Purposes: 
 The A21H Type operating temperature ≤ 200 ° C air, ammonia, liquefied petroleum gas (LPG) and other media equipment and pipeline. The A21W Type applicable to operating temperature ≤ 200 ℃ or corrosive gas or liquid equipment and pipeline A21F type applies to liquefied petroleum gas, liquefied petroleum gas (LPG) and other equipment and pipeline operating temperature ≤ 200 ° C, as overpressure protection device. Exceed the allowable value when the internal pressure equipment and piping, valves automatically open, then the whole amount of emissions; when the pressure is reduced to the specified value, the valve automatically closes to ensure the safe operation of equipment and piping. 
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