Reference Manual

4−3
Figure 4-3. Typical Cavitation Damage
W1350
where:
F
L
= K
m
Ǹ
F
F
= rc
N
1
= units factor
Cavitation
Closely associated with the phenomenon of
choked flow is the occurrence of cavitation. Simply
stated, cavitation is the formation and collapse of
cavities in the flowing liquid. It is of special
concern when sizing control valves because if left
unchecked, it can produce unwanted noise,
vibration, and material damage.
As discussed earlier, vapor can form in the vicinity
of the vena contracta when the local pressure falls
below the vapor pressure of the liquid. If the outlet
pressure of the mixture is greater than the vapor
pressure as it exits the valve, the vapor phase will
be thermodynamically unstable and will revert to a
liquid. The entire liquid-to-vapor-to-liquid phase
change process is known as “cavitation,” although
it is the vapor-to-liquid phase change that is the
primary source of the damage. During this phase
change a mechanical attack occurs on the
material surface in the form of high velocity
micro-jets and shock waves. Given sufficient
intensity, proximity, and time, this attack can
remove material to the point where the valve no
longer retains its functional or structural integrity.
figure 4-3 shows an example of such damage.
Cavitation and the damage it causes are complex
processes and accurate prediction of key events
such as damage, noise, and vibration level is
difficult. Consequently, sizing valves for cavitation
conditions requires special considerations.
Figure 4-4. Comparison of High and Low Recovery Valves
A3444
The concept of pressure recovery plays a key role
in characterizing a valve’s suitability for cavitation
service. A valve that recovers a significant
percentage of the pressure differential from the
inlet to the vena contracta is appropriately termed
a high recovery valve. Conversely, if only a small
percent is recovered, it is classified as a low
recovery valve. These two are contrasted in figure
4-4. If identical pressure differentials are imposed
upon a high recovery valve and a low recovery
valve, all other things being equal, the high
recovery valve will have a relatively low vena
contracta pressure. Thus, under the same
conditions, the high recovery valve is more likely
to cavitate. On the other hand, if flow through
each valve is such that the inlet and vena
contracta pressures are equal, the low recovery
valve will have the lower collapse potential
(P
2−
P
vc
), and cavitation intensity will generally be
less.
Therefore, it is apparent that the lower pressure
recovery devices are more suited for cavitation
service.
The possibility of cavitation occurring in any liquid
flow application should be investigated by
checking for the following two conditions:
1. The service pressure differential is
approximately equal to the allowable pressure
differential.
2. The outlet pressure is greater than the vapor
pressure of the fluid.