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Turbo-hydraulic pumps are:

centrifugalpumps,

Propellerpumps,

andjetpumps.

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Analysis of turbo-hydraulic pumps:

is a problem involving fundamental principlesof hydraulics.

Positive-displacementpumps:

move fluid strictly by precise machinedisplacements such as a gear system rotating

within a closed housing (screw pumps) or a

piston moving in a sealed cylinder (reciprocalpumps).

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Analysis ofpositive-displacementpumps

involves purely mechanical concepts and does

not require detailed knowledge of hydraulics.

This chapter will only treat the first category,

which constitutes most of the water pumps

used in modern hydraulic engineering systems.

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Centrifugal Pumps

Modern centrifugal pumps basically consist oftwo parts:-

1. the rotating element( commonly calledthe impeller);

2. the housingthat encloses the rotatingelement and seals the pressurized liquidinside.

The poweris supplied by a motor to the shaft ofthe impeller.

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The rotary motion of the impeller creates acentrifugal forcethat enables the liquid to enter

the pump at the low-pressure region near thecenter (eye}of the impeller and to move alongthe direction of the impeller vanes toward thehigher-pressure region near the outside of thehousing surrounding the impeller.

The housing is designed with a graduallyexpanding spiral shape so that the enteringliquid is led toward the discharge pipe withminimum loss while the kinetic energy in theliquid is converted into pressure energy.

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Cross section of a centrifugal pump

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5.3 Jet (Mixed-Flow) Pumps

Jet pumps capitalize on a high-pressure stream of fluid.

The pressurized fluid ejects from a nozzle at high speed

into a pipeline transferring its energy to the fluid requiring

delivery.

Jet pumps are usually used in combination with a

centrifugal pump, which supplies the high-pressurestream, and can be used to lift liquid in deep wells.

They are usually compact in size and light in weight.

They are sometimes used in construction for dewatering

the work site.

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Because the energy loss during the mixing

procedure is heavy, the efficiency of the jet pump

is normally very low (rarely more than 25%).A jet pump can also be installed as a booster

pump in series with a centrifugal pump. The jet

pump may be built into the casing of the

centrifugal pump suction line to boost the watersurface elevation at the inlet of the centrifugal

pump as shown schematically in Figure 5.7. This

arrangement avoids any unnecessary installation

of moving parts in the well casing, which is

usually buried deep below the ground surface.

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Jet (Mixed-Flow)

Pumps

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5.4 Selection of a Pump

The efficiency of a pump depends on:

the discharge, head, and power requirement of thepump.

The approximate ranges of application of each type

of pump are indicated in Figure 5.8.

The total head that the pump delivers its dischargeagainst includes the elevation head and the head

losses incurred in the system.

The friction loss and other minor losses in the

pipeline depend on:

the velocity of water in the pipe (Chapter 3),

and the total head loss can be related to the discharge

rate.

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For a given pipeline system (including the pump), aunique H-Q curve can be plotted, as shown in

Example 5.3, by computing the head losses forseveral discharges.

In selecting a particular pump for a given system,the design conditions are specified and a pump is

selected for the range of applications. The H-Q curve is then matched to the pump

performance chart (e.g., Fig 5.9 and 5.10) providedby the manufacturer.

The matching point , M, indicates the actual workingconditions.

Selection process -----example 5.3

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Head,

discharge, and

power

requirement ofdifferent pumps

Selection of a Pump

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Pumpselection

Chart

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Characteristic curve

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Characteristic curve

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5 5 Pumps in Parallel or in

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5.5 Pumps in Parallel or in

Series

the efficiency of a pump varies with:

the discharge rate of the pump and the total head overcome by the pump.

The optimum efficiency of a pump can

be obtained only over a limited range ofoperation (see Figure 5.10).

To install a pumping station that can beeffectively operated over a large rangeof fluctuations in both discharge andpressure, it may be advantageous toinstall several identical pumps at the

station.

When several pumps are connected in

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When several pumps are connected in

parallel in a pipeline, the discharge is

increased but the pressure head remains the

same as with a single pump. It should be noted that two identical pumps

operating in parallel may not double the

discharge in a pipeline because the total headloss in a pipeline is (Hp aQ2).

The additional resistance in the pipeline will

cause a reduction in the total discharge.

Curve B in Figure 5 1 1 schematically

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Curve B in Figure 5. 1 1 schematically

shows the operation of two identical pumps

in parallel. The joint discharge of the two

pumps is always less than twice thedischarge of a single pump.

Pumps connected in seriesin a pipeline will

increase the total output pressure, but thedischarge will remain approximately the

same as that of a single pump. A typicalperformance curve for two pumps connected in series is shown

by curve C in Figure 5.11.

