Bolt two flat-surface flanges or two raised-surface flanges of the same size together with a gasket (sold separately) to create an access point in a pipe line.
Slip-on weld flanges have no internal stop; slide a pipe through the flange and weld on both sides. Cast flanges are also called MSS flanges.
304 stainless steel flanges offer very good corrosion resistance, and 316 stainless steel flanges have excellent corrosion resistance.