There are two ways of expressing NPSH relative to a centrifugal pumping system.
Excerpt from the August 2020 Pumps & Systems article by Gary Dyson
To make the term net positive suction head (NPSH) more accessible to pump engineers who may not understand how to design an impeller for NPSH or the exact details of the physics, I have tried to simplify it:
- NPSH is a measure of the absolute pressure energy present in a liquid. Pump engineers use this energy to help “feed” the fluid into the eye of the first-stage impeller. Pumps generally do not suck.
- NPSH is the sum of the total static plus kinetic pressure minus the liquid vapor pressure at the pump suction nozzle or impeller entry, which is expressed in terms of head.
There are two ways of expressing NPSH relative to a centrifugal pumping system:
- NPSHa—The net positive suction head available is the measurement of the amount of fluid pressure energy available from the system at the pump impeller inlet.
- NPSHr—The net positive suction head required is the measurement of the amount of fluid pressure energy required by the pump.
The NPSH available to the pump should be more than what the pump requires. If there is not enough NPSHa, the pump will cavitate. As a result, the performance and reliability can be compromised.
NPSHr curves, as provided by the pump manufacturer, are generated using the data collected during a pump performance test. Determining NPSHr requires testing the pump over a series of carefully controlled, constant flow points and varying suction conditions in a test facility. The test lab operator sets the flow and then begins to introduce a vacuum on the suction side of the pump. This reduces the suction pressure in controlled increments. While reducing the suction pressure, the discharge pressure is closely monitored.
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