When would you use a spectacle blind instead of a standard blind flange

A spectacle blind, also known as a spectacle flange or figure 8 blind, is a specific type of blind flange with a unique design that features two discs connected by a handle or bar. Each disc has a solid plate on one side and an opening on the other. Spectacle blinds are used in specific situations where a standard blind flange may not be suitable, primarily for the following purposes:

1. Flow Control and Change: Spectacle blinds are used to control the flow in a pipeline. By rotating the spectacle blind, you can change the position of the openings to either block flow (closed position) or allow flow (open position). This makes them ideal for applications where the direction or volume of flow needs to be controlled.

2. Isolation and Maintenance: Spectacle blinds are used when it is necessary to isolate a section of a pipeline for maintenance, repair, or cleaning while maintaining the integrity of the rest of the system. The closed position of the spectacle blind seals off the flow, and the open position allows for unobstructed flow.

3. Preventing Contamination: Spectacle blinds are helpful in applications where contamination or mixing of different fluids or substances is a concern. By closing the spectacle blind, you can prevent cross-contamination between two sections of the pipeline.

4. Multiple Fluids or Media: In systems that handle multiple fluids or media, a spectacle blind can be used to divert the flow from one stream to another as needed.

5. Reducing Pressure Drop: In some cases, a spectacle blind can be used to reduce pressure drop in a system. By partially opening the spectacle blind, you can allow flow through one side while reducing the pressure drop compared to a fully closed standard blind flange.

6. Draining or Purging: Spectacle blinds are often used for draining or purging a section of a pipeline. By setting the spectacle blind in the open position, you can ensure that the contents of the pipeline are completely drained or purged.

In summary, a spectacle blind is chosen over a standard blind flange when flow control, isolation, maintenance, and specific handling of fluids are essential in a piping system. The ability to change the position of the openings by rotating the spectacle blind handle or bar allows for versatile and controlled operation of the flow.