1. Blow-out proof stem: In order to prevent the stem from flying off, resulting in abnormal rising of the inner pressure of valve, the shoulder is fixed at the lower part of the stem. Moreover, for the purpose of preventing leakage resulting from burnout of packing set of the stem in a fire, thrust bearing is set at the contact position of the shoulder at lower part of the stem and valve body. And an inverse seal seat is formed which will prevent leakage and avoid accident.
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