NPSHA in LP tanks
NPSHA in LP tanks
(OP)
Can anyone explain why the two pressures in a closed tank cancel out??
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RE: NPSHA in LP tanks
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.
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RE: NPSHA in LP tanks
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.
RE: NPSHA in LP tanks
RE: NPSHA in LP tanks
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.
RE: NPSHA in LP tanks
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.