what is Pumps process Minimum Flow? 1

[usharma007]

Please explain me pump minimum flow should be less than process minimum flow .If it get more than process minimum flow how to control that flow. Also please explain me why the shut off head should be 5 % in normal operation & 10 % for parallel operation ?

 

[LittleInch]

Pump min flow comes from the vendor. He doesn't know what your process is so just gives you information. So long as you don't operate pump below pump min flow then you should be OK

More flow is controlled either by speed control of the pump (VSD) or control valve downstream of the pump throttling the pump and reducing flow

Normally API 610 pumps require no more than 10% head rise from duty point to shut off head. Don't know why this is 5%
Parallel operation should be no different to "normal"
This is to stop pressure rising too far on no flow / start-up and creates a steady head / pressure during operation.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[usharma007]

thanks for your suggestion but as per my knowledge pumps minimum flow should not be the more than the process minimum flow, and for shut off API 610 say for parallel operation minimum shut off head should not be less than 10 %,i want to know why 10 % shut off head is required.

 

[DubMac]

For parallel pumping, you want to select a "steeper" curve to keep pumps from "wandering" up and down the curve as they seek to equalize pressures with each other. This can happen with flat or drooping curves in parallel.

 

[LittleInch]

Pump minimum flow should be less than process min flow so that you don't operate the pump in a region (below pump min flow) where it will start to heat up if used there for a long time.

Why 10% (and not say 15%) - don't know, but is a measure of a higher quality centrifugal pump and has been shown over time to be a number that works well and doesn't cuase any substantial issues in terms of overpresusre of piping in the event of no flow.

Dubmac is exactly correct - paralell pumping of even "identical" pumps are rarely equal, but the steeper the curve, the more likley it is that the share of flow between towo or more pumps is more evenly split

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DubMac]

From a pump design standpoint, minimum flow is determined by maximum permissible temperature rise in the pump. This number typically comes from dusty old files of former repairs, i.e. empirical data.

The old equation for Temperature rise in a pump (don't know why a new one would be any better) is:
Temp Rise, DegF= (TDH,ft./778) * ((1/Efficiency at point considered) - 1)

If a manufacturer of a certain pump says the max permissible temp rise is 20 degrees, then you can backcalculate your minimum flow using the performance curve.

 

[usharma007]

thanks all for sharing your experiences, please give me more exploration on parallel operation shut off .