There are many methods used in the process industries for placing gas detectors. These methods range from pure rule-of-thumb approaches and semi-educated guesses all the way to full quantitative risk analysis with computerized tools for calculating coverage provided by the detector arrays. In oil and gas production, particularly offshore, one of the more common methodologies is to simply put detectors on a grid. The advantage of the grid approach is that it is somewhat deterministic and leads to consistent and predictable designs.
Given that a grid type approach is desirable to some engineering and operating companies, the next question that is typically asked is what should the grid size be? A number that many people will run into through a quick literature search or conversations with colleagues is 5 meters. While a number is generally easy enough to get hold of, the basis for that number is typically not present. As a result, other grid sizes are often utilized depending on the situation and “gut feel” of the project manager. And unfortunately, sometimes these figures lead to inappropriate and even comical results. Detector grid spacings that I have seen range from a three dimensional 3-meter grid – which resulted in nearly 2,000 detectors for a fairly small facility, all the way to no detectors being placed at all… On the large spacing end, 10 meters is a spacing figure that can also be commonly found.
Since the spread of grid spacing figures has a somewhat wide range, and the spacing number is critical in balancing required risk mitigation and cost of installation. As a result, having a firm grasp on what number is required and why is critical for gas detection system design.
The most referenced document discussing gas detection grid spacing is Offshore Technology Report – OTO 93 002 – Offshore Gas Detector Siting Criterion, Investigation of Detector Spacing. This report was funded and is distributed by the UK Health and Safety Executive. It is available for download from their web site (www.hse.gov.uk). The fundamental concept behind gas detection philosophy, as defined in the HSE report, is that any gas cloud that is sufficiently large that if ignited it will create an explosion that will cause significant damage should be detectable by the installed gas detection array.
The HSE report, and conventional wisdom, has agreed that a “significant” explosion is one where the flame front of the ignited gas cloud reaches a greater than 100 meters per second, which then will result in a peak overpressure in the resulting shock wave of more than 150 millibar. Explaining in a bit more detail, when a gas cloud ignites the oxidation reaction that generates the flame travels at a speed that is determined by the material being combusted and the confinement and obstructions surrounding the cloud. More confinement traps and builds pressure and obstructions quickly generate turbulence, reducing laminar drag resulting from surface tension (the same reason that golf balls have dimples). The following figure shows pictures from tests that were run in the US by NASA that show flame fronts as they are being generated. 
The HSE report went through a host of literature where flame speed and overpressure were measured in experimental trials. The trials covered a range of conditions, including methane and propane as the hydrocarbon source, and blockage rations ranging from 0-40%. Upon review of all of the data HSE determined that for blockage ratios of up to 30-40%, which is typical of a congested offshore production platform, cloud sizes that are less than 6 meters in length are not expected to result in damaging over pressures resulting from explosion. As a result of this determine, the HSE subsequently recommended employing a 5 meter grid for offshore oil production.
Of course, these results are really customized for offshore production where methane is the species of concern. If other chemicals such as Propane, or worse yet Ethylene, are the concern, much smaller clouds can result in significantly more damage. On the other hand, large open facilities such as refinery tank farms could have much larger clouds (10 meters or more) that will not result in significant damage because there is a lack of confinement and obstructions. A general rule of thumb has evolved that says 1) in dense process areas, 5 meter grids are acceptable, 2) if highly confined areas or areas where the chemical of concern has a greater ability to cause damage, smaller (e.g., 4 meter) grids should be considered, and 3) in open on-shore process areas a wider 10 meter spacing is more appropriate.
The next figure shows a layout of a 5 meter grid for a typical well bay of an offshore platform.
You can see that the spacing is reasonably good, but the grid sometimes results in odd locations, where either equipment is not present, or no feasible means of actually mounting the detector are available. Using the Kenexis Effigy Fire and Gas Mapping tool to calculate the coverage (using a 5 meter critical cloud size) you will also see that the coverage is quite good.
While the design of the gas detection layout is good with the grid spacing layout shown here, more sophisticated techniques, such as gas detection mapping based on the critical cloud size will allow better coverage to be achieved with fewer detectors and also allow novel technologies such as open path. This results in better safety performance at a lower cost. Enhancing the gas detector grid approach will be the topic of an upcoming magazine article. I’ll keep you posted on where and when it will be available.