Selection of LNG vacuum pipes and fittings?
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We are currently building an L-CNG refueling station, and when installing the pipelines, we faced the issue of choosing vacuum pipes and fittings. I have seen two types of vacuum tube connections: in one type, the straight sections are made of vacuum tubes, while the elbows are not; an expansion tube is added at the junction between the straight sections and the tube ends, and the elbows are insulated using thermal insulation materials. The second type is probably the one that is commonly used in China at present: the flanged vacuum tube, where the pipelines are connected using vacuum flanges. But I’m not sure how to handle the pipe ends in this case How does this connection method take into account the cold contraction of pipes? I would also like to ask what materials are currently used for the interior and exterior components of the vacuum tube? Is 0Cr18Ni9 okay?
0Cr18Ni9 can definitely be used. The second option is better, because it’s impossible to manufacture vacuum tubes on-site; they must be ordered from the manufacturer, which is quite troublesome.
The advantage of low-temperature elastic insulation materials is that, regardless of the complexity of the pipes, insulation systems can be fabricated on-site according to the shapes of flanges, tees, and elbows, without having to worry about joints or moisture barriers. It provides a very effective heat-insulating effect for LNG pipelines and equipment.
Under normal circumstances, flange connections are highly recommended for vacuum pipes (the flange connections referred to here are specialized plug-and-socket flanges); after all, there is a significant difference in the amount of gas generation between vacuum flange connections and welding. However, welded connections are recommended in the following cases: 1. When the pipe diameter is greater than 100; 2. For high-pressure pipes; 3. For toxic or flammable and explosive substances. LNG falls into the category of flammable and explosive substances, so it is advisable to use welded connections. Furthermore, there are many LNG sites with limited space and close pipe spacing, which are also not suitable for socket flanges
The second option is better; vacuum tube fittings offer good sealing performance, and bellows can be used in the pipelines to address thermal contraction