Adjusting The Sample Gas Flow Rate - Ametek 241CE II User Manual

Adjusting the Sample Gas Flow Rate

The gas sample flow rate needs to be properly adjusted for your application.
During each measurement cycle, the solenoid is closed. The main purge flow
through the solenoid is also shut off. During the Purging/Hold Stage, the
solenoid opens and the sample gas flows through the sample system and
Measuring Cell. This stage allows previously condensed liquids to evaporate,
while obtaining a new representative gas sample. During this stage, the gas
purge flow rate must be high enough (e.g.,1.0–5.0 Nl/minute or 2.1–10.6 SCFH)
to completely purge the sample system and Measuring Cell within the allotted
Purge time.
To set the flow rate, use the pressure regulator on the backpan (Figure 2-1). If
this adjustment has been done properly during installation, no further adjust-
ment should be necessary.
Because high flow rates result in increased pressure drops through the
Membrane Filters, do not set the purge flow rate greater than 5.0 Nl/minute
(10.6 SCFH). The higher pressure tends to force liquids through the mem-
branes. The purge flow rate setting is especially important if hydrocarbon
and/or water droplets are present in the gas stream. Hydrocarbons have low
surface tension and can be forced through Membrane Filters more easily than
high surface tension liquids such as glycol, alcohols, and water. Liquid carry-
over through the filter can result in measurement problems. It becomes dif-
ficult – or even impossible – to detect condensing hydrocarbons when there is
already hydrocarbon liquid present.
The sample flow rate and the Purge time can be easily adjusted. Therefore, it is
relatively easy to obtain a reasonable balance between the two variables. The
first thing to determine is whether to operate the analyzer with a bypass flow
(or "trickle purge") during the measuring cycle. Although the solenoid shuts off
the main purge flow during the measure cycle, opening the Bypass Valve will
allow a small flow rate during measurement (approximately one-tenth of the
purge cycle flow). This bypass flow tends to improve accuracy and response
time because there is always a small, but constant flow of sample gas flowing
through the sample system and Measuring Cell.
PN 903-8797, Rev A
If you measure the flow rate at the Vent Line, you will find that it is sub-
stantially higher during the Purging/Hold Stage because the Purging/
Hold Stage flow rate (1.0–5.0 Nl/minute (2.1–10.6 SCFH) is added to
the trickle purge flow rate (Bypass Valve is open).
Installation and Start-Up
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