Flammable gas detectors

Waste facihties should be designed to prevent explosions in sewer systems and typically are comprised of suitable traps, vents, clean-outs, collecting chambers, etc. Flammable gas detectors are installed in sewers to warn of ha2ardous concentrations, and inert gas blanketing of closed process sumps generally is advisable. 

Industrial use of flammable gas detectors Storage of highly flammable liquids... 

The main rises of a sintered metal flame arrester are in the sensing heads of flammable gas detectors and in flame arresters for gas welding (oxyacetylene) equipment. 

If a flammable gas or vapour is present, a pre-fire condition may be identified by a flammable-gas detector. Fire detection may be by ... 

If a flammable gas or vapour is present, a pre-fire condition may be identified by a flammable-gas detector. This will actuate an alarm at a fraction of the LFL. Banks of detectors may be installed at high or low level depending, in part, upon the gas density. Fire detection may be by ... 

Each of these steps might be performed either by direct action of operations or emergency response personnel or by automatic systems. An example of the latter might be an array of toxic or flammable gas detectors that might trip an emergency shutdown system that closes remotely actuated block valves and vents off the process pressure to a flare if two adjacent sensors read above a predetermined vapor concentration. 

Health and Safety Executive (HSE), Guidance Note CS1, Industrial Use of Flammable Gas Detectors. HMSO, London, U.K., 1987. 

Catalytic (Pellistor) Flammable gases Air Measures the heat output due to the catalytic oxidation of flammable gas molecules. A stream of the sample is passed over the sensor which is usually a ceramic bead impregnated with Pt or Pd. The temperature variations in the sensor due to reaction are monitored. Dependent on individual design. Flammable gas detector. Usually portable... 

Semiconductor Flammable and other gases Air Two small coda of platinum wire are fixed side by side within a bead of semiconductor material of diameter 2-3 mm. The sample gas flows over the bead and molecules of gas are adsorbed on the surface of the semiconductor—thereby changing the resistance of the semiconductor as measured between the two coib. One coil is also employed as a heater to ensure that the adsorption is reversible. The device is sensitive but has poor selectivity. Dependent on individual design. Flammable gas detector. Inexpensive... 

A multi-disciplined team interviewed witnesses and performed tests, and the evidence was followed to a sound conclusion. Investigators sent the explosion meters (flammable gas detectors) to the instrument shop for examination. The welding torch was also inspected. Experts found both the torch and the meter behaved satisfactorily. There was speculation that some aquatic life deteriorated and formed methane, but that was not the case. They concluded that the fuel was dissolved methane from the well water system that was routed to the pump seal. Trace amounts of dissolved methane (about 0.02 percent by volume) accumulated in the well water stream flowing through the 3/8-inch-diameter stream on the... 

Figure 10-21 Prooftesting a flammable gas detector in a tank farm.

With such good intentions, what could go wrong After completion of maintenance, Pump B was put into service. After 45 min of operating time, major problems appeared. Flammable gas detectors alarmed, warning that a large amount of process fluid was escaping containment in the Pnmp B area. The cloud ignited. 

There are a number of ways by which fire can be detected. The three principal fire detection methods involve sensing the heat, either as actual temperature or as rate-of-rise of temperature, the presence of flame or the presence of smoke. There is one other detection method which identifies a pre-fire condition and that is a flammable gas detector whose use is limited to places where flammable gases are likely to be present. 

A flammable gas detector is designed to measure the amount of flammable gas in the atmosphere and relate it to the upper and lower flammable limits. The gas mixture is drawn over a catalytic surface where oxidation, i.e. combustion, takes place. The combustion causes a rise in temperature of the surface which is measured by a decrease in its electrical resistance. The instruments have to be calibrated for the particular gas of interest but for petrol vapours pentane or heptane are used as the reference gas. The readings are usually displayed in terms of percentage of the lower explosive limit. 

To overcome the problems of gas-sensor failure due to poisoning of the elements, several manufacturers have used specific filter devices in front of the elements. For example, filters to remove alkyl lead compounds from petrol vapour have included paradichlorobenzene and fibrous silica (3). However, the most commonly used material is based on active charcoal. Such filters (4) effectively absorb many common poisons and inhibitors including silicones, alkyl lead compounds and halogen-containing species. However, along with the absorption of potentially harmful vapours, carbon filters will also absorb hydrocarbons. This effect in practice limits the use of such filters to detectors for C3 and lower hydrocarbons. They cannot be employed in general flammable gas detectors. 

Combustible gas detectors are required whenever there is a possibility of a hazard to life or property caused by the accumulation of combustible gases. There are mainly two types of combustible/flammable gas detectors that are often used in most industries. These are the low-cost catalytic bead sensor and infrared sensor [26,27]. 

Portable fire extinguishers (carbon dioxide or chemical types), or other fire protection or suppression systems or devices must be available for fire emergencies at storage installations. Only trained personnel should be allowed to operate fire extinguishers. A flame-producing device must never be used for detection of flammable gas leaks. Either a flammable gas detector or compatible leak detection solution should be used. 

Flammable gas detectors should not be used to detect toxic gas and vice versa. Dedicated detectors should be installed. 

Associated with each of these demands that may cause the hazardous event were various protective systems. These were either hardware (e.g., ventilation system, flammable gas detectors) or procedural (e.g., instructions on allowable maintenance activities). For any particular demand to lead to the hazardous event, all the protective systems designed to protect against that demand must have failed to perform. Again, this failure may be a hardware failure (e.g., the gas detector has drifted out of calibration) or a human error issue (e.g., the flammable gas detector alarm warned of a flammable leak but Ihe operator failed to take appropriate action). 

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