FAQ

Question:

Do centrifugal pumps require more Net Positive Suction Head (NPSH) margin at higher elevations than at sea level?

Answer:

NO. The recommended NPSH margin for a centrifugal pump does not change at higher elevations. The NPSH required (NPSHr) by the pump also does not change with altitude.

Altitude does, however, affect NPSH available (NPSHa)

NPSHa = P_a - P_v + H_s - H_f
9.8 x SG 9.8 x SG

Where:

Pa = atmospheric pressure in kpaA
Pv = vapour pressure of liquid in kpaA
Hs = static height from water level to pump centreline (on suction side) in metres
Hf = friction loss is suction pipework in metres

Therefore, as atmospheric pressure is decreased at higher altitudes, the NPSHa reduces.

This will not create any pump problems provided the NPSHa at altitude is greater than the NPSHr by the pump. A typical minimum margin is 0.5 metres.