Gas detection
Gas analysers: extractive or in-situ?
There are two main options for gas analysis in industrial processes or emissions; the analyser can be located inside the gas stream (in- situ), or a sample can be removed from the gas stream (extractive) and transferred to an external gas analyser. As a gas analyser developer and manufacturer with a history that dates back to 1977, Signal Group predominantly focuses on extractive analysers, and in the following article, the company’s managing director, James Clements, explains why.
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parameter(s) are necessary? How quickly should data be provided? What are the process conditions? and so on. Proponents of in-situ analysers might
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claim that extractive analysers present higher risks due to the perceived potential for condensation in the sample line. Gases could dissolve in this moisture and go unmeasured, they say, and moisture could conceivably run into the analyser causing damage to metallic components. The manufacturers of extractive gas analysers and sampling systems are obviously aware of this, so monitoring systems are designed to prevent it from happening. For example, sample lines and measurement cells are heated to remove any possibility of condensation, and only non-corrodible materials are allowed to come into contact with a sample. In-situ analysers are also exposed to risks;
mainly because sensors are directly exposed to untreated, hot and potentially corrosive process gas. In addition, in-situ systems may
irstly, it is important to be clear that that one method is not necessarily better than another, because the choice depends on the application. Where is monitoring required? Which
be exposed to vibration which could cause misalignment of optical components, and/or the measurement point may be within a classified hazardous area, which would complicate design and installation work. If the monitoring requirement is for a
single parameter in a ‘clean’ gas flow within an industrial process for which real-time data is used for process control; in-situ analysers are likely to be ideal. This is mainly because in-situ analysers can be lower in cost, and would be capable of delivering faster results. Nevertheless, there are many other (probably the majority of) applications in which an extractive analyser would be preferable, and this is the reason why Signal Group has focused on developing extractive gas analysers for the last 45 years.
MULTIPLE SAMPLING POINTS An in-situ analyser might be suitable where only one measurement point is required, but another measurement point doubles that cost; a third trebles, and so on. In contrast, an extractive analyser can be multiplexed – connected to a multipoint sampling system so that one gas analyser can take measurements from multiple points in the same line or from different lines, to dramatically lower the cost per measurement location.
January 2023 Instrumentation Monthly
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