Bend classification is based on its radius of curvature, which can be categorized 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, denoted as R=1.5D. Short-radius bends have a radius of curvature equal to the pipe's outside diameter, indicated as R=D. In this formula, D is the bend diameter, and R is the radius of curvature. If categorized by pressure class, there are approximately seventeen types, which are consistent with American 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 wide variety of bend types. When placing an order, the order form often uses the following notation: for example, "LR STD 90° 8" indicates a long-radius bend with a pressure class of STD and an 8" angle; or "SR XS 45° 4" means a short-radius bend with a pressure class of XS and a 4" angle. The above is a general overview of bend classification.