When calculating NPSH, which pressure should be used?

The appropriate pressure to use when calculating Net Positive Suction Head (NPSH) is absolute pressure. This is because NPSH calculations are concerned with ensuring that the pressure at the pump's suction inlet is sufficient to prevent cavitation.

Absolute pressure includes both the atmospheric pressure and the gauge pressure, providing a true measure of the pressure available at the suction side of the pump. By using absolute pressure, the calculations can accurately reflect the actual conditions experienced by the liquid being pumped, factoring in variations such as changes in atmospheric pressure or the elevation of the pump.

Other types of pressure, like gauge pressure, only reflect the pressure relative to atmospheric pressure and can be misleading in scenarios where there's a risk of cavitation. Dynamic pressure refers to the pressure associated with fluid motion and is not relevant in evaluating the suction conditions of a pump. Static pressure, while relevant in certain contexts, does not account for the overall pressure conditions impacting the liquid's state concerning cavitation occurrence. Thus, absolute pressure is crucial for obtaining a reliable NPSH calculation.