Area monitoring
Many of the fixed type toxic and combustible gas detectors in usage in industry today, are used for area monitoring. What do we mean by area monitoring? We do a strategic placement of these gas detectors in locations that will have the explosive vapor or gas present, in case of a leak. Obviously, we cannot place these gas detectors at all possible places, as this will be very costly, not only in terms of the cost of acquiring these gas detectors and their associated panels, wiring, controls, etc, but also because each gas detector has a maintenance cost associated with it.
Each gas monitor requires periodic calibration with a test gas, replacement of sensor and/or electronics and other maintenance from time to time. We cannot avoid this cost as otherwise the gas monitor will fail to function properly. Just having a gas monitor physically present in an area, may at the most satisfy a factory inspector, or insurance surveyor, it cannot actually detect anything, if not maintained well. Hence, we designate certain areas of our plant or facility as being the most vulnerable to a gas leak. We then pin point the place where the gas is most likely to be present immediately after a leak. We then ensure that vat least we have a gas detector at that point to monitor such a leak.
Placement is the key to safety at a reasonable cost
In the UK, there is a concept known as ALARP (As low as reasonably possible). This indicates that we can spend only so much on safety as to yield a benefit that is more than the cost of the spending. Thus, it is not possible to have swarms of gas detectors all over a plant, as it would cost a fortune to install and maintain them. Maybe the number of such gas detectors would be many more than the number of conventional instruments like pressure or flow transmitters in the plant! Thus strategic placement of gas monitors is the key to achieving safety under an ALARP principle. How does one therefore go about the placement?
Different strokes for different folks
As the saying goes, we employ different types of placement strategies for different types of gas detectors. Therefore, for those gases and vapors that are heavier than air, we select places that are closer to the floor or grade, for lighter gases like Hydrogen, we select higher levels where the gas is most likely to go after a leak. For a gas like Carbon Monoxide which is only slightly heavier than air, one should mount the sensor at approximately 1.6 meters above the ground level for best results.
The next step is to pinpoint potential leakage sources. These could be the outlet of a Pressure Safety relief valve, or flange joints at the inlet or outlet of a large reactor or other process vessel. All such points should be marked on a layout drawing of the plant, before we visit the site. Note that in this case, we are not targeting fugitive emissions, but only places where there can be a large amount of sudden release of a toxic or explosive gas. Other places include the inlet/outlets of blowers, gas turbines, ventilation equipment, etc.
After this study is complete, we can visit the site and actually see if our marked locations are logically correct. If it is an old site, talk to the operators or other plant personnel who can have an idea about the likely points of gas leakage. Once these are ready, we can mark these on drawings.
Next Steps
Next, select the gas detectors based on the amount of coverage for each point/area. If the area is small, a point type gas monitor will do. If the gas is expected to diffuse into the gas detector, nothing else is necessary. If however, it is felt that the gas may not easily diffuse into the detector, we may need additional forced sampling (like small vaccum pumps and collection funnels) to route the samples from the surrounding area into the gas detector. If the area is larger however, we will need an area monitor like an Infrared open path gas detector. These types of detectors cover a large open path (which is the gap between the infrared source and detector) that can cover hundreds of meters of an area. Ultimately the placement will depend on the likelihood of the gas, the type of gas monitors used and the total budget that can be allocated to the project.
Optimization
After the gas detectors have been ordered, installed and placed in the designated spots, the next stage is do a continuous performance monitoring. Are the detectors really detecting gases? Have leaks taken place that were not detected? If so, why? Were these not detected due to wrong placement or the gas detectors themselves were not functioning? Answers to these will enable you to optimize the placement further and ensure that your gas monitors do the thing that they are best at-protecting the plant, people and the environment from undesirable events.
Sam. P. Whittle is a technical expert on gas detectors and gas monitors. He is an engineer with several years of experience in various industries. At present he is on the panel of subject matter experts of Abhisam Software
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