Emerson Rosemount 370XA, System Reference Manual

Page: 56 / 209

370XA Gas Chromatograph System Reference Manual
APRIL 2014 7P00370-H01
3 - 29
The following image shows the sample gas connector (1).
The sample pressure entering the gas chromatograph sample
conditioning system should be between 15 and 30 PSI (100 and 200
kPa). If the pressure in the pipeline is higher than this, regulate the
sample pressure to this pressure with a dual stage regulator. Pressure
regulation should occur immediately after the probe, or be combined
with the probe (a "regulator probe") because any extended lengths of
sample line before the pressure regulator adds significant
lag time ,
which is the time taken for the sample entering the probe to reach the
analyzer oven.
Note that when the pressure of a gas is reduced, the temperature of
the gas will decrease. This is called the
Joules-Thompson effect . If the
temperature is reduced below the sample's hydrocarbon dew point,
the higher hydrocarbons will begin to condense and be removed from
the gas phase, which changes the composition of the gas. The
analyzed sample will no longer accurately represent the flowing gas
stream.