Use:

A21H type are used for the equipment and pipelines with such media as air, ammonia and petroleum at a working temperature less than 200℃.A21W type are used for the equipment and pipelines containing corrosive gases or liquidsat a working temperature less than 200℃.A21F type are used for the equipment and pipelines of liquefied gas and petroleum gas at a working temperature less than 80℃.They function as anoverpressure protection device.