A spectacle blind flange, also known as a figure-8 blind flange, is a type of flange that consists of a pair of metal discs that are joined by a central section, resembling the shape of a pair of glasses or spectacles. One of the discs is solid, while the other has a hole or opening in the center. The discs can be rotated so that either the solid disc or the disc with the hole is aligned with the pipeline, allowing or blocking the flow of fluid or gas.

Spectacle blind flanges are commonly used in pipelines where temporary or permanent isolation is required. They are often used to allow for maintenance or repair work to be carried out on one section of the pipeline while the other section remains in operation. The spectacle blind flange can be rotated to open or close the flow, depending on the required operation.

Spectacle blind flanges are available in various sizes, pressure ratings, and materials, including carbon steel, stainless steel, and alloy steel. They are typically used in high-pressure applications, such as oil and gas pipelines, chemical processing plants, and power generation facilities.

Proper installation and torque of the bolts are crucial to ensure a tight and leak-free seal. It is also important to ensure that the spectacle blind flange is rotated to the correct position to allow or block the flow of fluid or gas.