NPSH Calculator
NPSH available is the pressure margin a pump has at its inlet, above the point where water starts to boil. This works it out from the surface pressure, the vapor pressure, the height of the water and the losses in the suction pipe.
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- Works offline after your first visit
- Saves on this device
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How it works
Surface pressure minus vapor pressure, divided by the density of water times gravity, gives a head in length. Add the static head when the water sits above the pump, and take away the friction losses in the suction line.
Water density is taken as 1000 kilograms per cubic metre, so the result is for water. Other liquids need their own density.
Worked example
With 14.7 psi on the water surface, 0.34 psi of vapor pressure, 3 ft of static head and 2 ft of suction losses, the NPSH available is 34.12 ft.
Questions
What does NPSH available mean?
It is the head of liquid pressure at the pump inlet, above the vapor pressure. The pump's required NPSH, listed by its maker, has to be lower than this number for the pump to run without cavitating.
Why does water temperature matter?
Warmer water has a higher vapor pressure, which cuts the margin. Enter the vapor pressure at the water's actual temperature.
Is this enough to choose a pump?
No. It gives the available margin only. Pump selection also compares the required NPSH on the pump data sheet and the full losses in the system.
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