Sizing - Pietro Fiorentini

Cg and Kg valve coefficient
Sizing the Control Valve
Choise of the valve is usually on the basis of Cg valve and Cg flow rate coefficients.Cg coefficient corresponds
numerically to the value of air flow in SCF/H in critical conditions with full open valve operating with an upstream
pressure of 1 psia at a temperature of 15°C.KG. coefficient corresponds numerically to the value of natural gas
flow rate in Stm/h in critical conditions with full open valve operating with an upstream pressure of 2 bar abs at a
temperature of 15°C. Flow rates at full open position and various working conditions, are bound by the following
formule where:
Pu = inlet pressure in bar (abs) Q = flow rate in Stm/H
Pd = outlet pressure in bar (abs) KG, Cv, Cg = valve coefficent
1 > When the Cg and KG values of the control valve are known, as well as Pu and Pd, the flow rate can be
calculated as follows:
1.1 > in non critical conditions:
1.2 > in critical conditions:
Reflux 919 - Syncroflux - VLM
Q = KG (Pu - Pd) Pd Q = 16,8 x Cv x Pu x sin 106,78
KG
Q x
= 2
2 > Vice versa, when the values of Pu, Pd and Q are known, calculate the values of Cv, Cg or KG with:
KG =
Q
Pd ( Pu - Pd )
Cv =
Pu - Pd
Pu Q= 16,8 x Cv x Pu Q= 0,526 x Cg x Pu (valid for Pu ≥ 2 x Pd)
Q
.16,8x
Pux
sinx
106,78
Pu - Pd
((
Pu
(valid for Pu < 2 x Pd)
2.2 > in critical conditions: x Q
KG =
(valid for Pu ≥ 2 x Pd)
Pu
2
Q
Q
Cv =
Cg =
16,8 x Pu
0,526 x Pu
Pu
(valid for Pu < 2 x Pd)
Q
A oversizing of 20% on calculated values is raccomanded. Cg formulae give flow rate values more correct while
KG formulae give values 5% higher than real ones only in noncritical conditions. In the case of noise limitation level
a speed at the outlet flange of 130 m/sec. it is also raccomanded. Above formulae are valid for natural gas with a
relative specific gravity of 0,61 compared to air and temperature of 15° C at inlet. For gases with different relative
specific gravity (S) and temperature t (in °C) ), value of flow rate calculated as above, must be adjusted multiplying
by:
175.8
Fc =
S x ( 273.15 + tu )
Reflux 919 - Syncroflux - VLM
Nominal diameter (mm)
Size (inches)
Cg flow coefficient
KG flow coefficient
Cv flow coefficient
Q = 0,526 x Cg x Pu x sin
25
1"
575
605
18
50
2"
2200
2335
69
106,78
80
3"
4937
5194
154
Pu - Pd
100
4"
8000
8416
250
Pu
Cg =
150
6"
16607
17471
519
200
8"
25933
27282
810
250
10"
36525
38425
1141
( (
( (
0,526 . xPux
sinx
106,78
Pu - Pd
((
Pu