A thorough discussion on the material selection for instrument air supply pipes

Let me put my opinion forward first – in my view, seamless stainless steel pipes should be used. I’m not sure if it’s correct

Seamless pipes and galvanized pipelines are used; tees with diameter changes are available from the main pipe to the branches, all of which are finished products.

This post was last edited by zxb1990 on 2016-3-11 at 21:44. The gas supply pipes are made of stainless steel, and the pipes are connected to each other using flanges.

Our company’s main gas supply pipes are made of carbon steel, while the branch pipes are made of galvanized steel. For the sections leading to the valves, stainless steel and copper pipes are used. Even with different materials and sizes, it is all achieved through reducers and tees.

Stainless steel, carbon steel, and copper pipes can be selected as needed, with threaded connections between them. The gas supply section is generally made of carbon steel, while the part near the valve is made of stainless steel or copper pipes.

Whether a pipeline is seamless depends mainly on pressure, which can be determined through calculations; however, the instrumentation field generally does not carry out such calculations. When designing, design institutes generally require that the main air path of the compressed air system be seamless; consequently, the branch pipes leading to the valves are also seamless

Regarding the material selection for instrument air supply pipes in the petrochemical industry: 1. According to regulatory requirements, galvanized welded steel pipes are sufficient for main pipes and branch pipes under normal circumstances. Design institutes generally design it in this way: from the branch pipe to each point where gas is used (the valves), the 8mm gas supply pipeline between the gas supply ball valve and the valves is usually made of 304 stainless steel, and the gas supply ball valve itself is also made of 304 material ； Other branch pipes and main pipes are all made of galvanized welded steel pipes. 2. Galvanized welded steel pipes are not seamless pipes; the so-called “seam” is created through welding. Therefore, welding is not allowed during the construction of seamless pipes. When galvanized welded steel pipes are welded, their galvanizing layer is damaged, and they can no longer provide rust protection in those areas. This can lead to rusting of the gas supply pipelines, resulting in contamination of the gas supply and subsequent malfunctions in the instruments. Secondly, when welding galvanized welded steel pipes, it is easy to cause cracks in the welds of the pipes themselves. Therefore, galvanized welded steel pipes are all connected using threaded connections, that is, a threading machine is used on-site to thread the ends of the steel pipes. 3. In some northern regions, due to the extremely low temperatures in winter, the drying effect of the gas source is poor; water content in the gas source is high, leading to severe corrosion. As a result, the lifespan of galvanized welded steel pipes is short. Many clients require that gas supply pipelines be made of stainless steel, and this is also the practice in many projects in those northern areas. Special areas require differentiated treatment; when owners have requests, their demands and practical issues should also be respected. 4. Although the specifications require the use of galvanized welded steel pipes, these are merely minimum requirements; it is better and also acceptable to use stainless steel pipes, although the cost is higher. As long as the owner approves, why not use them?

The gas source doesn’t allow welding, right? Use a swivel joint for connection

Seamless steel pipes, galvanized pipes, stainless steel pipes, and even hoses can all be used; it depends on the application scenario

In our company, the instrument air pipelines are made of carbon steel pipes (connected by flanges) starting from the power source; their diameter is around DN100 or more. These pipes branch off to various workshops, where their diameter becomes DN50. After passing through the buffer tanks in those workshops, they remain carbon steel pipes, while the branch pipes become galvanized pipes. The main pipes and branch pipes are connected using flanges; no welding is used thereafter. The branch pipes are generally 4/4 inch galvanized pipes, all connected by threading. Upon reaching the locations where air is needed, stainless steel air supply ball valves are used, followed by stainless steel 8mm air supply pipes, all of which are seamless pipes. For connecting gas supply pipes of different diameters, reducers are used; for branches with the same diameter, tees and similar components are employed – all of these are standard parts……