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 outside diameter, i.e., R = 1.5D. Short-radius elbows refer to those with a curvature radius equal to the pipe's outside 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 90° 8" indicates a long-radius elbow with a pressure grade of STD, a 90° bend, and an 8" diameter; or "SR XS 45° 4" indicates a short-radius elbow with a pressure grade of XS, a 45° bend, and a 4" diameter. The above is a general classification of elbows.