Understanding cavitation: friend or foe?

We are going to discuss the destructive side of cavitation, a dynamic force almost always unwanted and unexpected.

09/15/2014


Courtesy of Spirax SarcoWe are going to discuss the destructive side of cavitation, a dynamic force almost always unwanted and unexpected.

To start, let’s repeat the definition of cavitation: the creation of a vapor bubble through a change in energy level in a liquid and then a rapid collapsing of that same bubble with additional changes in energy level. On the dark side of cavitation, the energy level required to cause the cavitation happens usually by the motion of liquid in transporting of the fluid. We are moving liquid from point A to point B. The possibility of creating cavitation is probably the last thing on our mind but it becomes very important when a centrifugal pump or a control valve fails due to unintended damage caused by cavitation, mostly through breaking off fragments of metal from the pump or valve.

To understand the cause of these unintended consequences, we need to understand Bernoulli’s principle, for it is the cause for most of the problems in unintended cavitation generation. Bernoulli’s principle states that the pressure of a fluid decreases as the velocity increases and visa-versa. This is a pretty simple statement, but what does it mean, and how does it relate? The key component here, relative to cavitation, is the change of pressure (caused by the changes in fluid velocity). If the pressure falls below the vapor pressure of the liquid it undergoes a phase change and becomes a vapor. If the pressure then increases to a point above the vapor pressure, the bubble collapses. It is the collapsing of the bubble that is destructive. Actually the bubble collapses with a violent implosion. If the violent implosion happens in the middle of a mass of fluid such as the pot on your stove, nothing happens except for some high pitched sounds. If however, the violent implosion occurs against the side of a piece of metal on a continuous basis and over a period of time, severe and costly metal damage will occur.

It is a bit hard to imagine how liquid and vapor can destroy metal. Try imagining the combination of implosion, corrosion and erosion. The violent cavitation implosion occurring against a piece of metal creates high frequency vibrations and eventually causes micro-fissures in the surface of the metal. It also removes the microscopic coating on the metal which protects the parent metal. Examples include iron oxide or rust on a carbon steel surface. When the oxide is removed by cavitation more oxide is created from the parent metal and repeats over and over until there is no more parent metal left. The micro-fissures also continue to grow until the surface becomes metallic sponge-like. At this point, chunks of the weakened, spongy metal may be removed by the still violent, continuous implosion. The result is the removal of metal to the point that the equipment no longer functions and must be repaired or replaced.

Let us first look at centrifugal pump cavitation. Cavitation will occur at the inlet to the pump, at the eye of the impeller, if the Net Positive Suction Head (NPSH - the net fluid pressure at the inlet) is lower than the vapor pressure of the liquid. The eye of the impeller adds rotational velocity to the fluid and Bernoulli’s Principal shows additional pressure reduction at that point. A vapor bubble will form at the eye of the impeller and start the cavitation process. As the vapor bubble moves radially with the flow of liquid, the pressure of the liquid/vapor increases due to centrifugal force. When the liquid pressure exceeds the vapor pressure of the fluid, a violent cavitation implosion will occur against the impeller and ultimately destroy the impeller if not rectified in a timely manner.

Prevention of cavitation in a centrifugal pump starts with obtaining a pump curve from the manufacturer and determining the NPSH required for the liquid flow rate. Then the piping and feed system are designed to so that the NPSH available (as determined by the design) is greater than the NPSHR. Alternately, one picks a pump that has a lower NPSHR then is available in the piping system (NPSHA) where the piping system already exists. Centrifugal pumps have different NPSHRs and vary with motor speed. The proper piping/system design or the right selection of pump will go far to render your pumping system to run smooth. The detail of the calculation of NPSHA (available) is a study in itself and is available elsewhere. The importance of this article is to make you aware that ignoring potential cavitation in centrifugal pumps amounts to the roll of the dice.

Cavitation in liquid control valves also may have dire consequences with self-destruction. Liquid flowing up to the entrance of a control valve travels at a given velocity. The velocity then increases as the fluid passes through the seat and plug, with the velocity then going back to its original velocity (assuming the inlet and outlet piping size is the same) after flowing out of the valve. Once again, applying Bernoulli’s principal, it indicates that the pressure will drop as the liquid passes through the annulus of the plug/seat, with the reduced cross sectional area. If the liquid pressure at that point drops below the vapor pressure of the liquid, a vapor bubble will form and the first part of cavitation will occur. As the pressure recovers after the liquid passes through the valve, the pressure of the liquid vapor stream will rise above the vapor pressure of the liquid and the bubble will collapse with a violent implosion. The damage cycle is similar to cavitation occurring in a centrifugal pump.

Standard mathematical approaches provided by valve manufacturers will help you predict cavitation problems and help you avoid them with the proper valve selection. The important point is to be aware of the possible dangers of cavitation occurring in a liquid control valve and getting help from the valve manufacturer for proper selection.

We have been dealing here mainly with cavitation that has fairly small vapor bubbles and its subsequent collapse but has much destructive power in its small package. There are other forms of this type of destructive cavitation but the two discussed here are the most common. Space here does not allow the delving into macro-cavitation nor vapor column separation. This is the stuff that creates major waterhammer and the destruction of pipe.

So, is cavitation a friend or foe? The answer can be either. If you want cavitation for cleaning purposes, for example, then you induce pressure fluctuation through high frequency vibrations, a friend. If one ignores the potential negative implications in a liquid system with self-induced pressure fluctuation then you have a foe, for sure. Intelligence can always overcome brute force.

Content provided by Spirax Sarco, originally published in Steam News. Edited by Anisa Samarxhiu, Digital Project Manager, CFE Media, asamarxhiu@cfemedia.com 



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