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Engineering
G-5
Valve Sizing Formulas
The Most Important Variables to Consider
When Sizing a Valve:
1. What medium will the valve control? Water? Steam?
What effects will specic gravity and viscosity have
on the valve size?
2. What will the inlet pressure be under maximum load
demand? What is the inlet temperature?
3. What pressure drop (differential) will exist across the
valve under maximum load demand?
4. What maximum capacity should the valve handle?
5. What is the maximum pressure differential the valve
must close against?
When these are known, a valve can be selected by
formula (Cv method) or water and steam capacities
tables which can be found in the Valves section, pages
A-6 through A-10. The valve size should not exceed the
line size.
The following denitions apply in the following formulas:
Cv Valve ow coefcient, U.S. GPM with P = 1 psi
P
1
Inlet pressure at maximum ow, psia (abs.)
P
2
Outlet pressure at maximum ow, psia (abs.)
∆P P
1
— P
2
at maximum ow, psi
Q Fluid ow, U.S. ∆M
W Steam ow, pounds per hour (lb./hr.)
S Specic gravity of uid relative to water @ 60°F
K 1 + (0.0007 x °F superheat), for steam
K
r
Viscosity correction factor for uids (See Page G-6)
S
∆P
√
S
∆P
√
Formulas: Remarks:
1. For liquids (water, oil, etc.):
Cv = Q
Cv = K
r
Q
Specic gravity correction
is negligible for water below
200°F (use S=1.0).
Use actual specic gravity
S of other liquids at actual
ow temperature.
Use this for uids with
viscosity correction fact.
Use actual specic gravity
S for uids at actual ow
temperature.
2. For steam
(saturated or superheated):
Cv = WK
2.1
√
∆P (P
1
+ P
2
)
Cv = WK
1.82 P
1
Use this when P
2
is greater
than 1/2P
1
Use this when P2 is less than
or equal to 1/2P
1
Control Valves
Selection and Sizing
Sizing a Valve