Carbon Steel Spectacle Blind Flange — Factory Direct Supplier | Huaxia-Steel
Carbon steel spectacle blind flanges (paddle blinds) are a single-piece figure-8 device used to positively isolate sections of piping or equipment during maintenance. One end is a solid disc (the "blind"), the other end is a ring (the "open" or "spacer"). Rotating the spectacle between the flanges either blocks or allows flow without removing the assembly. Huaxia Steel manufactures figure-8 blinds in ASTM A516 Gr.70N and ASTM A105, sizes 1/2" to 24", ASME classes 150 to 2500, with EN 10204 3.1 / 3.2 certification.
1. Product Overview
A spectacle blind (also called a paddle blind, figure-8 blind or bleeder ring) is a positive isolation device installed between two flanges. Unlike a blind flange that must be unbolted and removed for line opening, the spectacle simply rotates 180° within the bolt circle to switch between the blank (closed) and the ring (open) positions. A bleeder / tell-tale hole in the ring allows controlled draining or venting when transitioning from open to closed.
Spectacle blinds are mandatory in many process plants for positive isolation during maintenance — they provide visible confirmation that the line is isolated, and they remain in place during normal operation. ASME B16.48 covers the dimensional standard; the related ASME B16.5 flange class defines pressure-temperature rating.
- Positive line isolation — visible at a glance whether the line is open or closed
- Faster turnaround — no need to unbolt a blind flange to open the line
- Reduced gasket change-out cost — the spacer ring avoids compressing the gasket during normal operation
- Compatible with any ASME B16.5 or B16.47 flange specification
- Typical use in hydrocarbon, chemical, marine and power plant service
2. Material Grades
| Grade | Standard | Typical Application |
|---|---|---|
| ASTM A516 Gr.70N | Pressure vessel plate, normalised | Most common — plate carbon steel with good weldability and low-temp toughness |
| ASTM A516 Gr.65N | Pressure vessel plate, normalised | Lower strength option |
| ASTM A515 Gr.70 | Pressure vessel plate, not normalised | Higher temperature service |
| ASTM A105 / A105N | Forged carbon steel | For smaller sizes or where forging is preferred |
| ASTM A350 LF2 | Low-temperature forged carbon steel | Cryogenic service down to −46°C (−50°F) |
| ASTM A387 Gr.11 Cl.2 | Chromium-moly alloy plate | High-temperature service |
The majority of carbon-steel spectacle blinds are cut from ASTM A516 Grade 70 plate in the normalised condition (Gr.70N). The plate form is preferred because it can be machined to large diameters more economically than forging. Above 24" NPS, spectacle blinds are almost exclusively plate.
3. Standard Size Range and Pressure Class
Huaxia Steel spectacle blinds are produced per ASME B16.48 (for pressure classes) in the following standard size and class combinations:
| Pressure Class | Size Range (NPS) | Manufacturing Standard |
|---|---|---|
| Class 150 | 1/2" – 24" | ASME B16.48 + ASME B16.5 |
| Class 300 | 1/2" – 24" | ASME B16.48 + ASME B16.5 |
| Class 600 | 1/2" – 24" | ASME B16.48 + ASME B16.5 |
| Class 900 | 1/2" – 24" | ASME B16.48 + ASME B16.5 |
| Class 1500 | 1/2" – 24" | ASME B16.48 + ASME B16.5 |
| Class 2500 | 1/2" – 12" | ASME B16.48 + ASME B16.5 |
For NPS above 24", ASME B16.47 Series A or B spectacle blinds are manufactured to order. Larger sizes (up to 60" NPS) are supplied as fabricated pieces from rolled plate with full penetration welds.
4. Dimensional Standards (ASME B16.48)
- Outside diameter (OD): matches the mating flange OD per ASME B16.5 / B16.47
- Center-to-center dimension (hub length): per ASME B16.48
- Web thickness (center bar): thicker than standard ring gasket per ASME B16.48
- Ring width: matches the gasket seating width of the mating flange
- Blind thickness: calculated to ASME Section VIII Div. 1 for the design pressure
- Bleeder / tell-tale hole: standard 1/2" NPT or 3/4" NPT on the spacer ring
- Handle / lifting eye: standard for installation and rotation
5. Facing Types
| Type | Description | Typical Application |
|---|---|---|
| Raised Face (RF) | 1.6 mm (1/16") or 3.2 mm (1/8") raise, serrated finish | Standard — matches Class 150 / 300 RF flanges |
| Flat Face (FF) | No raise, flat sealing | Matches FF flanges; rare for spectacle blinds |
| Ring Type Joint (RTJ) | RTJ groove per ASME B16.5 | Matches Class 600 and above RTJ flanges |
| Male & Female (M&F) | Tongue-and-groove facing | Self-aligning high-pressure service |
For Class 600 and above service, spectacle blinds are manufactured with RTJ grooves to match the corresponding RTJ flanges. The RTJ design provides metal-to-metal sealing for high-pressure hydrocarbon and steam systems.
