Chemical & Process Technology: Facts About NPSH - Cavitation Even NPSHa More than NPSHr ?

Saturday, September 12, 2009

Facts About NPSH - Cavitation Even NPSHa More than NPSHr ?

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Earlier post "Relationship between NPSHa & NPSHr", Process engineer must always ensure the operating pressure along the pump (from suction to discharge) always higher than fluid vapor pressure. Net positive suction head (NPSH) is used to check if cavitation will occur. Process engineer must always ensure available Net positive suction head (NPSHa) is always higher than pump required Net positive suction head (NPSHr). In recent discussion with some engineers, there was some doubt or confusion on a simple statement. Should pump still cavitate eventhough Net Positive Suction Head available (NPSHa) higher than Net Positive Suction Head required (NPSHr) ?

Yes. Cavitation can exist eventhough NPSHa is above the NPSHr of a centrifugal pump. Based on Hydraulic institute definition of NPSHr, a NPSHr of a pump is the level of NPSHa that three percent (3%) reduction in total discharge head of the pump caused by flow blockage from cavitation vapor in the impeller eye. Pump manufacturers design their pumps based on this definition. There is still situation where head drop is below 3%. Nevertheless, it is believe this definition was based suction energy sufficient low (below 3% head drop) and will not results cavitation which is detrimental to pump internal i.e. impeller.

A few interesting facts about NPSHa and NPSHr

Cavitation occur when NPSHa is above NPSHr, however it is not reach detrimental level
Possible achieve 100% head when NPSHa = 1.05 to 2.5 times NPSHr
Zero cavitation when NPSHa = 2 to 20 times HPSHr, subject to suction energy, present of air, erosive & abrasive material, etc however
Common Zero cavitation when NPSHa = 4 times NPSHr
NPSHa in the range of 1.1-1.3 of NPSHr for low suction energy pump design
NPSHa in the range of 1.3-2.5 of NPSHr for high suction energy pump design
Minimum one (1) meter NPSHa above NPSHr

Above may be used as guildeline in determining margin between NPSHa and NPSHr. However, higher margin shall be included if air and abrasive material is present in the pumping fluid.