Can a pumps suction gauge ever read below NPSHr, taking into account atmospheric pressure

[BronYrAur]

This is more of a theoretical question, but it is based on some readings I an getting.

If I have a pump that requires 5.7 psi NPSHr operating at sea level (14.7 psi), the lowest I should theoretically be able to see is -9 psi on a suction gauge, correct? Anything more negative would cause the water to flash. I assume that would either cause the gauge to not properly read, or it would increase the pressure as steam takes up more volume.

I am just wondering if it is possible to ever see a number that is lower than -9 psi (atmospheric - NPSHr)on a suction gauge in this situation?

 

[RVAmeche]

If you don't meet the NSPHR requirements for the pump you have cavitation in the pump, not necessarily in the piping. Note you should try to provide at least 3 ft of additional head over the NPSHR to avoid cavitating.

If you have very excessive losses in the suction piping it may drop below the vapor pressure for the water at the given temperature, which would cause it to flash in the piping.

 

[BronYrAur]

Yes thank you for those comments. I understand what you're saying which is why I'm asking my question. Is it theoretically possible to drop below the pressure which would cause cavitation? It seems to me like it would be impossible to get a suction pressure lower than that. I understand that we would never want to see such a pressure. I just want to know if it's possible to achieve it?

 

[LittleInch]

I think you fundamentally misunderstand what NPSHR is.

It's a pump thing related to cavitation inside the pump but not fkashing of the liquid in the pipe.

So your pump might be able to operate below the NPSHR figure but will gradually lose the ability to produce differential head. If you can lower the pressure on the discharge then you might see lower pressures but the pump will almost certainly be cavitating madly.



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

 

[BronYrAur]

No I understand what npshr is and that it's entirely a function of the pump. But if I am cavitating inside the pump, can I still pull a stronger negative in the suction piping? It would seem that once I reach this point, I would not be able to pull any more of a negative in the piping.

 

[LittleInch]

Yes but only if you can reduce discharge pressure. Maybe not by much but depends on the pump.

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

 

[Artisi]

Yes, you can keep lowering the inlet pressure to the point when the water will start to boil and put the pump off-prime, not recommended of course.

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.)

 

[nondimensional]

Bernoulli says that high velocity is low pressure. As fluid accelerates into the impeller right at the inlet, pressure drops locally creating a flashing effect. This may not be happening in the suction pipe where velocity is relatively constant and relatively low.