The numerical difference between the NPSHa and NPSHr, i.e., NPSH Margin = NPSHa – NPSHr. The NPSH margin can also be expressed as the ratio NPSH Margin = NPSHa/NPSHr. The Hydraulic Institute NPSHa Margin Work Group recommended NPSH margins ranging from zero to four times the NPSHr (NPSHa ÷ NPSHr = 1 to 5) to prevent cavitation from occurring. The higher values apply to pumps with high to very high values of suction energy, or the product of the diameter of the pump suction nozzle, pump rpm, and the suction specific speed.
For hydrocarbon liquids: NPSHa/NPSHr = 1.1 to 1.2 or NPSHA – NPSHr > 3.3 ft. or 1.5 m, whichever is greater.
For water: NPSHa/NPSHr = 1.1 to 1.5 or NPSHa – NPSHr > 4.9 ft. or 1.5 m, whichever is greater.
If possible, add a safety margin on top of these recommended values to handle upset conditions.