The blind flanges is a flages without a bore. It is used to close off the ends of a piping system and/or pressure vessel opening. It also permits easy access to the interior of a line of a line or vessel once it has been sealed and ust be reopened.
This square flanges are normally refered to as high-pressure under oil pressure. There are two kinds of square flages, one has a groove for inserting O-ring, the other without groove.
This flange uses with valve etc.
In case of valve miss-operating, it works as performing double-safety equipment.
It was manufactured for easy using under open/shut and closing.