6
■EPDT-179c■
5.4.2 Calculating a nominal size
Determine the required Cv value by using the formula shown below and then the
corresponding nominal size based on the Cv value.
To calculate a Cv value (for selecting nominal size when valve-inlet pressure is 2.5 kPa up
to 200 kPa):
If P
2
>P
1
/2, If P
2
≤
P
1
/2,
94
.
2
)
P
P
(
P
G
)
t
273
(
Q
Cv
2
1
+
+
=
Δ
Δ
Δ
Δ
1
P
55
.
2
G
)
t
273
(
Q
Cv
+
=
P
1
：
Inlet pressure
(kPa
･
A)
Q
：
Max. gas flow rate (Nm
3
/h)
P
2
：
Reduced pressure
(kPa
･
A)
t
：
Gas temperature
(
℃
)
∆
P
：Ｐ
1
－Ｐ
2
(kPa)
G
：
Specific gravity (relative to air)
Cv
：
Cv value for a specific nominal size
Nominal Size
15A
20A
25A
Cv value
1.5
2.0
3.0
For selecting a nominal size in the case of inlet pressure is in a range 200 kPa up to 400 kPa,
refer to Table 1.
