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How to determine NPSHa for two pump in parallel

417 (Mechanical) (OP)
17 Aug 01 7:01
Hi, Can you please advise on how to calculate the head losses for two centrifugal pump connected in parallel to a common suction line? And also the NPSHa for the pump. The flow rate require is 500 liters/sec and the pump is 100m away from a reservior 5 m above the pump. The water is pump to two tanks one 200m away at 30m and the other 300m away at 50m. Currently the selected suction pipe size is 500mm. Thanks.

Regards,
417
troubleshooter (Mechanical)
21 Aug 01 14:19
Assuming both parallel pumps are identical, you calculate normally to find the absolute pressure at the pump suctions, using the actual flow in the common line and then dividing the flow as it physically does, calculating losses in each branch to find the suction pressure available at each pump suction.  They are unlikely to be equal, and even if they are, it would be wise to use 60% flow in each branch, even though you expect closer to 50%, as the pumps and impellers are NEVER identical, and the flow will be slightly higher on one.  You just don't know which one it will be.

Remember that the NPSHr is more critical with a single boiling point fluid (like water) than it is with a multi-boiling point fluid (like oil), as cavitation will come on very quickly with single boiling point fluids, and slowly with multi boiling point fluids.
rusman (Chemical)
27 Aug 01 1:26
Is those two pumps are running concurently or as a standby operation (one running, and one standby?, 2 x 100 %).

If the later is the case, line suction line size need to be sized for both pumps are running, ie suction line need to be sized for 1000 l/s (1 m3/s). And the pressure drop shall be sized accordingly.

Thanks

rusman
Senior Process Engineer
Kuala Lumpur, Malaysia
man@rwsb.po.my
stanier (Mechanical)
27 Aug 01 2:00
Suggest you invest in AFT's Fathom software and model the piping and pump curve. www.aft.com for a demo download.

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