Reference Manual

4−6
Figure 4-8. The implosion of cavitation vapor cavities is rapid, asymmetric and very energetic. The mechanics of
collapse give rise to high velocity liquid jets, which impinge on metallic surfaces. Ultimately, the metal fatigues and
breaks away in small pieces.
E0111
For choked flow:
Q
max
+ F
I
C
v
P
1
* F
F
P
v
G
Ǹ
(43)
Valve Material Damage
Cavitation damage is usually the most
troublesome side effect plaguing the control valve
industry. It does not take many examples of such
damage to fully demonstrate the destructive
capabilities of cavitation.
Typically, cavitation damage is characterized by
an irregular, rough surface. The phrase
“cinder-like appearance” is used frequently to
describe cavitation damage. It is discernible from
other types of flow damage such as erosion and
flashing damage which are usually smooth and
shiny in appearance. This next section will deal
with cavitation damage, although most of the
comments can also apply to flashing damage. A
comparison of figures 4-3 and 4-6 illustrates these
differences.
While the results of cavitation damage are all too
familiar, the events and mechanisms of the
cavitation damage process are not known or
understood completely in spite of extensive study
over the years. There is general agreement,
however, on a number of aspects of the process
and consistency in certain observations.
Cavitation damage has been observed to be
associated with the collapse stage of the bubble
dynamics. Furthermore, this damage consists of
two primary events or phases:
1. An attack on a material surface as a result of
cavitation in the liquid.
2. The response or reaction of the material to the
attack.
Any factor that influences either of these events
will have some sort of final effect on the overall
damage characteristics.
The attack stage of the damage process has been
attributed to various mechanisms, but none of
them account for all the observed results. It
appears that this attack involves two factors that
interact in a reinforcing manner:
1. Mechanical attack
2. Chemical attack
There is evidence indicating the almost universal
presence of a mechanical attack component which
can occur in either of two forms:
1. Erosion resulting from high-velocity microjets
impinging upon the material surface.
2. Material deformation and failure resulting from
shock waves impinging upon the material surface.
In the first type of mechanical attack a small,
high-velocity liquid jet is formed during the
asymmetrical collapse of a vapor bubble. If
orientation and proximity of the jets is proper, a
damaging attack occurs on the metal surface as
shown in figure 4-8. This is the most probable
form of mechanical attack. High-speed
cinematography, liquid drop impingement
comparisons, and various analytical studies
support its presence.
The second type of mechanical attack, shock
wave impingement, does not appear to be as