How to Procure Figure-8 Blinds for High-Pressure Piping Systems

What Is a Figure-8 Blind?

A figure-8 blind (also called a spectacle blind or 8-shaped blind) is a piping component that combines a solid plate (blind) and a spacer plate (open) in one unit, shaped like the number 8. It allows quick isolation or opening of a pipeline by simply rotating the device. This design eliminates the need for separate gaskets and bolting during maintenance, saving time and labor. Common in high-pressure and high-temperature systems, figure-8 blinds are critical for safety and operational flexibility.

Key Applications in Heavy Industry

Figure-8 blinds are used across industries where positive isolation is required:

• Chemical & Petrochemical: Isolating reactors, heat exchangers, or pumps during maintenance.
• Oil & Gas: Pipeline pigging, metering skids, and manifold isolation.
• Power Generation: Steam lines, feedwater systems, and boiler isolation.
• Shipbuilding & Offshore: Ballast systems, fuel lines, and fire mains.
• Aerospace Test Stands: High-pressure gas or liquid handling systems.

Their ability to withstand extreme pressures (up to Class 2500 or higher) and temperatures makes them indispensable in these sectors.

Critical Standards to Confirm in Your RFQ

When procuring figure-8 blinds, always specify the governing standard. The most common is ASME B16.48 for line blinds. However, confirm with your engineering team:

• Design Standard: ASME B16.48, API 6A, or DIN/EN equivalents.
• Pressure Class: Class 150, 300, 600, 900, 1500, or 2500.
• Facing: Raised face (RF), ring-type joint (RTJ), or flat face (FF).
• Size: Nominal pipe size (NPS) or DN, including wall thickness schedule.

For non-standard applications, consider MSS SP-44 or custom designs.

Required Inspection and Documentation

Reputable manufacturers provide the following documents. Always request them during quotation:

For critical services, request third-party inspection (e.g., Lloyds, DNV, or SGS).

RFQ Checklist: What to Send Before Quotation

To get accurate lead times and pricing, include the following in your request:

• Quantity and size: NPS and schedule.
• Pressure class and facing type.
• Material specification: ASTM A105 (carbon steel), A182 F304/F316 (stainless steel), or nickel alloys (e.g., Inconel 625).
• Service conditions: Design pressure and temperature, fluid type (e.g., hydrogen sulfide, chlorine).
• Special requirements: NACE MR0175 for sour gas, cryogenic service, or high-cycle operation.
• Coating/plating: Optional zinc coating or PTFE lining.
• Delivery location and Incoterms.
• Required certifications and inspection agency.

Common Quality Risks and How to Avoid Them

• Improper face finish: Raised face finish (125-250 AARH) must match gasket requirements. Specify finish on RFQ.
• Wrong thickness: The blind thickness must equal the spacer thickness (per ASME B16.48). Verify before production.
• Material mix-up: Request MTCs and perform PMI (Positive Material Identification) on delivery.
• Misalignment of bolt holes: Ensure bolt circle diameter matches flange standard (ASME B16.5).
• Lack of traceability: Require stamping of part numbers on each blind.

Choosing the Right Material and Facing

Material selection affects corrosion resistance, strength, and cost:

Facing: Raised face is standard for most applications. For API 6A service, use RTJ facing with appropriate ring gasket.

Conclusion

Figure-8 blinds are simple yet critical components. To ensure safety and reliability, specify the correct standard, material, and facing, and always request full documentation. A detailed RFQ package helps manufacturers deliver exactly what you need.

For your next project, consider sourcing from Hebei Haihao Group. We manufacture figure-8 blinds to ASME B16.48, API 6A, and custom requirements, with full traceability and third-party inspection options. Contact us for a quote.