Quick NPSHA calc from suciton pressure and vapour pressure

[r0nz3y]

Quick question for you.

Is NPSHA approximately calculated by subtracting suction pressure and vap p= ressure (gauge)?

I'm creating a trip logic for a butane pump which transfers from our butane sphere.

I got a tag which calculates vapour pressure based on temperature and I hav e pressure indication on the suction at the pumps.

Since NPSHA = static head + surface head - vap pressure - frictional losses

Static head = suction pressure
Surface head = 0
Vap pressure = calculated
Frictional losses ~ 0 (PT at pump suction)

I'm going to set the logic to trip when 0 + Suction pressure (psig) - Vapour pressure (psia) - 14.2psi] < NPSHr (2psig)

Thanks for the feedback!

 

[sshep]

r0nz3y,

I think your NPSHa is basically set by the tank level (literally the ft above pump center line). I guess that the pressure in the sphere is basically the vapor pressure, unless the tank is refrigerated and blanketed. Surely your pump suction line is sized for minimal friction.

You might be better to trip the pump on level. If you are trying to take too much flow, the NPSHr may limit how far down you can draw the level. Reduce the flow at the end to pump out to minimum level.

That is how I see it anyway.

best wishes,
SShep

 

[GHartmann]

I believe sshep has explained it correctly. Since your butane is always in equilibrium with the tank temperature it always has a vapor pressure equal with the tank pressure. Thus the only safety above NPSHr is the liquid level (minus the friction losses).

I always prefer working in "feet of liquid" and that helps keep me straight. Using psi (as is done on the API data sheets) AND feet of liquid can often lead to confusion/errors. I can't recall ever seeing a pump NPSHr expressed in psi.

G.Hartmann
BSChE NC State University

 

[LittleInch]

r0nz3y,

From what I can glean from your post, in your particular case (or any other actually thinking about this more clearly) all you need to do is measure pump inlet pressure, subtract vapour pressure and convert to metres or feet head all in absolute units.

According to your calculation you seem to have a liquid which has a vapour pressure equal to / in equilibrium with the head pressure if it is a non externally pressurised system. If you actually pressurise your sphere above the vapour pressure of butane, this means that for a true calculation you need to have this pressure converted to head in absolute units and the vapour pressure include dinthe calcualtion

However that is for your initial design calculation. Once you have a pump inlet pressure then NPSHA is that pressure in absolute unite minus the vapour pressure at the inlet temperature converted to head as the inlet pressure will already have taken account of any pressurisation of the liquid or any friction losses.

Make sure all your units are the same - I would recommend using absolute, i.e. I think your 2psig for NPSHR looks very high - 2 psia seems more likely - roughly 2.3 m / 7.7 ft for liquid butane

Therefore your calculation is inlet pressure(psig) + 14.5 - vapour pressure (psia)> NPSHR (converted to psia) (2 psia??)

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way