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NPSHA vs NPSHR

NPSHR is defined as Net Positive Suction Head Required and NPSHA is defined as Net Positive suction head available. NPSHR is a function of the design of the pump, whereas NPSHA is a function of the design of the system, properties of the liquid, and static pressure at the suction of the pump. Net Positive Suction Head is measured in terms of feet of water.

For a pump to work properly, NPSHA must be greater than NPSHR: you must have more suction head available in the system than required by the pump.

When NPSHA is less than NPSHR, the pump will either cavitate and operate at a reduced performance condition, or the pump will not operate at all.

The difference between NPSHA and NPSHR is defined as the NPSH operating margin.  This margin must be positive (greater than 0) in order for the pump to operate properly. The required NPSH operating margin varies depending on the application and design characteristics of the pump. A good “rule of thumb” for NPSH operating margin for general service, water pumps is 2 to 5 feet—this will vary depending on the suction energy of the pump.

As the NPSH operating margin approaches 0, the pump may become less stable; however, some specially designed pumps will exhibit stable operation with an NPSH operating margin close to 0.  Non-Clog type pumps will generally require a larger NPSH operating margin than Clear Liquid type pumps; this is because non-clog pumps have a fewer number of impeller vanes than clear liquid pumps.

NPSHA is calculated as follows:

 NPSHA =  Absolute pressure (in feet of liquid) at the surface of the liquid supply

      - Friction Losses in Suction Pipe (including entrance losses and fittings)

      - Suction Lift in feet (subtract for suction lift; add for flooded suction)

      - Vapor Pressure (head in feet corresponding to the vapor pressure of the          liquid at the given liquid temperature)

  • Note: Absolute pressure at the surface of the liquid is equivalent to barometric pressure for an open tank or the absolute pressure existing in a closed tank.

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