NPSH check Cavitation occurs when the pressure at the impeller inlet falls below the vapour pressure. The check compares the NPSH available in the system with the NPSH required by the pump.

Input (Example from chap. 3.3.4)

Air pressure above the suction water level p_b

mbar

Medium temperature determines the vapour pressure p_D (table 3.3-2)

Density of the medium ρ

kg/m³

Static suction head h_geo: + positive suction head / − suction lift

Losses in the suction pipe H_v,s

NPSH required by the pump NPSH_R from the pump curve

Municipal wastewater has an annual average temperature of about 20 °C. Air pressure drops by approx. 12 mbar per 100 m of altitude.

Result

NPSH margin (NPSH_A − NPSH_R)

– m

NPSH available in the system NPSH_A = – m

–

NPSH_A = (p_b − p_D) / (ρ · g) + h_geo − H_v,s
Margin = NPSH_A − NPSH_R

Reference	Technical book, chap. 3.3.2 · The NPSH value
Example	Chap. 3.3.4 · Worked example NPSH
Limits	Valid for steady-state operation. During start-up and at part load or overload, NPSH_R is higher.

Specific speed Affinity laws Pressure surge (Joukowsky) Switching volume & starts per hour

back to overview

Background knowledge on coated pumps