NPSHR convention
NPSHR convention
(OP)
This could be easy. An engineer came to me with a KSB pump specsheet which gave NPSHr in units of meters H2O for a pump that pumps light hydrocarbon at well above 100C. The question was: what temperature (density) is the meters of H2O referencing. She wants to convert this to meters of suction fluid to compare with her NPSHa calc.
I have to admit I think it would be some ambient temperature water as per a test rig, in which case the difference between references like 20C or 60F would be negligible, but is there a convention for this sort of (stupid in my opinion) reporting? A quick google gave me no hint of this (meters H2O) convention being in normal use.
Thanks and best wishes,
sshep
I have to admit I think it would be some ambient temperature water as per a test rig, in which case the difference between references like 20C or 60F would be negligible, but is there a convention for this sort of (stupid in my opinion) reporting? A quick google gave me no hint of this (meters H2O) convention being in normal use.
Thanks and best wishes,
sshep





RE: NPSHR convention
Any comment on that thinking?
You would think that after 25 years in the industry I wouldn't have any questions about a concept like NPSHr, but apparently not.
thanks again,
sshep
RE: NPSHR convention
From somewhere I recall that using water based test data for design of hydrocarbon systems is slightly conservative because of water's low molecular mass. When a HC vaporises it generates less volume than water does because the vapor is more dense. But HC's tend to have lower heats of vaporisation, so this would counter that argument.
Katmar Software
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RE: NPSHR convention
It is easy enough to convert it to a gradient of 9.807 kPa/m (0.433 psi/ft) by multiplying density times g (and fixing the units). Then with the gradient of your actual pumped fluid you can recreate the NPSH-a chart.
David
RE: NPSHR convention
I have to this point been led to understand that the NPSH reductions for hydrocarbon fluids afforded via the Hydraulic Institute Standards are - at least in part - due to their significantly lower Vg/Vf ratio. In the case of vapour bubble collapse during cavitation, much less energy is released by a hydrocarbon liquid than for water. The cavitation still occurs but the damage is much less severe.
Regards,
SNORGY.
RE: NPSHR convention
RE: NPSHR convention
The NPSH requirements of centrifugal pumps are normally determined on the basis of handling water at or near normal room temperatures. Lab tests (various parties) have shown that pumps handling certain gas free hydrocarbon liquids will operate satisfactorily with harmless cavitation and less NPSH than required for cold water.
"Cameron Hydraulic Data", Ingersoll Rand, 1984
The reference has a chart (p.1-46) graphically showing NPSH reductions for propane, isobutane, butane, and methanol. No reduction shall exceed the smaller of 50% of the NPSH for cold water or 10 ft.
e.g. Pump has stated NPSH of 16 ft on cold water. Service will be propane at 55 F with a vapor pressure of 100 psia. The chart shows a reduction of 9.5 ft. The corrected NPSHR is 8 ft (50% of the NPSHR for water).
WEC
RE: NPSHR convention
"Centrifugal pumps don't care about density", but it is true just for DH and not NPSHR.
The other diagram to convert NPSHR(w) to NPSHR(any other liquid)is Fig 12-5 of GPSA.
If you want to be more conservative, you can skip converting it.
RE: NPSHR convention
best wishes,
sshep
RE: NPSHR convention
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RE: NPSHR convention