ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
449
Vacuum,
Inches of
Mercury
Boiling
Point or
Steam
Temp-
erature
Deg F
Specific
Volume
(V),
cu. ft/lb
å…¬
V
(For
valve
sizing)
Maximum
Allowable
Pressure
Drop, psi.
29
25
20
15
14
76.6
133.2
161.2
178.9
181.8
706.00
145.00
75.20
51.30
48.30
26.57
12.04
8.672
7.162
6.950
0.23
1.2
2.4
3.7
3.9
12
10
8
6
4
2
187.2
192.2
196.7
201.0
204.8
208.5
43.27
39.16
35.81
32.99
30.62
28.58
6.576
6.257
5.984
5.744
5.533
5.345
4.4
4.9
5.4
5.9
6.4
6.9
Gage
Pressure,
psig
0
1
2
3
4
212.0
215.3
218.5
221.5
224.4
26.79
25.20
23.78
22.57
21.40
5.175
5.020
4.876
4.751
4.626
7.4
7.8
8.4
8.8
9.4
5
6
7
8
9
227.1
229.8
232.3
234.8
237.1
20.41
19.45
18.64
17.85
17.16
4.518
4.410
4.317
4.225
4.142
9.8
10.4
10.8
11.4
11.8
10
11
12
15
20
239.4
241.6
243.7
249.8
258.8
16.49
15.90
15.35
13.87
12.00
4.061
3.987
3.918
3.724
3.464
12.4
12.8
13.4
14.8
17.4
25
30
35
40
45
266.8
274.0
280.6
286.7
292.4
10.57
9.463
8.56
7.826
7.209
3.251
3.076
2.93
2.797
2.685
19.8
22.4
24.8
27.4
29.8
50
55
60
65
70
297.7
302.6
307.3
311.8
316.0
6.682
6.232
5.836
5.491
5.182
2.585
2.496
2.416
2.343
2.276
32.4
34.8
37.4
39.8
42.4
75
80
85
90
95
320.0
323.9
327.6
331.2
334.6
4.912
4.662
4.445
4.239
4.060
2.216
2.159
2.108
2.059
2.015
44.8
47.4
49.8
52.4
54.8
100
110
120
130
140
337.9
344.1
350.0
355.2
360.9
3.888
3.595
3.337
3.12
2.923
1.972
1.896
1.827
1.766
1.710
57.4
62.3
67.4
72.3
77.4
150
160
170
180
190
366.2
370.6
375.5
379.6
383.9
2.746
2.602
2.462
2.345
2.234
1.657
1.613
1.569
1.531
1.495
82.3
87.4
92.3
97.4
102.3
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).
C
v
=
(1 + 0.00075 x 0) x 750 x
63.5 5.6
=
3637.5
63.5 x 2.37
= 24.17
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
temperature 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
characteristic. 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:
C
v
=
(1 + 0.00075s)Q V
63.5 h
C
v
=
750 x 4.85
63.5 2
= 40.5
This larger valve (2 psi drop) has a steeper curve that is
further away from the desired linear valve characteristic.
See LINEAR VALVE under VALVE SELECTION for more
information.
Fig. 20. Effect of Pressure Drop in Steam Valve Sizing.
100%
0% 100%
STEM TRAVEL
VALVE OPENING/
STEAM FLOW
C2337
Cv = 41
Cv = 25
LINEAR VALVE
CHARACTERISTIC
(continued)
Table 5. Properties of Saturated Steam.
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