Bend classifications are based on their 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 equal to 1.5 times the pipe's outside diameter, denoted as R=1.5D. A short-radius bend has a radius equal to the pipe's outside diameter, indicated as R=D. In the formula, D represents the bend diameter, and R represents the radius of curvature. When categorized by pressure class, there are approximately seventeen types, which are the same as the 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 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" bend at 90°; or "SR XS 45° 4" denotes a short-radius bend with a pressure class of XS and a 4" bend at 45°. The above is a general overview of bend classifications.