Bend classification is based on its curvature radius, which can be categorized into long-radius bends and short-radius bends. Long-radius bends refer to those with a curvature radius equal to 1.5 times the pipe outer diameter, i.e., R = 1.5D. Short-radius bends refer to those with a curvature radius equal to the pipe outer diameter, i.e., R = D. In the formula, D is the bend diameter, and R is the curvature radius. If categorized by pressure class, there are approximately seventeen types, which are the same as 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 the most commonly used. By angle, there are 45-degree bends, 90-degree bends, and 180-degree bends. This results in a variety of bend types. When placing an order, the order form often uses the following notation: for example, "LR STD 90o 8" indicates a long-radius bend with a pressure class of STD and a 90-degree 8" bend; or "SR XS 45o 4" indicates a short-radius bend with a pressure class of XS and a 45-degree 4" bend. The above is a general overview of bend classifications.