The classification of elbows is based on their radius of curvature, which can be divided into long-radius elbows and short-radius elbows. Long-radius elbows refer to those with a radius of curvature equal to 1.5 times the pipe's outer diameter, i.e., R = 1.5D. Short-radius elbows refer to those with a radius of curvature equal to the pipe's outer diameter, i.e., R = D. In the formula, D is the elbow diameter, and R is the radius of curvature. If classified by pressure rating, 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° elbows, 90° elbows, and 180° elbows. This results in a variety of elbow types. When placing an order, the following notation is often used: for example, "LR STD 90° 8" indicates a long-radius elbow with a pressure rating of STD and a 90° 8" bend; or "SR XS 45° 4" indicates a short-radius elbow with a pressure rating of XS and a 45° 4" bend. The above is a general classification of elbows.