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Saturday, May 10, 2008

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The Net Positive Suction Head (NPSHa) available to a centrifugal pump is a function of suction liquid operating pressure, fluid vapor pressure, fluid temperature, fluid elevation at suction drum, fluid dynamic at pump suction, suction pressure drop, etc. Increases NPSHa would reduce the potential of cavitation in pump.

Following equation define NPSHa :

NPSHa = Hp + Hs - Hf - Hv - Hvp

where
Hp - pressure head
Hs - static head gain
Hf - frictional loss
Hv - velocity head
Hvp - vapor pressure head

From above equation, there are few ways to increase NPSHa :
a) Increase suction line size to reduce frictional loss
b) Rearrange and /or redesign suction pipe work to minimise bends, valves and fittings
c) Raise suction vessel to increase static head
d) Increase & maintain pressure in suction vessel to increase suction pressure head
e) Reduce fluid vapor pressure i.e. subcool fluid by dropping it temperature
f) Increase suction velocity to increase suction velocity head
Step (a) to (c) are common apply to raise NPSHa as they can be implemented easily. Step (d) & (e) involve new equipment & control devices and directly increase CAPEX and OPEX of a project. Generally not advisable to apply unless all efforts are implemented.

Increasing suction velocity would increase suction velocity head. However, it potentially lead to higher friction loss. Generally the suction velocity head is very small as compare to increases in frictional loss.


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posted by Webworm, 2:19 PM

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