Harper International Article_6272: Cavitation

The Cavitation Challenge

 It sounds like a term a dentist would use, “cavitation,” and in a way it’s rather like the destructive effect of tooth decay. But in a water distribution system, cavitation destroys valves, slowly over time, and like tooth decay, you might never know until it becomes very painful and expensive to fix.

While relatively harmless at lower flows and pressures, cavitation can be extremely damaging when it affects valves that are required to undergo extreme pressure differentials and/or high velocity flow conditions. These types of conditions create large changes in pressure and flow, and create thousands of destructive vapor bubbles that collapse rapidly. When this occurs in close proximity to any metal surface, damage to the valve and related piping can take place and – over time – will result valve failure (see photo above).

In addition, the condition creates excessive noise and vibration, and it can cause hearing loss in plant personnel if they are subjected to it for extended periods of time.

There are solutions: Valves installed in series to gradually step down the pressure are effective, as are single valves equipped with anti-cavitation trim, and reduced ported valves. All are effective, but only careful analysis and skillful specification can pinpoint which valve or valve combination will be best suited to combat cavitation while providing optimum flow control under the system’s specific operating conditions.

The standard approach to fight cavitation is to use valve/orifice plate combinations or multiple valves installed in series to achieve pressure reduction. In some cases, a valve equipped with anti-cavitation trim will be the preferred method to eliminate the potential for noise and damage. Finally, a reduced ported (600 Series) valve with a configuration designed to keep cavitation from damaging the valve is available as a third solution to this problem.

There are different advantages to each option. When a pressure drop is too high and is permitting cavitation to form within a single valve, a two-stage pressure reduction lowers the pressure across each valve and eliminates the condition of cavitation . In the event of any one valve failure, partial pressure reduction is still available.

The use of one anti-cavitation trimmed valve, offers virtually cavitation free operation; along with reduced noise and vibration. This trim, constructed of 316 stainless steel uses cast radial slots (for which there is a patent pending) to create a larger flow-path than is possible with drilled holes, typically employed by other valves on the marketplace. It is important to note, that when the cavitation trim is used, compared to the standard full ported valve, the flow factor (Cv) is somewhat reduced, thereby limiting the total flow for the valve. It is critical to carefully select not only an appropriate valve size but one that offers the correct flow factor for the application. A final installation note: A valve with caviation trim in use is visually identical to standard valves and require extra care in installation so that they don’t end up in the wrong place.

The 600 Series Hytrol Valve may be used where higher flows are required, but cavitation trim may restrict flow too much, and single-stage pressure reduction is still desirable. It has a flow factor that is higher than the cavitation trim, but lower than the full ported hytrol. It’s internal porting allows for greater clearances during the formation of cavitation, away from the valve body, providing a higher level of cavitation resistance than it’s full ported cousin.

With the implementation of Cla-Val’s cavitation analysis software, CLA-CAV, it is possible to identify the most correct valve for the application in question.