52 …56
MC • Edition 04.19 • No
. 278 349
53 … 56
Type of gas
Natural gas
City gas
LPG
Air
dv
0.65
0.47
1.67
1.00
f
1.24
1.46
0.77
1.00
Air density
spec. weight 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 indicated volume flow refers to natural gas with a density 0.81 kg/m
3
or to air with a density of 1.24 kg / m
3
at
15 °C under standard conditions. In case of different types of gases, a conversion of the volume flow according to the equation
below is carried out.
16.3 Valve flow volume coefficient K
G
Type
DN
K
G
-value
FRM 100065...
65
2600
FRM 100080...
80
3200
The valve flow volume coefficient K
G
f FRM is equal to the flow volume for a completely open firing valve with an absolute
inlet pressure of p
u, abs.
= 2.01325 bar and absolute outlet pressure of p
d, abs.
= 1.01325 bar. The K
G
-value value refers to natural
gas with a density ratio of d= 0.64 according to a standard density of p
n
= 0.83 kg/m
3
and gas inlet temperature of t= 15 °C
The mass flow through a nozzle increases at constant upstream pressure with sinking pressure downstream of the nozzle,
until it reaches its maximum at critical pressure ratio and remains constant from that moment on.
At constant outlet pressure, further increase of the upstream pressure causes a mass flow increase through the regulator.
Therefore, to calculate the mass flow through a nozzle, a distinction is made between two ranges:
a) subcritical or critical pressure ratio
b) supercritical pressure ratio
with
Q
N
= power of the regulator under standard 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
< 0,53
p
d, abs.
p
u, abs.
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
u
+1,013)
