CDP301 Dew-Point Tester User’s Manual
Michell Instruments 9
INSTALLATION
For proper sample system operation, Michell Instruments' recommendations are:
• The sampling point from the process line should be taken from the top
of the process line. If a radial probe is used, the orice should face
downstream.
• It is recommended that Viton is used for all O-rings.
• The internal volume of the impulse tubing connecting between the process
line and this product should be as small 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 lowest point in the system.
• It should be standard procedure to isolate this product during shutdowns
or when plant problems are being experienced and to adequately purge
the supply lines before restarting.
• The relatively large area of surfaces and internal volume of inline
components can be particularly troublesome if contamination is
experienced. Prolonged purging, or stripping and cleaning, followed by
re-purging with gas may be necessary to remove the contamination.
• Avoid sample gas streams that are already very close to the dew point or
which have dispersed liquid within them. In such cases, sampling from
fast loops and/or from downstream of existing catch pot/coalesce systems
is always preferred.
Failure to observe these recommendations will potentially cause problems of
contamination as well as causing consequential inaccurate, unreliable and inconsistent
measurement results. 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.
2.4.2 Considerations when setting sample pressure and flow
In-line particulate ltration is recommended to avoid contamination of the mirror surface
and generally within the dew-point tester. A 15 micron (or smaller) sintered stainless
steel lter is suitable.
Pressure within the dew-point sensor cell is indicated on the display of the CDP301.
Be aware that the ow restrictor tted to the gas inlet connection will cause a small
pressure drop between supply pressure and analysis pressure within the dew-point
sensor cell. Process pressures less than 10 barg (145 psig) cannot be sampled.
Set the sample ow rate to 0.5 – 1.0 Nl/min
The sample gas outlet should be vented to atmosphere (where permitted and in
accordance with safety procedures at the location of use) or exhausted into a low-
pressure are system (<3barg/43.5psig back-pressure).
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