NPSHA
NPSHA
(OP)
Hopefully somebody could correct me here as I must be doing something wrong. The suction pressure on this vertical pump is 106psig and the vapour pressure is 100psig. The suction pressure was taken off of the centrifugal pump datasheet. Subtracting the vapour pressure from the suction pressure, which I assume takes into account the loss from the pipes and fittings and is the actual pressure going into the pump, 106-100=6psid which is equivalent to 18ft of head, given the S.G. of the fluid is 0.77.
Is it fair to say the NPSHA is 18ft? Reason why I ask is, when the end user was queried on the NPSHA, they said it was only 3ft but confirmed the pressure values they provided were correct???
How so? Also, question#2..is it possible to be have the vapour pressure very close to the suction pressure but still not cavitate as the NPSHA>NPSHR?
Is it fair to say the NPSHA is 18ft? Reason why I ask is, when the end user was queried on the NPSHA, they said it was only 3ft but confirmed the pressure values they provided were correct???
How so? Also, question#2..is it possible to be have the vapour pressure very close to the suction pressure but still not cavitate as the NPSHA>NPSHR?





RE: NPSHA
NPSHR curves are in ft of water so you have 6 * 2.31 or nearly 14' of water for NPSHA. Do you know what the NPSHR for the pump is?
RE: NPSHA
TD2K
NPSHR curves are in feet.
NPSHR curves require head of product being pumped.
Not some equuivalent head of water, unless the product is water.
[b]RS82[/]
Please NOTE! vapor pressures are customarily given as ABSOLUTE pressures.
Pressures must be converted into feet of product.
NPSHA includes atmospheric pressure, or, if the fluid is contained within a closed tank or vessel, it includes the absolute pressure inside that closed tank or vessel
NPSHA = the equivalent head of (Pa - Pv + Pn)
Pa = atomospheric pressure, psiA
Pv = vapor pressure, psiA
Ps = suction pressure, psiG
Density of Water = 62.4 pcf
NPSHA = Ha - Hv + Hn Note! This is at the pump suction inlet.
Ha = atmospheric Head = 14.7 psiA *144/(62.4 * 0.77) = 44.06 ft
Pv = vapor Head = 100 psiA * 144/(62.4 *0.77) = 299.7 ft
Pn = pressure
at pump suction = 106 psiG * 144/(62.4 *0.77) = 317.7 ft
NPSHA = 44.06-299.7+317.7 = 62.04 ft NPSHA
The problem with high vapor pressure products is that when pipe pressure pressure reaches vapor pressure, they vaporize, even though the pump may calculate as haveing sufficient head, if the fluid remained as a liquid. That often happens on the way to the pump, as these products are actually stored at their vapor pressure inside the tank. There's not much head remaining to work with (usually only the head from liquid surface to tank take-off nozzle) before vaporization is occuring.
Also be careful of where the suction pressure is actually taken. If it was really taken at the tank, you must subtract pipe flow losses, but can add any height of fluid level over the pump Centerline, or subtract that distance if fluid level is below pump centerline. If the pump is near the tank, some people think tank pressure is a close enough of an approximation to pump suction pressure.
Since this is a vertical pump, I assume the pressure is suction pressure at the actual intake into the pump. That is apparently 106 psiG or 120.7 psi Absolute. Well above your ABSOLUTE vapor pressure of 100 psiA.
http://www.calculatoredge.com/calc/NPSH.htm
What would you be doing, if you knew that you could not fail?
RE: NPSHA
I'll let the OP comment if the 100 psig vapor pressure is a typo or if he's already converted from psia.
Checking the NPSHA using a suction pressure is a nice way to eliminate the head effect and any line losses, suction strainer, etc. However, any changes in vessel level and/or suction line losses (such as a plugged suction screen) is going to affect the available NPSHA.
RE: NPSHA
What would you be doing, if you knew that you could not fail?
RE: NPSHA
The vapour pressure was 114psia which I then converted to gauge.
One more question for you folks..I was told by a pump vendor that even though the suction pressure is very close to the vapour pressure, there is nothing to worry about as there is sufficient NPSHA..if NPSHA is a function of vapour pressure, then how is it sufficient?? Is he wrong?
RE: NPSHA
What would you be doing, if you knew that you could not fail?
RE: NPSHA
"I was told by a pump vendor that even though the suction pressure is very close to the vapour pressure, there is nothing to worry about as there is sufficient NPSHA"
It's really the margin between NPSHA and NPSHR you need to review. If I have 12' of NPSHA and my pump needs 3' NPSHR, I'm good. If I have 12' of NSPHA and my pump needs 10' NPSHR, I'm going to be worried.
RE: NPSHA
rmw
RE: NPSHA
I would not say that pump manufacturers routinely lie about NPSH3, but rather the topic is widely misunderstood even applications folks among pump manufacturers and data is routinely misrepresented by the end user.
To TD2K's point, the important factor is the difference between NPSH3 and NPSHa because of the way inwhich NPSH3 is actually measured. Because NPSH3 is actually based on a 3% head loss, then technically, cavitation has already begun somewhere between the NPSHa, and NPSH3 values. The inception of cavitation will be just before the differential head of the pump begins to decrease relative to the pump's suction head when the pump company develops their "Knee curves."
RS82, I am not sure if the pump manufacturer did misunderstand the pressure inwhich you provided to him. However, if he meant that the near limiting case of 0.01[ft] - 0.00[ft], means that there you have 0.01[ft] of NPSHa, then the pump manufacturer's understanding of NPSH3 calculation, measurement uncertainty, and design safety margin are clearly flawed. Of course, this is the limiting case.
RE: NPSHA
The only time a pump vendor *might* lie about NPSHR curves is to artificially reduce the NSS so a selection is not rejected due to a specified NSS limit. They *might* say pump needs more NPSHR than it really does, which would make the selection conservative.
If it went the other way, they (pump vendor) would have to deal with the problems through warranty. There is simply no advantage.
RE: NPSHA
Therefore we tested all pumps that we intended to use on our product and found out that precious few NPSHR curves were accurate. We learned to routinely add 2-3 feet. Except for Carver Pumps. With the Carver's that we bought if they said 6 ft NPSHR, we built it 6 feet over the pump suction. For the rest we tested and added margin.
Did they lie, or were they just that bad? Carver's results made me think the former but I don't discount the latter.
rmw
RE: NPSHA
What would you be doing, if you knew that you could not fail?
RE: NPSHA
rmw
PS: I do know that we didn't test for HPSHR at all.
RE: NPSHA
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: NPSHA
What would you be doing, if you knew that you could not fail?
RE: NPSHA