Bend classifications are based on their curvature radius, which can be categorized into long-radius and short-radius bends. Long-radius bends refer to those with a curvature radius equal to 1.5 times the pipe's outer diameter, denoted as R=1.5D. Short-radius bends have a curvature radius equal to the pipe's outer diameter, R=D. In this formula, D represents the bend diameter, and R is the curvature radius. When categorized by pressure class, there are approximately seventeen types, which align with 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 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 specification often takes the form: for example, "LR STD 90° 8" indicates a long-radius bend with a pressure class of STD and a 90° bend in 8" diameter; or "SR XS 45° 4" indicates a short-radius bend with a pressure class of XS and a 45° bend in 4" diameter. The above is a general overview of bend classifications.