Swinging the plate around a single bolt swaps the blank side into the line for positive isolation or the ring side for normal operation, while keeping the flange gap the same. Because the unused half sticks out, anyone walking by can see which state the line is in. In large sizes the two halves are separate pieces, called a paddle blind and paddle spacer.
Flip the spectacle blind to closed on the fuel gas line and hang the tag on it, then we can open the burner.
Always install new gaskets on both sides when swinging a spectacle blind; the old ones are crushed to the old gap.
Also heard as
- figure 8 blind
- spec blind
- line blind
Related terms
Spade
A solid blank plate with a handle, slipped between flanges to positively isolate a line.
Double block and bleed
Isolation using two closed seats in series with a vent or drain between them to prove the isolation holds.
Raised face
A flange facing where the gasket surface stands slightly proud of the bolt circle area, concentrating gasket load.