Siemens SITRANS SL, Руководство по эксплуатации, Страница: 193 / 212

SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04
195
Appendix
A
A.1 Gas flow calculations
Gas flow calculations are slightly complex because gases are compressible fluids whose
density changes with pressure. In this application we are dealing with choked flow. The
outlet pressure is less than one half of the inlet pressure and the gas reaches sonic velocity
in the valve. A further decrease in outlet pressure does not increase the flow.
The flow (q) of instrument air at P
1
= 6000 hPa and T
1
= 25 °C (77°F) through the needle
valve when it is fully opened would then be a little more than 110 Nltr/min.
The figure below shows the flow (q) as a function of the up-stream pressure (P
1
). There are
two parameters in the diagram, the process pressure (P
p
) and the temperature of the purge
gas which is assumed to be air. The diagram shows the flow through a system with a total C
v
of 0.1. The flow increases linearly with C
v
.
q = the flow rate on the low pressure side [Nl/min.]
C
v
= the flow coefficient (0.1 for the needle valve in our standard sensor)
P
1
= the inlet absolute pressure [hPa]
G
g
= the gas specific gravity (air = 1.0)
T
1
= the upstream temperature [°C]
P
p
= process pressure [hPa]
q (C
v
= 0.1) [Nltr/min]
P
1
[x1013 hPa]
140
120
100
80
60
40
20
0
7 6 5 4 3 2 1 0
T
1
= 100 °C
T1 = 25 °C
P
p
= 1013 hPa
P
p
= 2026 hPa
P
p
= 3039 hPa
Figure A-1 Purge air flow rate
Note
This figure represents a general context between pressure and flow rate.
The maximum purge flow rate of the SITRANS SL on process side is limited to 50 Nl/min.