what is NPSH ?

Answer Posted / anand swami

net positive suction head NPSH can be defined as the excess
of inlet total head over the head equivalent to vapour
pressure of pumped liquid.


If water close to boiling point enters a pump, it is liable
to flash to steam at the low pressure area at the eye of the
pump impeller. If this happens, bubbles of steam are formed
as the pressure drops below the water vapour. When the
pressure rises again, these bubbles will collapse and water
flows into the resulting cavity at a very high velocity.

This is known as 'cavitation'; it is noisy and can seriously
damage the pump.

To avoid this problem, it is essential to provide the best
possible Net Positive Suction Head (NPSH) to the pump so
that the static pressure is as high as possible. This is
greatly aided by locating the feedtank as high as possible
above the boiler, and generously sizing the suction pipework
to the feedpump

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