Stainless steel pipes are used for many industrial, domestic and commercial applications. The main areas of usages are the structural engineering and fluid transfer. There are liquids and gases that are transported through the pipes. The gas pipelines is one of the major applications where steel pipes are used. There are different types, varieties and grades of stainless steels that are used in gas pipe lines.
Requirements for pressure gas pipelines
Gas pipelines are carried out over land or under water. The distance, pipeline size, depth and throughput capacities are some factors that decide which pipe could be used in the piping. Depending on the application, the pressure requirement can go anywhere from 70 bars to 200 bars generally. Also, the gases are of oxidizing or reducing capacity. So the pipelines have to be made in materials that reduce or resist oxidation and reduction. When supplying, the pipes should remain corrosion resistant for longer periods of time and without embrittlement and cracking. So the selection of the material grade for different types of gas pipes depends on the requirements of the application.
Material grades for gas pipes and fittings
Stainless steel grades such as 304L, 316 and Inconel are used in high pressure piping fittings. The fittings need to be strong enough to hold the pipes and at the same time retain the original mechanical properties of the piping material. The high pressure gas pipes can be of various pressure ratings. The fittings have to provide the same pressure rating in order to maintain integrity of the pipes. The grade 304 is a very cost effective material for fittings, but the corrosion resistance and oxidation resistance properties are lower than other grades. It has the minimum tensile strength of 505MPa and minimum yield strength of 215MPa. It is a chromium nickel alloy and is suitable for low grade pressure pipe fittings.
The 316 is a chromium nickel and molybdenum alloy. So the fittings made up of this grade are more corrosion resistance, especially to chloride ion corrosion cracking. These are suitable for underwater pipes with medium level of pressure because they have a tensile strength of 580MPa and yield strength of 290MPa.
The Inconel or nickel chromium cobalt molybdenum alloys are very strong, very much corrosion resistance and in addition oxidation and corrosion resistant. There are different grades in the Inconel family and they have very high tensile and yield strengths. For example, the Inconel 625 has 880MPa tensile strength and 760MPa yield strength. These properties make the Inconel fittings the best choice for high pressure gas fittings.
However, the carbon steel pipes are widely used for gas pipes applications because of their optimum cost and performance with gas lines. Depending on the carbon content, they are categorized as low carbon, mild carbon, medium carbon and high carbon steels.
The production types of fittings
There are different types of fittings such as butt weld, socket weld, threaded, flanged, spigot and buttress fittings; each has its own specified application properties. The buttweld fittings are most common where the socket weld fittings are better sealed with the socket and the welding made to connect the pipes. Welded fittings can seal the gaps. The application of filler material and professional welding by conventional and modern methods can eliminate the chances of leaking. Wrought pipe fittings are the best for the gas line applications. These are created by impact force to form a fitting from one piece of raw material. So there are no welded joints, deformations, weak spots etc. in the fittings. This allows for the fittings to be highly efficient in connecting the pipes together.
So, the best fit for high pressure gas fittings would be wrought fittings that are made up of high pressure material such as Inconel. But is always the requirement of the application and the cost efficiency that determines what type and grade of fitting could be used.
