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