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APPENDIX
Select a linear valve providing close control with a capacity
index of 4 and meeting the required pressure and tempera-
ture ratings.
NOTE: For steam valves downstream from pressure
reducing stations, the steam will be superheated
in most cases and must be considered.
EXAMPLE 2:
In Figure 19, a linear valve (V1) is needed for accurate flow
control of a steam coil that requires 750 pounds per hour of
steam. Upstream pressure in the supply main is 5 psig and
pressure in the return is 4 in. Hg vacuum minimum.
Fig. 19. Linear Valve Steam Application.
Use the steam valve C
v
formula to determine capacity index
for Valve V1 as follows:
Where:
Q = Quantity of steam required to pass
through the valve is 750 pounds per hour.
h = The pressure drop across a valve in a
modulating application is found using:
h = 80% x (Pm – Pr)
and:
Pm = Upstream pressure in supply main is 5
psig.
Pr = Pressure in return is 4 in. Hg vacuum.
NOTE: 1 in. Hg = 0.49 psi and 1 psi = 2.04 in. Hg.
Therefore,
4 in. Hg vacuum = –1.96 psig.
h = 0.80 x [5 – (–1.96)]
= 0.80 x 6.96
= 5.6 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 (5 psig upstream + 14.7 psi)
= 0.50 x 19.7 psia
= 9.9 psi
The pressure drop (h) of 5.6 psi is used in calculating
the C
v
, since it is less than the critical pressure drop
(h
critical
) of 9.9 psi.
V = Specific volume (V) of steam, in cubic feet
per pound at average pressure in valve
(P
avg
)
:
Pavg =
=
The specific volume of steam at 2.2 psig is 23.54 and
the square root is 4.85.
63.5 = A scaling constant.
s=0
Substituting the quantity of steam, specific volume of steam,
and pressure drop in the C
v
formula shows that Valve V1
should have a C
v
of 24.17 or the next higher available value
(e.g., 25).
NOTE: If P
avg
is rounded off to the nearest value in
Table 5 (2 psi), the calculated C
v
is 24.30.
Select a linear valve providing close control with a capacity
index of 25 and meeting the required pressure and tempera-
ture ratings.
EXAMPLE 3:
Figure 20 shows the importance of selecting an 80 percent
pressure drop for sizing the steam valve in Example 2. This
pressure drop (5.6 psi) approximates the linear valve charac-
teristic. If only 30 percent of the available pressure drop is
used (0.30 x 6.96 psi = 2.10 psi or 2 psi), the valve C
v
becomes:
VALVE VI
C2336
5 PSI
SUPPLY
1.96 PSI
(VACUUM)
RETURN
STEAM
COIL
30% PRESSURE DROP, Cv = 41
80% PRESSURE DROP, Cv = 25
C
v
1 0.00075s+()QV
63.5 h
--------------------------------------------------=
Pm
h
2
---–
5
5.6
2
-------– 5 2.8– 2.2psig==
C
v
1 0.00075 0×+()750 4.85××
63.5 5.6
---------------------------------------------------------------------------=
3637.5
63.5 2.37×
--------------------------- 24.17==
C
v
1 0.00075s+()QV
63.5 h
--------------------------------------------------=
C
v
750 4.85×
63.5 2
------------------------- 40.5==
Appendix A: Valve Selection and Sizing