TURBOMACHINES : Pumps Performance

8/3/2019 TURBOMACHINES : Pumps Performance

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TURBOMACHINES

PUMPS PERFORMANCE


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Net Positive Suction Head (NPSH)

On the suction side of a pump, There is a possibility ofcavitation occurring within the pump due to low

pressures. cavitation occurs when the liquid pressure at a given

location is reduced to the vapor pressure of the liquid .

This causes loss in efficiency as well as structural

damage to the pump. Potential for cavitation is characterized with the

difference between the total head on the suction side,near the pump impeller inlet,

And the liquid vapor pressure head,

reference for the elevation head passes through thecenterline of the pump impeller inlet.


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Net Positive Suction Head (NPSH)

This difference is called the net positive suction headNPSH

There are actually two values of NPSH of interest.

NPSHR , required NPSH, that must be maintained, orexceeded, so that cavitation will not occur.

Determined from the above equation

NPSHA , available NPSH, which represents the head that

actually occurs for the particular flow system. This value can be determined experimentally, or

calculated if the system parameters are known.


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Absolute pressures are normally used since the vapor

pressure is usually specified as an absolute pressure. Forproper pump operation it is necessary that

as the height of the pump impeller

above the fluid surface, z1, isincreased, the NPSHA isdecreased.

Therefore, there is some criticalvalue for z1 above which the pump

cannot operate without cavitation.

if the reservoir is above the

pump, z1 will be negative inand the NPSHA will increaseas this height is increased.


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Pump Curves

Pump manufacturers supply performance curves for

each of their pumps. These are normally referred to aspump curves. These curve are generally developedusing water as the reference fluid.

The following can be read directly from a pump curve:

Head vs. flow rate information for any fluid

Pump efficiency for any fluid

Pump horsepower for system operating with water


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Pump Performance Curves

DevelopedHead

ImpellerDiameter

Efficiency

Flow Rate

NPSH

Horsepower


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ExampleQ = 300 gpm

Di= 10

Head(ft) = 95 ft

(%) = 70

P(hp) = 10HP


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Power Input

fluid

water

fluid

water

fluidGrSp

P

P..

Note: A less dense fluid requires less horsepower


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NPSH

Do not use NPSH to size or select a pump unless all elsefails. Pump selection is governed by H vs. Q requirementsof system. When NPSHA is too small, it might be increasedby:

Increasing source pressure (not usually feasible) Cooling liquid to reduce vapor pressure (not usually

feasible) Raise elevation of source reservoir

Lower elevation of pump inlet Raise level of fluid in reservoir


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EXAMPLE

Estimate(a)the design-point discharge,(b)the water horsepower, and

(c)the head if b1 = b2 =1.75 in.

Given are the following data for a commercial centrifugal waterpump:


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From the inlet-velocity diagram, Fig.

with

the discharge is

Solution Part (a)The angular velocity is

the tip speeds are


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Example -Part (b)

The outlet radial velocity follows from Q

This enables us to construct the outlet-velocity diagram as in Fig.

given

The tangential component is


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Part (b)

The power is then computed from Eq.with

at the design point


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Part (c)

The head is estimated from


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If NPSHA Cant Be Increased

If the pump must be modified to achieve proper NPSH:

Larger slower-speed pump

Double suction impeller

Larger impeller eye

Oversized pump with an inducer


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Pump Selection from Many Choices ofCharacteristic Curves

1. Examine pump curves to see which pumps operatenear peak efficiency at desired flow rate. Thissuggests some possible pipe diameters.

2. Compute system head requirement for a few

diameters.

3. Compute V for some diameters. For water V in therange of 1 10 ft/s is reasonable (see ahead).

4. Re-examine pump curves with computed head andpipe diameters. This may give a couple of choices.

5. Pick pump with highest efficiency.


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Remember

Maximize pump efficiency

Power input (hp) should be minimized if possible

Selected impeller diameter should not be largest or

smallest for given pump. If your needs changeswitching impellers is an economical solution

NPSH required by the pump must be less thanNPSHA

O


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Effect of Operating Pumpsin Series, in Parallel

Pumps can be arranged in series or in parallel to provide

for additional head or flow capacity. When two pumps are placed in series, the resulting

pump performance curve is obtained by adding heads atthe same flowrate.

both the actual head gained bythe fluid and the flowrate areincreased, but neither will bedoubled if the system curveremains the same.

