Michell Instruments Promet EExd User Manual page 20

INSTALLATION
The following points should be considered when installing the sample gas supply line:
PTFE tape is recommended for pipe connections. Solvent based pipe thread sealant
should not be used, as condensable components or contaminates can be leached during
the curing period.
It is recommended that Viton is used for all O-rings.
Care and attention to the position and installation of the piping will minimize problems
caused by avoidable contamination of the measurement system. The most common
cause of difficulty is the accumulation of liquid in impulse lines during a shutdown
period. If the measurement system has not been isolated on restart-up, condensate
can be displaced into components and associated pipe work within the measurement
system.
If this event follows a period when process lines may have been contaminated by non-
hydrocarbons, e.g. glycol, corrosion inhibitors, etc., the problem is magnified. Similarly,
difficulty will be encountered in sample gases carrying liquids, including hydrocarbon
liquids.
Our recommendations are:
• The sampling point on the process line should be on the top of the pipe. If
a radial probe is used, the orifice should face downstream. Sample should
be taken from middle 1/3 pipe internal diameter.
• The internal volume of the impulse tubing between the process line and
any sampling system should be as low as possible (sample lines should
be kept as short as possible) to minimize response lag time to changing
process conditions.
• Piping should be lagged and/or trace heated if ambient temperatures
could cause the sample gas to fall below its dew-point temperature.
• A drain valve should be placed at the low point (if any) in the system.
• It should be standard procedure to isolate the measurement system
during shutdowns or when plant problems are being experienced. The
supply lines must be fully purged before restarting.
• The relatively large area of surfaces and internal volume of pressure
regulators can be particularly troublesome if contamination is experienced.
Prolonged purging with gas may be necessary to remove the contamination.
Stripping and cleaning, followed by purging of the system, is preferred.
• Avoid sample gas streams that are already very close to the dew point
or which have dispersed liquid (not necessarily hydrocarbon) burden. In
such cases, sampling from fast loops and/or from downstream of existing
catch pot/coalesce systems is always preferred.
NOTE: Failure to observe these recommendations will potentially cause
problems of contamination as well as causing consequential inaccurate,
unreliable and inconsistent monitoring. If a top-entry sample point
is not available, extra attention should be given to the design of the
sample line installation to avoid unwanted contamination.
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Promet EExd User's Manual
97090 Issue 16.1, November 2019