Combustible Gas Detection
Gas detection is employed to detect the presence of potentially flammable vapor/air mixtures before they reach the Lower Flammable Limit (LFL) and become a potential source of fire or explosion. Gas detectors used for fire protection differ from the detectors used for industrial hygiene monitoring which function in low concentration ranges such as parts per million. Gas detection systems for fire protection are used widely in the chemical industry in both process and storage areas. Combustible gas detectors are often used for areas of a facility that are subject to accidental flammable vapor releases (potential leak sources) or where accumulations of flammable vapors could come in contact with ignition sources. They are used to detect large releases, where the potential for a vapor cloud exists, and to detect smaller releases from potential leak sources. These areas include, but are not limited to, the following:
• Selected pump or compressor facilities and liquid hose transfer stations
handling highly flammable fluids.
• Tank car and tank truck facilities handling highly flammable fluids.
• Electrical centers or control rooms in the vicinity of potential large flammable gas releases.
• Air inlets to ventilation and pressurization systems in classified areas and in the vicinity of accidental large flammable gas releases.
• Aerosol filling areas.
• Ditches, trenches, sumps, and other low points where heavy flammable vapors could accumulate.
Most gas detection system set points are chosen based on a percentage of the LFL, with a two-tiered approach (one alarm for warning and manual response and a higher set point alarm to initiate predetermined emergency measures) being common. Alarms are generally initiated at 10-30% of the LFL and the high level, shutdown, or other action initiated at 30-50% of the LFL. Detection systems are sometimes used to shutdown processes or equipment and activate inerting, fire protection (water spray, water curtains, etc.), and emergency ventilation systems (Greenawalt n.d.). Emergency shutdown of electrically energized equipment in the presence of combustible gas could cause an explosion since electrical arcing produced from opening energized contacts could be an ignition source. Set points should be chosen to ensure that equipment is deenergized before vapors reach dangerous levels.
The location of gas detectors should take into consideration such factors as the gas density, wind direction analysis, gas detector sensitivity, potential ignition sources, and potential flammable release points. Detectors may be provided either on an area basis, or located to detect leaks from specific locations, or a combination of the two philosophies. There are no standards for gas detector placement, and it is important that detectors be located in accordance with the manufacturers recommendations. Access for maintenance and calibration must also be considered when locating detectors since an inaccessible detector is unlikely to be maintained.
There are several types of combustible gas detectors used for fire protection. Most fall into two general categories, spot types and sampling types. Spot type detectors are generally faster than sampling detectors since the gas does not have to travel through the sample tube from the sampling point to the detection instrument. The most common detectors include diffusion head type catalytic oxidation (spot type), infrared spectroscopy (sampling type), and thermal incineration (sampling type). Catalytic oxidation detectors oxidize the gas, heating an internal element and changing its electrical resistance. Infrared detectors detect a change in infrared radiation as higher gas concentrations absorb more infrared radiation. Incineration detectors pass the gas though a constant flame and detect the increased heat generated by a combustible gas. Diffusion head type catalytic detectors are generally recommended unless they are not compatible with materials that may be released.
Gas detectors have potential problems. Catalytic detectors are subject to catalyst poisoning. Exposure to silicones, lead, sulfur, and other materials can quickly make the detectors inoperable. Exposure to chlorine, chlorinated compounds, or corrosive materials can reduce the life of the sensor and cause maintenance problems. Calibration can also be a problem, since calibration of many detectors is dependent on the specific gas to be detected. Catalytic type detectors exposed to hydrocarbons such as ethane, ethylene, and propane at 20% of their LFL may give different results for each gas. Thermal incineration detectors contain an ignition source. Absorption of infrared radiation by a gas depends on the gas’s molecular structure, and therefore infrared detectors are designed for use with specific gases. The concentration indicated by detectors may not be accurate when several different gases may be released or a gas mixture is present. Some detectors are also sensitive to changes in humidity, temperature or atmospheric pressure.
Gas detectors should be maintained and calibrated periodically in accordance with the manufacturer’s instructions. In hazardous areas, the detector housing should be opened only when safe conditions can be ensured. Portable gas sensors or sniffers should be used to determine the possible presence of flammable vapor-air concentrations before opening the sensor housing.