A "blind" is just a piece of metal right? More complex than you might think…

What are "blinds"?

Blind flanges, commonly referred to as "blinds", are solid disks that are used to block off / isolate a section of piping or a piece of equipment in a system. They are typically shown on P&IDs as either of the following depictions:

When would you install a blind?

There are multiple uses for blind flanges such as blocking off a section of pipe, sealing a nozzle on a vessel that is not used, adding new pipes safely, isolating sections of pipe to safely perform remedial tasks, etc. Depending on your company's isolation standards/philosophies, a blind flange may be required as a form of positive isolation in order to perform work.

Is there a difference between a "blind" and a "spec blind"?

Spectacle blinds, or commonly referred to as "spec blinds", have a similar function of blocking flow but offer the flexibility to rotate and switch to the open side of the spectacle and re-open the flow path. One half is a typical blind whereas the other half has an open path to mimic a short piece of piping. Although it serves the same isolation functionality as a blind, spectacle blinds are typically used for temporary isolation because of their flexibility. Spectacle blinds can be shown in either their open position or closed position on P&IDs as shown below:

An important aspect to consider when installing blind flanges or spectacle blinds is to ensure they are rated for the same pressure as the piping. There is potential for overpressure if a lower pressure specification blind is installed in a high pressure system.