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Pumps in Parallel or Series

The efficiency of two (or more) pumps operating in

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The efficiency of two (or more) pumps operating in

parallel or in series is almost the same as that of the

single pump based upon discharge.

The installation can be arranged with a separate motorfor each pump or with one motor operating two (or

more) pumps.

Multipump installations could be designed to perform

either in-series or in-parallel operations with the sameset of pumps. Figure 5.12 is a typical schematic of such an

installation.

For series operations, valveA is opened and valves B

and C are closed; for parallel operations, valve A isclosed and valves B and C are open.

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5.6 Cavitation in Water Pumps

One of the important considerations in pump installationdesign is the relative elevation between the pump andthe water surfacein the supply reservoir.

Whenever a pump is positioned above the supplyreservoir, the water in the suction line is under pressurelower than atmospheric. The phenomenon of cavitationbecomes a potential danger whenever the water

pressure at any location in the pumping system dropssubstantially below atmospheric pressure.

To make matters worse, water enters into the suctionline through a strainer that is designed to keep out trash.This additional energy loss at the entrance reduces

pressure even further.

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A common site of cavitation is near the tips of

the impeller vanes where the velocity is very

high.

In regions of high velocities much of the

pressure energy is converted to kinetic energy.

This is added to the elevation differencebetween the pump and the supply reservoir, hp,

and to the inevitable energy loss in the pipeline

between the reservoir and the pump, hL. Those

three items all contribute to the total suctionhead, Hs, in a pumping installation as shown

schematically in Figure 5.13.

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The value of Hs must be kept within a limit so that thepressure at every location in the pump is always above

the vapor pressure of water; otherwise, the water will bevaporized and cavitation will occur.

The vaporized water forms small vapor bubbles in theflow. These bubbles collapse when they reach theregion of higher pressure in the pump. Violent

vibrations may result from the collapse of vapor bubblesin water. Successive bubble breakup with considerableimpact force may cause high local stresses on themetal surface of the vane blades and the housing.These stresses cause surface pitting and will rapidly

damage the pump.

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To prevent cavitation, the pump should

be installed at an elevation so that the

total suction head is less than thedifference between the atmospheric

head and the water vapor pressure

head, or

(Patm/g - Pvap/g ) > Hs

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Cavitation

in Water

Pumps

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5.7 Specific Speed and Pump Similarity

The selection of a pump for a particular service is based on therequired discharge rate and the head against which the discharge is

delivered. To lift a large quantity of water over a relatively small elevation (e.g.,

removing water from an irrigation canal onto a farm field) requires ahigh-capacity, low-stage pump.

To pump a relatively small quantity of water against great heights(such as supplying water to a high-rise building) requires a low-

capacity, high-stage pump. The designs of these two pumps are verydifferent.

Generally speaking, impellers of relatively large radius and narrowflow passages transfer more kinetic energy from the pump intopressure head in the flow stream than impellers of smaller radius andlarge flow passages. Pumps designed with geometry that allows waterto exit the impeller in a radial direction impart more centrifugalacceleration to the flow than those that allow water to exit axially or atan angle. Thus, the relative geometry of the impeller and the pumphousing determine the performance and the field application of aspecific pump.

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Dynamic analysis (Chapter 10) shows thatcentrifugal pumps built with identical proportionsbut different sizes have similar dynamic

performance characteristics that areconsolidated into one number called a shapenumber. The shape number of a particular pumpdesign is a dimensionless number defined as

where w is the angular velocity of the impeller inradians per second, Q is the discharge of thepump in cubic meters per second, g is thegravitational acceleration in meters per second

squared, and Hp is the total dynamic head inmeters that the pump develops.

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In engineering practice, however, the dimensionlessshape number is not commonly used. Instead, mostcommercial pumps are specified by the term specific

speed. The specific speed of a specific pump design(i.e., impeller type and geometry) can be defined in twodifferent ways. Some manufacturers define the specificspeed

of a specific pump design as the speed an impeller

would turn if reduced in size enough to deliver a unitdischarge at unit head. This way, the specific speedmay be expressed as Eqn (5.22).

Other manufacturers define the specific speed of a

specific pump design as the speed an impeller wouldturn if reduced enough in size to produce unit powerwith unit head. This way, the specific speed isexpressed as Eqn(5.23)

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Normally, the specific speed is defined at the optimum

point of operational efficiency.

In practice, pumps with high specific speedsaregenerally used for large discharges at low-pressure

heads,

while pumps with low specific speedsare used to deliver

small discharges at high-pressure heads.

Centrifugal pumps with identical geometric proportions

but different sizes have the same specific speed.

Specific speed varies with impeller type. Its relationship

to discharge and pump efficiency is shown in Figure

5.14.

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