Filter
Top Products
260
2. Determine the critical pressure drop:
h
critical
= 50% x Pma
Where:
Pma = Pressure in supply main in psia (absolute
pressure)
psia = psig + 14.7
Use the smaller value h or h
critical
when calculating C
v
.
Two-Position Applications
Use line sized valves whenever possible. If the valve size must
be reduced, use:
h = 20% x (Pm-Pr)
Where
Pm = Pressure in supply main in psig or psia
(gage or absolute pressure).
Pr = Pressure in return in psig or psia. A
negative value if a vacuum return.
STEAM VALVE SIZING EXAMPLES
EXAMPLE 1:
A two-way linear valve (V1) is needed to control high-pres-
sure steam flow to a steam-to-water heat exchanger. An
industrial-type valve is specified. Steam pressure in the sup-
ply main is 80 psig with no superheat, pressure in return is
equal to atmospheric pressure, water flow is 82.5 gpm, and
the water temperature difference is 20F.
Use the steam valve C
v
formula to determine capacity index
for Valve V1 as follows:
Where:
Q = The quantity of steam required to pass
through the valve is found using the
converter valve formula:
Where:
gpm = 82.5 gpm water flow through exchanger
TD
w
= 20F temperature difference
0.49 = A scaling constant
Substituting this data in the formula:
Q = 808.5 pounds per hour
h = The pressure drop across a valve in a
modulating application is:
h = 85% x (Pm-Pr)
Where:
Pm = Upstream pressure in supply main is 80
psig.
Pr = Pressure in return is atmospheric
pressure or 0 psig.
Substituting this data in the pressure drop formula:
h = 0.80 x (80 – 0)
= 0.80 x 80
= 64 psi
The critical pressure drop is found using the following for-
mula:
h
critical
= 50% x (psig + 14.7 psi)
h
critical
= 0.50 x (80 psig upstream + 14.7 psi)
= 0.50 x 94.7 psi
= 47.4 psi
The critical pressure drop (h
critical
) of 47.4 psi is used in cal-
culating C
v
, since it is less than the pressure drop (h) of 64
psi. Always, use the smaller of the two calculated values.
V = Specific volume (V) of steam, in cubic
feetper pound at average pressure in
valve (P
avg
):
Pavg =
=
The specific volume of steam at 56.4 psig is 6.14 and
the square root is 2.48.
63.5 = A scaling constant.
Substituting the quantity of steam, specific volume of steam,
and pressure drop in the C
v
formula shows that the valve
should have a C
v
of 4.6.
NOTE: If P
avg
is rounded off to the nearest value in
Table 5 (60 psi), the calculated C
v
is 4.5 a negli-
gible difference.
C
v
1 0.00075s+()QV
63.5 h
--------------------------------------------------=
Q gpm TD
w
× 0.49×=
Pm
h
2
---–
80
47.4
2
----------– 80 23.6– 56.4psig==
C
v
1 0.00075 0×+()808.5 2.48××
63.5 47.4
-------------------------------------------------------------------------------=
1745.6
63.5 6.88×
--------------------------- 4.6==
Appendix A: Valve Selection and Sizing