Reference Manual

4−8
F
F
= the liquid critical pressure ratio factor. Can be
obtained from the following equation:
F
F
+ 0.96 * 0.28
P
v
P
c
Ǹ
K
c
: Cavitation coefficient. A valve parameter
dependent upon valve style and trim. It predicts
the beginning of cavitation related damage and
vibration problems for a particular valve/trim style.
DP
Cavitation
+ K
c
(P
1
* P
v
)
A
r
: Application ratio. A cavitation index which is
dependent upon the actual service conditions. It
indicates the presence of flashing or potentially
cavitating services.
A
r
+ (DP
Flow
)ń(P
1
* P
v
)
K
i
: Incipient cavitation coefficient. A valve
parameter which predicts the point of initial
generation and collapse of vapor bubbles.
(Specific values of K
i
are generally not available).
DP
IncipientCav.
+ K
i
(P
1
* P
v
)
Valve Selection Coefficient Criteria and
Selection Procedure
1. Determine ΔP
Flowing
(ΔP
Flow
)
2. Calculate A
r
a. If A
r
≥ 1.0, the service is flashing.
b. If A
r
≤ 1.0, the service is potentially
cavitating.
3. Use ΔP
Flow
and A
r
in conjunction with the K
c
values of valve trim ΔP limits and K
c
indices, as
well as other valve selection criteria (P
1
, temp.,
style, etc.), to make a valve selection.
The cavitation coefficient (K
c
) is based upon valve
type and pressure drop limit. Select a valve/trim
that has a ΔP limit higher than the service ΔP
Flow
and a K
c
higher that the service A
r
.
Application Guidelines
Guidelines (including A
r
and K
c
ratios) were
developed to help select the proper valve
construction when cavitation is present. These
guidelines are intended to provide valve selections
free of cavitation related material and vibration
damage over the long term. The guidelines do not
indicate an absence of cavitation. Thus, noise due
to cavitation may still be present. If noise is a
concern, use hydrodynamic noise prediction to
assist in selecting a valve.
The following restrictions apply to these
guidelines:
D Water only
D Customer requirements that may require use
of different guidelines
Examples:
D Long maintenance intervals
D Very low noise requirements
D Different fluids
D Corrosive an/or erosive environment
D Installation limitations
D Valve usage rate
These guidelines will aid in selecting a valve and
trim designed to help prevent cavitation damage
and thus offer long term valve life in potentially
cavitating services.
For detailed cavitating service valve selection
guidelines, please contact your local sales office.
Additional Guidelines
D For all valve styles and sizes, applying
backpressure to the valve can eliminate cavitation.
This solution is most effective when the service
conditions do not vary widely.
D Fluids information:
— Cold water is the most common problem
fluid.
— Pure component fluids, similar to water,
can also cause problems.
— Fluid mixtures, like that of pulp stock, can
be less damaging even when the numbers
indicate cavitation is present. Experience is
most useful here.
These guidelines have been constructed from a
broad base of experience. There are undoubtedly
exceptions to these guidelines and, as always,