Rated and Normal pump flow

Rated and Normal pump flow

By Mr. Bharat Kadu, process Engineer at Jacobs Engineering, Mumbai.

For single stage centrifugal pump, if my rated flow is 135% higher than the normal flow; then what about my BEP and pump actual performance in the field? I am not having pump curves, we are just deciding over the flow design margin as our desing flows are 20% higher than normal, add on that we have taken more 10% margin of desing flow. is worth designing pump this way?
I am quite worried about pump performance and efficiencies!

Lionel S. • If the rated flowrate is (too) much higher than the BEP, then:
* reliability falls (due to increased wear and recirculation)
* pump efficiency decreases
* power consumption goes up (increasing the flowrate by 10% increases the friction losses by 21% in turbulent flow).
* NPSH Required increases.

So I suggest you not to over-specify the pump's flowrate and specify only the design flowrate (normal flowrate + design margin, usually between 10 and 25% to allow for calculation errors - to be checked with your Process Design Basis/Criteria document).

The manufactuer will then select the closest model pump that fits these conditions.

After, once you get the selected pump curve from the pump manufacturer, check that the rated flowrate (which is the actual capacity realized by the actual, supplied centrifugal pump when running at the design speed and TDH) falls betwen 80% and 110% of the BEP flowrate (caution: this range varies from pump to pump). In any case the BEP should be between the rated point and the normal operating point on the pump curve.

S. L. Abhyankar -->

@Bharat Kadu - I notice that you are Process Engineering at Jacobs Engineering, Mumbai. Taking clue from that I would like to point out that your phrase "Is it worth designing pump this way?" is wrong. I take it that you are designing a system and are wanting to work out the purchase specification for the pump to work in the system.

The terms RATED and NORMAL flow have specific significance for the system-designer. To speak by numbers, say, the NORMAL system-requirement is 100 m3/h. But you would like the system to work for an increased output of 135 m3/h. So with 35% margin over NORMAL flow, you may specify RATED flow for the pump to be 135 m3/h.

Coming to specifying the head, if the static head in the system is 20m. If piping is selected for Hf = 5m at NORMAL flow of 100 m3/h, at RATED flow of 135 m3/h Hf will increase in square proportion. It will become 5 * (1.35)^2 = 5 * 1.8225 = 9.1125. Then RATED head will have to be 29.1125 m, say 29.5 m

NORMAL system-requirement is 100 m3/h, 25 m. RATED duty will be 135 m3/h, 29.5 m. Power requirement will be higher by the ratio (135*29.5) / (100*25) = 1.593 !

If piping is selected to have Hf = 5 m at 135 m3/h, at NORMAL flow of 100 m3/h will be 5 / 1.8225 = 2.7435 m. Then also -

NORMAL system-requirement is 100 m3/h, 22.7435 m. RATED duty will be 135 m3/h, 25 m. Power requirement will be higher by the ratio (135*25) / (100*22.7435) = 1.484 !

Capital cost of piping when selected for Hf = 5 m at 135 m3/h will be higher. But if piping is selected for Hf = 5 m at 100 m3/h, the Operating cost to the user will be higher !

Pump-vendor will offer pump to get best possible efficiency at RATED duty in purchase specification. But it may not be the BEP of the pump. Effect on performance in actual operation will depend upon where the RATED and NORMAL duties will be with reference to the BEP of the pump.