NPSH for submerged pump

Yes, the vapor pressure of the pumped fluid must still be considered.

thank you for your reply,

if this pump is submerged in vessel(closed drain vessl),which code is telling how we calculate the minimum level of liquid in the vessel

regards

I am not sure that any code governs this. Basically you consult the pump manufacturer's data for NPSH_required (sometimes shown on the pump curves) and compare it with the NPSH_available at various liquid levels, and then you make sure the liquid level is sufficient.

I don't remember the best Web sources for this, but if you search "NPSH" you will probably find many good explanations. My favorite is from Cornell Pumps in Portland, OR, USA, but I'm not sure it is available on the Web.

If your search doesn't pan out, you can ask again, and I will try to add to this.

thank you

Good rule of thumb is to maintain the minimum level in the tank such that the NPSH available is always 1 meter greater than the NPSH required by the pump.

It is a rule of thumb to be taken into consideration while we design and/or operate the pump: the pump suction pressure shall not be equal to nor less than the vapor pressure of liquid to avoid boiling of liquid which causes pump cavitation.

There is no difference in calculation of NPSHa (available) for a submerged pump and a surface mounted pump.Here the static head assumes + sign as opposed to the - sign it has in case of pumps with suction lift ( surface mounted pumps).

If the Pump located inside a tank that is NOT open to atmosphere and is under partial vacuum,only then there would be a need to go into the details.Vapour pressure of liquid is always an important factor in determining the NPSHa.The liquid and its pumping temperature will decide the vapour pressure.

If is an open (to) air vessel/tank, you already have the benefit of at least 10mWC plus the height of liquid above the impeller level.Only very large flow rated pump will ever need NPSHr above 10 m -this will be very rare case ( e.g.hot liquid pumping) .

If you had stated the actual condition and the make/size of pump,I would have been glad to help.(liquid data,pumping temp also needed) for calculation.

Chuck Cowlagi

Try a tesla pump and get rid of the cavitation problem.

Interesting--

Tesla as in Nicola Tesla turbine/pump? Or Tesla the Italian mfr of submersible pumps? In the first case, what exempts it from cavitation issues?

Tesla as Nikola Tesla, the serbian inventor. This type of pump does not have blades. Cavitation is caused by blades that create points with low pressure in the fluid/ liquid: then the liquid vapourises, creating bubbles. The bladeless tesla pump unses friction to accelerate the fluid. It is a laminar flow across the discs resulting in the absence of cavitation. The fact is that is recommended for the discs to be as smooth as possible- mirror like.

If you just google a little you can find out a lot about it.

I am not sure about it doing away with NPSH induced cavitation altogether as you seem to imply, but I do know for sure that the efficiency is awfull.

As far as I know NPSH translates into Net Positive Suction Head. Doesn't this represent the maximum height above the open surface at which the pump can be installed for being able to obviously "suck"? And then when submerged the surface of the fluid is higher than the suction vent. Isn't the fluid rushing into the pump so the pump does not need to suck it anymore? There is even a "negative head".

Well for Submerged pumps the criteria of NPSHa is not important as it will be completely sumberged in the liquid...

Dear Mr.mostafaalkholy.

The Answer for your question

for 1st Part - NO.

for 2nd Part - YES.

DHAYANANDHAN.S.