BLIND
A blind flange (sometimes called a blank flange) is a solid, circular metal disk used to block the end of a piping system, a valve, or a pressure vessel opening. Unlike most other flanges, it has no center bore to allow fluid flow.
Key Functions
- Pipeline Termination: Permanently seals off the end of a line when no further connections are needed.
- System Isolation: Blocks off specific sections of a pipe for maintenance, repair, or cleaning while keeping the rest of the system operational.
- Pressure Testing: Acts as a secure barrier during hydrostatic or pneumatic testing to check for leaks and structural integrity.
- Access Point: Provides an easy-to-open entryway for future system inspections or expansions.
Pressure & Temperature Ratings
Specifications for these flanges include Pressure Classes (150, 300, 400, 600, 900, 1500, and 2500).
- Maximum Working Pressure: This is not a fixed number; it decreases as the operating temperature increases.
- Material Influence: Common materials include ASTM A105 (Carbon Steel) and ASTM A182 (Stainless Steel). Stainless steel typically has different pressure-temperature limits than carbon steel.
Both socket weld and blind flanges are governed by ASME B16.5 for sizes up to 24 inches. Their specifications ensure compatibility with standard piping systems by defining precise dimensions for diameter, thickness, and bolt patterns.
1. Key Dimensional Specifications (Class 150)
The following table summarizes common physical specifications for small-to-medium sizes under the Class 150 pressure rating:
| Nominal Pipe Size (NPS) | Outside Diameter (O) | Min. Thickness (T) | No. of Bolt Holes | Bolt Circle (BC) |
|---|---|---|---|---|
| 1/2" | 3.50" | 0.38" | 4 | 2.38" |
| 1" | 4.25" | 0.50" | 4 | 3.12" |
| 2" | 6.00" | 0.69" | 4 | 4.75" |
| 4" | 9.00" | 0.88" | 8 | 7.50" |
| 8" | 13.50" | 1.06" | 8 | 11.75" |
- Standard Reference: ASME B16.5 provides these benchmarks to ensure any flange from any manufacturer will fit a standard pipe.
- Bore Specifics: For socket weld flanges, the internal bore must match the pipe's outside diameter to allow insertion into the recessed socket.
Facing Specifications
Flanges are specified by their "face" type, which determines the gasket required:
- Raised Face (RF): The most common; features a 0.06" (1.6mm) raised portion for Class 150/300.
- Flat Face (FF): Used mostly for low-pressure cast iron or bronze systems to prevent cracking during bolting.
- Ring Type Joint (RTJ): Machined with a groove for high-pressure/temperature sealing with a metal ring.
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