User Manual SERVOPRO PLASMA Trace N
2
Analyzer
APPENDIX 6
77
SAMPLING LINE SIZE, (AN-01)
It is a common practice in air separation plants to
use ¼" O.D. line for sampling system. In older
installation it is also common to have sample
pressure regulator close to the analyzer sample
inlet. This can lead to long lag time limiting the
speed of response of an analytical system. For
example, sample line of 100 feet of ¼" O.D. with
.190" I.D. (typical copper line) has an internal
volume of .02 cubic feet. If the sample flow is 1
SCFH (475 sccm) and we assume the line to be at
atmospheric pressure, it will take 1 minute and 12
sec. to travel down this line. If the same line is
made of 1/8" O.D. with .085" I.D., the internal
volume becomes equal to .004 cubic feet. This is
5 times less volume. It will take 24 seconds to
travel down the line with 1 SCFH. We have
assumed a line pressure equals to the atmospheric
one. If the line is pressurized at 1 atmosphere
there will be twice the volume of gas into the line
so, twice the time will be required for a sample to
go through this line.
One may thought to increase the sample flow
through the ¼" O.D.line to overcome this
problem. If a bypass flow is set to 10 SCFH the
time will be decrease by a factor of 10. But after
one year of operation this result in 87600 cubic
feet of gas thrown away. If this gas is pure argon,
this gives around 97 cubic feet of liquid. This is
also equivalent to 350 gas
cylinders (250 cubic feet size cylinder). The older
type of analyzer system for trace nitrogen
measurement uses 2 to 4 SCFH of sample flow
(for silent electric discharge type) or 1.48 cubic
feet (700 sccm) for ion mobility type. Servomex
Canada analyzer works with a default sample
flow of 75 sccm. The flow can be set as low an 25
sccm if required.
In conclusion, we are recommending the use of
1/8" O.D. stainless steel line for sampling lines.
Furthermore, 1/8" O.D. line are available in coil
of 500 or 1000 feet for long run. There is no need
for fittings or welding. Also, installation cost is
minimum since 1/8" O.D. line are easily installed.
A sample line of 100 feet made of 1/8" O.D. and
.085" I.D. connected to a nitrogen source at 1
PSIG and vented to the atmospheric pressure will
have a flow of 475 sccm (1 SCFH). Well enough
to supply sample gas to a K3000. It will require
10 psig for 1000 feet long sample line.
The sample pressure regulator must be installed as
close as possible from the sample connection
point. The pressure will be adjusted to the
minimum value required to have the proper flow
into the analyzer. Such sampling system will have
a faster response time, better leak integrity, less
operation cost.
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