Check that a pump will not cavitate. Enter the water temperature, tank and suction conditions, and the calculator gives NPSH available, compares it with the pump's NPSH required, and shows the maximum suction lift.
The pump runs without harmful cavitation only if NPSHa is comfortably above the NPSHr on its curve. ANSI/HI 9.6.1 recommends a margin of at least 0.6–1.5 m, or a margin ratio of 1.1 to 1.5 or more depending on the application.
| Temperature | Vapour pressure | Vapour head | (Patm − Pv) head at sea level |
|---|---|---|---|
| 10 °C (50 °F) | 1.23 kPa | 0.13 m | 10.21 m |
| 20 °C (68 °F) | 2.34 kPa | 0.24 m | 10.11 m |
| 30 °C (86 °F) | 4.25 kPa | 0.43 m | 9.94 m |
| 40 °C (104 °F) | 7.38 kPa | 0.76 m | 9.65 m |
| 50 °C (122 °F) | 12.35 kPa | 1.27 m | 9.18 m |
| 60 °C (140 °F) | 19.95 kPa | 2.07 m | 8.44 m |
| 70 °C (158 °F) | 31.20 kPa | 3.25 m | 7.31 m |
| 80 °C (176 °F) | 47.41 kPa | 4.98 m | 5.66 m |
| 90 °C (194 °F) | 70.18 kPa | 7.41 m | 3.29 m |
Hot water leaves very little NPSH: at 80 °C a pump drawing from an open tank has only about 5 m of pressure head before losses, and at 100 °C none. Hot water and boiler feed pumps therefore need a flooded suction with the tank well above the pump; see the Boiler Feed Pump Calculator.
For the full duty point use the Pump Head Calculator and the Water Pipe Pressure Drop Calculator.
NPSHa = (absolute pressure on the water surface − vapour pressure) ÷ (ρ·g) + height of the water above the pump centreline − suction pipe losses. Use a negative height for suction lift.
NPSHa is what the installation provides at the pump suction. NPSHr is what the pump needs, from its test curve. NPSHa must exceed NPSHr by a safe margin to avoid cavitation.
At sea level with cold water the theoretical limit is about 10.3 m, but after vapour pressure, suction losses and the pump's NPSHr the practical lift is usually 4–7 m, and less at altitude or with warm water.
Yes. Atmospheric pressure falls by about 1.2 m of water head for every 1,000 m of altitude, which reduces NPSH available by the same amount for an open tank.
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