Article
Closing a valve is not always enough to make a line safe for maintenance. Valves can pass. When a plant needs positive isolation, it fits a line blind between two flanges. The most common forms are the spectacle blind, the paddle blank and the ring spacer.
The three main types
- Spectacle blind (figure-8 blind) – a solid disc and a ring joined by a web. One side blocks the line, the other lets flow pass. To change, the bolts are loosened and the blind is rotated.
- Paddle blank – a solid plate with a handle, inserted between flanges to block the line. Used for larger sizes where a spectacle blind becomes too heavy.
- Paddle spacer / ring spacer – an open ring with a handle that keeps the flange gap filled when the paddle blank is removed.
Why spectacle blinds are popular
A spectacle blind always stays in the line, so the open or closed status is visible from a distance and the matching piece is never lost. It is the preferred choice up to the size where the weight becomes difficult to handle.
Standard
ASME B16.48 covers line blanks in sizes NPS 1/2" to 24" for Classes 150 to 2500. It sets thickness, outside diameter, web dimensions and handle details so that the blind fits standard ASME B16.5 flanges.
Facing and thickness
| Mating flange facing | Blind facing |
|---|---|
| Raised face (RF) | Flat, sits within the bolt circle |
| Ring type joint (RTJ) | Ring grooves machined on both sides, or oval/octagonal ring formed on the blind |
Thickness increases with size and class. Always check that the flanges and bolts can accommodate the blind thickness; longer stud bolts are often needed.
Materials
Blinds are usually machined from plate or forgings in the same material group as the piping: ASTM A516 Gr 70 or A105 in carbon steel, ASTM A240 or A182 304L/316L in stainless steel, and duplex grades for seawater service.
A complete description
Spectacle Blind 6" Class 300 RF, ASTM A240 316L, ASME B16.48 – 2 nos
Frequently asked questions
What is a spectacle blind used for?
It provides positive isolation between two flanges, and can be rotated to switch the line between open and closed.
Which standard covers spectacle blinds?
ASME B16.48 covers spectacle blinds, paddle blanks and spacers for NPS 1/2" to 24".
When is a paddle blank used instead of a spectacle blind?
For larger sizes and higher classes where a spectacle blind would be too heavy to handle safely.
Key takeaways
- Line blinds give positive isolation that valves cannot.
- Spectacle blinds stay in the line and show status clearly.
- Specify size, class, facing, material and ASME B16.48.
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