The leakage of bellows stop valve is generally divided into internal leakage and external leakage, and there are many reasons for the leakage. Endocrine factor is generally the failure caused by the destruction of the sealing surface by solid impurities in the liquid medium. Combined with the analysis of field use, the causes of valve stem leakage are mainly related to the valve structure, working environment and operation mode.

1. Structure:

The valve rod adopts the double sealing structure of bellows and packing. The bellows is welded with the inner ring and outer ring to form a bellows assembly, and then welded with the valve rod and guide body to block the outward leakage of medium through the valve rod. The up-down movement distance of the valve stem moves upward from the flat key I until the upper end face of the flat key I contacts the lower end face of the lower packing. As the lower packing is made of PTFE, if the valve is opened excessively, the flat key I will be embedded in the packing, the valve can no longer be opened and closed, and the bellows assembly will be killed, resulting in medium leakage along the valve stem. The tensile length and compression of the bellows are determined according to the stroke of the valve. If the limit tension or compression is exceeded, the bellows may be damaged, resulting in small braided pipe rupture and seal failure. Therefore, it is necessary to increase the limit device for opening and closing the valve, so that the bellows can expand and contract within the design range all the time during the opening and closing of the valve.

2. Working environment:

The bad working environment of bellows stop valve is also one of the reasons for its failure. As the bellows stop valve installed in the ship works in the salt fog environment for a long time, and there is often condensate dripping on the upper part, resulting in the corrosion of the upper thrust ball bearing. In serious cases, it will lead to contact corrosion with the valve stem.

3. Operation:

If the bellows stop valve is opened or closed excessively in use, the valve may not meet the relevant performance requirements. When the valve is excessively closed, the sealing surface may be crushed, resulting in sealing failure when the valve is reused. When the valve is opened excessively, the bellows assembly may be operated, resulting in valve leakage.

Failure analysis and improvement of bellows stop valve:

After analyzing the existing problems of the valve, its structure, operation and environment are improved. First, increase the limit welfare of flat key I, and add a 3mm thick stainless steel packing pad to the lower packing of PTFE to prevent the packing from getting dirty and limiting the valve stem.

When the valve is closed, if the valve cannot be closed, it shall not be forced to close, but open the valve, wash it with fluid for a period of time, and then close it again and again. If it still cannot be closed, check the grinding sealing surface. When the valve is opened, when the valve is opened to a certain height and the valve stem encounters small resistance, try not to continue forced opening, so as to prolong the service life of the valve stem and its sealing components.

For the stop valve with thrust ball bearing on the upper part of the valve stem, during installation, try to avoid condensate falling directly on the valve stem, resulting in corrosion of the valve stem.