Describe the suction side: the tank, the water temperature, how high the liquid level is above or below the pump, and the friction in the suction pipe. The calculator shows NPSH available and the margin to the pump's NPSH required.
NPSH available
Margin over NPSHr
Pressure head at liquid surface
Vapor pressure head
How NPSH available is calculated
A pump draws liquid into its eye at low pressure. If that pressure falls below the vapor pressure of the liquid, bubbles form and collapse: cavitation. NPSH available tells you how far the inlet pressure stays above the vapor pressure:
NPSHa = (ps − pv) ÷ (ρ · g) + z − hf
ps
absolute pressure on the liquid surface (air pressure for an open tank)
pv
vapor pressure of the liquid at its temperature
z
height of the liquid level above the pump inlet (negative for suction lift)
hf
friction loss in the suction line
Worked example
An open tank at sea level holds water at 20 °C, 2 m above the pump, and the suction line loses 0.5 m. Air pressure gives 101.3 kPa ÷ (998 × 9.81) = 10.35 m; vapor pressure takes off 0.24 m. NPSHa = 10.35 + 2 − 0.5 − 0.24 = 11.6 m. With the same water at 80 °C the vapor pressure head is 4.97 m and NPSHa falls to 7.2 m.
NPSHa = pressure head at the liquid surface + static height of the liquid above the pump − friction loss in the suction line − vapor pressure head of the liquid. All terms are in meters or feet of the liquid being pumped.
What is the difference between NPSHa and NPSHr?
NPSH available is a property of your system. NPSH required is a property of the pump at a given flow, taken from its curve (usually the point with 3% head drop). NPSHa must be higher than NPSHr, or the pump cavitates.
How much NPSH margin do I need?
A common rule is NPSHa at least 1 m (3 ft) or 10 to 35% above NPSHr, whichever is larger. ANSI/HI 9.6.1 recommends ratios from 1.1 to 2.5 depending on the application; hot water and high-energy pumps need the most.
Why does hot water cause cavitation?
Vapor pressure rises fast with temperature: about 2.3 kPa at 20 °C, 47 kPa at 80 °C and 101 kPa at 100 °C. At 100 °C an open tank gives almost no NPSH, so the liquid must stand well above the pump.