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4.5 Nominal size selection calculation formula
An appropriate nominal size can be calculated by obtaining Cv value for the operating conditions
in question, as shown below.
• Calculation formula for Cv value
P
1
P
2
2
P
1
P
2
2
● Rated Cv value table
Nominal size
Cv value
[Example]
The flow rate of the pressure reducing valve is calculated under the following conditions: Nominal
size 15A, saturated steam, Inlet pressure of 0.6 MPa = 0.7 [MPa A], Reduced pressure of 0.4 MPa
= 0.5 [MPa A], you can find that Cv value of 15A is 5.0 from the chart. The following formula is
established.
P
=
W
* Use a safety factor of 80 to 90% for nominal size selection in view of friction losses in piping.
Wk
=
Cv
+
138
P(P
P
1
2
Wk
Cv =
120P
1
15A
20A
25A
5.0
7.2
10.9
=
P
(
. 0
1
=
(
. 0
) 5
2
2
+
138
Cv
P
(
P
P
1
2
k
)
32A
40A
50A
14.3
18.8
32
) 7
)
138
5
0 .
=
9
W: Max. steam flow rate [kg/h]
P
: Inlet pressure [MPa･A]
1
P
: Reduced pressure [MPa･A]
2
ΔP: P
- P
[MPa]
1
2
k: 1+0.0013× {superheated steam
temp.[°C] - saturated
steam temp. [ °C]}
65A
80A
100A
54
70
108
+
0
2 .
0 (
7 .
. 0
) 5
=
1
338
kg /
h
EPDT-021a
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