PUMPE & VENTILATORI

Circulatory System

1

Pumps and pumping stations

3.1 Introduction

To transport water through pipes energy has to befed to the water. The energy is needed to overcomethe dynamic friction losses in the pipe. Also energyis needed to compensate differences in level betweenthe beginning and the end of a pipe (lift energy).Basically a pump is a piece of equipment to feedenergy to a water flow. Two types of pumps can bedistinguished:o Pumps capable of lifting water from one free sur-

face to another: open pumps or Archimedeanscrews (fig. 3.1).

o Pumps capable of feeding energy to water in com-bination with a closed pipe: centrifugal or impellerpumps (fig. 3.2).

Pumps are used for instance to pump water out ofthe ground, to overcome level differences in treat-ment processes, to transport drinking or seweragewater over large distances in combination with pipes

or to dispose of rain water from polders. Numerousother applications of pumps can be given, but theywon’t be dealt with in this chapter, as they are lessrelevant for the actual bulk transport of water.

Pumps that work in combination with pipes can becategorised in displacement pumps and impellerpumps. The impeller pumps can be further catego-rised based on the type of impellor in Radial flowpumps, mixed flow pumps and axial flow pumps.All types of pumps will be described in this chapter.

3.2 Pump characteristics

The hydraulic properties of a pump can be describedby some characteristics:o Q-H curveo Efficiency curveo Power curveo Net Positive Suction Head (NPSH) curve.Each of the characteristics is explained in the nextsections for propeller pumps.

3.2.1 Q-H curveThe Q-H curve is the relation between the volumeflow and the pressure at a constant speed of the pumpcrank. The H in the curve is the difference in energylevel between the suction and the pressure side ofthe pump. Q-H curves will be given by the manufac-turer of the pump and can normally be consideredas a simple quadratic curve.

An example of a pump curve is given in figure 3.3.

Fig. 3.3 - Pump curve

Volume flow Q

Pre

ssu

re H

pump curve

Fig. 3.2 - Centrifugal pump

Incomingflow

Impeller

Outgoing pressurisedflow

crank

Incomingflow

Impeller

Outgoing pressurisedflow

crank

Fig. 3.1 - Archimedean screw

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CT5550 - Water Transport

3.2.2 Efficiency curve (fig. 3.4)The hydraulic efficiency of the pump with the motor isgiven with the efficiency curve. The hydraulic efficiencyis the relation between the absorbed hydraulic energy(pressure and velocity) and the provided mechanicalenergy at the pump crank including the power effi-ciency of the motor.

3.2.3 NPSH curveThe Net Positive Suction Head curve is the relationbetween the volume flow Q and the needed marginbetween the energy level at the suction side of thepump and the vapour pressure of the water to pre-vent too much cavitation in the pump (fig. 3.5).At the suction side of a pump negative pressures,i.e. pressures below the atmospheric pressure, canoccur, especially when the actual weir of the pumpis above the level of the reservoir the water is drawnfrom. (see paragraph XX) This negative pressure islimited to the actual vapour pressure of the fluid atthe current temperature. If this allowable negativepressure is subsided, cavitations will take place inthe pump. Although a small amount of cavitationswithin a pump cannot be avoided, this should be lim-ited. The NPSH requirements of a pump give theselimitations. The effect of cavitation is extra wearingof the pump, which can be unacceptable. The bub-bles that are formed at the suction side of the pumpwill be pressurised at the outlet side. The pressu-rised bubbles will act as actual grains or small stones

and wear the material out.To avoid unacceptable cavitations the available NPSHshould be larger or equal to the needed NPSH. Theavailable NPSH is defined as:

0available a vNPSH H h h= + −

With Ha = suction head at the impellerho = Atmospheric pressurehv = Vapour pressure

2

2av

H hg

= +

with h the pressure at the impeller entrance, v thevelocity of the water at the impeller entrance.The varous pump curves are provided by the pumpmanifacturer.

