1. Failure occurs on every connection. Cryogenic valves fall into this category of failures: valve sealing and freezing, valve seat and piping connection flange leaks, leaking tin filler screws at both ends of valve screw sleeves. Stem packing is usually located in the packing groove of the valve stem near the wall of the cold box. If the packing is unevenly packed, not firm, or the stem is not straight or round, cryogenic liquid or gas can leak along the packing gap. External heat and cold transfer freezes moisture from the air in the packing, which freezes the valve stem. In this case, only steam or hot water is used to heat the injector and open and close the valve. However, when the valve is opened or closed, the water accumulated in the packing can freeze again. Because valve switching is cumbersome, fire valves often twist the valve stem, damaging the handwheel.
Therefore, after a major overhaul of the valve, the gasket must be installed and tightened evenly and the compression nut tightened. Leaks in the valve stem packing should also be checked during the overall leak test of the air separation unit and this should be addressed prior to cold running. Common causes of flange leakage are non-smooth, flat sealing surfaces, poor pipe compensation, uneven bolt tightening, and improper bolt material. The end of the threaded sleeve on the outside of the valve stem is connected by a tin-filled thread, which is prone to cracking and leakage after long-term use. If the leak pressure test finds a leak, it is best to pull out the valve stem, refill it with tin and tighten, and it is best to use silver soldering. Cryogenic valves are equipment that requires frequent maintenance, high standards, strict requirements, and careful maintenance and use during the normal operation of an air separation plant.
Cryogenic valve
2. The top of the valve fails. The valve top (valve head) of this type of valve is removably attached to the valve stem so that both sealing surfaces can be "self-adjusted" when the valve is closed. Check gaskets can damage titanium, nickel, zirconium, monel alloy valves after long-term use, which can cause the locknut to loosen and the valve top to fall off when the valve is opened. At this point, the valve loses its closing function. Damaged locknuts may be due to improper material, use cold, brittle metal instead. Another common obstacle is that the top of the valve is not closed tightly, or the valve is leaking.
A common cause is that the top of the valve and the sealing surfaces of the valve seat are crushed by hard objects such as silicone, metal shavings, welding slag, and other hard objects, creating dents. In this case, marine valves often expend a lot of effort to close the valve, exacerbating the pressure loss. The centerline of the valve stem may not be perpendicular to the sealing surface (valve surface) of the valve seat, or the valve top surface and valve surface may wear over long-term use, and the valve may leak.
3. Valve stem failure. A more common hindrance is thread wear on the valve stem and screw sleeve. The valve does not close due to the ceramic valve. Generally, the threads on the stem are less likely to wear, while the female threads on the threaded sleeve (brass) are more prone to damage. The reason could be that the force when opening the valve is too high, or that the threads are difficult to twist even after opening and closing, and the threads are seized.
Occasionally, the screw may be completely "bited" and the valve may only open and not close. To close the valve, push the stem inward with a temporary bolt, wait for repair, and then repair. To prevent this obstacle, it is important not to use excessive force when opening and closing the valve, and also not to use excessive force when opening and closing the valve
