How to Specify Figure-8 Blind Flanges for Pipeline Isolation: A Procurement Guide

Understanding Figure-8 Blanks and Spacers

Figure-8 blind flanges, also known as figure-8 blanks or spectacle blinds, are critical components in piping systems for positive isolation. They consist of two discs connected by a slender web, one solid and one with an opening (spacer). By rotating the flange, operators can either block or allow flow without removing the device. This design makes them ideal for applications requiring periodic maintenance, cleaning, or testing.

EPC engineers and procurement specialists must specify figure-8 blinds correctly to ensure safety, compliance, and long-term reliability. Below we outline the key factors to consider when sourcing these components.

Key Standards for Figure-8 Blinds

Figure-8 blind flanges are manufactured to international standards. The most common is ASME B16.48 (Line Blanks). Other relevant standards include:

• ASME B16.5 (for flange dimensions and facing)
• ASME B16.47 (for large diameter flanges)
• BS 1560 (British standard, equivalent to ASME)
• ISO 17292 (for metal ball valves, often referenced for blank dimensions)

When specifying, confirm the following:

• Pressure class (150, 300, 600, 900, 1500, 2500#)
• Facing type (raised face, flat face, ring joint)
• Nominal pipe size (NPS) and schedule
• Material grade (carbon steel, stainless steel, alloy steel)

Material Selection and Pressure Ratings

The material shall match the piping system and service conditions. Common choices:

• Carbon steel (ASTM A105) for general service
• Stainless steel (ASTM A182 F304/F316) for corrosive or cryogenic service
• Alloy steel (ASTM A182 F11/F22) for high-temperature or sour service

Pressure ratings follow ASME B16.48, which aligns with ASME B16.5 for flange ratings. For example, a Class 300 figure-8 blind has a pressure-temperature rating identical to a Class 300 flange.

Inspection and Documentation Requirements

Procurement should mandate the following documentation from the manufacturer:

• Mill test reports (MTR) for all materials
• Pressure test certificate (if applicable)
• Positive material identification (PMI) report for alloy materials
• Dimensional inspection report confirming thickness, face-to-face length, and gasket surface finish
• Visual inspection report (no cracks, pits, or defects)
• Heat treatment chart (if required by specification)

For critical services, third-party inspection (e.g., by SGS, Bureau Veritas) may be required.

Quality Risks to Watch

1. Incorrect thickness: The solid blind must have sufficient thickness to withstand design pressure. Verify against ASME B16.48 tables.
2. Poor gasket surface finish: Rough finishes cause leakage. Specify 125-250 micro-inch RMS for raised face.
3. Mismatched facing: Ensure the blind facing matches companion flanges (e.g., RF to RF, RTJ to RTJ).
4. Wrong web length: The spacer and blind sections must align accurately with the pipeline centerline.
5. Improper material: Verify MTRs to avoid using carbon steel in corrosive service or non-impact tested material in low temperatures.

RFQ Checklist: What to Send Before Quotation

To get accurate pricing and lead time, provide this information:

• Quantity (pieces)
• Size (NPS) and pressure class
• Facing type (RF, FF, RTJ)
• Material specification (ASTM A105, A182 F304, etc.)
• Standards required (ASME B16.48, etc.)
• Inspection requirements (NDE, PMI, third-party)
• End use (piping material and service conditions)
• Delivery terms (Incoterms) and timeline

Why Choose Hebei Haihao Group?

Hebei Haihao Group has extensive experience manufacturing figure-8 blinds to ASME, DIN, and other international standards. Our quality control includes MTR verification, dimensional checks, and surface finish inspection. We work with EPC buyers to ensure compliance with project specifications.

For a prompt quotation, please send the above checklist to our sales team. We will respond with detailed technical data and competitive pricing.