NPSH Net Positive Suction Head

Overview

Engineering reference data for NPSH Net Positive Suction Head in miscellaneous.

Key Formulas

Unit Conversion

$y = x \cdot k$

Multiply by conversion factor.

Linear Interpolation

$y = y_1 + \frac{(x - x_1)(y_2 - y_1)}{x_2 - x_1}$

Estimate between two known points.

Percentage

$p = \frac{\text{part}}{\text{whole}} \times 100\%$

Part as fraction of whole.

Variables

Available vs Required NPSH

Available NPSH (NPSHa)

The Net Positive Suction Head available from the system to the suction side of a pump is denoted NPSHa. It can be estimated during system design or determined experimentally.

For a pump lifting fluid from an open tank:

$\text{NPSHa} = \frac{p_{atm}}{\gamma} - h_e - h_l - \frac{p_v}{\gamma}$

Where:

• $p_{atm}$ = atmospheric pressure (Pa, psi)
• $\gamma$ = specific weight of the liquid (N/m³, lb/ft³)
• $h_e$ = elevation from surface to pump (m, in) — positive if pump is above tank
• $h_l$ = head loss in suction pipe (m, in)
• $p_v$ = vapor pressure at operating temperature (Pa, psi)

Required NPSH (NPSHr)

The NPSHr is the minimum suction head required by the pump to prevent cavitation for safe and reliable operation. This value is determined experimentally by the pump manufacturer and provided in pump documentation.

Critical design rule: The available NPSHa must always exceed the required NPSHr:

$\text{NPSHa} > \text{NPSHr}$

A safety margin is recommended to account for variations in operating conditions.

System Design Considerations

Effect of Pump Position

Pump above the tank:

• Elevation $h_e$ is positive
• NPSHa decreases as pump elevation increases
• At some elevation, NPSHa reaches zero and fluid begins to evaporate

Pump below the tank:

• Elevation $h_e$ is negative
• NPSHa increases as pump is lowered
• Lowering the pump always increases NPSHa (accounting for additional pipe losses)

Practical Recommendations

• Locate the pump in the lowest possible position
• If possible, increase the static pressure in the system
• Oversize and simplify piping on the suction side to limit friction and dynamic losses
• Lowering the pump is especially important when pumping fluids near their evaporation temperature

Water Vapor Pressure and Suction Head

Altitude Effects on Suction Lift

Common Fluid Vapor Pressures

Interactive Charts

References

• Original Source