Control Valves and Cavitation

[Esam]

When a fluid passes a valve the fluid velocity increases and the pressure drops according the Bernoulli equation.

[h=Cavitation]3[/h] If the speed through the valve is high enough, the pressure in the liquid may drop to a level where the fluid may start bubble or flash. The pressure recovers sufficiently and the bubbles collapse upon themselves.

Cavitation may be noisy but is usually of low intensity and low frequency. This situation is extremely destructive and may wear out the trim and body parts of the valve in short time.

• The Cavitation Number An introduction to and a definition of the Cavitation Number.
• Cavitation - an Introduction

[Esam]

[h=Application Ratio]3[/h] A common way to characterize potential cavitation conditions is the "applications ratio" (or "the incipient cavitation index") and can be expressed as

AR = p_i - p_o / (p_i - p_v) (1)
where
AR = Application Ratio
p_i = inlet pressure, absolute
p_o = outlet pressure, absolute
p_v = vapor pressure of the fluid, absolute

For application ratios above 1 - the fluid flashes. This is not the same as cavitation, but the closer the ratio is to 1, the higher the potential for cavitation.
Note! With an increasing fluid temperature the possibility for cavitation increases.
[h=Example - Flashing Water]4[/h] If we know the boiling point and the absolute pressure of a fluid (Steam Table with saturated steam properties) the minimum outlet pressure from a valve to avoid flashing can be calculated.
For an application ratio of one, equation (1) can modified to

AR = 1
= p_i - p_o / (p_i - p_v)
or transformed
p_o = p_v

Using "Steam Table" with saturated steam properties we can conclude that

• for a water temperature of 17.51 ^oC and absolute inlet pressure of 1 bar - the minimum outlet pressure is 0.02 bar to avoid flashing
• for a water temperature of 81.35 ^oC and absolute inlet pressure of 1 bar - the minimum outlet pressure is 0.5 bar to avoid flashing
• For a water temperature of 99.63 ^oC and absolute inlet pressure of 1 bar - the minimum outlet pressure is 1 bar to avoid flashing

Note! Flashing is not the same as cavitation. Due to local conditions in a valve cavitation may start on much higher outlet pressures.
[h=Multi Stage Control Valves]3[/h] Cavitation can be avoided by using more than one control valve or more convenient - a multistage control valve.

As illustrated above the "vena contracta" is much lower for a single stage valve than a multi stage valve. Depending on the pressure drop and the temperature of the fluid its possible to avoid cavitation conditions using more than one stage in a valve.