ASTM A182 F51 UNS S31803 Duplex Spectacle Blind Flange– ASME B16.48 Figure-8 Blank

UNS S31803 Duplex Spectacle Blind, also known as a Figure-8 Blank, is an operational line blank used for positive isolation of piping systems.

Manufactured from astm_a182_f51_uns_s31803_duplex_2205 and dimensioned to ASME B16.48, spectacle blinds are commonly installed between ASME B16.5 flanges in oil & gas, offshore, petrochemical, chemical and marine piping systems.

Super Duplex UNS S32750 spectacle blinds are also available for more demanding chloride and seawater environments.

UNS S31803 Spectacle Blind Specification

What Is a Spectacle Blind?

A spectacle blind combines two connected sections:

• a solid blind used to stop flow;
• an open spacer used to allow flow.

The two sections form the characteristic figure-8 shape.

When the solid section is positioned between the mating flanges, the line is mechanically isolated. When the open section is installed in the flow path, the pipeline remains open.

This design provides a clear visual indication of whether the line is open or isolated.

UNS S31803 vs UNS S32750

UNS S32750 provides higher resistance to pitting and chloride corrosion than S31803.

Material selection should be based on the project material specification, operating temperature, chloride level and actual service environment.

ASME B16.48 Spectacle Blind

ASME B16.48 is the principal ASME standard covering operational line blanks such as:

ASME B16.48 specifies requirements including dimensions, pressure classes, facing configurations, tolerances and marking.

Spectacle blinds are installed between compatible piping flanges, commonly manufactured to ASME B16.5.

Facing Types

Spectacle blinds may be supplied with different sealing configurations depending on the mating flange and piping class.

The spectacle blind facing must be compatible with the mating flange and gasket arrangement.

Testing and Inspection

Standard supply requirements should be distinguished from project-specific inspection.

Typical material and dimensional verification includes:

• Material identification and traceability
• Dimensional inspection
• Visual inspection
• Material certificate review

Additional inspection may include PMI, PT, UT, ferrite measurement, EN 10204 3.1/3.2 certification or third-party inspection when specified by the purchaser.

These additional tests should not automatically be treated as mandatory ASME B16.48 requirements.

Applications

UNS S31803 Duplex Spectacle Blinds are commonly used in:

• Oil & gas processing
• Offshore platforms
• Petrochemical plants
• Chemical processing
• Marine piping
• Water treatment
• Process equipment isolation
• Maintenance isolation points

Their main purpose is to provide visible and positive mechanical isolation during maintenance, shutdown or process changeover.

How to Order a Duplex Spectacle Blind

A complete RFQ should specify:

Size + Pressure Class + Facing + Material + Standard + Quantity + Certification / Inspection Requirements

Example:

SPECTACLE BLIND / FIGURE-8 BLANK, NPS 8, CLASS 150, RF, ASME B16.48, UNS S31803 DUPLEX STAINLESS STEEL, EN 10204 3.1

For Super Duplex:

SPECTACLE BLIND, NPS 8, CLASS 150, RF, ASME B16.48, UNS S32750 SUPER DUPLEX, EN 10204 3.1

Frequently Asked Questions

What is a spectacle blind flange?

A spectacle blind is an operational line blank consisting of a solid blind and an open spacer connected together in a figure-8 shape.

Although commonly searched as a “spectacle blind flange,” it is installed between two flanges rather than functioning as a conventional bolted flange.

What standard covers spectacle blinds?

ASME B16.48 covers spectacle blinds, paddle blinds and paddle spacers used as operational line blanks.

What is the difference between a spectacle blind and a blind flange?

A spectacle blind is installed between two mating flanges and can be changed between open and closed positions.

A blind flange is directly bolted to a flange connection to close the end of a piping system.

What is the difference between a spectacle blind and a spade and spacer?

A spectacle blind combines the blind and spacer into one connected component.

A spade and spacer provide the same basic isolation functions but are separate components.

Is UNS S32750 available?

Yes. UNS S32750 Super Duplex Spectacle Blinds can be supplied for applications requiring higher resistance to aggressive chloride and seawater environments.

ASTM A182 F51 UNS S31803 Duplex Spectacle Blind Flange

Related Products

UNS S31803 Duplex Paddle Blind
Solid line blank for positive piping isolation.

UNS S31803 Duplex Paddle Spacer
Open spacer used to maintain pipeline flow.

UNS S32750 Super Duplex Spectacle Blind
Figure-8 blank for more aggressive chloride and offshore environments.

ASTM A182 F51 Duplex Flange
Forged duplex UNS S31803 piping flange.

ASTM A182 F53 Super Duplex Flange
Forged super duplex UNS S32750 piping flange.

UNS S31803 Duplex Spectacle Blind Supplier

We supply UNS S31803 Duplex Spectacle Blinds / Figure-8 Blanks manufactured to applicable ASME B16.48 requirements for oil & gas, offshore, petrochemical, marine and industrial piping projects.

We work with qualified manufacturers and can support project-specific requirements including EN 10204 3.1 certification, PMI, NDT, ferrite measurement and third-party inspection when specified.

Send us your NPS, pressure Class, facing, material, quantity and project specification for technical review and quotation.