NPSHa in a vacuum application
NPSHa in a vacuum application
(OP)
Looking for confirmation on an NPSHa calculation involving a centrifugal pumping from vacuum vessel.
Fluid: Water @ 180 DegF
Fluid Density: 0.970
Vapor Pressure @ 180 DegF: 7.51 PSIA
Vessel: 28 in Hg Vacuum
Approx Friction Losses: 0.5 ft
Water Operational level above pump centerline: +7.5 ft
Fluid: Water @ 180 DegF
Fluid Density: 0.970
Vapor Pressure @ 180 DegF: 7.51 PSIA
Vessel: 28 in Hg Vacuum
Approx Friction Losses: 0.5 ft
Water Operational level above pump centerline: +7.5 ft





RE: NPSHa in a vacuum application
RE: NPSHa in a vacuum application
RE: NPSHa in a vacuum application
RE: NPSHa in a vacuum application
NPSHA = 2,29 m (suction head) – 0,15 m (friction loss) – 5,28 m (vapour pressure) + 9,67 m (vessel) = 6,53 m
RE: NPSHa in a vacuum application
RE: NPSHa in a vacuum application
RE: NPSHa in a vacuum application
rmw
RE: NPSHa in a vacuum application
RE: NPSHa in a vacuum application
I think the OP needs to clarify what they mean, in absolute pressure terms, because the way it is written doesn't quite make sense.
At least, not to me.
Perhaps the original post was supposed to mean '28 in Hg(abs), a vacuum' as opposed to '28 in Hg vacuum'.
RE: NPSHa in a vacuum application
Thanks to everyone for all the inputs. I have also got confirmations from pump vendors that it NPSHa is 7ft based on the scenerio.
RE: NPSHa in a vacuum application
I would like to know how your pump vendor has computed 7 ft with the data presented.
RE: NPSHa in a vacuum application
Where all the terms are in feet of liquid.
With the Vessel @ 28" Hg Vacuum and the liquid vapor pressure @ 7.51 PSI Absolute, you would not have any liquid in the vessel (only vapor)!!!
Since you will have liquid in the tank, the pressure on the surface of the liquid will be equal to the vapor pressure.
Therefore, the NPSHa = Static Suction Head – Friction Loss.
Kindly review the attachment from Goulds Pumps and contact our office if you require further information.
RE: NPSHa in a vacuum application
RE: NPSHa in a vacuum application
Vapor pressure and level of vacuum in the tank will determine the liquid level, no need for a calculator when you have a "real life" liquid level.
RE: NPSHa in a vacuum application
If not, the calculations are required.
And, TenPenny, the use of the mercury vacuum term is ambiguous. Some people use in Hg as the "back pressure" or the operating pressure, meaning you read the vacuum value directly from a steam or water table, and others use it to mean how much below atmospheric the vacuum is - in other words you have to take the vacuum stated and subtract it from 29.92 to obtain the pressure.
You never quite know until someone gives a point of reference. I've dealt with it for years and it never gets easier.
rmw