is compressed to pressures up to 5,000 psi and is commonly known as compressed natural gas (CNG). Natural gas is odor ized with mercaptan to yield the familiar “rotten egg” smell. Natural Gas Vehicle: A motor vehicle with a combustion engine fueled by natural gas. Paths of Migration: The flow paths by which natural gas travels as it dissipates throughout a structure. Pressure Relief Device (PRD): A valve activated by high temperature or pressure used to eliminate overpressure in a vehicle fuel tank or system. A temperature-activated valve is designed to open at a given temperature (for example in the event of a fire) and remain open.
Once the device is activated, the valve remains permanently open and cannot be reset to the closed position. A pressure-activated valve is designed to open at a given pressure and will remain open until the overpressure is released. This type of valve is resettable and will reseal when the lower prescribed pressure is realized. Pressure Release Valve (PRV): A type of valve used to control or limit the pressure in a system or vessel. A PRV is usually resettable and when activated by high pressure will open to release the overpressure until a lower prescribed pressure is realized, then reseal.
PRVs are usually found on stationary tanks and equipment and can also be used for vehicle pressure regulators. PRVs are also known as safety relief valves (SRVs). Upper Flammability Limit (UFL): The maximum concen tration of a combustible gas that will ignite; sometimes referred to as the upper explosive limit (UEL). This is generally accepted to be 15% natural gas in air by volume at atmospheric conditions. COMPRESSED NATURAL GAS VEHICLE MAINTENANCE FACILITY MODIFICATION HANDBOOK IV