Bend classification methods, divided by their curvature radius, include long-radius bends and short-radius bends. Long-radius bends refer to those with a curvature radius equal to 1.5 times the pipe's outside diameter, i.e., R = 1.5D. Short-radius bends refer to those with a curvature radius equal to the pipe's outside diameter, i.e., R = D. In this formula, D is the bend diameter, and R is the curvature radius. If classified by pressure grade, there are approximately seventeen types, which are the same as 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 bend angle, there are 45° bends, 90° bends, and 180° bends. This results in a variety of bend types. When placing an order, the order often uses the following notation: for example, "LR STD 90o 8" indicates a long-radius bend, pressure grade STD, and a 90° 8" bend; or, "SR XS 45o 4" indicates a short-radius bend, pressure grade XS, and a 45° 4" bend. The above is a general classification of bends.