If graded according to the pressure level, there are about 17 species, which are the same as the American pipe standards: sch5s, sch10s, Sch10, Sch20, Sch30, sch40s, STD, Sch40, Sch60, sch80s, Xs;sch 80, Sch100, Sch120, Sch140, Sch160, XXS, the most commonly used is STD and XS two kinds. According to the angle of the elbow,