Among the fittings of the structure tightened with screws such as 2 double rings compression and bite rings type, metal gasket type, metal C ring type, etc., those with a structure that keeps the fitting's airtightness at a contact surface other than the screw is called Threaded Connection Fittings. In general rule 6, article 1, item 35 of High pressure gas safety law, it is stated that, "The piping, pipe fittings and valves for gas equipment that use Toxic gas should be done by welding. However, in situations where welding is not appropriate, it can be used alternatively flange or threaded connection fittings that have strength necessary for safety. (Cone & thread type fittings are limited to being used in compressed hydrogen gas stations)