Expanding flanges, or expander flanges, are used to increase the bore of the pipeline from a specific point or to connect pipes to other mechanical devices such as pumps and valves. This type of flange is a weld neck flange with a larger bore on the non-flanged end. The run size can be safely increased only by one or maximum two sizes (example: from 2 to 3 or maximum 4 inches). Expander flanges are a lighter, cheaper and faster solution to expand the run pipe size vs. the combination of a butt weld reducer and a weld neck flange.
Reducing flanges, otherwise called reducer flanges, have an opposite function than expander flanges, i.e. they are used to decrease the bore of a pipeline. The run pipe reduction can be only limited (one or two sizes maximum).