The pump manufacturer will state the NPSHr, Net Positive Suction Head required, to prevent cavitation or vaporization of the liquid in the pump. The NPSHa, Net Positive Suction Head available, must be greater than that required by the pump.
For a basic understanding of Net Positive Suction Head, you must think in terms of absolute pressure in feet. To determine the NPSHa, the Net Positive Suction Head available at the pump inlet, you first determine the pressure above the liquid. As atmospheric pressure varies with altitude (and weather), the site elevation must be known. For this discussion, we will be at sea level and pumping water.
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