4 replies to this topic

[stu]

Dear Sir,
Please clear me the following doubts regarding vapor pressure in NPSH calc.
we have to maintain the suction pressure normally above the Vapor presssure of the liquid to avoid cavitation.To ensure that we have to subtract the Vapor pressure in NPSH Calculation, is it correct?
If it is so how does the vapor pressure opposes the vessel's liquid surface pressure, in general the vapor generated in the system also increases the system pressure is it?
Please clear my confusion regarding this.Whats the exact reason to substract the vapor pressure of the liquid in calculating NPSH .
thanks & Regards.
Stu

[gvdlans]

Did you read the Wikipedia page? http://en.wikipedia.org/wiki/NPSH

The reason for subtracting the vapour pressure is that the pressure in the pump impeller must stay above this vapour pressure otherwise the liquid will start to boil ==> cavitation.

[djack77494]

we have to maintain the suction pressure normally above the Vapor presssure of the liquid to avoid cavitation.To ensure that we have to subtract the Vapor pressure in NPSH Calculation, is it correct?

Yes, stu, that is correct.

how does the vapor pressure opposes the vessel's liquid surface pressure, in general the vapor generated in the system also increases the system pressure is it?

This question is awkwardly worded, and I'm not sure what you mean. IF you have a saturated liquid, then the pressure above the liquid surface IS the vapor pressure of the liquid. If not, then the vessel's pressure is not solely determined by the liquid's vapor pressure. Have I answered this question?

Whats the exact reason to substract the vapor pressure of the liquid in calculating NPSH .

You are trying to determine if the fluid, when it reaches the region of lowest pressure within the pump, will be a liquid or if it will begin to vaporize. Vaporization = pump cavitation = undesirable and potentially destructive condition, so you want to design to avoid this possibility. Check out the link suggested by Guido, or try a search on this or other technical websites to receive more information on this topic than you can read in a lifetime.

[KR]

Hi Stu,

I would say, you are subtracting vapor pressure as vapor pressure is exerting in opposite direction to the pressure exerted above liquid level (assume half filled closed vessel. Here molecules of vapor at the liquid interface try to push the air above it ). Therefore, by subtracting vapor pressure, you will come to know how much NET pressure available on liquid surface (needless to say, you are adding static head and subtracting frictional losses to arrive NPSH). Again all pumps come with NPSHr (NPSH required) therefore NPSHa (NPSH available) should be around 2-3 ft greater than NPSHr (in general).

Hope it helps!

[fallah]

you will come to know how much NET pressure available on liquid surface

NET pressure available on pump inlet (not liquid surface).