3.3 Classification of pumps

A pump can feed energy to water, which can be ex-pressed in the pressure or lift of the water or in thevolume flow. Based on the combination of pressure/lift and volume flow a classification of pumps can bemade, which can be quantified with the specific speed(see previous paragraph):

High pressure, low flow High pressure, high flow • Drinking water transport • Sewerage water transport

Pres

sure

/lift

Low pressure, low flow • Dosing pumps • Drainage pumps

Low pressure, high flow • Surface water intake • Rain water discharge

Volume flow

Fig. 3.4 - Pump and efficiency curve

Volume flow Q

Pef

orm

ance

pump curve Efficiency curve

Fig. 3.5 - Pump efficiency and NPSH curve

Volume flow Q

NP

SH

pump curve eff. curve NPSH curve

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Pumps and pumping stations

Based on the classification the type of pump neededcan be determined.

3.2.1 Archimedean screwThe Archimedean screw is used in situation that largequantities of water have to be pumped from one freesurface level to another with a level difference of afew meters. A typical use of Archimedean screws isdrainage of polder areas to pump out large volumesof storm water (fig. 3.6). The screw can also be usedif water is polluted with debris as wood, plants andother floating objects.

The capacity of the screw is dependant of the head H(difference between fluid surfaces), the slope of thescrew with the horizontal α the diameter of the screwD and the diameter of the casing d, the number ofblades and the pitch S and the rotating speed of thescrew n.The basic formula for estimating the capacity of anArchimedean screw is

3Q knD= with , , ,S d

k f nD D

α =

Some typical values of k are given in table 3.1

3.3.2 Displacement pumpsA positive displacement pump works following theprinciple of figure 3.7. Several types of displacementpumps are available all working following the princi-ple that a fixed amount of fluid is “encapsulated” andpushed to the pressure side of the pump. Anotherexample is shown in figure 3.8.

Displacement pumps are mostly used for ‘difficult’ flu-ids like very high viscous fluids or for applications ahigh pressure is needed. The pump curve is typicallyvery steep (figure 3.9). This means that a more or lessconstant volume flow is produced with a range of pres-sures.

Fig. 3.6 - Picture of Archimedean screw

pressureside

H

asuction sideh

Screw

d

D

pressureside

H

asuction sideh

Screw

d

D

Fig. 3.7 - Displacement pump

pressureside

suctionside

suctionvalve

pressurevalve

plunjer

sealing

power

pressureside

suctionside

suctionvalve

pressurevalve

plunjer

sealing

power

Table 3.1 - k-values for 3 and 4 blade screws

d/D a = 22O a = 26O a = 30O

S=1D S=1,2D S=0,8D S=1,0D S=1,2D S=0,8D S=1,0D 0.3 0.331 0.336 0.274 0.287 0.286 0.246 0.245 0.4 0.350 0.378 0.285 0.317 0.323 0.262 0.271 0.5 0.345 0.380 0.281 0.317 0.343 0.319 0.287 0.6 0.315 0.351 - .0300 0.327 - 0.273

Fig. 3.8 - Displacement pumpopen (left) andclosed (right)

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CT5550 - Water Transport

3.3.3 Impellor pumpsImpellor pumps are by far the most used pumps. Aindicative figure is that of all the pumping capacity90% is of the impellor type, covering 75% of allpumps. Most important reason is the broad applica-tion possibilities in combination with relative lowmaintenance and high efficiency.

The impellor pump has an axle in a bearing with oneor more (multi stage pump) impellors with a numberof blades. The principle is that the rotation of theimpellors accelerates the fluid and ‘fling’ the water inthe house, which is connected to the pipes. Figure3.10 gives a principle drawing of an impellor pump.The axle can be motored with all kinds of enginesvarying from conventional combustion engine to solarpower driven motors.Three different blades are possible (fig. 3.11):o Centrifugal impellor with axial inflow and radial or

tangential outflowo Mixed flow impellor with axial inflow radial, axial

and tangential outflow

o Axial flow impellor with axial inflow and axial tan-gential outflow

The different types are illustrated in figure 3.11.

Figure 3.12 gives a five stage centrifugal pump. Theblades of this pump are closed.Figure 3.13 gives a two stage ground water abstrac-tion pump. The motor is under the stages and alsosub merged. This is a common way of powering thesekind of pumps. Long shafts are difficult to balanceand sensitive to failure and are not preferred to a di-rect connection between pump and motor.

