Valve Magazine - Summer 2009

CONTROLLING CAVITATION

pressure and velocity. This is accounted for in Bernoulli’s energy equation and also satisfies the continuity equation for conservation of mass. As a fluid passes from a large flow area to a smaller area, the pressure will drop and the velocity will increase. As the fluid exits a small area and enters a larger area, the pressure partially recovers as the velocity decreases. Each time this occurs, energy is lost to friction, resulting in lower and lower final pressures upon each subsequent expansion.

Figure 6. Typical valve recovery coefficients. Values will vary with specific makes, models and percentage open.

Values apply to water at nominal test pressures. Scale factors are not included here. ** Choking may not occur, or valve may be designed to operate choked. Note: These values are supplied by the manufacturer of the valve types used for this illustration.

holes. This method can only control low levels of cavitation and does not eliminate it. The resulting collision of the streams causes the cavitation bubbles to implode in the middle of the flow away from the cage and control surfaces. The surrounding liquid absorbs

much of the bubble collapse energy.

Sudden Expansion and Contraction – With every sudden change, either through an expansion or a contraction in volume, energy losses occur as total energy is converted back and forth between

Frictional Losses in Small Passages – When a fluid flows through many small passages, the boundary surface area that a viscous fluid comes in contact with increases friction near the boundary surface, which can cause significant energy to be lost, resulting in additional pressure reduction.

Turbulent Mixing – When two or more streams converge from wide (oblique) angles, considerable energy is lost in the

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