and Process Sample Transport Systems for Analyzer Applications
This selection is too many times done by a seat of the pants .we
did it this way before selection method. The task is to make
sure that the sample we are measuring is truly representative of the
process.In the case of timed interval sampling (GCs
commonly use this) we should utilize a fast loop (a continuous flow
of sample either back to the process and/or to a flare).By tapping a fast loop stream with the smallest possible diameter
tubing will greatly reduce the resonance time. These considerations
are especially critical in the instances whereby the sample extracted
is liquid and is subsequently vaporized.Due to the volumetric increase in the phase change it is possible
to be measuring a sample that is many days old!
Since most sample systems control the sample flow to approximately 1 liter/minute
you must also consider the volumetric expansion of gas samples.
A two times pressure drop will result in twice the volume.
Simple formulas for calculating
the lag time in minutes:
of Thumb Tubing Volumes:
(min) = Line Length (ft) / Velocity (ft/min)
Lag (min) = Line Volume (cc) / Flow Rate (cc/min)