Such failures include "cooling leakage" and freezing of valve packing, leakage of valve seat and pipe connecting flange, and leakage of valve screw sleeve at both ends of tinned thread. The stem packing is usually located in the packing groove of the stem close to the cold box wall. When the packing is not uniform or tight, and the valve stem is not straight or round, cryogenic liquid or gas will leak along the packing gap. As a result of cold transfer to the outside, the water in the air will freeze on the packing, thus freezing the valve stem. In this case, the valve can only be opened and closed by heating the filling with steam or hot water. However, after opening and closing the valve, the water accumulated in the packing will freeze again. The valve stem is usually twisted and the handwheel is broken due to the hard work of valve opening and closing.
Therefore, after the maintenance of the valve, the packing should be evenly and tightly packed, and the compression nut should be tightened. During the whole leakage test of the air separation unit, the leakage at the valve stem packing should also be checked. Before the cold start, this problem should be thoroughly solved to prevent the occurrence of larger faults. The common causes of flange leakage are that the sealing surface is not smooth, the sealing surface is not uniform, the pipeline compensation is insufficient, the bolts are not evenly tightened, and the bolt material is improper, etc. Both ends of the external thread sleeve of the valve rod are filled with tin, and the tin is easy to crack and leak after long-term use. During the pressure test of leakage, if such leakage is found, it is better to take out the valve stem, refill tin and tighten it, and silver welding is preferred.
