The plate flange
has a circular disk that is welded onto the end of a pipe and allows it to be bolted to another pipe. Typically used in fuel and water pipelines, the two flange plates will be bolted together with a gasket in between them. The flange plate will have bolt holes all around the perimeter and will be used to create junctions, tees and joints.
When building a pipeline, the lengths of the pipes used are not always known. By manufacturing the pipe separate from the flange plate, the welders can cut the pipe to length and weld a flange plate in place to join the pipes at any needed length. The plates can also be welded to the pipe on a slight bias, allowing two pipes to be joined that may not be precisely lined up.