Centrifugal impellors can be open or closed. The pic-ture of the five stage pumps shows closed blades. Infigure 3.14 a picture of an open blade in the form af apropellor is shown.

Figure 3.15 gives an example of a cut open pump withan open blade.

Fig. 3.12 - Five stage centrifugal pump

Fig. 3.11 - Different types of impellor pumps

Centrifugal pump

Mixed flow pump

Propellorpump

Centrifugal blade

Mixed flow blade

Propellor bladeFig. 3.9 - Steep pump curve displacement pump

Pre

ssur

e

Volume flow

Steep pumpcurvedisplacementpump

Weir inlet

Suctionshield

Pressureshield

Sealing

Axle

WeirWeir outlet

House

Weir inlet

Suctionshield

Pressureshield

Sealing

Axle

WeirWeir outlet

House

Fig. 3.10 - Principle impellor pump

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Pumps and pumping stations

The axial propellor pumpsare best applied for large vol-ume flows and low heads(less than 1 ato or 10 me-ter). The application formixed flow pumps is in therange of 3 to 30 meters headand is characterised with afairly constant efficiencywith varying heads.Centrifugal pumps are usedwhen high heads are re-quired (above 20 meters).With serial or multi stagepumping heads up to 1000meter can be reached.

3.3.4 Specific speedTo classify geometricallysimilar pumps, the numeri-cal quantity ‘specific speed’has been adopted. Specificspeed is the speed requiredfor delivery of unit flowagainst unit head; it will varyin accordance with the sys-tem of units used

Specific speed

3 4

n QNs

H=

N is the pump impellerspeed in rpm; Q is outputat maximum efficiency (m3/s); H is delivery head atmaximum efficiency (m) ascan be read from the effi-ciency curve. Specificspeeds are an indication ofwhat type of impeller pumpcan be used for a specificapplication.Following the definition ofspecific speed a pump witha low specific speed will

generally give a low volume flow with a high pressure.A radial flow pump will be most suited. A high specific

Fig. 3.14 - Propellor

Fig. 3.15 - Glass pump

Fig. 3.13 - Ground waterpump withtwo stages

Effi

cien

cy in

%

centrifugal

Mixedflow

axial

spec. speed

volu

me

flow

in m

3 /s

Pre

ssur

e in

m

speed in r/min

Effi

cien

cy in

%

centrifugal

Mixedflow

axial

spec. speed

volu

me

flow

in m

3 /s

Pre

ssur

e in

m

speed in r/min

Fig. 3.16 - Relation between specific and actual speed,

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CT5550 - Water Transport

speed will result in a relative large volume flow at arelative low pressure, resulting in a preference for anaxial pump.

Figure 3.16 gives a relation between specific speed,actual speed, pressure, volume flow and efficiency.

3.3.5 Working principle impellor pumpsThe rotation of the axle accelerates the fluid addingkinetic energy to the fluid. Within the house or diffuserof the pump this kinetic energy is partly transformedto static pressure, increasing the head of the fluid.At the suction side of the impellor the fluid acceler-ates as well and pressure at that point will be low-ered, following the laws of Bernoulli. Energy is con-served and at the suction side of the pump the pres-sure of the fluid will be partly transformed to kineticenergy. This requirement is expressed in the Net Posi-tive Suction Head (NPSH) of a pump. At high volumeflows, the NPSH will increase, because the kineticenergy at the suction side will be higher due to thehigher fluid velocity.

The form of the blades on the impellor affects the char-acteristics of the pump. Different shapes of bladesare given in figure 3.17. Every blade has its own set ofpump curves.

3.4 Application of pumps

3.4.1 Pump and network characteristicsThe performance of a pump is described by the Q-H-curve, giving the relation between the volume flow

and the head realised by a single pump (figure 3.3).

On the other hand the network served can be char-acterised with the pipe characteristic. Every set orsystem of pipes can be virtually represented by asingle pipe with equivalent hydraulic features. In theend a complete network can hydraulically be repre-sented by one pipe, called the network characteristic.The hydraulic losses in this characteristic pipe arequadratic proportional to the velocity, and thus volumeflow, in that pipe.

