Flame arrestors are safety devices for handling combustible gases. They prevent gas from burning back into a device and exploding. Flame arrestors are used in storage tank vents, fuel pipelines, gas storage cabinets, and exhaust systems for internal combustion engines. They are also used with Davy lamps, light sources that are used in coal mines. Firedamp or minedamp, a name for a group of flammable gases that includes methane, are found typically in bituminous coal mines. Flame arrestors may also be used with equipment for processing over-proof rums and other sugar-based liquors that contain higher-than-usual percentages of alcohol. Flame arrestors are designed to force the front of a flame through channels that are too narrow to allow the flame to spread. These passages may be regular or irregular, and made of wire mesh or sintered metal. Venturi flame arrestors are an active type of device that restricts the mixture of hydrocarbons and air in the delivery pipe to prevent flashback from moving upstream. Flashback in the direction of the flow may still occur, however. Although even a partially-closed valve can produce an adequate velocity to prevent flashback, flame arrestors that are Venturi-shaped feature a significantly lower decrease in pressure. The effectiveness of Venturi flame arrestors depends upon gas flow, however, so techniques for measuring flow and add a makeup gas such as nitrogen are often used. In addition to Venturi devices and other active products, flame arrestors include passive devices such as inline flame arrestors. Mechanical flame arrestors are a type of passive device filled with metal or ceramic. These materials absorb heat from a flashback and reduce it to a temperature below ignition to stop the spread of flame. With a suitably low flow-rate for a mixture of hydrocarbons and air, a flame that reaches the face of the flame arrestor may become stable. Heating of the flame arrestor’s body still occurs, however. If the temperature rises sufficiently, the upstream side may reach the ignition temperature. Consequently, some passive flame arrestors include a temperature switch or other active element on the flame side. If these safety devices detect an elevated temperature, an alarm sounds so that the gas flow can be interrupted. Flame arrestors are used in gas pipe lines, gas storage tanks, exhaust systems, storage cabinets, and with vent arrestors. They are also used with welding equipment, for explosives storage, and to contain tank vapors. Typically, tank vapors are found in flare stacks, flammable liquid storage emergency tank vents, feeds to burners, and flammable liquid-filling and discharging systems. Flame arrestors are also used to manage tank vapors in hazardous environment engine protection systems, chemical and petrochemical process plants, coal mine ventilation systems, sewage digester plants, gas pressure regulators, gas analysis instrumentation, nuclear waste treatment, fuel delivery, fuel cells, and vacuum pumps.