Honeywell MS4103 Manual Do Utilizador
261
APPENDIX
Select a linear valve providing close control with a capacity
index of 4 and meeting the required pressure and tempera-
ture ratings.
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.
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.
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 Cv formula to determine capacity index
for Valve V1 as follows:
Where:
Q
Q
=
Quantity of steam required to pass
through the valve is 750 pounds per hour.
through the valve is 750 pounds per hour.
h
=
The pressure drop across a valve in a
modulating application is found using:
modulating application is found using:
h
=
80% x (Pm – Pr)
and:
Pm
Pm
=
Upstream pressure in supply main is 5
psig.
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.
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:
mula:
h
critical
= 50% x (psig + 14.7 psi)
hcritical= 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 Cv, since it is less than the critical pressure drop
(hcritical) of 9.9 psi.
the Cv, since it is less than the critical pressure drop
(hcritical) of 9.9 psi.
V
=
Specific volume (V) of steam, in cubic feet
per pound at average pressure in valve
(P
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 =
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 Cv formula shows that Valve V1
and pressure drop in the Cv formula shows that Valve V1
should have a Cv of 24.17 or the next higher available value
(e.g., 25).
NOTE: If Pavg is rounded off to the nearest value in
Table 5 (2 psi), the calculated Cv 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.
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 Cv
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 Cv
becomes:
VALVE VI
C2336
5 PSI
SUPPLY
1.96 PSI
(VACUUM)
RETURN
STEAM
COIL
30% PRESSURE DROP, Cv = 41
80% PRESSURE DROP, Cv = 25
Cv
1 0.00075s
+
(
)Q V
63.5 h
--------------------------------------------------
=
Pm h
2---
–
5 5.6
2
-------
–
5 2.8
–
2.2psig
=
=
Cv
1 0.00075 0
×
+
(
) 750 4.85
×
×
63.5 5.6
---------------------------------------------------------------------------
=
3637.5
63.5 2.37
×
--------------------------- 24.17
=
=
Cv
1 0.00075s
+
(
)Q V
63.5 h
--------------------------------------------------
=
Cv
750 4.85
×
63.5 2
-------------------------
40.5
=
=
Appendix A: Valve Selection and Sizing