In the rapid development of the LNG industry, the demand for its supporting cryogenic valves is increasing day by day. Because LNG is flammable, explosive, and ultra-low temperature, and LNG plants are usually located in a seaside salt spray environment, higher requirements are placed on the selection and design of cryogenic valves. At present, most of the domestic LNG cryogenic valves still need to be imported, and some cryogenic valves for severe working conditions are still monopolized by foreign countries, such as J-T valves, breathing valves for large LNG storage tanks, large-diameter, high-pound valves, etc. Although the domestic material design meets the low temperature requirements, the pressure bearing and sealing performance of some parts still need to be improved.
LNG cryogenic valve selection key technology
General low temperature technical requirements
Cryogenic or ultra-low temperature valves involved in the LNG field should meet the following basic requirements:
①The low temperature valve needs to lengthen the bonnet, and the bonnet should be an integral bolt connection structure;
②It should be equipped with an extended valve stem to ensure that the packing is at ambient temperature, and all valve stems are of anti-blowout structure.
③The valve trims should be easy to disassemble, especially the valve seat can be maintained online and easily replaced, the trims and seals can be quickly disassembled from the top of the valve, and can be repaired online.
④The valve with soft sealing structure should adopt fireproof design.
⑤ All ball valves should be equipped with pressure relief devices.
⑥ Ball valves and globe valves with external sealing structure should have external sealing performance.
⑦The valve body, internal parts, bonnet, valve stem, actuator, etc. must be designed with anti-static structure.
Cryogenic valve selection considerations
【1】Material selection of pressure-bearing parts
Material quality is an important indicator to measure the strength, reliability and service life of the valve. At present, austenitic stainless steel is recommended for cryogenic valves for LNG. Commonly used grades for valve body are A351 Gr.CF3M, A182 Gr.F316L, A182 Gr.F304L; common grades for valve core are A351 Gr.CF8M, A182 Gr.F316, A182 Gr .F304.
[2] Selection of sealing surface type and material
Cryogenic valves for LNG usually adopt a hard sealing structure. The material of the valve core (or valve plate) is not less than 316 stainless steel. In the case of high pressure difference or prone to cavitation or flash evaporation, consider using hard stainless steel or Stellite alloy. On the valve core (or valve plate) and valve seat Surfacing STL alloy. LNG low temperature ball valve seat seal is recommended to use metal seal + embedded soft seal, and the soft seal material can be polytrifluorochloroethylene (PTFCE). Stem packing is generally recommended to adopt the structure of “lip seal + graphite”.
[3] Reasonable determination of the extension length of the valve stem
For LNG ultra-low temperature medium, the extension of the valve cover is a key design indicator. 80% of the valve leakage in the on-site LNG project is due to the fact that the manufacturer did not configure enough extended valve covers in order to save costs. BS6364 and Shell’s MESC SPE77/200 have different requirements for this length. Shell’s elongation requirements are strict, but only relying on increasing the elongation requirements to ensure that the packing does not freeze is lack of field basis. It is recommended to take the requirements of BS6364 for non-cold box valves as the basis, and comprehensively consider material selection, local environmental dew point value and various factors. Based on various factors such as the manufacturer’s project experience, the extension amount is selected through thermal analysis and actual test conclusions.
[4] Design considerations for valve piping accessories
Application Status of Cryogenic Valves in LNG Field - Zorgon (Zhejiang) Automation Technology Co.,Ltd. (cnzorgon.com)
www.cnzorgon.com/application-status-of-cryogenic-valves-in-lng-field.html
