A closed valve stops flow. It does not prove the line is dead. Valves pass, valves get opened by somebody who did not read the tag, and a handwheel position does not tell you what the seat is actually doing.
When a person is going to enter a vessel, break into a line or do hot work near one, safety procedures want a solid plate bolted between two flanges. That is positive isolation: a physical barrier that cannot pass, and whose presence can be confirmed by looking at it.
The hardware
Spectacle blind
Two discs on a common web, one solid and one bored through, shaped like a pair of spectacles. Also called a figure eight blind.
Spread the flanges, swing the plate over, bolt back up. The line is either positively blanked or fully open. Two advantages follow: the position is visible from the ground, and the plate lives permanently in the line so it cannot be lost or fitted somewhere it does not belong.
It is the heaviest and most expensive option, and it is standard on connections that get isolated as routine, such as furnace, reactor and vessel inlets.
Paddle blind and paddle spacer
A paddle blind is a single solid disc on a handle. A paddle spacer is a single open ring on a handle, made to the same thickness.
They work as a pair. The blind goes in when the line is isolated, the spacer goes in when it is running, and the joint keeps the same face to face dimension either way. Lighter and cheaper than a spectacle, and much easier to handle at large sizes.
The weakness is administrative rather than mechanical. A loose plate can be misplaced, and a spacer can be installed where a blind was intended. Plants that use paddles need a register and a tagging discipline to go with them.
Ring spacer
An open ring that fills an axial gap. It takes up the space of a removed component, sets a required face to face dimension, or pairs with a paddle blind. Thickness is whatever the gap requires, and it should be marked on the part so nobody has to measure it in the field.
Line blind valve
A purpose-built assembly holding both a blank and a spacer, which swaps between them without unbolting the line. Much more expensive than a spectacle blind, and justified where the line is isolated often enough that breaking the joint each time costs real time.
Thickness is a calculation, not a lookup
The most common ordering error is assuming the blind should match the flange thickness, or ordering by size and class without stating the differential pressure.
A blind is a flat unstayed head under pressure. Required thickness comes from the differential pressure, the diameter, the material allowable stress at temperature, and the governing design code. Because it scales with diameter, thickness climbs fast on large lines.
Give us the differential, not just the class. A 36 inch blind at 50 psi and a 36 inch blind at 600 psi are completely different parts. Send size, differential pressure, temperature, material and design code, and we will calculate it and put the thickness on a drawing for approval.
The spacer carries no differential, so its thickness is set by the gap, normally matching the blind it pairs with.
The bolt length nobody remembers
Inserting a blind or a spacer into an existing joint adds its thickness plus an extra gasket. The studs that were fine before are now too short.
This gets discovered at two in the morning during an outage, with a crane on standby. Work it out beforehand:
- Existing stud length
- Plus the blind or spacer thickness
- Plus one additional gasket thickness
- Plus enough thread engagement at both ends
Order the long studs with the blinds, and keep them together. Plants that store blinds without their matching studs end up improvising, and improvised bolting on an isolation joint is exactly the wrong place to improvise.
Other details that matter
- Handles and lifting lugs. A large blind is a heavy awkward plate. Specify the handle, and on the big sizes specify lifting lugs. Otherwise fitters improvise, and that is where people get hurt.
- Marking. Size, rating, thickness, material and heat number stamped on the handle, where it stays readable with the plate installed. An unreadable blind gets used on the wrong line.
- Face type. A raised face joint needs the blind sized for the raised face gasket. A ring joint needs a grooved blind, which is a machined part, not a cut plate.
- Storage. Blinds live outdoors for years between uses. Coat them, rack them, and keep the marking legible.
- Register. Know how many you have, what size and rating, and where they are. This is a safety-critical inventory.
Choosing between them
| Situation | Choose |
|---|---|
| Isolated routinely, position must be visible | Spectacle blind |
| Large diameter, weight is a handling problem | Paddle blind and matching spacer |
| Isolated very frequently, downtime is costly | Line blind valve |
| Filling a gap, no isolation duty | Ring spacer |
| Permanent blanking of a dead connection | Blind flange |
Full detail on ring spacers and blinds.