Help with NPSHA Calc - my numbers need to be checked
Help with NPSHA Calc - my numbers need to be checked
(OP)
I am attempting to calculate the NPSHA for a centrifugal pump in a system I am designing. I have checked over my calculations and come up with a number that looks to be technically correct but does not seem intuitively correct. I want to get a second opinion to see if I did something wrong.
Inputs:
Liquid is water at 80F and sea level
Flow is 4.2 l/min
Pump inlet is 1” ID
Tank level height above the pump inlet varies from 3 ½ feet to ½ foot
The run from the tank to the pump is in 2 sections:
First section is 15 feet of 1” ID Buna hose
Second section is 2 feet of 1” 316 SS pipe with (1) 90 degree elbow and (1) ball valve which is fully open when the system is running
The numbers I came up with are Average Re = 4100 and NPSHA = 36 feet.
The one area I am unsure of is the supply tank. The tank is a 30 gallon cylinder lying on its side. It is filled through an opening at the top which is closed with a screw-on lid. The lid is not airtight so I included atmospheric pressure in the above calculation. If I use 0 feet for atmospheric head then the NPSHA comes out 2.29 feet. So a more intuitive answer would be something less than 34 feet of atmospheric head (34 feet atmospheric = 36 feet NPSHA).
Inputs:
Liquid is water at 80F and sea level
Flow is 4.2 l/min
Pump inlet is 1” ID
Tank level height above the pump inlet varies from 3 ½ feet to ½ foot
The run from the tank to the pump is in 2 sections:
First section is 15 feet of 1” ID Buna hose
Second section is 2 feet of 1” 316 SS pipe with (1) 90 degree elbow and (1) ball valve which is fully open when the system is running
The numbers I came up with are Average Re = 4100 and NPSHA = 36 feet.
The one area I am unsure of is the supply tank. The tank is a 30 gallon cylinder lying on its side. It is filled through an opening at the top which is closed with a screw-on lid. The lid is not airtight so I included atmospheric pressure in the above calculation. If I use 0 feet for atmospheric head then the NPSHA comes out 2.29 feet. So a more intuitive answer would be something less than 34 feet of atmospheric head (34 feet atmospheric = 36 feet NPSHA).





RE: Help with NPSHA Calc - my numbers need to be checked
You should use your minimum level of 0.5 ft for the tank level.
At these flowrates in a 1" pipe your pressure drop will be negligible.
The vapor pressure of water at 80F is about 1.2 ft and needs to be subtracted from the NPSHa.
On this basis I get a worst case NPSHa of 31.5 ft (=32.2+0.5-1.2)
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RE: Help with NPSHA Calc - my numbers need to be checked
cheers,
gr2vessels
RE: Help with NPSHA Calc - my numbers need to be checked
RE: Help with NPSHA Calc - my numbers need to be checked
If you are at sea level, you are taking a maximum head of 34 feet. Subtact the vapor pressure of water at 80F which is 0.5 psia or 1.16 feet. Depending on line losses (which I haven't calculated) and your liquid head (0.5 to 3.5 feet), 36 feet NPSHA is not impossible
RE: Help with NPSHA Calc - my numbers need to be checked
The only comment I would think of refers to dissolved air. If the liquid pressure at the pump's eye is lower than that at the source, dissolved air may desorb and create problems when the volume of flashed gas is greater than, say, 3%.
There was an interesting article in the Chemical Engineering issue of July 26, 1982, titled Accounting for dissolved gases in pump design worth reading.
RE: Help with NPSHA Calc - my numbers need to be checked
I set up the system exactly as designed except that the pump was not connected. I simply opened the valve and let the liquid flow out the pipe that would be connected to the pump inlet and measured the flow rate over the liquid height range. I measured flow rates from 8 gal/min at 16 inches above the pump inlet to 5 gal/min at 10 inches above the pump inlet. My application is requiring a maximum pump output of 1.1 gal/min so this seems to verify that the system is more than sufficient.
The pump curve indicates a NPSHR of 10 feet at the low end of the flow rate range (which is still a factor of 2 higher than my application) so the difference between my NPSHA and the NPSHR correlates the simple flow test so I think I am comfortable with the set-up.
Thanks for the responses!