Spectacle Blinds, Figure 8 Blanks, and Line Blinds: The Flange Isolation Family Explained
There is a reason every refinery, chemical plant, and pipeline has a drawer full of what look like giant steel glasses. Spectacle blinds, figure 8 blanks, paddle blinds… whatever nicknames you would like to use… these ASME line blinds are the unsung heroes of process isolation. When a valve alone is not safe enough to separate a crew from what is in the pipe, one of these blinds is what gets installed. This guide covers the four common styles, how to spec them to your flange class, and how to avoid the mistakes that turn a simple isolation into a rework job.
Why Positive Isolation Exists
A closed valve is not necessarily positive isolation. Valves leak, seats erode, actuators fail. For hot work, confined-space entry, hydrotest isolation, or any scenario where somebody is going to be on the downstream side of a line that used to contain something nasty, OSHA and API 510 usually require a physical blank between the crew and the process. A paddle or spectacle blind drops a solid piece of steel across the bore so nothing can pass through, full stop. It does so while taking up minimal space as well.
Blinds are specified under ASME B16.48, “Line Blanks,” covering spectacle blinds, spacers, and paddle styles from ½” through 24″ in classes 150# through 2500#. For wellhead service, blinds are sized to API 6A flanges. Sizing has to match the flange class exactly because the blind sees the same pressure the flange does.
The Four Styles
Spectacle Blinds
A spectacle blind, sometimes called a spectacle plate or figure-8 blind depending on the region, is a single piece of steel machined into two circles connected by a web. One circle is solid (the blank). The other is open with a bore matching the pipe ID (the spacer). The whole thing looks like a pair of glasses, which is where the name comes from.
Spectacle blinds are installed permanently between two flanges and rotated between positions as needed. Blank side in line: positive isolation. Rotate 180 degrees: process flows normally. You never lose the blind because it stays bolted in between the joint.
Why they’re great: plants with frequent, predictable isolation needs (catalyst changes, heat exchanger cleaning, relief valve testing) use these when space is limited. The downside is that the web sticks out past the flange OD and can get in the way physically if you need to work around either side of the assembly.
Figure 8 Blanks (also: Figure 8 Plates)
“Figure 8 blank” and “spectacle blind” refer to the same geometry and the same part. Regional and industry preference drives which name gets used, and you will see both terms on refinery drawings. If a drawing says “figure 8 blank,” treat it as a spectacle blind and confirm ID, thickness, bolt hole pattern, pressure class, and material grade.
Paddle Blinds and Paddle Spacers
A paddle blind (also called a line blank or slip blind) is half of a spectacle: a single circle of steel with a handle (the “paddle”) sticking out past the flange OD. Slide it in when you need to blind the line, pull it out and replace it with a paddle spacer when you need to open flow.
Paddles come in pairs: one blank, one spacer. They are easier to install on crowded piping because you only have to break the flange enough to slide a thin plate in and out, not rotate a full spectacle. The downside: you have to store the spare somewhere. Lose the spacer and you cannot put the line back in service without finding a new one.
Line Blinds (Quick-Release / Hinged Blinds)
For high-cycle isolation, full spectacle or paddle setups are slow. Every operation means breaking bolts, swinging or sliding the blind, and re-torquing. That adds up on lines that get isolated weekly.
Line blinds are engineered assemblies that ride between two flanges with a built-in mechanism (hinged, cam-operated, or slide-operated) that lets an operator change position without fully disassembling the joint. Consider them for hydrogen service, catalyst regen, relief valve isolations, and anywhere crews are breaking the same joint more than a couple of times a year.
Spec’ing a Blind to Your Flange
A blind is only as safe as the match to the flange it sits between. Four things have to line up:
Bore size. The spacer (open) side needs to match the pipe inside diameter, not the nominal pipe size. For standard weight (Sch 40) in common sizes they are close enough, but for schedule 80, XXS, or non-standard bores you have to spec the ID.
Pressure class. A spectacle blind installed between 300# flanges has to be rated 300#. Using a 150# blind in a 300# system is a failure waiting to happen. ASME B16.48 thickness tables scale with class.
Bolt circle and hole pattern. The outer circle of the blind has to match the flange bolt circle so the bolts pass through cleanly. This is why confirming the flange standard matters (B16.5 vs. B16.47 Series A vs. Series B change the hole pattern on larger sizes).
Face and gasket. Blinds can be supplied flat face or raised face to match the flange. Gasket thickness on either side has to allow the blind to seat. For RTJ flanges there are specific RTJ-compatible blind geometries since a normal flat spectacle does not have the groove.
Get any of these four wrong and the blind either will not bolt up, will not seal, or will not hold pressure. Our existing Complete Guide to Blind Flange Types covers the close cousin of these parts (standard blind flanges used at dead ends) in more detail.
Style Comparison at a Glance
| Style | Permanent or Removable | Typical Use | Handling |
| Spectacle Blind | Permanent between two flanges | Predictable, occasional isolation | Rotate 180 degrees between blank and spacer |
| Figure 8 Blank | Same as spectacle blind (terminology) | Same as spectacle blind | Same as spectacle blind |
| Paddle Blind / Spacer | Removable (pair: blank + spacer) | Crowded piping, infrequent isolation | Slide in/out, requires a stored spare |
| Line Blind (hinged/cam) | Permanent engineered assembly | High-cycle isolations, critical service | Operator-actuated without full disassembly |
Materials and Certs
Spectacle blinds and paddles are typically supplied in the same material as the flange they install against: A516-70 plain plate for standard carbon, A516-70 normalized and charpy tested for low-temperature service, F316/316L for stainless, F22 or F91 for chrome-moly high-temp. For sour service under NACE MR0175, specify the blind material and hardness to match, and confirm on the MTR.
Blinds themselves are usually machined from plate for practicality purposes, and do not require on-site welding when properly specified.
Stock, Sizing, and How to Order
Texas Flange carries spectacle blinds, paddle blinds, and spacer rings in carbon, stainless, and alloy grades from ½” through 24″ in 150# through 2500#. Larger sizes through 60″ and higher-pressure custom blinds are available on lead time. When requesting a quote, give us:
Size (nominal pipe size), class, flange standard (B16.5, B16.47 Ser A, Ser B, or API 6A), face type (RF, FF, RTJ), material grade, bore (standard or specific), and quantity. If you have a piping drawing or an iso with the blind already identified, attach it to the RFQ and we will match everything from there.
The Bottom Line
Spectacle blinds and their cousins do not show up in the process flow diagram, but they are the reason a maintenance crew can open a vessel without wondering what might be flowing into it. Pick the style that matches your isolation frequency, spec the class and bore to match the flange, and keep the matching spacer in the same drawer as the paddle blind. If you need blinds quoted or help matching a blind geometry to an unusual flange, get in touch with us and we will have pricing and lead time back same day.
Texas Flange & Fitting Supply | 281-484-8325 | texasflange.com
