NPSHa vs NPSHr
NPSHa vs NPSHr
(OP)
Hello:
I have an application where the vapour pressure (in gauge) is slightly higher than the pump suction pressure. This information was retrieved from the pump datasheet. However there is a postive difference between the NPSHa and NPSHr. How does this work? Shouldn't the pump be cavitating in the suction pipe before it comes into the pump? How can the NPSHa be higher than NPSHr? Just trying to understand as pumps is not my cup of tea.
Thanks!
I have an application where the vapour pressure (in gauge) is slightly higher than the pump suction pressure. This information was retrieved from the pump datasheet. However there is a postive difference between the NPSHa and NPSHr. How does this work? Shouldn't the pump be cavitating in the suction pipe before it comes into the pump? How can the NPSHa be higher than NPSHr? Just trying to understand as pumps is not my cup of tea.
Thanks!





RE: NPSHa vs NPSHr
Please supply figures or attach scan of the data sheet and units so we can see where you're going wrong. If vapour pressure is in guage, not bara or psia then your product is boiling....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: NPSHa vs NPSHr
RE: NPSHa vs NPSHr
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: NPSHa vs NPSHr
Sorry for the lack of info.
RE: NPSHa vs NPSHr
RE: NPSHa vs NPSHr
Hence the VP could indeed be higher than the pump suction pressure, but when you add it all together the end pump suction head is still above NPSHr
I suspect what you saw was something like Vapour pressure / head say 7m and NPSHR 3m / NPSHA 5m
If your liquid is say 5m above pump suction and friction losses 4m and your tank is at atmospheric pressure at sea level (say equivalent to 10m), then NPSHA is
5 + 10(atmosphere in liquid terms -7 - 3 = 5m NPSHa
At the pump suction NPSHa always needs to be greater than NPSHr
As vapour pressure rises you run the risk of cavitation unless your other elements (tank pressure / liquid height) more than compensate for that plus friction losses.
Makes more sense now?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: NPSHa vs NPSHr
That available energy is made up of two components: the pressure head due to elevation difference, and what is left of the pressure originally applied to the liquid (either atmospheric or from another source); nothing else. Calculating that final component is where all the trouble comes from.
Sometimes working backwards from the impeller entrance helps to "get it".
That is all I will add to this discussion on NPSH, which by the way has been going on as long as there have been impellers.
RE: NPSHa vs NPSHr
NPSHa/r has been done to death in eng-tips and other forums, suggest you do a bit of research on the subject for a complete understanding.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)