BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Pressure on the liquid surface (absolute)
- What it is
- The absolute pressure acting on the free surface of the liquid the pump draws from: local atmospheric pressure for an open tank, or the vessel pressure for a closed one.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.000001 kPa to 100000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.3 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher surface pressure raises NPSH available.
- Get it right
- Enter absolute pressure, not gauge: an open tank is at atmospheric pressure, not zero.
What this means: Vapour pressure at pumping temperature
- What it is
- The pressure at which the liquid boils at its pumping temperature, from steam tables or the liquid’s data sheet.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0 kPa to 100000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.34 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher vapour pressure lowers NPSH available.
- Get it right
- It rises steeply with temperature, so use the hottest the liquid will be.
What this means: Liquid density
- What it is
- The mass per unit volume of the liquid at its pumping temperature, used to convert the pressures into metres of head.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 25000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 998 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A denser liquid turns the same pressure difference into less head.
- Get it right
- Use density at the pumping temperature.
What this means: Liquid level above pump centreline (negative =…
- What it is
- The vertical distance from the pump centreline up to the liquid surface in the suction tank. If the surface is below the pump, the value is negative.
- Units and format
- Enter a number in m: metres. The form accepts -100 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with -2 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Each metre higher adds a metre to NPSH available.
- Get it right
- Use a negative value for a suction lift; use the lowest liquid level expected.
What this means: Suction line losses
- What it is
- The head lost to friction in the suction pipe, fittings, strainer and valves at the duty flow, from a pipe friction calculation.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Each metre of loss removes a metre from NPSH available.
- Get it right
- Enter it as metres of head, not bar; include a partly blocked strainer if that can occur.
What this means: Pump NPSH required at duty
- What it is
- The net positive suction head the pump needs at the duty flow, read from the pump maker’s curve.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the margin and the pass or fail line.
- Get it right
- Read it at the actual duty flow; it rises as flow increases.
What this means: Required margin
- What it is
- The extra head above the pump’s requirement that your standard or the pump maker asks you to keep to avoid cavitation.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 100 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line.
- Get it right
- Take it from your engineering standard or the pump supplier, not from the example.
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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