Dungs 279042, Instruction Manual, Page: 71 / 72

70 … 72
MC • Edition 05.20 • Nº 279 663
71 … 72
16.2 Flow volume coefficient of the valve K
G
Type of gas
Natural gas
City gas
GLP
Air
dv
0.65
0.47
1.67
1.00
f
1.24
1.46
0.77
1.00
Specific mass of the air
Spec. mass of the gas used
f =
°V
gas used
= V
air
x f
°
Density
[kg/m
3
]
0.81
0.58
2.08
1.24
The maximum flow rate indicated refers to natural gas with a density of 0.81 kg/m
3
or to the air with a density of 1.24 kg/m
3
at 15 °C under normal conditions. In the case of different types of gases, a conversion of the volume flow according to the
equation shown below is carried out.
The flow coefficient of the valve K
G
of the FRM is equal to the flow rate with the actuator fully open, with an absolute inlet
pressure of p
u, abs.
= 2,01325 bar and an absolute outlet pressure of p
d, abs.
= 1.01325 bar. The value K
G
refers to natural gas
with a relative density of d = 0.64, corresponding to a standard density of p
n
= 0.83 kg/m
3
and and a gas inlet temperature
of t = 15 °C
The mass flow through a nozzle increases with a constant inlet pressure and a decrease in pressure after the nozzle, until it
reaches its maximum when the pressure ratio is critical and thereafter remains constant. With constant outlet pressure, an
additional increase in inlet pressure causes an increase in the mass flow through the regulator. For this reason, to calculate
the mass flow that flows through a nozzle, two areas are distinguished:
a) sub-critical or critical relative pressure
b) super-critical relative pressure
where
Q
N
= regulator power under normal conditions
Q
N
(p
d
+1.013)*(p
u
-p
d
)
K
G
=
Q
N
*2
(p
u
+1.013)
K
G
=
p
d, abs.
p
u, abs.
≥ 0.53
p
d, abs.
p
u, abs.
< 0.53
Abbreviation Description
p
d
[bar] Outlet pressure
p
d, abs.
[bar] Outlet pressure as absolute pressure ( p
d
+1,013)
p
u
[bar] Inlet pressure
p
u, abs.
[bar] Inlet pressure as absolute pressure ( p
d
+1,013)
Model Thread Ø6 Ø8 Ø10 Ø12 Ø14 Ø16 Ø20 Ø25 Ø30 Ø35
1", 1 1/2" M22 34 61 87 115 144
DN25 M30 34 61 87 115 144
2", DN40 M45 105 279 396 546
DN50 M56 151 282 412 575 737 900