What Does Net Positive Suction Head Required (NPSHR) Mean?
Net positive suction head required (NPSHR) is the pressure or energy required for the liquid in a pump to overcome the friction losses from the suction nozzle to the eye of the impeller without causing vaporization.
For normal pump operation the NPSHR is usually less than the available NPSH in order to avoid cavitation erosion and corrosion.
Corrosionpedia Explains Net Positive Suction Head Required (NPSHR)
The net positive suction head required is determined and provided by the pump manufacturer as a standard performance curve on the gallons per minute range.
NPSHR is unique for each pump and depends on:
- Pump design
- Pump size
- Operating conditions
The NPSHR is determined in a test rig where a system is run in a closed loop in which the flow, total head and power consumed are measured. A vacuum pump is used to lower the pressure at the suction tank so as to provide a low head at the pump suction. The pressure in the suction tank is lowered until a drop of 3% is measured. At this point the NPSH is calculated and recorded as the NPSHR for that operating point. This test is done at several operating points and heated coils are used to increase water temperature and vapor pressure, and hence lower the NPSH as needed.
A 3% total head drop criteria is used to set the NPSHR. For proper pump functioning, the NPSH available must be higher, and recommended to have a 15% margin or 5 ft of absolute water above the NPSHR.
If the NPSHR is greater than the NPSHA, the liquid vaporizes. The vapor bubbles collapse (implode) as they encounter increased pressure when exiting from the impeller, leading to cavitation.
The NPSHR forms the basis of the NPSHA calculations and should be observed in order to avoid:
- Cavitation erosion
- Noisy and slow operation
- Damaged pumps