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Considerations of pressure parameters vs. trim fluid velocity in valve liquid applications.
When selecting control valves, is the efficiency of the pressure parameter more important than valve trim fluid velocity? There's been some important debate on this point recently in the valve manufacturing community.
Although the trim exit velocity needs to be considered in control valve selection, it does not describe the whole physical phenomena that occur inside a control valve. Hence, the use of the trim velocity approach may not be reliable in solving many of the problems that may occur in a control valve and, in many cases, may not provide the most economical solution.
Cavitation
In a control valve handling a pure liquid, cavitation may occur if the static pressure of the flowing liquid decreases to a value less than the fluid vapor pressure. At this point, continuity of flow is broken by the formation of vapor bubbles. Since all control valves exhibit some pressure recovery, the final downstream pressure is generally higher than the orifice throat static pressure. When downstream pressure is higher than vapor pressure of the fluid, the vapor bubbles revert back to liquid. This two-stage transformation is defined as cavitation.
The implosion of the vapor bubbles can cause local pressure waves up to 100,000 psi. Also, during cavitation, fluid micro-jets are formed due to asymmetrical bubble collapse. The combination of the high intensity pressure waves and micro-jet impingement on valve surfaces can cause severe damage. Cavitation damage leads to rapid deterioration of the valve plug and seat and causes valve body damage as well (see photo). It also may lead to noise and vibration problems and poses a potential safety hazard. Therefore, it is necessary to understand and to prevent this phenomenon, particularly when high-pressure drop conditions are encountered.
Three parameters should be considered for predicting cavitation in a control valve: valve inlet pressure (P1), the valve outlet pressure (P2), and liquid vapor pressure...