The operating point is at (A)for one pump and moves to(B) for two pumps in series.


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Effect of Operating Pumps - in Parallel For two identical pumps in parallel, the combined

performance curve is obtained by adding flowrates at the

same head, as shown in Fig

The flowrate for the system willnot be doubled with the additionof two pumps in parallel (if thesame system curve applies).

However, for a relatively flatsystem curve, as shown inFig. a significant increase inflowrate can be obtained as

the operating point movesfrom point (A) to point (B).


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Dimensionless Parameters and Similarity Laws

Characteristics of pumps are usually determinedexperimentally,

dimensional analysis and similitude considerations will beuseful in the study and documentation of thesecharacteristics.

The principal, dependent pump variables are the actual

head rise, ha, shaft power, WShaft, and efficiency, .

incompressible fluids - compressibility effects notConsidetred


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head rise coefficient.

power coefficient

Reynolds number that

represents the relativeinfluence of viscous effects.


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At high Reynolds numbers, the effect of the Reynolds

number can be neglected.

The relative roughness, /D, can also be neglected in pumpssince the highly irregular shape of the pump chamber isusually the dominant geometric factor rather than thesurface roughness.

All pertinent dimensions, li scaled by a common length scale

The dependent pi terms are functions of only Q/D3, sothat

Di i l P t d Si il it L


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CQ= Q/D3 the flow coefficient

These three equations provide the desired similarityrelationships among a family of geometrically similarpumps.



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It follows then

where the subscripts 1 and 2 refer to any two pumpsfrom the family of geometrically similar pumps.

If two pumps from the family are operated at the same value of flowcoefficient

With pump scaling lawsit is possibleto experimentally determine the performance characteristics of onepump in the laboratory and then use these data to predict the corresponding characteristicsfor other pumps within the family under different operating conditions.


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shows some typicalcurves obtained for acentrifugal pump (12inch dia, at 1000 rpm.

shows the results plotted interms of the dimensionlesscoefficients, CQ, CH, CP,and .

Dimensionless Parameters and Similarit La s


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From these curves shown in Figure

the performance of different-sized, geometrically similar

pumps can be predicted, as can the effect of changingspeeds on the performance of the pump from which thecurves were obtained.

It is to be noted that the efficiency, , is related to theother coefficients through the relationship = C

Q

CH

C-1P

.


Special Pump Scaling Laws


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Two special cases related to pump similitude commonlyarise.

In the first case we are interested in how a change in theoperating speed, , for a given pump, affects pumpcharacteristics. For the same flow coefficient (andtherefore the same efficiency) with D1 = D2 (the same

pump)

The subscripts 1 and 2 now refer to the same pump

operating at two different speeds at the same flowcoefficient.



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Also

Thus, for a given pump operating at a given flowcoefficient, the flow varies directly with speed, the headvaries as the speed squared, and the power varies asthe speed cubed.

These scaling laws are useful in estimating the effect ofchanging pump speed when some data are availablefrom a pump test obtained by operating the pump at aparticular speed.



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In the second special case we are interested in how achange in the impeller diameter, D, of a geometrically

similar family of pumps, operating at a given speed,affects pump characteristics.

for the same flow coefficient with 1 =2


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Specific Speed


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Specific Speed

A useful pi term can be obtained by eliminating diameterD between the flow coefficient and the head rise

coefficient.

The dimensionless parameter Ns is called the specificspeed.

In dimensional form.

Suction Specific Speed


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Suction Specific Speed

In dimensional form

With an analysis similar to that used toobtain the specific speed pi term, the

suction specific speed, Ss can beexpressed as

This dimensionless parameter is useful in determining therequired operating conditions on the suction side of the pump.


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Power Input


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Power Input

For fluids other than water:

WmP

min

s

hps

lbf t

f t

lb

gal

ft

min

galq

lb

lbft

g

gH

hpP f

m

m

f

c

60550

48.7

1

)(

3

3

W

m

TURBOMACHINES : Pumps Performance

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