Margin

Do pumps suck?

The suction source and elevation stay the same. A partly closed suction valve is opened. Can available NPSH improve even though source pressure did not increase?

Workbench · generic example — not this plant

Change one term. Code computes.

Density 1000 kg/m³, g = 9.80665. Generic example — not this plant. Display, control, and receipt use the same value.

Source P abs (kPa)101.325
Elevation z (m of liquid)2
Vapor pressure (kPa abs)0.8
Suction loss (m of liquid)0.5
NPSHR at this flow (m of liquid)2

Source head: 10.33 m
Vapor head: 0.08 m
NPSHA: 11.75 m
Margin (NPSHA − NPSHR): 9.75 m

Calc, judgment, shop notes

Evidence · verified

What verified this edge

verified · Hydraulic Institute
ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin
2024 update summary
NPSHA is a suction-system head balance; NPSHR is a pump characteristic at a specified flow, speed, and liquid; their difference is NPSH margin.
Applies. Rotodynamic pumps. General mechanism, not this plant’s feet.
Limits. Does not give this pump’s NPSHR at a flow. Operating-point flow can move NPSHA via losses.
Open source
verified · Hydraulic Institute
Pump fundamentals — pump curves
Pump-curve NPSHR is defined at a specified rate of flow, speed, and pumped liquid.
Applies. Curve reading. Not a field actuation.
Limits. Vendor curve for the installed impeller is the project record.
Open source