The valves on this part of the cryogenic pipeline mainly include cryogenic stop valves and cryogenic check valves. To avoid low-temperature brittle damage, the main material of cryogenic valves is austenitic stainless steel, the valve internals must meet the requirements of low temperature resistance, and the valve disc is welded with hard alloy. In order to stabilize the valve size, improve the valve sealing performance, and eliminate the stress caused by organizational changes, the valve and its parts need to be cryogenically treated. The method of cryogenic treatment is to place the valve at a low temperature of -196℃ for a certain period of time, and then take it out and place it at room temperature to restore it to room temperature.
The welding parts of the valve pressure-bearing parts should be subjected to welding process assessment and Charpy impact test according to the standard, and the low-temperature impact value should meet the regulations. Cryogenic valves have special structural designs. Cryogenic valves adopt a long-necked bonnet structure, and the bonnet extension section should meet the requirements of the gasification space so that the working temperature of the valve stem packing meets the use conditions. To prevent condensed water from entering the bonnet insulation layer, a water receiving tray is usually set at the neck of the bonnet extension section.
For low-temperature valves with double valve seats, there is a closed middle cavity when the valve is closed, and the liquid in the middle cavity may be damaged due to gasification. Therefore, a pressure relief hole is set on the gate, ball or valve seat to discharge the high-pressure medium in the closed middle cavity. The diameter of the pressure relief hole should not be less than 3mm, and the pressure relief direction should be the upstream high-pressure side. The pressure relief direction should be marked in a conspicuous position on the valve, preferably on the isolation water tray.
The pressure relief direction of the valve is different from the medium flow direction of the valve. In the past, during the on-site installation of low-temperature valves, some construction workers took the pressure relief direction of the valve as the medium flow direction when installing the valve. If a problem was found during the verification process, the valve would be removed and reinstalled.
