Chapter 9 - Net Positive Suction Head (NPSH)

Abstract

Net positive suction head (NPSH) is defined as “the total suction head in feet of liquid absolute determined at the suction nozzle and referred to datum less the vapor pressure of the liquid in feet absolute.” One can determine the net positive suction head required (NPSHR) of a particular pump design through calculations. Usually, pump manufacturers test the pumps to find the NPSHR. The test basically consists of finding where the liquid's vapor pressure exceeds the NPSHA when the liquid enters the eye of the impeller, causing the pump to cavitate. The NPSHR test consists of running a pump at constant speed while throttling the pump suction line until the discharge pressure drops. At that point, the pump starts to cavitate. Incipient cavitation happens as soon as the discharge pressure begins to drop—called 0 percent head drop. Because this point is difficult to mark, the NPSHR is recorded at a 3 percent head drop. Points are plotted from 0 capacities to end-of-the curve capacity.