KEVOS® MAKEOS · Hydraulics
Pump NPSH Available
Is the net positive suction head available enough to prevent cavitation?
Net positive suction head available from suction surface pressure, vapour pressure, static height and suction losses, compared with the pump’s required NPSH plus a margin.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Pressure on the liquid surface (absolute) | kPa | Absolute pressure on the liquid surface (atmospheric about 101.3 kPa at sea level; tank pressure for closed vessels). |
| Vapour pressure at pumping temperature | kPa | Vapour pressure of the liquid at its temperature (water 2.34 kPa at 20 °C, 47.4 kPa at 80 °C). |
| Liquid density | kg/m³ | Liquid density at its temperature. |
| Liquid level above pump centreline (negative = below) | m | Height of the liquid surface above the pump centreline; negative for suction lift. |
| Suction line losses | m | Friction and fitting losses in the suction line at duty flow (see the pipe friction tool). |
| Pump NPSH required at duty | m | NPSH required from the pump curve at the duty flow. |
| Required margin | m | Safety margin your standard requires above NPSHr. |
Method and formulas
- NPSHa = (p_s − p_v) ÷ (ρ g) + z − h_f, pressures in Pa.
- Margin = NPSHa − NPSHr.
- Example: water at 20 °C with a 2 m suction lift gives NPSHa = 7.3113 m.
Limitations
- NPSHr on pump curves corresponds to a 3% head drop; cavitation damage can begin above it.
- Dissolved gas and temperature rise reduce the real margin.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a pump installation for cavitation
- Decide pump elevation or tank level
- Assess hot liquid duties
Who it is for
- Hydraulic and process engineers
- Maintenance engineers
- Design engineers
What you get
- NPSH available
- Margin over NPSH required
Interpreting the result
- Raise the tank, lower the pump, enlarge the suction pipe or cool the liquid to gain NPSH.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027. Topic reference; no source tables or recommended settings are reproduced.
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