AODD Pumps- NPSH ISSUE

[shashankTurbomachine]

We all understand that NPSH has little significance in case of AODD as the air is forced to AODD and not lifted up

Still bidder furnish NPSH curve.

OFF LP TRANSFER PUMP
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CAPACITY: 13gpm
DIFF Press: 25.9 psi
Max. Suct. Press: 50.2 psig
NPSHA: 9.3ft
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Bidder furnished NPSHR of 7.5 ft.
Project specs calls for NPSH test in case the margin is less than 3 ft.
Here is the catch!
Does even "NPSH" have relevance in case of AODD pumps. Test is a separate issue altogether.

 

[SandCounter]

Yes, it has relevance with any liquid pump. All fluids have a maximum height they can be drawn from vacuum. For water it is generally about 10 meters before it starts to boil due to low pressure in the suction. I'm not sure what "OFF LP" means but it may be that the fluid the specs reference has a lower vapor pressure than water.

I used to count sand. Now I don't count at all.

 

[LittleInch]

~Shasank.

you seem to completely misunderstand how and what an AODD pump is. Perhaps this might help.

https://www.youtube.com/watch?v=4kAR0WEwBbY

It's still an positive displacement pump so the fact it's motive power is air has no impact whatsoever on the fluid being pumped.

So of course NPSH has relevance.

The issue with AODD and other PD type pumps is that NPSH is not uniform during the stroke cycle. It is common for such pumps to describe an acceleration head as the fluid first rushes to enter the chamber. If you've only got 9ft NPSHA, this might become important.

Something like this post might help.

http://www.eng-tips.com/viewthread.cfm?qid=320165

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

For an AODD pump: NPSH matters to ensuring that the pump is capable of delivering the required capacity, but doesn't matter much to whether or not the pump will survive in service.

That is different with a centrifugal pump, where cavitation occurring when there is insufficient NPSHA can result in damage to the pump in addition to potential loss of prime and capacity loss.

Remember that the N in NPSH stands for "net", meaning that the suction head must be positive relative to something- the vapour pressure of the liquid being pumped.

 

[georgeverghese]

DD pumps are more tolerant of intermittent loss in NPSH margin than centrifugal pumps, as may occur during startup and during process upsets.

However, for normal ops, a decent NPSH margin is also required for DD pumps - this margin should also account for acceleration head losses in the pump suction due to pulsating flows. Else you can expect delivery flow deficencies (and cavitation too for metallic DD pumps). The effect of this NPSH margin loss may be less than that on a centrifugal.