What is NPSH3, and what are the methods for determining the NPSH3 of a rotodynamic submersible pump? NPSH3 is the net positive suction head available to a pump under test at a constant rate of flow when the pump head is decreased by 3 percent as a result of cavitation caused by a decreasing available suction head. Sometimes NPSH3 is referred to as net positive suction head required (NPSHr). However, a pump’s NPSHr must be higher than the NPSH3 for the pump to operate without head reduction, and it may need to be higher than the NPSH3 for long-term reliable operation. For more information on the required margins above NPSH3, refer to ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin. The NPSH is established through the datum elevation of the impeller. The flow toward the pump must be uniform and free of undue disturbances. A pump tested with suction piping may require a flow-straightening device before entering the pump. Arrangements for cooling or heating the liquid in the loop may be needed to maintain the required temperature.