22

50,08262

L v LH Q

D g Dλ

λ∆ = =

The pump is used to compensate the dynamic en-ergy losses in the pipes.

A network characteristic gives the required pressureat the pump that guarantees a minimum pressure ata critical point in the network. The network charac-teristic represents the hydraulic behaviour of thenetwork between the pumping station and the char-acteristic pressure point in the network. If a networkis represented in one pipe the pipe characteristic canbe visualised as is shown in figure 3.18.

In a graph this results in the quadratic curve shown infigure 3.18.

Combination of the Q-H curve and the pipe charac-teristic gives the optimal working point of the combi-nation pump-network, which is represented in figure3.19.The intersection of the two curves is the working point.At this volume flow the exact minimum pressure isreached in the network attached to this pump.If another volume flow is demanded in this network,for instance a lower volume flow, than the working

Fig. 3.18 - Network characteristic

H network

H (Q2)

H (Q1)

H (Q3)

?H (Q1)

?H (Q2)

?H (Q3)

QoutQin

H network

H (Q2)

H (Q1)

H (Q3)

?H (Q1)

?H (Q2)

?H (Q3)

QoutQin

Fig. 3.17 - Different types of blades

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Pumps and pumping stations

point will shift on the pump curve. The extra pressurewill be the extra pressure in the characteristic point inthe network (see figure 3.20). This means that in thenetwork the pressure is higher than the minimum re-quired according to the network characteristic.The manufacturer of the pump will provide the pumpcurve as a product specification. One must bear inmind that the pump curve only reflects the pump per-formance in its own and not the performance withinthe lay-out of the pumping station, for instance in com-bination with a non-return valve.The network characteristic can theoretically be cal-culated by combing the pipes in the network to onepipe only. Using network models and a calculationprogram is a more feasible option.

3.4.2 Pump lay outThe volume flow in a network will vary over the hoursof the day, the days of the week and even the sea-sons in a year. Typical demand curves for drinkingwater are given in figure 3.21.

The variation in demand amounts to a factor 5 or higher

given the fact that the average demand can vary with afactor 2 over the seasons (see also the lecture notesCT3420). The situation for a pressurised sewer pipesis more or less the same, with an even higher fluctua-tion as a result of rainwater. In times of heavy rainfallthe maximum capacity of the system will be used.Usually this results in a direct waste of diluted sewer-age water on surface water.This varying flow pattern cannot be covered with onepump. Theoretically a pump and a network have onlyone ideal working point. In reality always more pumpswill be installed to cover the pipe characteristic asgood as possible with the combination of pumps.

Pumps can be put in parallel as is shown in figure3.22. Pumps in parallel give a different composite curveas pumps in series as sown in the different pictures.The result of pumps in parallel is more flow with apressure less than the maximum pressure of a singlepump. This is demonstrated in figure 3.22.

A picture of pumps in parade is given in figure 3.23.

Fig. 3.19 - Working point

Volume flow Q

Pre

ssur

e Pump curve

Network characteristic

Workingpoint

Volume flow Q

Pre

ssur

e Pump curve

Network characteristic

Workingpoint

Fig. 3.20 - New working point

Volume flow Q

Pre

ssur

e

New workingpoint

Pump curve

Network characteristic

Less volume flow

More pressure

Fig. 3.21 - Demand curves

0

0,5

1

1,5

2

hours of the day

Qac

tual

/Q a

vera

ge

Qaverage

Fig. 3.22 - Pumps in parallel

Q

Q2Q

Volume flow Q

Pre

ssur

e H

Network char.Single pump

curve

double pump curve

Working points

Two pumps in parallel

Q

Q2Q

Q

Q2Q

Volume flow Q

Pre

ssur

e H

Network char.Single pump

curve

double pump curve

Working points

Two pumps in parallel

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CT5550 - Water Transport

Pumps can also be configured in series. This alsogives a composed pump curve as is demonstrated infigure 3.24.

In series pumps can also be placed, for instance arow of windmills, all pumping the water to a higherlevel. The result of pumps in series is that pressureis added. This means that higher pressures are pos-sible, but not more flow than from the single pump.

