NPSHa in a vacuum application

[krupnikas]

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

 

[micalbrch]

I had to convert your values into metric values but if I'm not wrong, I'll get approx. 6.6 m for NPSHA.

 

[ione]

I confirm the value quoted by micalbrch

 

[krupnikas]

Converts to about 21.6ft. Seems way too much. I was coming out with around 7ft. Its the vacuum that that causes me the confussion in the equations.

 

[ione]

Vacuum is not a negative value, when speaking of NPSH always refer to absolute pressure

NPSHA = 2,29 m (suction head) – 0,15 m (friction loss) – 5,28 m (vapour pressure) + 9,67 m (vessel) = 6,53 m

 

[micalbrch]

Correct unless we misunderstood "28 in Hg Vacuum".

 

[TenPenny]

28 in Hg vacuum means approx 0.9 psia, to my reading.

 

[rmw]

28" HG vacuum means that it isn't a deep vacuum. If it were 0.9 psia, the water would flash off (to a temperature that I am not going to go look for a steam tables to find out what it is). I believe 28 in hg vacuum means 28" hga. Check the saturation temperature for that pressure and see if it is at or above 180F.

rmw

 

[ione]

I agree with rmw: 28 in Hgg pressure (0.942 psia) implies a saturation temperature of 99.7 °F, being water at 180 °F it would definitely flash.

 

[TenPenny]

Atmospheric pressure is 0 in Hg vacuum. A perfect vacuum is 29.92" Hg vacuum.

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'.

 

[krupnikas]

This is a distillation unit and the water in the vessel will become vapor as it re-enters the unit.

Thanks to everyone for all the inputs. I have also got confirmations from pump vendors that it NPSHa is 7ft based on the scenerio.

 

[ione]

krupnikas,

I would like to know how your pump vendor has computed 7 ft with the data presented.

 

[krupnikas]

Net Positive Suction Head Available = Pressure on Surface of Liquid + Static Suction Head – (Vapor Pressure + Friction Loss)
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.

 

[ione]

Wow, TenPenny interpreted it the right way

 

[1gibson]

Yes, static head - friction losses = NPSHA (ignoring velocity head)

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.

 

[rmw]

If that liquid is at saturation pressure that is.

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