for Example 3Solution for Example 3Solution for Example 3Solution for Example 34. Largest flowrate achieved by a fully open valve

HP

10 m

2 bar

Globevalve 15 m

4 mMin liquid level

3.854 bar

( ) ( )LSLDSDSD

P HHZZg

PPH +++

=

Data available Data unavailable

( ) ( )1LSLD

2

1

2

2LSLD HHQ

QHH +

=+ Q

HSystem

1128098


Problem for revision 1Problem for revision 1Problem for revision 1Problem for revision 1A liquid of density 950 kg/m3 is to be pumped from a suctionvessel at 3 bar to a receiving vessel at 30 bar. The liquid levelin the suction vessel is 9 m below the pump and the level in thereceiving vessel is 15 m above the pump. At a flowrate of 200

m3

/h, the dynamic head losses of the whole piping system,including the resistance of a fully open throttling valve for flowcontrol, are equivalent to a head of 30 m of liquid. The kineticenergy change can be omitted in this system. Thecharacteristics of the pump at a speed of 3000 rpm is given inthe following table.

250340403440450Head (m)

200150100500Flowrate (m3/h)


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Lecture 5 - Pump Selection