Difference btwn NPSH and NPSHA
Difference btwn NPSH and NPSHA
(OP)
I came across these two definitions for Net Positive Suction Head and Net Positive Suction Head Available:
NPSH = p(s)/y + v(s)^2/2g - p(v)/y
and
NPSHa = p(atm)/y - h(e) - h(l) - p(v)/y
where:
p(s) = static pressure in fluid
y = specific weight of fluid
v(s) = velocity of fluid
g = acceleration of gravity
p(v) = vapor pressure
h(e) = elevation from surface to pump
h(l) = head loss
Correct me if I am wrong, but these two formulas should be representative of the same quantity, theoretically. This is, of course, the vessel from which the fluid is being pumped is at atmospheric pressure. Could someone confirm or explain this?
p(s)/y + v(s)^2/2g = p(atm)/y - h(e) - h(l)
I appreciate all the help.
NPSH = p(s)/y + v(s)^2/2g - p(v)/y
and
NPSHa = p(atm)/y - h(e) - h(l) - p(v)/y
where:
p(s) = static pressure in fluid
y = specific weight of fluid
v(s) = velocity of fluid
g = acceleration of gravity
p(v) = vapor pressure
h(e) = elevation from surface to pump
h(l) = head loss
Correct me if I am wrong, but these two formulas should be representative of the same quantity, theoretically. This is, of course, the vessel from which the fluid is being pumped is at atmospheric pressure. Could someone confirm or explain this?
p(s)/y + v(s)^2/2g = p(atm)/y - h(e) - h(l)
I appreciate all the help.





RE: Difference btwn NPSH and NPSHA
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RE: Difference btwn NPSH and NPSHA
RE: Difference btwn NPSH and NPSHA
The only two that are important are NPSHa and NPSHr. You don't calculate NPSHr, you get it from the mfgr's pump data, so now there's only one you have to calculate, NPSHa, which must be >= NPSHr.
Use absolute pressures. (If tank is open or closed, it makes no difference to the equation, as long as you calculate the tank's absolute pressure correctly). Vapor pressure is always absolute.
NPSHa = p(on_fluid_surface)/y - p(v)/y + v(s)^2/2g + h(e) - h(l)
I've substituted absolute pressure on_fluid's_surface for p(atm).
The sign convention for h(e) is positive when the fluid surface is above the pump and negative when below the pump CL.
Dropping the velocity term is common as it's almost always relatively small, not worth calculating, and yields a conservative result.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
RE: Difference btwn NPSH and NPSHA
BigInch, my understanding is that the velocity term is by definition not part of the equation you mention.
Cilliers
RE: Difference btwn NPSH and NPSHA
RE: Difference btwn NPSH and NPSHA
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
Although it might not be applicable for your application you should keep that in mind: If you have a reciprocating pump installed the acceleration head must be considered, too. Although acceleration is part of the "pump's problem", the acceleration head is part of the NPSHA calculation.
RE: Difference btwn NPSH and NPSHA
RE: Difference btwn NPSH and NPSHA
Velocity head is included in the definition of Total Suction Head, Total Discharge Head, Total Head, and by reference to Total Suction Head in the definition of NPSH, is also included in NPSH as well. This is found in the Pump Handbook - Second Edition by I.J. Karassik, W.C. Krutzsch, W.H. Fraser, J.P. Messina, Published by McGraw-Hill 1986, Section 13.3 in which they also say conforms to the same definitions and units of the Hydraulic Institute.
Velocity head is included in the definition of NPSH in "Centrifugal and Axial Flow Pumps", Second Edition, Section 2.6 by A.J. Stepanoff published by John Wiley & Sons 1957.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
1) What references say:
NPSH = Total suction pres - Vap pres
= P_suction/y + v_suction^2/2g - P_sat/y
So you add the velocity per the references.
2) Usual method of doing calc:
P_suction/y + v_suction^2/2g + H_loss = P_vessel/y + H_Elev
Thus:
NPSH = P_vessel/y + H_Elev - H_loss - P_sat/y
Your equation can be correct if H(l) includes the recoverable velocity to pressure conversions (for example if entrance K=1.5 instead of 0.5), but this is not usually done.
Cilliers
RE: Difference btwn NPSH and NPSHA
For a point after all losses have been accounted for.
hs = ps/γ + vs2/(2g)
where
hs = suction head close to the impeller
ps = static pressure in the fluid close to the impeller
γ = specific weight of the fluid
vs = velocity of fluid
g = acceleration of gravity
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
It is not just your understanting, it's the way things are.
CJKruger,
Usually BigInch's understanding is the "right" understanding.
RE: Difference btwn NPSH and NPSHA
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
ione, did you actually compare the equation posted by BigInch on 12 Jan 10 13:39 to the ones in the link posted by you ?
Cilliers
RE: Difference btwn NPSH and NPSHA
http://www
RE: Difference btwn NPSH and NPSHA
You probably know by now that I never agree to disagree. If there is some error in my understandings, I prefer to get it correct. Please be clear in your comments, because at this point I have little idea of what you mean by all the questions and what your point eventually might be. Additionally if you could point out my supposed error using either my or ione's reference equations, rather than typing out new ones (esp. without supplying definitions of all variables), it would be of help.
Thanks
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
The equation posted by you is:
NPSHa = p(on_fluid_surface)/y - p(v)/y + v(s)^2/2g + h(e) - h(l)
I believe the correct equation is:
NPSHa = p(on_fluid_surface)/y - p(v)/y + h(e) - h(l)
Cilliers
RE: Difference btwn NPSH and NPSHA
CJK, It would have been a lot easier on me, if you just said that. Sorry it took me so long to figure out what you were after. That green dust on the floor under the bed was obviously kryptonite.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Difference btwn NPSH and NPSHA
you were right V2/2g != 0 (not equal) since V=Q/A where Q is the flow rate and A is the Cross sectional area at pump inlet. Your reference point should always be the pump inlet where you measure the static & dynamic heads (i.e. the total head) as well as the vapour head.
In most cases V can be ignored. If you prefer you can still calculate it using V=Q/A.
Regards,
RE: Difference btwn NPSH and NPSHA