A Buyer’s Guide to Figure-8 Blinds (Spectacle Blinds) for Piping Systems

What Are Figure-8 Blinds?

Figure-8 blinds, also known as spectacle blinds or spectacle flanges, are a type of isolation device used in piping systems. They consist of two metal discs joined by a narrow section: one disc is solid (blind), and the other has a hole (spacer). The shape resembles the number “8,” hence the name. When installed between flanges, rotating the device allows either the solid side to block flow or the open side to allow passage. This provides a positive, verifiable isolation point for maintenance, testing, or emergency shutdown without dismantling piping.

Applications and Industries

Figure-8 blinds are widely used in industries requiring reliable fluid isolation:

• Oil and Gas: Upstream and downstream pipelines, refineries, and petrochemical plants.
• Chemical Processing: Where toxic, corrosive, or flammable fluids must be safely isolated.
• Power Generation: Steam and cooling water systems.
• Water and Wastewater: Treatment plants and distribution networks.
• Food and Beverage: Hygienic piping requiring quick isolation for cleaning.

Their key advantage is the ability to switch between open and closed positions without removing the device, saving time and reducing leak paths.

Standards to Confirm

When procuring figure-8 blinds, ensure they comply with relevant international standards. Common standards include:

• ASME B16.48: Standard for line blanks (including spectacle blinds) in steel piping.
• ASME B16.5: Pipe flanges and flanged fittings – defines flange dimensions and pressure ratings.
• ASTM A105 / A182 / A240: Material specifications for carbon steel, alloy, and stainless steel.
• API 6A: For wellhead and tree equipment (if used in upstream oil and gas).

Always verify the applicable standard with your engineering team and confirm the pressure-temperature rating (e.g., Class 150, 300, 600, etc.).

Inspection Documents Required

For quality assurance, request the following documentation from your supplier:

• Mill Test Reports (MTRs) for raw materials.
• Heat treatment records (if required by standard).
• Dimensional inspection reports verifying flange face thickness, overall length, and bolt hole alignment.
• Pressure test certificates (if stipulated).
• Third-party inspection reports (e.g., from SGS, BV, or Lloyd’s).
• ISO 9001:2015 certification of the manufacturer.

RFQ Checklist for Figure-8 Blinds

To receive accurate quotations, include in your RFQ:

• Quantity and size (pipe nominal diameter and flange rating).
• Face-to-face length (often per ASME B16.48).
• Material specification (carbon steel, stainless steel, alloy).
• End connections (raised face, RTJ, flat face, etc.).
• Service fluid and operating pressure/temperature.
• Any special coating or lining requirements.
• Required standards (ASME, API, etc.).
• Delivery terms and destination.

Common Quality Risks

Be aware of these potential issues when sourcing figure-8 blinds:

• Material mixing: Verify correct grade via PMI (Positive Material Identification).
• Incorrect dimensions: Especially face thickness and handle length, which affect fit between flanges.
• Poor surface finish: Can cause leakage at gasket contact surfaces.
• Inadequate heat treatment: May lead to brittle failure under pressure.
• Lack of traceability: Without MTRs, material integrity cannot be assured.

Information to Send Before Quotation

To expedite the quoting process, provide the supplier with:

• Complete piping specifications: size, class, material, and end preparation.
• A simple sketch or P&ID extract showing the blind location.
• Expected operating conditions (pressure, temperature, fluid).
• Any project-specific requirements (e.g., NACE MR0175 for sour service).
• Your preferred shipping terms (FOB, CIF, etc.).

RFQ CTA

Need reliable figure-8 blinds for your next project? Hebei Haihao Group manufactures spectacle blinds in carbon steel, stainless steel, and alloys per ASME B16.48 and other standards. With strict quality control and global shipping, send your RFQ to us for a competitive quotation.