NPSHa vs NPSHr
NPSHa vs NPSHr
(OP)
In the reference books there are formulas for calculating the NPSHa that indicate an adjustment to the pressure absolute should be made to account for an increase in elevation (if the pump is to operate at elevation). When pumping water it seems to small as to be negligible. Also, it would seem reasonable that is the NPSHr is established for conditions at sea level then the NPSHr would need to be corrected for the operation at elevation so in the end it would all be a moot point. What is everyone else's take on this?





RE: NPSHa vs NPSHr
RE: NPSHa vs NPSHr
RE: NPSHa vs NPSHr
NPSHr won't change as this is a function of pump design and remains constant for the flow and speed etc under consideration.
RE: NPSHa vs NPSHr
RE: NPSHa vs NPSHr
NPSHa = Ha - Hvpa (+ or - Hst) - Hfs
During pump testing NPSHr is corrected for atmospheric pressure at sea level (not all test facilities are at sea level), temperature using std water (from memory 60 F) and any S.G. correction etc.
RE: NPSHa vs NPSHr
Let's consider a suction pressure of 2 psiG. Assume water and a vapor pressure of 0 psia.
At sea level, atmospheric pressure (14.7 psia) converts to a head of 33.9 feet. A suction pressure of 2 psig would convert to 4.61 ft Absolute suction pressure = 14.7 + 2 = 16.7 psia. Total suction head available at sea level = 33.9 + 4.61 = 38.51 ft
At 10,000 feet atmospheric pressure would be about 1/2, so say 8 psia, or some 17 feet. A suction pressure of 2 psig would convert to absolute suction pressure of 2 + 8 = 10 psia and 23.07 feet.
If you need an NPSHr of 25 feet, that converts to 10.83 psia
At sealevel you'd have 16.7 psia, or 38.51 ft, > 25 ft, so you're OK.
At 10,000 ft you'd have an absolute suction pressure of 10 psia, or 23.07 ft < 25 ft, so you're NOT OK.
If vapor pressure was significant, then you'd have to subtract that vapor pressure from the absolute suction pressures above. Vapor pressure always decreases available NPSHa.
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"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSHa vs NPSHr
RE: NPSHa vs NPSHr
When the fluid level is over the pump suction level, the
NPSH available can be calculated as follow:
NPSHa = Atmospheric Pressure + Static Height – Total Friction Losses (in the suction side) – Vapor Pressure of the fluid at the temperature being pumped
NPSHr, in most of case, is a value that the manufacturer of the pump can give you.
In order to avoid cavitation NPSHa > NPSHr
Regards
Luis Sisniegas
RE: NPSHa vs NPSHr
"And if the NPSHr is affected, then way do we not have to account for the impact of elevation on the NPSHr when considering an installation at elevation. "
NO - The only correction that needs to be done is to NSPH A THAT varies with elevation of the pump.
"At one level I understand that the pump is just adding energy to the flow stream, consequently the conditions outside of the pump do not affect what the pump does, i.e., Tdh, BEP, flow and of course NPSHr. THAT is true, but that is a differential energy added to what is available at the pump's elevation (NPSHa) which is affected by atmospheric pressure.
"But then if the environment is extended to the extremes would not there be changes in the pump performance; if the pump was to operate on the moon would the parameters of the pump performance remain unchanged?" The pump peformance (the addition of differential energy) is unchanged. What changed is the part about differential energy added to what initial energy. At the Moon, atmospheric pressure is 0, but the moon is not a good example, as the Moon still has some gravity and would give you some pressure head, if the tank were for instance, elevated. Deep outer space would be a better example, without gravity at all and where NPSHa would total 0 psia - suction loss - vapor pressure, clearly not possible, which would indicate that the tank had better be pressurized and have a diaphram to force the fluid towards the pump.
**********************
"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSHa vs NPSHr
**********************
"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSHa vs NPSHr
RE: NPSHa vs NPSHr
**********************
"Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSHa vs NPSHr
First up - pumps don't "suck" that's unless of course they aren't working like you expected - then they really suck while you sort out the problems
Secondly - in a fully primed and running pump, the pressure at the impeller eye is reduced to below atmospheric pressure - this is a function of design.
Third - the pressure on the pumped product measured at the at the pump inlet must be higher than the eye pressure, this can be from from atmospheric pressure or an overhead source or a combination of both - otherwise flow can't take place.
fourth - at a given flow rate, the reduced pressure at the impeller eye will be "Y" - this is shown as NPSHr, this doesn't change due to atmospheric pressure external to the pump case - therefore the inflow needs to be at a pressure exceeding "Y" NPSHa so that cavitation doesn't take place.
Hence the need to factor in atmospheric pressure into your NPSHa calc's along with temp (vapour pressure), viscocity, friction and entry losses.