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Chemical Process Technology

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Friday, May 9, 2008

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Earlier post "How Pump Cavitation Sound and Looks Like ?" and "Why Cavitation is Destructive ?" showed and discussed on how cavitation is formed and how destructive is cavitation. Some images shown impeller damaged by cavitation in "Damages by Cavitation".

This post will look at pressure profile from pump suction to pump discharge and present the relationship between Available Net Positive Suction Head (NPSHa) due to pump system configuration and Required Net Positive Suction Head (NPSHr) as imposed by a pump itself.

Bubble formation occurs at a point where the fluid operating pressure is lower than fluid vapor pressure, and bubble collapse or implosion occurs at a point where the pressure is increased to the vapor pressure. In general, cavitation occur at pump suction with lowest possible operating pressure.

Below figure shows a typical pressure profile in a centrifugal pump. As pumping fluid passing pump, operating pressure drop due to frictional lose (Entrance loss (path A-C). Once the liquid enters pump chamber, it will experience serious turbulence cause by impeller. Major Turbulence Friction Entrance Loss is expected along path C-D. Once the fluid reach point D, impeller generated large centrifugal force and acting on the liquid. The energy is transferred from pump impeller to liquid and increase liquid velocity and operating pressure. As the liquid is leaving the pump exit chamber, fluid velocity is reduced (expansion) velocity head is converted to pressure head base on Bernoulli principle (by Daniel Bernoulli) . This will further increase the fluid operating pressure (path D-E)

Below figure shows as fluid B with low vapor pressure below lowest operating pressure in pump, NO cavitation occur. However, fluid A with high vapor pressure, as the operating pressure lower than fluid vapor pressure bubble form. Once fluid passing the impeller, operating pressure increased will cause bubble collapse (sometime called implosion) once the operating pressure above the vapor pressure. Above phenomenon occur in a very short time and it cause several things happen at once : · Bubbles collapsed when they pass into the higher regions of pressure, causing noise and vibration· Loss in capacity. · No longer build the same head (pressure) · Efficiency drops· Damage to many of the components i.e. chamber, impeller, etc.

One shall understand that pump chamber and impeller design will serious affect the entrance friction loss and turbulence loss caused by impeller. Refer to below figure. Pump A having high entrance friction loss and turbulence loss results cavitation occurred. However, pump B shows low entrance friction loss and turbulence loss, operating pressure is always above vapor pressure and NO cavitation will occur.

Process engineer must always ensure the operating pressure along the pump always higher than fluid vapor pressure. Generally 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).


The following chart illustrate the relationship between NPSHa & NPSHr

As NPSHr is subject physical construction of pump (by manufacturer), it is not much a Process Engineer can do other than specifying the requirement and selection of correct pump. However, Process engineer can put extra effort to increase NPSHa.

Note : Above only illustrates cavitation for flashing perspective.

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posted by Webworm, 3:31 PM


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