This standard stipulates the type and size of flat welded steel pipe flanges with concave and convex necks with nominal pressure PN of 1.6, 2.5, 4.0 MPa and nominal pressure PN of 5.0, 11.0, 15.0, 26.0 MPa. This standard is applicable to flat welding steel pipe flanges with concave and convex necks with nominal pressure PN1.6~PN26.0MPa.