Special structure of cryogenic valve
1) The valve body should be able to withstand the expansion and contraction caused by temperature changes, and the structure of the valve seat will not be permanently deformed due to temperature changes.
2) Valve cover: The structure of the long neck valve cover is used to protect the stuffing box. The long neck bonnet structure also facilitates the winding of the cold insulation material.
3) Valve flap (gate): The gate valve adopts a flexible shutter or an open gate; the flat valve seat of the shut-off valve and the needle valve adopt a plug-shaped valve flap. These structural forms maintain a reliable seal regardless of temperature changes.
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4) Stem: The valve stem needs to be chrome-plated, nickel-plated or nitride to improve the surface hardness of the valve stem, prevent the valve stem from packing with the packing and packing, and damage the sealing packing, causing the stuffing box to leak.
5) Gasket: The gasket selection should consider the low temperature properties of the material, such as compression resilience, preload, tightening pressure distribution and stress relaxation characteristics.
6) Stuffing box and packing: The stuffing box cannot be in direct contact with the low temperature section, but is placed at the top of the long neck valve cover, so that the stuffing box is located far away from the low temperature zone, working under the temperature environment above 0 °C, improving the stuffing box The sealing effect. The stuffing box adopts a two-stage packing structure with an intermediate metal isolation ring. The grease can be added from the middle of the stuffing box to form an oil seal layer to reduce the packing box pressure difference.
7) Upper sealing: The cryogenic valves are all provided with an upper sealing structure, and the upper sealing is to be deposited with cobalt-chromium-tungsten carbide, and is ground after finishing.
8) Seat, disc, (shutter) sealing surface: The closing part of the cryogenic valve is made of cobalt chromium tungsten carbide surfacing mechanism. The soft seal structure has a large expansion coefficient of polytetrafluoroethylene and becomes brittle at low temperatures, so only a low temperature valve having a temperature higher than -70 ° C is used, but polytrifluoroethylene can be used for a low temperature valve of -162 ° C.
9) Middle flange bolts: 1): Bolts should have sufficient strength because the bolts work under repeated loads and often break due to fatigue.
2): Because the bolt is easy to cause stress concentration at the root of the thread, it is better to use a bolt with a full thread structure.
10) Cold insulation: Also known as the heat insulation board, it is a circular plate welded to the neck portion under the stuffing box. Improve the insulation effect.
11) Measures to prevent abnormal pressure rise: After the valve is closed, some liquid will remain in the valve chamber. As time increases, these liquids absorb back the heat in the atmosphere, rise back to normal temperature and re-gasify. After gasification, its volume expands rapidly, about 600 times more, resulting in extremely high pressure and acting inside the valve body. This phenomenon is called abnormal boosting and is a peculiar phenomenon of low temperature valves. When an abnormal boost occurs, the shutter is pressed against the valve seat, causing the shutter to fail to open. The high pressure will flush out or break the packing of the middle flange gasket; it can also cause deformation of the valve body and the valve cover, so that the sealing performance of the valve seat is significantly reduced.
In order to prevent abnormal boosting, the following measures are generally adopted for the construction of low temperature valves:
1) Set the pressure relief hole, also known as the pressure balance hole or the vent hole.
2) Set the outlet pipe or install a safety valve on the valve to discharge abnormal high pressure.
Technical requirements for cryogenic valves
Cryogenic treatment: A method of immersing parts in a low temperature liquid nitrogen tank for a certain period of time to reduce the deformation caused by temperature difference and metallographic structure, thereby improving the sealing performance of the valve at low temperatures.
1) Cementing of hard alloys on the sealing surfaces of the valve disc and seat shall comply with JB/T 6438. When the use temperature is lower than -100 °C, after surfacing, it should be subjected to cryogenic treatment, that is, it is immersed in liquid nitrogen at -196 °C for 2-6 hours before grinding, and then taken out for natural treatment, and then ground and assembled.
2) For low temperature valves below -100 °C, the valve body, valve cover, valve disc, valve seat, valve stem and other parts should be cryogenically treated before finishing, that is, the parts are immersed in a liquid nitrogen tank for cooling. When the temperature of the part reaches -196 ° C, the temperature is started for 1-2 h, and then the outside of the box is taken out to the normal temperature, and the cycle is repeated twice.
Cryogenic valve cryogenic treatment
Test method for cryogenic valves
1. Normal temperature test: The normal temperature test should meet the requirements of GB/T13927, JB/T9092, API598 and API 6D.
2. Low temperature test: The low temperature test of the low temperature valve is carried out after passing the normal temperature test.
Before the test, the water and grease in the valve body should be removed, and the thermocouple connected to the test should be connected with the valve. The temperature of the valve body and the valve cover should be monitored during the test. The low temperature test cooling medium is a mixture of liquid nitrogen and alcohol or liquid nitrogen, and the test medium is helium.