Answer Posted / learner
Quoting from "Masoneilan Control Valve Sizing Handbook",
Bulletin OZ1000 7/00, Page 6 and 7.
Cavitation, a detrimental process long associated with
pumps, gains importance in control valves due to higher
pressure drops for liquids and increased employment of
high capacity valves (e.g., butterfly and ball valves).
Cavitation, briefly, is the transformation of a portion of
liquid into the vapor phase during rapid acceleration of
the fluid in the valve orifice, and the subsequent collapse
of vapor bubbles downstream. The collapse of vapor
bubbles can cause localized pressure up to 7000 bar
(100,000 psi) and are singly, most responsible for the
rapid wear of valve trim under high pressure drop
conditions.
Cavitation leads to rapid deterioration of the valve
body plug and seat. It also leads to noise and vibration
problems and as well, poses a potential safety hazard.
It is, therefore, necessary to understand and to prevent
this phenomenon, particularly when high pressure drop
conditions are encountered.
Cavitation in a control valve handling a pure liquid 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 pressure recovery in a valve is a function of its
particular internal geometry. In general, the more
streamlined a valve is, the more pressure recovery is
experienced. This increases the possibility of cavitation.
The pressure drop in a valve at which cavitation is
experienced is termed as critical pressure drop. Full
cavitation will exist if actual pressure drop is greater
than critical pressure drop, and if the downstream pressure
is greater than fluid vapor pressure.
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