NPSHA calculation
NPSHA calculation
(OP)
I am checking the NPSHA relation with the data, included in the Mechanical datasheet of pump, which is used in a petrochemical site.
the NPSHA=8.7 m is insisted in the datasheet, but the Psuc is
Psuc,norm=2.16 bara,
Psuc,max=2.18 bara
Psuc,min=1.06 bara
Psuc,des=2 barg
Pump design temperature=85 C
Pumping Temperature=40~50 C
Vap. Pressure=0.12 bara (at pumping T)
Relative Density=0.988
when calculating the NPSHA using the above quantities, the resulted NPSHA differs from the one, included in the datasheet.
would you please inform me which one is true?
NPSHA=(Psuc-Pvap)/(1000*S.G.*g)
the NPSHA=8.7 m is insisted in the datasheet, but the Psuc is
Psuc,norm=2.16 bara,
Psuc,max=2.18 bara
Psuc,min=1.06 bara
Psuc,des=2 barg
Pump design temperature=85 C
Pumping Temperature=40~50 C
Vap. Pressure=0.12 bara (at pumping T)
Relative Density=0.988
when calculating the NPSHA using the above quantities, the resulted NPSHA differs from the one, included in the datasheet.
would you please inform me which one is true?
NPSHA=(Psuc-Pvap)/(1000*S.G.*g)





RE: NPSHA calculation
You have a minimum of (1.06 -0.12) barg + 1 bara of = 1.94 BarA of NPSHA. That's equivalent to some 20 meters of NPSHA, so the 8.7 m would seem to be wrong.
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: NPSHA calculation
Johnny Pellin
RE: NPSHA calculation
Data sheets didn't use to be that way, having only one box for each value and that required that the specifying engineer know what he was doing and preselect the worst case, or most conservative operating point and write down those corresponding values. Today it seems that they just want to copy all the data from the computer run and paste it into the boxes on the data sheet, no matter how disassociated those data values might be from any given actual operating point.
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: NPSHA calculation
The NPSHR must be provided by the pump manufacturer and the NPSHA is provided by the pump purchaser.
May be that the 8.7 m be the NPSHR instead the NPSHA.
casflo
RE: NPSHA calculation
You only provided the pressure detail. NPSHA calculation need to take elevation diff.between liquid level and pump impeller center line and also suction piping and fitting losses into the consideration.
Your Psuc, des=2.0barg as relate to Psuc,max=2.18 bara certainly is a mistake.
RE: NPSHA calculation
NPSH = S - Hfs - Hi Ps - Pvp
where:
S = static head between suction liquid level and pump centerline
Hfs = friction loss in suction line
Hi = entrance loss in suction line
Ps = pressure on suction liquid surface
Pvp = vapor pressure of liquid
all units in feet of liquid
RE: NPSHA calculation
Left symbol out of equation, should be:
NPSH = S - Hfs - Hi + Ps - Pvp
RE: NPSHA calculation
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: NPSHA calculation
As my experience ,the suction pressure in pump data sheet is normally the pressure on the surface of the liquid or suction vessel pressure. The OP may have misread the data as indicated in the 2 values. It would be help a lot if the OP can post the data sheet.
Psuc,des=2 barg = 3.03barA is > Psuc,max=2.18 bara
DubMac, you are mixing head and pressure in your formula.
RE: NPSHA calculation
I should have been clearer that the same units should be used throughout the equation; either head in units of feet, or pressure in units of PSI. Convention in the industry uses "feet of head" for NPSH calculations.
There is no difference between head and pressure; the quantity is just expressed in different units; pressure in PSI, head in feet of liquid.
And as we all know from our Pumps 101, to convert from one to the other:
Head, ft. = (2.31 X PSI) / S.G. of fluid
RE: NPSHA calculation
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso