Blind flanges is a type of flange that is bolted to other flanges and not welded to the pipe. They are used to seal or close the end of a pipe piece and openings in various systems in the apparatus construction. When pressure vessels have been built, all openings will be closed with blind flanges. This closed system can then undergo a pressure test.
In some cases blind flanges can also be used to machine other flanges. They can be drilled and tapped into a reducing threaded flange. A reducing slip-on flange is also typically machined from a blind flange.