NPSHA in LP tanks

[am35martin]

Can anyone explain why the two pressures in a closed tank cancel out??

 

[25362]

It is included in the definition of NPSHA:

The difference between the total absolute head available at the pump inlet and the head that corresponds to the vapor pressure of the pumped liquid.

Visit, for example, thread407-51468

 

[Montemayor]

This query really belongs in the Pump engineering Forum. This subject has been dealt with perhaps more than 20 times in the past that I know of. Allow me to presume two things:

1) By "LP" you mean LPG (or a saturated liquid, in equilibrium)
2) You understand what I mean by a saturated liquid in equilibrium.

If the above is correct, then we can look at the basic equation for NPSHa:

NPSHa = (Static head + surface pressure head) – (the vapor pressure of the pump fluid) – (the friction losses in the pump suction piping, valves and fittings).

The equation is taken from

http://www.mcnallyinstitute.com/11-html/11-12.html

I think you can see that the two terms, surface pressure head and the vapor pressure of the pump fluid, cancel out. I believe your question is answered.

 

[am35martin]

Thank you for the information. Yes it is a LPG but in the aerosol industry we call it LP (Liquid propellant) I understand how to calulate NPSH and use related tabls and charts. What I can't seem to grasp is the physics behind the pressure cancellations. If I have a tank of A108 and the tank pressure is 108 PSI at 60F how is it that this does not aid in pushing the liquid to the pump. I experimented by filling a 5 gallon tank to 80% capacity with A108 (tank pressure 65 psi) and connected it to a 1000 gallon tank filled with A31 at 15 psi. The 5 Gallon tank was below the inlet of the 100 gallon tank. When I opened the valve, the A108 emptied into the A31 tank. I the liquid head wa sthe only force then the 5 gallon tank should not have emptied in the 1000 gallon tank.

 

[rbcoulter]

Looks like semantics is the problem here.

Your initial question was not specific enough to obtain a meaningful response. Yes, the vapor pressure does aid in pushing the liquid to the pump; however, this is usually nullified by the vapor pressure of the fluid. Remember this is a saturated fluid that will start to boil as soons as it encounters less pressure that its equilibrium storage pressure that you mentioned. This often happens as a fluid enters the suction side of a pump or compressor. So the only extra pressure that is available is the level of the fluid above the pump. This extra pressure is needed to ensure that fluid stays liquid inside the pump. The vapor pressure does push the fluid but it does not always push it in liquid form.

 

[25362]

NPSH is a statement of the minimum suction conditions to prevent cavitation, ie, vaporization.

Have a look at the name: net positive suction head. Net above what ? Net above the vapor pressure.

Thus to any pressure acting on the free surface of the liquid in the storage tank one must deduct the liquid's vapor pressure when estimating the NPSH. If both pressures are equal they cancel out. See the formula posted by Montemayor.