Expanding flanges are used to increase the bore of the pipeline from a specific point to another or to connect pipes to other mechanical devices such as pumps, compressors, and valves that have different inlets sizes.
The expanding flange represented in the picture is a welding neck flange with a larger bore on the non-flanged end.
Expanding flanges can be used to increase the run pipe bore only by one or maximum two sizes and not more.
Expander flanges are a cheaper (and lighter) solution compared to the combination of a buttweld reducer and a standard flange.
The most common materials for expanding flanges are A105 (high-temp. carbon steel), A350 (LTCS) and ASTM A182 (stainless steel and above).
Pressure ratings and dimensions of expanding flanges are in accordance with the ANSI/ASME B16.5 specification and are available with raised or flat face (RF, FF).