The analysis of existing gas detection systems has shown that the primary limiting factor in the effectiveness of a system is incorrect detector placement. This factor alone outweighs the probability of failure on demand of the individual system components (sensors, logic solvers, and final elements). Incorrect detector placement can be so detrimental that the system cannot even be credited as an effective independent protection layer.
Gas detector location has historically been selected based on rules of thumb and experience. Common rules have been to place detectors:
at breathing height for toxic gases
one to two feet above ground for gases heavier than air
above the leak source, or as high as possible, for gases lighter than air
near the ground for cryogenic conditions
near air ductwork intakes, or room outlets
in areas accessible for maintenance
away from locations that can be damaged by general maintenance
where to place gas detector
The optimal detector location will vary from plant to plant. What is appropriate for a congested offshore platform will be different than for a batch chemical plant with multiple recipes, or a refinery, or a sour production well. Inconsistent approaches have often been found. Existing facilities that have been analyzed have been found to have significant gaps in detector coverage.
There has been a growing interest in determining the effectiveness of gas detection systems in a quantitative manner. Our understanding of gas dispersion, and the ability to model and predict the release behavior, has grown significantly. Two approaches have been developed for detector placement; geographic coverage, and scenario-based coverage.
Geographic coverage places detectors on a uniform grid. Geographic methods can result in more detectors than are necessary. In addition, geographic methods are based on low or medium reactive materials such as methane or propane. Geographic methods are not suited for high reactivity materials which can achieve a detonation. Furthermore, geographic placements can lead to higher installation and operating expenses. As a result, many companies prefer to use scenario-based coverage over geographic methods.
Scenario-based coverage places gas detectors based on computer dispersion modeling. Scenario model selection involves identifying a variety of leak points, hole sizes, and leak directions. The optimal number of detectors can then be placed in the optimal locations. There are limitations of what a gas detection system can reasonably be expected to do, beyond having a highly effective detector coverage. Beyond having a highly effective detector coverage, there are limitations of what a gas detection system can reasonably be expected to do. What is the effectiveness of the mitigation system? To achieve an overall performance of SIL (safety integrity level) 1 or higher, a system would require detector coverage over 90%, and mitigation effectiveness over 90%. To achieve SIL 2 would require both numbers be greater than 99%. This would be over specifying potential performance. ISA-TR84.00.07 advises that a system not be considered an independent protection layer if either value is less than 90%, as SIL 1 will not be possible in such a case.
3D modeling incorporating wake effects from buildings can show a gas plume reaching areas that may not be immediately intuitive, such as air handlers on the back side of a building. Room ventilation patterns may also cause non-intuitive gas behaviors. Using scenario-based coverage dispersion modeling may increase the initial project cost, but it has been shown to offer a lower overall project cost due to reduced detector quantities and reduced maintenance. It also provides a quantitative basis for documenting the rationale behind detector placement decisions. Benefits include reducing life cycle costs, reducing risk to onsite plant personnel, and reducing risk to offsite public receptors.
To learn more about this topic, read the full paper “How Can I Effectively Place My Gas Detectors” by clicking here.