Library / Hydraulics, open channels and fluid power / HYD-36
Hydraulics, open channels and fluid power
Available net positive suction head at a pump inlet
Head at the pump inlet above the vapour pressure of the liquid, from the static absolute suction pressure, vapour pressure and mean inlet speed.
- Ref
- HYD-36
- Rev
- v1
- Field
- Hydraulics, open channels and fluid power
- Result
- NPSHa [m]
Net positive suction head available is the head at the pump inlet above the liquid’s vapour pressure: NPSHa = (ps − pv)/(ρ g) + Vs²/(2g), using absolute pressures. A pump needs NPSHa above its required NPSH, with a margin, to avoid cavitation, which damages the impeller and drops the head. Vapour pressure rises quickly with temperature, so hot liquids have much less margin.
Head at the pump inlet above the vapour pressure of the liquid, from the static absolute suction pressure, vapour pressure and mean inlet speed. It must exceed the pump’s required value with a margin.
Calculator
- Pressure head above vapour, (ps − pv)/(ρ g)7.849 m
- Velocity head at the inlet, Vs²/(2g)0.2039 m
- Margin over a required NPSH of 5 m3.053 m
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- The pump inlet has 8.053 m of head above vapour pressure, of which 0.204 m is velocity head.
- Against a required NPSH of 5 m the margin would be 3.05 m; a ratio of NPSHa to NPSHr of 1.1 to 1.3 or more is commonly asked for.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (m) | Local step | Change in result |
|---|---|---|---|---|---|---|
| ps Absolute static pressure at the suction tap | 1,000 to 1e7 Pa | 0 | 1,019 | 1,019 | +10% | +10.1 % |
| pv Vapour pressure of the liquid | 100 to 1,000,000 Pa | −93.58 | 8.349 | 101.9 | +10% | −0.38 % |
| g Gravitational acceleration | 1 to 30 m/s² | 2.633 | 79 | 76.37 | +10% | −9.09 % |
| Vs Mean speed at the inlet | 0 to 30 m/s | 7.849 | 53.72 | 45.87 | +10% | +0.532 % |
| ρ Liquid density | 500 to 2,000 kg/m³ | 4.128 | 15.9 | 11.77 | +10% | −8.86 % |
Sweep of Vapour pressure of the liquid
| Vapour pressure of the liquid (Pa) | Available NPSH (m) |
|---|---|
| 100 | 8.349 |
| 316.2 | 8.327 |
| 1,000 | 8.257 |
| 3,162 | 8.036 |
| 10,000 | 7.339 |
| 31,620 | 5.135 |
| 100,000 | −1.835 |
| 316,200 | −23.88 |
| 1,000,000 | −93.58 |
Sweep one variable
Variables and how each one contributes
Available NPSH
What the equation gives you. Everything else feeds it.
Absolute static pressure at the suction tap
The absolute static pressure at the suction tap, in Pa. Raises the head in direct proportion to the pressure. Raising it by 10 % (to 88,000) moves the result by +10.1 %, from 8.053 to 8.869 m.
Vapour pressure of the liquid
The vapour pressure of the liquid at the pumping temperature, in Pa. A higher vapour pressure lowers the available head. Raising it by 10 % (to 3,300) moves the result by −0.38 %, from 8.053 to 8.022 m.
Liquid density
The liquid density, in kg/m³. A denser liquid gives less head for the same pressure. Raising it by 10 % (to 1,100) moves the result by −8.86 %, from 8.053 to 7.339 m.
Gravitational acceleration
The gravitational acceleration, in m/s². Lowers the head in inverse proportion. Raising it by 10 % (to 10.79) moves the result by −9.09 %, from 8.053 to 7.321 m.
Mean speed at the inlet
The mean speed at the inlet, in m/s. Raises the head through the velocity-head term. Raising it by 10 % (to 2.2) moves the result by +0.532 %, from 8.053 to 8.096 m.
Worked example with the values above
- Pressure head: (ps − pv)/(ρ g) = (80,000 − 3,000) / (1,000 × 9.81) = 7.8491 m.
- Velocity head: 2² / (2 × 9.81) = 0.20387 m.
- NPSHa = 8.053 m.
Assumptions and limits
- Steady incompressible flow with the pump curve and system curve both of quadratic form.
- Ideal blade-guided flow for the velocity-triangle results, so the real head is lower.
Common mistakes
- Using gauge pressure in place of absolute pressure.
- Using the vapour pressure at the wrong temperature.
- Forgetting that the available value must exceed the required value with a margin, not equal it.
Statistics and uncertainty
Vapour pressure changes by a factor of about two for every 15 to 20 K near ambient temperature, so the margin shrinks quickly with hot liquids, and suction level varies with tank level.
Where it is used
- Choosing pumps and finding the duty point on a pipe system.
- Checking cavitation margin and estimating head from impeller geometry.
Maths you need
- Pump curve: head against flow for a pump at a fixed speed
- System curve: head needed by the pipe system against flow
- Velocity triangle: the vector diagram of blade and fluid velocities