Centrifugal Pump Efficiency Question

[vermab]

It has been said to me that the efficiency of a centrifugal pump is increased if you increase the suction side head available.

I have to pump 1500 GPM water into an RO system requiring @ 850 psi of pressure, pump is operating @ 2,900 RPM and requires @ 20 psi suction head, so I have to provide a low pressure pump into the suction of this high pressure pump.

So my question.....Does the efficiency of the HP pump increases if you significantly increase the suction side pressure above the required NPSH?

Appreciate any help from pump experts subscribing to this forum.

 

[BigInch]

Unfortunately the efficiency of the pump will not increase if you increase suction pressure. The pump curve H-Q & E-Q curves are not affected by increasing suction pressure.

Increasing suction pressure might change the flowrate in the system and, if that flow is closer to the Best Efficiency Point of each pump, it is possible that the net pumping efficiency of the system will be improved, but that would not be solely the result of simply increasing suction pressure to the second pump alone.


"Make everything as simple as possible, but not simpler." - Albert Einstein (1879-1955)
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[khardy]

Well, for a given flowrate, bosting the suction pressure via a booster pump would lower the power consumption of the primary pump, but not the efficiency. However, the booster would likely consuem more than the power saved from the primary pump.

 

[BigInch]

khardy,

Bosting the suction pressure would not lower the power consumption of the primary pump. The differential head for any given flowrate through the primary pump would remain the same, hence power consumption at any given flowrate would also remain the same. If you look down below the pump H-Q curve, often you will see the power consumption, P-Q curve. As you will notice, that curve remains the same at any given suction pressure.



"Make everything as simple as possible, but not simpler." - Albert Einstein (1879-1955)
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[BigInch]

This chart plots power P-Q in the same frame as the H-Q plot. No suction pressure dependency.

"Make everything as simple as possible, but not simpler." - Albert Einstein (1879-1955)
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[Artisi]

Boosting the inlet pressure will only shift the operating point of the pump out along its operating curve - ie, make it a 2 stage operation - unless you change impleller diameter or restrict the discharge boosting the inlet pressure would be a negative step and serve no useful function.

 

[smckennz]

Lets split some hairs:
At a suction head equal to the NPSHR, the pump will, by definition, produce only 97% of the TDH that it would if the suction head was sufficiently high to prevent any trace of cavitation. A portion of the missing 3% will seen as a slight drop in absorbed power, but the remainder will be lost as heat. So strictly speaking, there will be a slight improvement in pump efficiency as the suction head increases above NPSHR. However any sensibly applied pump will have sufficient NPSHA/NPSHR margin to make the efficiency improvement either zero or too small to be measured.

Cheers

Steve