Reasons for leakage in low-temperature ball valves and reference methods for low-temperature testing

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There are mainly two situations: one is internal leakage； The second is external leakage. Internal leakage: The main reason for internal leakage in valves is the deformation of the sealing pair at low temperatures. When the temperature of the medium drops to a level that causes a phase change in the material, this results in volume changes, which in turn cause the sealing surfaces, which originally had high grinding precision, to warp and deform, leading to poor sealing at low temperatures. External leakage of valves: One cause of this is when the valve and the pipeline are connected using flanges; leakage occurs due to relaxation resulting from the asynchronous contraction of the materials involved in the gaskets, connection bolts, and other fittings at low temperatures. Therefore, the connection method between the valve body and the pipeline can be changed from flange connection to a welded structure, thereby avoiding leaks at low temperatures. The second is leakage at the valve stem and packing. Reference method for low-temperature testing of valves: 1. Preparation before testing – Remove oil stains from the valve components, wipe them clean, and assemble the valve in a clean environment free from dust and oil stains ； Tighten the bolts to the predetermined torque and pull force values, and record these values ； Connect a suitable thermocouple to the valve so that its temperature can be monitored throughout the entire test process. 2. Testing: Install the valve in the testing container and connect it properly, making sure that the valve packing is located at the top of the container, where there are no vaporized gases. Conduct an initial system verification test at room temperature using the specified medium gas at the maximum seat test pressure, to ensure that the valve is in proper condition, after which the testing can begin. Submerge the valve in liquid nitrogen for cooling; the liquid level should cover at least the junction between the valve body and the valve cover. Helium gas should be supplied to the valve throughout the cooling process. During the cooling process, the temperature of the valve is monitored using thermocouples installed in appropriate locations. The valve reaches stability at the test temperature. Use a thermocouple to measure the temperature to ensure that it is uniform throughout the valve. At the test temperature, an initial verification test is conducted using helium at the maximum seat test pressure. The seat pressure test is performed on the inlet side of the valve; for valves that can provide sealing in both directions, the test is carried out on each of the two seats separately. Place the valve in the open position, close the needle valve on the outlet side of the valve, and raise the pressure in the valve chamber to the seat testing pressure. Maintain this pressure as required, and check for leaks at the valve packing as well as at the connection between the valve body and the cover; there should be no leaks. Return the valve to room temperature and then conduct a sealing test at normal temperature: After the test is completed, clean and dry the valve; once it passes the inspection, it is sent out for sale. This is purely my personal opinion