Pipeline Pump Installation Height and NPSH: A Worked Example

A pipeline pump is not self-priming, and it can only lift water a limited distance on its suction side. With hot water, it may need to sit below the water level. The figure that decides this is NPSHr, which our selection table lists for every build.

The simple estimate

At sea level the atmosphere can support about 10.3 m of water. Take away the pump’s NPSHr, the suction pipe losses, the vapour pressure of the liquid and a safety margin of about 0.5 m. What is left is the most the pump can sit above the water surface.

Example 1: water at 20 °C

Pump 65-160, NPSHr 2.5 m. Suction pipe losses 1.0 m. Vapour pressure of water at 20 °C is about 0.24 m. Height = 10.3 − 2.5 − 1.0 − 0.24 − 0.5 ≈ 6.0 m above the water.

Example 2: the same pump on water at 80 °C

Water at 80 °C has a vapour pressure of about 4.8 m of head. Height = 10.3 − 2.5 − 1.0 − 4.8 − 0.5 ≈ 1.5 m. The allowed lift has fallen by more than three-quarters. Much hotter, and the pump must sit below the water surface.

Other things that reduce it

• Altitude: the atmosphere supports less water the higher the site
• Long or narrow suction pipe, many bends, a partly closed valve
• Running far to the right of the duty point, where NPSHr rises

NPSHr for every build is in the selection table.