6. Chemical Composition (ASTM A516 Gr.70N)
| Element | ASTM A516 Gr.70 (%) |
|---|---|
| Carbon (C), max | 0.31 (3/4" to 1-1/2"); 0.28 (2" to 4"); 0.27 (over 4") |
| Manganese (Mn) | 0.85 – 1.20 (3/4" to 1-1/2"); 0.85 – 1.20 (2" to 4"); 0.85 – 1.30 (over 4") |
| Phosphorus (P), max | 0.025 |
| Sulfur (S), max | 0.025 |
| Silicon (Si) | 0.15 – 0.40 |
7. Mechanical Properties (ASTM A516 Gr.70)
| Property | ASTM A516 Gr.70 Requirement |
|---|---|
| Tensile strength | 70 – 90 ksi (485 – 620 MPa) |
| Yield strength, min | 38 ksi (260 MPa) |
| Elongation in 8 in. (200 mm), min | 17% |
| Elongation in 2 in. (50 mm), min | 21% |
8. Common Applications
- Positive line isolation during maintenance in oil & gas and petrochemical plants
- Equipment isolation (vessels, columns, heat exchangers, pumps, compressors)
- Ship and marine piping systems (per IMO and classification society rules)
- Power plant boiler, turbine and auxiliary piping
- Chemical and pharmaceutical plants where frequent isolation is required
- Pipeline pig launcher / receiver isolation
- Hydrocarbon storage tank isolation
9. Manufacturing Standards
- ASME B16.48 — Steel Line Blanks
- ASME B16.5 — Pipe Flanges and Flanged Fittings (NPS 1/2 through NPS 24)
- ASME B16.47 — Large Diameter Steel Flanges (NPS 26 and above)
- ASTM A516 / A516M — Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature Service
- ASTM A105 / A105N — Forged carbon steel for piping components
- ASME Section VIII Div. 1 — Boiler and Pressure Vessel Code (for design pressure)
- MSS SP-45 — Bypass and Drain Connections
10. Quality Assurance and Inspection
Each lot of spectacle blinds is supplied with EN 10204 Type 3.1 mill certificate including:
- Plate / forging heat number and traceability
- Chemical composition (heat analysis)
- Tensile test, yield strength, elongation
- Charpy V-notch test (mandatory for A516 Gr.70N at −46°C / −50°F)
- Hardness test
- Dimensional inspection report (OD, web thickness, ring thickness, bleeder hole)
- Visual and surface finish inspection
- 100% magnetic particle examination (for higher classes)
- RTJ groove dimensional check (where applicable)
For critical service, third-party witnessed inspection by SGS, BV, TUV, Intertek or ABS can be arranged. PMI (Positive Material Identification) and hydrostatic testing of the bleeder plug are available on request.
11. Identification Marking and Tagging
Each spectacle blind is permanently marked with:
- Manufacturer’s name or logo
- Material grade (e.g. A516-70N)
- Size (NPS)
- Pressure class (e.g. CL 300)
- Heat number
- Direction indicator ("OPEN" / "SHUT" arrows on the ring and blind)
- Standard reference (ASME B16.48)
Direction indicators are critical safety features — operators rely on them to verify line status. The blind portion is also usually painted red and the spacer portion painted yellow or green for visual identification.
12. Packaging and Shipping
Spectacle blinds are individually wrapped in plastic or kraft paper, then packed in fumigated wooden crates or plywood boxes for small sizes. Large sizes (above 12" NPS) are shipped on wooden pallets, protected by wooden braces and corner protection. Each crate is labelled with size, class, material, heat number and MTC number. Tagging outside the crate includes the project name and PO number for traceability.
Standard lead time: 25–45 days for standard size and class; 45–75 days for non-standard combinations and large diameters. Sample lot of 2–5 pieces available for trial orders.
Need carbon steel spectacle blinds for a positive isolation requirement? Huaxia Steel manufactures A516 Gr.70N and A105 paddle blinds in Class 150–2500, NPS 1/2"–24" (and larger on request), with RF / RTJ facing and EN 10204 3.1 / 3.2 certificate. Email [email protected] with your size, pressure class, material grade, facing, quantity and delivery port, and we will respond with a quotation within 24 hours.
