Last time, we talked about some gas
analysis applications that emerged from a coal mine project in the southern USA. This
mine is equipped with an elaborate tube system which allows air samples to be
drawn from different parts of the mine to a central monitoring station.
The first instruments we provided to this
facility performed beyond expectations. This enabled further cooperation
between our companies on some additional projects. Here are the 3rd and 4th applications from this project.
Application #3 – Carbon Monoxide and
Hydrogen Research
The next request from this facility was to
expand the baseline air analysis. In detecting fire events in a distant part of
the mine, it was suspected that there would be brief spikes of ppm CO and ppm H2
coming in through the relevant areas of the tube bundle.
Previously, measurement of these gases was
accomplished using a Tedlar sample bag and an external laboratory located above
ground. This process took several days to complete which effectively negated
its value as a fire detection methodology. A continuous CO / H2 analyzer would
provide an immediate measure of these gases without the inconveniences
associated with the existing process.
The requested ranges were:
CO: 0 – 1,000 PPM
H2: 0 – 1,000 PPM
These gases and ranges are frequently
achieved in analyzers using electrochemical sensors. However, these sensors
have issues with mutual interferences. That is, CO provokes a response on the
H2 sensor, and H2 provokes a response on the CO sensor. It would be impossible
to obtain a reliable and accurate result where both gases were present in the
sample at the same time.
The first and obvious approach was to
switch the CO measurement methodology to IR (infrared). We have a long-path IR that is ppm CO
capable. This method eliminated one of the interference problems. The IR
reading will not be affected by the levels of H2 that were expected in the
sample.
We do not have access to an alternate
method for measuring ppm H2. However, we can utilize the stable output from the
IR CO detector to provide some compensation to the H2 reading. This will
subtract some of the CO effects from the displayed H2 result. However, it does
not remove the physical effects of the CO on the H2 sensor. A spike of CO would
provoke the expected response on the H2 sensor. The compensation circuit would
help, but afterwards, the H2 sensor would be slow to recover.
So we devised a calibration procedure which
allowed some ‘re-freshing’ of the H2 sensor to occur when needed. However, we
noticed that the CO / H2 effects only seemed to occur during the spike periods.
At the levels normally present in the mine, there did not appear to be any
adverse affects on the continuous analysis. We assume that even if a spike
occurred, the elevated readings would be valuable and the post-spike recovery
could be managed as needed.
As with the previous applications, this
analyzer required a reliable data-logging feature. We upgraded the data logger
to a unit with larger capacity and a more convenient data storage process. The
collected data is written directly to a SD card similar to what is used in many
digital cameras. The SD cards are swapped out weekly for subsequent data
analysis. We mounted the logger itself on the inside of the analyzer door. This
protected it and allowed easy wiring access.
 |
Data logger mounted on door of
gas analyzer and writes to SD memory card. |
Application #4 – Air Flow Research
Very low levels of SF6 (sulfur
hexafluoride) can be released in the mine as a tracer gas and tracked using the
tube bundle. By ‘sniffing’ for SF6 in the relevant tubes, the air flow in the
mine can be modeled. SF6 tends to have a persistent and detectable presence
even after a long time and at very low levels. This allows the air flow model
to be detailed and to capture subtleties not detectable by other means.
We have produced instruments for 0-100% SF6
for analysis of switch gear atmospheres. (Here is a link.) In large electrical switches where the
possibility exists of dangerous arcing, a blanket atmosphere of SF6 will quench
the arcs. To check the purity of the SF6 atmosphere, a gas analyzer can be
used. The measurement methodology on these models is thermalconductivity.
However, the mine tracer gas application
required a range of 0-50ppm. A percent scale instrument would not be useful in
this case. The thermal conductivity detector does not have ppm SF6 capability.
So we provided an infrared detector which can measure 0-50ppm SF6 in a
background of breathable ambient air.
The same style of data logger was also
integrated onto the door of the analyzer cabinet. Although it probably wasn’t
necessary, we tested the output of this detector in background of air with
spikes of H2 & CO as mentioned above in Application #3. No interferences or
disruptions to the SF6 reading were observed.
 |
Front view of continuous SF6 analyzer during
inspection phase. SF6 analysis on this instrument
done by infrared detector. |
So far, all of the equipment we have supplied into this facility has worked well. The applications discussed in this post were somewhat experimental for us. But are occasionally willing to step outside of our standard designs for projects that interest us. With patience and support from the end-user in this case, we managed to produce instruments that will hopefully make a useful contribution to mine safety research.
For information on these and other gas
analyzer systems, give Mike or Dave at Nova a call, or send us an e-mail.
1-800-295-3771
sales at nova-gas dot com
websales at nova-gas dot com
http://www.nova-gas.com/
