Basic Documentation

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Valve Sizing Formulas
Valve Reference
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
1
— P
2
at maximum ow, psi
Q Fluid ow, U.S. GPM
Qa Air or gas ow, standard cubic feet per hour (SCFH) at 14.7 psi
and 60°F
W Steam ow, pounds per hour (lb./hr.)
S Specic gravity of uid relative to water @ 60°F
G Specic gravity of gas relative to air at 14.7 psi and 60°F
T Flowing air or gas temperature (°F)
K 1 + (0.0007 x °F superheat), for steam
V
2
Specic volume, cubic feet per pound, at outlet pressure P
2
and
absolute temperature (T + 460)
K
r
Viscosity correction factor for uids (See Page I-4)
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 gases
(air, natural gas, propane, etc.):
Cv= Qa G(T+460)
1360 ∆P(P
2
)
Cv= Qa G(T+460)
660 P
1
Use this when P
2
is greater than
1/2P
1
.
Use this when P
2
is less than or
equal to 1/2P
1
.
3. 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
.
4. For vapors other than steam:
Cv= WK
63.4
When P
2
is less than or equal to
1/2P
1
, use the value of 1/2P
1
in place of P and use P
2
corresponding to 1/2P
1
when
determining specic volume V
2
.
S
∆P
S
∆P
V
2
∆P