3.4.3 Flow control and variable speedpumps

Because of the varying demands in a drinking waternetwork, a pumping station will consist of severalpumps. Switching pumps on and off will make differ-ent combinations of pumps possible in series or inparallel, which gives sufficient pressure and volumeflow in the network.As shown in the previous paragraph, pump curves canbe constructed using the individual pump curves ofthe separate pumps. In the network the pressure willvary when pumps are switched on and off. Especiallyduring night hours, when consumption is low, the pres-sure may rise. Too high pressure may lead to an el-evated level of leakage and may cause taps to startdripping. To maintain a constant level of pressure thepump can be throttled at the pumping station, for in-stance by using a valve. Effectively this means a higherresistance at the beginning of the pipe.

Another way of controlling the flow and pressure in anetwork is to install variable speed pumps. With avariable speed pump the power of the driving enginecan be controlled leading to a pump with a number ofpump curves.In figure 3.26 a set of pump curves of a variablespeed pump is drawn, together with a network char-acteristic.

Fig. 3.25 - Mills in serie

Lowermill

Uppermill

Polderlevel

Sea level

Fig. 3.26 - Variable speed pump

Volume flow

Pre

ssur

e

Different pump curvesof variable speed pump

Network characteristic

Volume flow

Pre

ssur

e

Different pump curvesof variable speed pump

Network characteristic

Fig. 3.23 - Multiple pumps in parallel

Fig. 3.24 - Pumps in series

Volume flow Q

Pre

ssur

e H

QQ Q

Network char

Single pumpcurve

Double pumpcurve

Workingpoints

Two pumps

in series

Volume flow Q

Pre

ssur

e H

QQ QQQ Q

Network char

Single pumpcurve

Double pumpcurve

Workingpoints

Two pumps

in series

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Pumps and pumping stations

3.4.4 Pumping station lay outFor the design of an actual pumping station severalinput parameters have to be known. At first the varia-tion in volume flows that will have to be dealt with bythe pumping station. For typical ‘utility purposes’ likedrinking water supply or sewerage water transport aforecast for the volume flows to be expected has to bemade. The growth of the area to be served has to beknown, for instance the number of drinking water con-sumers or the paved surface in case of a seweragewater collection. If it is difficult to make a more or lessrealistic forecast, this will have an effect on the lay outof the pumping station. Modular built up in compart-ments that can easily be added to the original build-ing is an opportunity. Another way is to have morepumps and pump combinations that can serve thedemand.The next input is the network characteristic of thenetwork to be served. This network characteristic canbe determined using measurements or network cal-culations. With the designed network or a calibratedmodel of an existing one (see next chapter) the net-work characteristic can be composed.The network characteristic together with the demandvariations, gives the range of pressures and volumeflows to be covered with the pumping lay out.

In the number of pumps to be installed one has tohave a certain redundancy. Because of normal main-tenance one or two pumps may be in service of can

break down for any reason. For maintenance reasonsone has to consider whether is it sensible to have thesame pumps or pumps with exchangeable parts.

Eventually the pumps have to be installed within apumping station. A schematic of a pumping stationwith several pumps in parallel is given in figure 3.27and 3.28 as an example of how to make a pump layout.

Hydraulic considerations will have to be made. Avoid-ing sharp corners is the leading principle. Make con-structions that are as smooth as possible. Firstly toprevent energy losses because of release of flow linesand introducing extra resistance in the system. Sec-ondly to prevent ‘dead’ corners in the system that cangive rise to water quality problems.

Fig. 3.27 - Pump layout Fig. 3.29 - Pump with non-return valve

Fig. 3.28 - Pump layout

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CT5550 - Water Transport

Fig. 3.31 - Pump with check valve

Extra attention should be paid to the suction side ofthe pumps. Of course the NPSH (net positive suctionhead) has to be taken into account, but also the riskof air entrapment due to vortexes should be consid-ered. An example of a vortex is given in figure 3.30.

Every pump in a pumping station will be equipped witha non-return valve. This is necessary to prevent waterto flow back when the pump is switched of or in thecase of parallel pumps to prevent ‘round pumping’.The check valve can be at the suction side or pres-sure side of the pump. Examples of both are given infigures 3.29 and 3.31.

Fig. 3.30 - Vortexes