What type of fitting is installed in a pipeline that either blanks off the pipe or allows full flow passage of a liquid?

The correct choice is a spectacle flange, which serves a dual purpose in pipeline systems. It has two sections: one that is a solid plate (the blank) and the other that is an opening that allows for fluid to flow through when required. This design enables operators to quickly change the flow path of a fluid in a pipeline without needing separate hardware to install or remove, providing efficiency during maintenance or system modifications.

The spectacle flange is particularly useful in situations where it may be necessary to isolate sections of a pipeline for repairs or safety checks. By simply rotating the flange, the operator can switch between isolating the section and allowing full flow, streamlining operations and minimizing potential downtime.

In contrast, other fittings mentioned do not offer this functionality. A pivot coupling is designed for flexibility but does not blank off flow. A blind flange effectively seals off a pipe but does not allow fluid to pass through at all. A quick-release coupling is intended for rapid disconnection and reconnection of hoses or pipes but does not serve the dual purpose of blanking off and enabling flow like the spectacle flange.