Approach channel. The passage or passages through which gas must pass from the cylinder to reach the operating parts of the safety relief device.

Combination frangible disc-fusible plug. A frangible disc in combination with a low melting point fusible metal, intended to prevent its bursting at its predetermined bursting pressure unless the temperature also is high enough to cause yielding or melting of the fusible metal.

Combination safety relief valve and fusible plug. A safety relief device utilizing a safety relief valve in combination with a fusible plug. This combination device may be an integral unit or separate units and is intended to open and to close at predetermined pressures or to open at a predetermined temperature.

Compressed gas in solution (acetylene). A nonliquefied gas which is dissolved in a solvent.

Discharge channel. The passage or passages beyond the operating parts through which gas must pass to reach the atmosphere exclusive of any piping attached to the outlet of the device.

DOT regulations. As used in these standards "DOT regulations" refers to the U.S. Department of Transportation Regulations for Transportation of Explosives and Other Dangerous Articles by Land and Water in Rail Freight, Express and Baggage Services and by Motor Vehicle (Highway) and Water, including Specifications for Shipping Containers, Code of Federal Regulations, Title 49, Parts 171 to 178.

Flow capacity. The capacity in cubic feet per minute of free air discharged at the required flow rating pressure of a safety relief device.

Flow rating pressure. The pressure at which a safety relief device is rated for capacity.

Frangible disc. An operating part in the form of a disc, usually of metal and which is so held as to close the safety relief device channel under normal conditions. The disc is intended to burst at a predetermined pressure to permit the escape of gas.

Free air or free gas. Air or gas measured at a pressure of 14.7 pounds per square inch absolute and a temperature of 60°F.

Fusible plug. An operating part in the form of a plug of suitable low melting material, usually a metal alloy, which closes the safety relief device channel under normal conditions and is intended to yield or melt at a predetermined temperature to permit the escape of gas.

Liquefied compressed gas. A gas which, under the charging pressure, is partially liquid at a temperature of 70°F. A flammable compressed gas which is normally nonliquefied at 70°F but which is partially liquid under the charging pressure and temperature, shall follow the requirements for liquefied compressed gases.

Nonliquefied compressed gas. A gas, other than a gas in solution which under the charging pressure, is entirely gaseous at a temperature of 70°F.

Operating part. The part of a safety relief device that normally closes the safety discharge channel but when moved from this position as a result of the action of heat or pressure, or a combination of the two, permits escape of gas from the cylinder.

Pressure opening. The orifice against which the frangible disc functions.

Pressurized liquid compressed gas. A compressed gas other than a compressed gas in solution, which cannot be liquefied at a temperature of 70°F, and which is maintained in the liquid state at a pressure not less than 40 p.s.i.a. by maintaining the gas at a temperature less than 70°F.

Rated bursting pressure. The maximum pressure for which the disc is designed to burst when in contact with the pressure opening for which it was designed when tested.

Reinforced fusible plug. A fusible plug consisting of a core of suitable material having a comparatively high yield temperature surrounded by a low-melting point fusible metal of the required yield temperature.

Safety relief device. A "safety relief device" is a device intended to prevent rupture of a cylinder under certain conditions of exposures. (The term as used herein shall include the approach channel, the operating parts, and the discharge channel.)

Safety relief device channel. The channel through which gas released by operation of the device must pass from the cylinder to the atmosphere exclusive of any piping attached to the inlet or outlet of the device.

Safety relief valve. A safety relief device containing an operating part that is held normally in a position closing the safety relief device channel by spring force and is intended to open and to close at predetermined pressures.

Set pressure. The pressure marked on the valve and at which it is set to start-to-discharge of a safety relief valve.

Start-to-discharge pressure. The pressure at which the first bubble appears through a water seal of not over 4 inches in the outlet of the safety relief valve.

Test pressure of the cylinder. The minimum pressure at which a cylinder must be tested as prescribed in DOT specifications for compressed gas cylinders 41 C.F.R. Ch. 1.

Yield temperature. The temperature at which the fusible metal or alloy will yield when tested in a fusible plug.