Bend types are categorized by their radius of curvature, which can be divided into long-radius and short-radius bends. Long-radius bends refer to those with a radius of curvature equal to 1.5 times the pipe's outside diameter, i.e., R = 1.5D. Short-radius bends refer to those with a radius of curvature equal to the pipe's outside diameter, i.e., R = D. In the formula, D represents the bend diameter, and R represents the radius of curvature. If categorized by pressure class, there are approximately seventeen types, which are the same as the US 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 commonly used. By bend angle, there are 45° bends, 90° bends, and 180° bends. This results in a wide variety of bend types, and when placing orders, the following notation is often used: for example, "LR STD 90° 8" indicates a long-radius bend with a pressure class of STD and a 90° bend of 8 inches; or "SR XS 45° 4" indicates a short-radius bend with a pressure class of XS and a 45° bend of 4 inches. The above outlines the general classification of bends.