Reference Manual

3−9
Figure 3-3. Liquid Critical Pressure Ratio Factor for Water
For valves installed with fittings attached:
DP
max(LP)
+
ǒ
F
LP
F
P
Ǔ
2
ǒ
P
1
* F
F
P
V
Ǔ
(29)
where,
P
1
= Upstream absolute static pressure
P
2
= Downstream absolute static pressure
P
v
= Absolute vapor pressure at inlet temperature
Values of F
F
, the liquid critical pressure ratio
factor, can be obtained from figure 3-3 or from the
following equation:
F
F
+ 0.96 * 0.28
P
v
P
c
Ǹ
 (30)
An explanation of how to calculate values of F
LP
,
the recovery factor for valves installed with fittings
attached, is presented in the preceding procedure
determining q
max
(the maximum flow rate).
Once the ΔP
max
value has been obtained from the
appropriate equation, it should be compared with
the actual service pressure differential (ΔP = P
1
−
P
2
). If ΔP
max
is less than ΔP, this is an indication
that choked flow conditions will exist under the
service conditions specified. If choked flow
conditions do exist (ΔP
max
< P
1
− P
2
), then step
five of the procedure for sizing valves for liquids
must be modified by replacing the actual service
pressure differential (P
1
− P
2
) in the appropriate
valve sizing equation with the calculated ΔP
max
value.
Note: Once it is known that choked flow conditions
will develop within the specified valve design
(ΔP
max
is calculated to be less than ΔP), a further
distinction can be made to determine whether the
choked flow is caused by cavitation or flashing.
The choked flow conditions are caused by flashing
if the outlet pressure of the given valve is less than
the vapor pressure of the flowing liquid. The
choked flow conditions are caused by cavitation if
the outlet pressure of the valve is greater than the
vapor pressure of the flowing liquid.
Liquid Sizing Sample Problem
Assume an installation that, at initial plant start-up,
will not be operating at maximum design
capability. The lines are sized for the ultimate
system capacity, but there is a desire to install a
control valve now which is sized only for currently