Bend classification is based on its radius of curvature, which can be categorized into long-radius and short-radius bends. A long-radius bend refers to a bend with a radius of curvature equal to 1.5 times the pipe's outer diameter, i.e., R = 1.5D. A short-radius bend refers to a bend with a radius of curvature equal to the pipe's outer diameter, i.e., R = D. In this formula, D is the bend diameter, and R is the radius of curvature. If classified by pressure class, there are approximately seventeen types, which are the same as the U.S. pipe standards, including: Sch5s, Sch10s, Sch10, Sch20, Sch30, Sch40s, STD, Sch40, Sch60, Sch80s, XS; Sch80, Sch100, Sch120, Sch140, Sch160, XXS, with STD and XS being the most commonly used. By angle, there are 45° bends, 90° bends, and 180° bends. This results in a variety of bend types. When placing an order, the specification often takes the following form: for example, "LR STD 90° 8" indicates a long-radius bend with a pressure class of STD and a 90° 8" bend; or "SR XS 45° 4" signifies a short-radius bend with a pressure class of XS and a 45° 4" bend. The above is an overview of the classification of bends.