Some designers believe that the wall thickness of some key parts of the same pipe system such as carbon steel elbows, tees, reducing pipes should be greater than that of the pipe due to stress concentration, which is not true. During field construction, even for pipes and elbows of the same pressure class, the wall thickness of the elbows is slightly larger than that of the pipes. If the thickness of the elbows is increased, the wall thickness of the two pipes will be inconsistent and they cannot be welded together well.
An elbow is a tube bent at an Angle (45° or 90° or 180°, etc.), with the outside being thinner in stretch than the inside. To eliminate this effect, make sure that the outer thickness of the pipe is required so that the elbow is made with a "thicker" tube. According to ASME B31.3 and ASME B16.9 and other standards, there is a formula to calculate the thickness of the elbow, one outer thickness and one inner thickness. Generally speaking, the thickness required by the inner side is larger than that of the outer side, which is generally 2 times that of the outer side. The reason is that the backarc of the elbow is the thinnest position, and it is also the most heavily scoured position. The wall thickness is increased only to increase the ability to bear pressure and resisting scour.
Every part of the pressure piping system such as the elbow, the valve has its own application standard, should follow the principle of equal strength, namely each element in the same pipe has the same adaptability of the medium, the same intensity and the same life (removable parts such as valve packing, gasket, except), it is mainly manifested on the material selection and application of the standard. For piping parts with nominal pressure classes, the same or similar application standards, including temperature-pressure should be used. the pipe with the same strength should be selected for the pipe elbows. Generally, carbon steel elbows are also made from pipes., as long as the thinnest part is greater than the "minimum wall thickness".