What Is a Figure 8 Blind?

Quick Answer

A figure 8 blind is another name for a spectacle blind—a figure-8-shaped steel plate with a solid blind disc on one end and a bored spacer ring on the other, connected by a web (bridge). It installs between two flanges and rotates 180 degrees to switch between open flow and full isolation. The term “figure 8 blind” comes from the component’s shape when viewed from the side. It is standardized under ASME B16.48.

How It Differs from a Paddle Blind

The figure 8 blind and the paddle blind (spade) serve the same isolation purpose but differ in design and operation:

Feature	Figure 8 Blind	Paddle Blind (Spade)
Shape	Two discs connected by bridge	Single solid disc with handle
Companion piece	Built-in (spacer ring is attached)	Separate spacer ring
Switching method	Rotate 180 degrees	Remove blind, insert spacer
Permanent installation	Yes—stays in the line	No—inserted/removed as needed
Visual indicator	Protruding disc shows open/closed status	Handle visible outside flange
Standard	ASME B16.48	ASME B16.48
Weight	Heavier (two discs + bridge)	Lighter (single disc)

Key Specifications

Property	Detail
Standard	ASME B16.48
Size range	NPS 1/2 to NPS 24
Pressure classes	150 through 2500
Materials	A105 (CS), A182 F304/F316 (SS), A182 F11/F22 (alloy)
Thickness	Same as a blind flange of the same NPS and class
Gaskets	Two required (one each side of active disc)
Bolts	Longer stud bolts to span blind thickness + gaskets

Thickness by Size and Class

NPS	Class 150 (mm)	Class 300 (mm)	Class 600 (mm)
4	6.4	9.5	15.9
6	7.9	12.7	20.6
8	9.5	14.3	23.8
10	11.1	17.5	27.0
12	12.7	19.1	31.8

Minimum thickness per ASME B16.48, A105 carbon steel.

Common Mistake

Piping isometrics must account for the additional face-to-face length introduced by the figure 8 blind and two gaskets. Failing to include this dimension in the piping isometric causes spool fabrication errors and field fit-up problems.

When to Use a Figure 8 Blind

• Frequent isolation switching: Turnaround shutdowns, unit isolation during partial plant operation
• Positive visual indication: The protruding unused disc shows the line status at a glance
• Hazardous/lethal service: Where valve leakage is unacceptable as the sole isolation method
• Hydrostatic testing: Isolating test sections from operating piping

Pro Tip

For large sizes (NPS 16+), the figure 8 blind becomes heavy and difficult to rotate. A Class 300 NPS 20 figure 8 blind in A105 weighs approximately 80-100 kg. Rigging provisions (davit or chain block) should be included in the piping design for safe rotation during turnarounds.

Read the full guide to flanges