With the development of society and the improvement of people’s awareness of environmental protection, more and more attention has been paid to clean energy. LNG is a kind of clean and efficient energy, which plays an important role in the production and life of the people. Therefore, LNG receiving stations and LNG transport ships have sprung up like mushrooms after a spring rain. LNG is the process of cooling gaseous natural gas to – 162 ℃ at atmospheric pressure to condense into liquid. Therefore, LNG pipeline must have high low temperature performance.
For low temperature LNG conveyor pipes, most of them are made of ultra-pure, low carbon, low sulfur and low phosphorus stainless steel at home and abroad. In recent years, double-brand stainless steel is very popular with LNG users, especially TP304/304L and TP316/316L. Double certificate steel not only has L grade corrosion resistance and low temperature performance, but also has high mechanical properties.
At present, the mainstream welding stainless steel pipes for LNG at low temperature at home and abroad are generally processed by automatic welding forming process, UOE forming process and JCO forming process.
Automatic welding unit is a fast, efficient and automatic production mode of welded pipe under the condition of non-thick wall. At present, most of the welding units use row rollers to shape sheet metal, then weld and heat treatment on line. Some of the welding units also integrate on-line ultrasonic wave of sheet metal, on-line ultrasonic wave of weld seam and self-welding seam. Advanced technology, such as dynamic tracking technology, can provide efficient manufacturing operations for LNG long-distance pipeline manufacturing and reduce the production period of manufacturers.
UOE forming technology is one of the most widely used, mature and quality low-temperature LNG pipe production processes in the world. Its main technology has been finalized.
JCO forming process is a new forming process in recent years. This forming technology is an organic combination of step-by-step pre-bending and NC bending of pipe body.
For LNG stainless steel welded pipe, because its working environment is in – 162℃ low temperature environment, so for LNG pipe, it must have high low temperature impact performance. At present, most design institutes, research institutes and manufacturers require that the low-temperature impact performance of LNG pipes should not be less than 80J, and the lateral expansion should not be less than 0.38mm according to ASME B 31.3.
For stainless steel welded pipes, the weld is the weak link of the pipe material. The quality of the weld directly affects the safety of the pipeline and even the transportation pipeline. Welding coefficient is one of the important factors to evaluate the quality of weld. For low temperature LNG welded pipe, the welding coefficient is Ej=1.0, and the welded joint must be full penetrating joint. After the completion of the welded joint, all welds must be 100% X-ray inspection. The welds must be free of defects such as impermeability, inclusion, undercut and crack, so as to ensure the stability of the welded joint at low temperature.