Gas flares or flare stacks are used to eliminate waste gas and to reduce pressures from non-waste gas that can strain equipment. These elevated, vertical conveyances are used with oil and gas wells, oil rigs, petroleum refineries, chemical plants, natural gas plants, and landfills. Gas flares that are used to remove waste gases eliminate substances that are unsuitable for use or transport. Flare stacks used with non-waste gases serve as safety systems by reducing gas pressures with pressure-relief valves. In addition to reducing equipment strain, these gas flares prevent gas processing equipment from becoming over-pressurized. During an emergency, gas flares or flare stacks can also help to burn out the total reserve gas. There are many different types of gas flares. Examples include ground flares, pit flares, elevated flares, air assist flares, and steam assist flares. Ground flares are either enclosed or equipped with an open-pit design. Most high-capacity enclosed flares are rectangular. Cylindrical products provide lower capacities. Pit flares are used with burn pits lined with refractory brick (firebrick), concrete, or gravel. With elevated flares, a waste gas stream is fed through a vertical chimney and then combusted at the top of the stack. Air-assisted flares provide smokeless combustion and generate little noise. These gas flares reduce smoke formation and flame luminosity at processing facilities without available steam. Steam-assisted flares, as their name suggests, require steam for smokeless combustion. Selecting gas flares requires an analysis of product specifications and application requirements. Gas pressures may be characterized as either high or low. Waste liquid phase is listed as mixed, liquid, or gas. Pit flares are used in low-pressure applications where the waste phase is mixed. If the waste is a bio-gas, then landfill gas flares may be used. Steam flares are designed for facilities that can provide a minimum cooling steam and that require a smokeless steam. Utility flares are also smokeless, but are designed for low radiation burning. Ground flares deliver low-visibility flame, but require adequate space. Other types of gas flares include low pressure flares, gas assisted flares, multi tip flares, offshore flares or boom flares, sonic flares, high pressure flares, low noise flares, and low radiation flares. In terms of applications, gas flares are used widely in the oil and gas, chemical, petrochemical, bio-gas, and steel industries. They are used with well drilling and at food processing plants; at oil rigs and gasoline refineries; and in bio-gas applications at landfills. Monitoring accessories for gas flaring systems include thermocouple sensors, UV flame sensors, remote flame sensors, and flue analyzers.