For those in the LNG industry and at Shaanxi Fuel Research Institute, please take a look (valve connection issues)
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The \"Design Code for Natural Gas Liquefaction Plants\" GB 51261-2019 came into effect on December 1, 2019. The editorial boards are composed of Shaanxi Fuel Research Institute and Sinopec Zhongyuan Petroleum Engineering Design Co., Ltd. Based on past usage experience and manufacturer surveys, I have doubts regarding clause 8.5.2 of the standards, and I hope those who are familiar with it will kindly share their insights. 8.5.2 Valves installed on pipelines carrying low-temperature flammable media shall be fire-resistant, and butt welding is preferred. What is the reason for using butt welding here? In actual manufacturing by manufacturers, as well as in previous projects and those low-temperature tank trucks, many of the low-temperature valves use socket welding; what is the reasoning behind this standard recommending butt welding? If butt welding is suitable for valves, what about fittings such as elbows and tees? Is it also welded together?
This post was last edited by Zhou Liuliuxu on 2020-6-17 at 13:56. Standardly speaking, “it is advisable to use” does not mean it is mandatory; it is merely a recommendation to give priority to such usage. Whether it is a butt-welded connection or a socket weld connection, the goal is to prevent leakage of the medium. The leakage of low-temperature flammable media poses significant safety risks (fire and explosion, injuries due to low temperatures), so leakage is not allowed. Flange connections may lead to leakage, hence they are also not recommended. Therefore, it is best to use butt-welded connections for elbows and tees as well. In petrochemical plants, I have seen that the elbows in the pipelines there were all welded directly to the pipes; I’m not quite sure about tees, but it seems they too were welded directly to the pipes. Valves and pumps, on the other hand, use flange connections, but since the media involved are not low-temperature media, leakage is less likely to occur. If there is any leakage, it can usually be detected during routine inspections. If the medium is liquefied hydrocarbons, it has a strong odor, so even a small amount of leakage can be detected. As for how to make the final choice, it is necessary to carefully check whether there are any regulations specified in the standards!
With socket welding, after a low-temperature valve fails and the old valve is cut out, the pipe length is reduced by a certain amount, which is quite troublesome; whereas with butt welding, the pipe length does not decrease. The customers of our company who purchase pneumatic low-temperature ball valves choose either butt-welded or threaded connections
It’s mainly experts; as long as they’re suitable for the task, they’re necessary, but it’s hard to communicate with them
Low-temperature liquid pipelines generally require 100% flaw detection, as it is difficult to perform flaw detection using socket welding.
Welding is used to address medium leakage caused by temperature gradient changes, while butt welding is chosen to ensure welding quality.
1. In petrochemical plants, valves supplied with spigot-and-socket short pipes have experienced pipe breakage, which can easily lead to serious accidents； 2. Welding of valves and short pipes together allows for RT inspection, whereas the socket connection method, due to the use of fillet welds, only permits PT inspection and not RT inspection.