Figure 8 Blind Flanges: Procurement Guide to Production Process and Standards

Figure 8 blind flanges (also known as spectacle blinds) are essential components in piping systems for isolating sections of a pipeline. They consist of a solid disc (blank) and a spacer ring (open) connected by a central web, resembling the figure 8 shape. This design allows quick switching between isolation and flow positions without removing the flange. For global EPC projects, proper procurement of these components is critical to avoid leaks, pressure failures, or non-compliance.

Production Process Overview

Understanding the manufacturing steps helps buyers evaluate supplier capabilities.

Material Selection

Materials must match the service conditions – pressure, temperature, and corrosiveness. Common grades include:

• Carbon steel: ASTM A105, A350 LF2 (low temperature)
• Stainless steel: ASTM A182 F304/F316
• Alloy steel: ASTM A182 F11/F22

Buyers should request material certificates (MTCs) and verify traceability to mill test reports.

Forming

Blanks are cut from plate or forged bar, then machined to final dimensions. CNC machining ensures accurate facing, bolt holes, and web thickness. For high-pressure designs (e.g., Class 900+), forging is preferred over plate cutting for better grain flow and strength.

Surface Treatment

Standard finishes include:

• Oiling or galvanizing for carbon steel (rust prevention)
• Pickling and passivation for stainless steel (corrosion resistance)
• Coating as per customer spec (e.g., epoxy, PTFE)

Testing and Inspection

Critical tests to request:

• Hydrostatic test (pressure rating)
• Dimensional inspection (per ASME B16.48)
• Non-destructive testing (UT, MT, or PT) for forged components
• Hardness test if required by material spec

Applicable Standards

Confirm with your supplier which standard they follow. ASME B16.48 is the most common for figure 8 blinds.

Key Procurement Considerations

Dimensions and Ratings

• Nominal pipe size (NPS) and pressure class (150, 300, 600, 900, 1500, 2500)
• Facing type: raised face (RF), flat face (FF), ring joint (RTJ)
• Web thickness: must be sufficient for handling and pressure

Material and Traceability

Always require:

• Material test reports (MTRs) from the mill
• Positive material identification (PMI) reports if alloy content is critical
• Certificate of compliance to ASTM/ASME specs

Quality Risks to Watch

• Web cracking during forming (poor forging practice)
• Dimensional mismatch with mating flanges (bolt hole alignment, OD, thickness)
• Incomplete heat treatment (for alloy steels)
• Surface defects (lamination, pitting)

RFQ Checklist

Before requesting a quotation, provide:

1. Quantity and size (NPS and class)
2. Material grade and standard (e.g., ASTM A105, ASME B16.48)
3. Facing type and finish (125-250 microinch RF, etc.)
4. Required tests: hydrostatic, NDE, PMI
5. Delivery schedule and packaging requirements (e.g., wooden crates for export)
6. Third-party inspection (e.g., by SGS, BV, or client representative)

Why Hebei Haihao Group?

Hebei Haihao Group has extensive experience manufacturing figure 8 blind flanges in carbon, stainless, and alloy steels. We follow ASME B16.48 and other international standards, providing full traceability from material to final inspection. Our quality control includes dimensional checks, material certification, and pressure testing as standard. Contact us with your project specifications for a competitive quotation and reliable delivery.