Honeywell AUTOMATIC CONTROL Engineering Manual page 459

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