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NPSH available (NPSHa) and vapor pressure
2

NPSH available (NPSHa) and vapor pressure

NPSH available (NPSHa) and vapor pressure

(OP)
Hello, this is my first time posting here. I hope this doesn't come off as a dumb question but here goes:

Premise
1.) NPSHa is the difference between stagnation pressure at the pump inlet, minus the vapor pressure associated with the temperature at the inlet.
NPSHa = Pstagnation - Pvapor

2.) Cavitation only happens when the stagnation pressure is less than or equal to the vapor pressure. In other words, cavitation starts to happen when Pstagnation=Pvapor , so cavitation starts to happen when NPSHa = 0

3.) The temperature at the inlet of the pump remains pretty much constant over various operating speeds, hence vapor pressure remains constant.

Questions

A. Why is NPSHr (NPSH required) anything other than it must be greater than 0? Because according to the premise, cavitation only starts when Pstagnation = Pvapor , so as long as the stagnation pressure is higher than the vapor pressure, no cavitation will occur.

B. Why is it said that NPSHr changes with pump speed? Recall that NPSH=Pstagnation-Pvapor , and the temp at the inlet is assumed to be constant, hence no change in vapor pressure at the inlet, so nothing should change. What am I missing?

Thank you

RE: NPSH available (NPSHa) and vapor pressure

Suggest you spend some time researching pump NPSHr, what, when, where,and why.
NPSHa is a fixed condition set by the installation, whereas NPSHr is set pump by design.

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.)

RE: NPSH available (NPSHa) and vapor pressure

NPSHa is measured at the inlet flange to the pump. If the fluid is going to move from this point into the suction eye of the impeller, the pressure must drop lower. Flow only goes from higher pressure to lower pressure. The amount of pressure needed to move the fluid from the inlet flange to the impeller eye is dependent on the design and operating conditions of the pump. It depends on flow rate and pump speed. This is generally determined experimentally. If a higher flow rate is required, more pressure drop is required. NPSHr must be tested at different flow rates to generate an NPSHr curve for the individual pump.

NPSHr is not the point where cavitation starts. Research "Incipient Cavitation". In order to be measured and documented, NPSHr is the point where the cavitation is sufficient to reduce the pump developed head by 3% (for most pumps) or 1% (for some high energy pumps, or if specified by purchaser). In either case, if NPSHr = NPSHa then there is already a significant amount of cavitation occurring. Thus, a margin is required in order to reduce the amount of caviation to an acceptable level.

Johnny Pellin

RE: NPSH available (NPSHa) and vapor pressure

(OP)
Thank you I understand now. I had totally forgotten that there is some space between the inlet of the pump (where the gauge is) and the eye of the impeller, hence there is a pressure drop from the inlet to the impeller. So since we can't really measure the pressure directly at the impeller, we just have to test to see what inlet pressure initiates cavitation at the impeller; then that inlet pressure minus the vapor pressure is the NPSHr at that pump speed.

RE: NPSH available (NPSHa) and vapor pressure

Cray.

Read what JJ is telling you.

NPSH is NOT cavitation point. It is simply the point at which differential pressure dec re ases by 3% because onset of cavitation is very difficult to measure but differential pressure is easy.

You could be spot on NPSHA = NPSHR and be busy cavitating like mad.

Always best to add at least 1m and 2 if you can

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

RE: NPSH available (NPSHa) and vapor pressure

(OP)
Ok I think I finally get it. NSPHr marks the point when the inlet pressure gets low enough relative to the vapor pressure to cause about a 3% decrease in pump TDH. Cavitation had started at something slightly higher than NPSHr. So basically, you want to stay above NPSHr by a reasonable amount; you don't want to see NPSHr as the lowest safe bound. The lowest safe bound is actually like 10% above NPSHr. Sorry I wasn't really listening, but I think I've got it straight now. Thanks for the help.

RE: NPSH available (NPSHa) and vapor pressure

Thats basically it.

It's always interesting asking a vendor if they have a cavitation curve as well as an npsh curve.

Sometimes it is close sometimes further away depending on flow.

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

RE: NPSH available (NPSHa) and vapor pressure

Cray25,

It should be:

NPSHa ≥ NPSHr
Where,
NPSHA = Psuction – Psaturation

NPSHa comes from the system while NPSHr comes from the pump. Note that you are comparing the differences of two pressures not the saturation pressure and suction pressure.

RE: NPSH available (NPSHa) and vapor pressure

Matt1309.

You really need to start your own thread and not hijack someone elses dead thread.

Mech eng - the thread ended 5 months ago....

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

RE: NPSH available (NPSHa) and vapor pressure


LittleInch:

Thank you for letting me know that.
I didn't notice the date of the post itself and just replied since it was on top of the posts. I wonder why Matt1309 didn't create a new post. I will let the Admins decide on what to do with last few responses.

RE: NPSH available (NPSHa) and vapor pressure

Sorry, my mistake! I really didn't mean to hijack somebody else's thread...
MechEng, thanks a lot for your answer

RE: NPSH available (NPSHa) and vapor pressure

Pump NPSHr is not just dependent on pump speed and flow. A lesser known fact is that it is also related to the density of the fluid being pumped. Pump manufacturers publish standard NPSHr curves based on water, and many users tend to incorrectly use these for other fluids also. If you cannot get the manufacturer to give you NPSHr plots for your service fluid, see Perry Chem Engg Handbook 7th ed for approximate corrections.

RE: NPSH available (NPSHa) and vapor pressure

NPSHA= Hstatic+ H atm + H velocity -H vapour pressure- Hfriction losses
As you see from the above equation that NPSHA is dependent on vapour pressure.
SUppose, the fluid is boiling the Hatm will be equal to H vapour pressure and both cancels out each other, as in case of boiler feed pump.
Hence BFP application the static head should be high and the velocity shall be more in order to acheive the the difference between NPSHA and NPSHR.
ALso note that in case of suction lift H static is negative.


Hope the above clarifies.

A well phrased question has answers in itself!!!

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