A slip on flange is designed to slip over the end of pipe. It allows for easy positioning before welding. Both the inside and outside of the pipe is welded on the slip on flange. See picture on the right. The pipe is generally inserted all the way, inside the slip on flange, leaving only the gap equivalent to wall thickness of pipe.
ANSI B16.5 (ASME B16.5), BS4504, BS 10, EN 1092-1, DIN, ASME B16.47, SANS (SABS) 1123