Adjusting The Sample Gas Flow Rate - Ametek 241CE II Operator's Manual

3-38 | Model 241CE II Hydrocarbon Dewpoint Analyzer

Adjusting the Sample Gas Flow Rate

The gas sample flow rate needs to be properly adjusted for your applica-
tion. 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/min 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
3-1). If this adjustment has been done properly during installation, no
further adjustment should be necessary.
 If you measure the flow rate at the Vent Line, you will find that it
is substantially higher during the Purging/Hold Stage because the
NOTE
Purging/Hold Stage flow rate (1.0–5.0 Nl/min (2.1–10.6 SCFH) is
added to the trickle purge flow rate (Bypass Valve is open).
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/min
(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 difficult – or even impossible – to detect condensing hydrocar-
bons when there is already hydrocarbon liquid present.
The sample flow rate and the Purge time can be easily adjusted. There-
fore, 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. Al-
though the solenoid shuts off the main purge flow during the measure
cycle, opening the Bypass Valve will allow a small flow rate during mea-
surement (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 sys-
tem and Measuring Cell.