Net Positive Suction Head (NPSH) is perhaps the most misunderstood concept in pumping technology. The main source of confusion stems from multiple uses of the term in different but related contexts.
One usage of the term NPSH is to denote the suction performance characteristic of the pump itself, independent of the system into which it is installed. In this context the terms often used are ‘NPSH Required’ or ‘NPSHR’. The usual industry standard criterion for NPSHR is the value of NPSH for which a 3% reduction in pump total head occurs. The term ‘NPSH3’ is also used for this. At this NPSH condition cavitation is taking place to the point where the pump head performance is falling off measurably. NPSHR varies with rate of flow and normally increases at a steep rate of climb beyond the pump’s best efficiency rate of flow. The pump manufacturer’s NPSHR versus flow curve may include margin above NPSH3. NPSHR is normally considered the lowest value of NPSH at which the pump can be operated. It is usual practice to provide a system design margin of NPSH above the pump’s NPSHR.
The term NPSH also denotes a dynamic liquid property in a given pump intake or piping system and in that context it is often referred to as ‘NPSH Available’ or ‘NPSHA’. It is the calculated absolute total head less liquid vapor head present at the pump suction nozzle. NPSHA normally decreases as the rate of flow in the system increases. The value of NPSHA is independent of the pump.
While there are, in fact, distinct definitions for NPSHR, NPSH3 and NPSHA – and, by the way, there are other NPSH terms – the general term ‘NPSH’ is often used interchangeably to mean any one of those and the reader is left to determine from context the correct technical meaning. The main idea is to realize whether the context of NPSH is relative to the pump or to the system.
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