A Guide to Blind Flanges: Types, Applications, and Sizes
Posted: Jan 31, 2026 | Posted in: Blogs
What Is a Blind Flange?
A blind flange is a flat disc with a bolted connection hub but no opening in the center. It's used to cap or isolate the end of a piping system, process vessel, or equipment. Blind flanges are essential components in any pressurized system.
Common Uses for Blind Flanges
System Isolation
Cap the end of a pressure line or vessel to prevent fluid loss and maintain system integrity. Blind flanges are bolted to a mating flange on the equipment.
Isolation and Testing
During hydrostatic testing or commissioning, blind flanges isolate sections of a pipeline so pressure testing can be done safely and accurately.
Future Expansion
A blind flange can be temporarily installed where a connection will be added later. When expansion is needed, the blind flange is removed and replaced with a spool or connection flange.
Termination Points
End caps for dead legs, branch lines, and systems that are intentionally closed to service conditions.
Types of Blind Flanges
Raised Face (RF) Blind Flanges
The most common type. Features a raised face (typically 1/4" or 1/8" above the bolt circle) that helps with gasket sealing. RF blind flanges are standard for ASME B16.5 applications.
Flat Face (FF) Blind Flanges
Flat surface with no raised face. Often used with soft gaskets or when isolation from a flat mating surface is required. Common in European (EN 1092-1) standards.
Ring Joint Groove (RTJ) Blind Flanges
Feature a groove machined into the sealing surface for a metal ring gasket. RTJ blind flanges are used in critical, high-pressure applications requiring superior sealing (API, petrochemical, subsea).
Design and Materials
Standard vs. Dished Design
Most blind flanges are flat. However, large blind flanges (typically 12" and above) may be dished (slightly concave on the pressure-facing side) to distribute stress more evenly under pressure and improve fatigue life.
Custom Machining
Blind flanges can be custom drilled for lifting lugs, reinforced with thicker material for extreme pressures, or machined with special features like drain holes, test ports, or vent holes.
Materials
Blind flanges are available in:
- Carbon Steel: ASTM A105, most common and economical
- Stainless Steel: 304, 316, for corrosive or sanitary applications
- Alloy Steel: A182 grades F5, F9, F11, F22 for high-temperature service
- Nickel Alloys: Inconel, Monel for extreme environments
Blind Flange Dimensions (Sample)
ASME B16.5 Carbon Steel Blind Flanges
| Size | Class 150 OD (in) | Class 150 Thickness (in) | Class 300 OD (in) | Class 300 Thickness (in) |
|---|---|---|---|---|
| 1" | 4.5" | 0.44" | 5.0" | 0.62" |
| 2" | 6.0" | 0.50" | 7.0" | 0.75" |
| 4" | 9.0" | 0.62" | 10.0" | 0.94" |
| 6" | 11.0" | 0.75" | 12.5" | 1.12" |
| 12" | 19.0" | 1.12" | 21.0" | 1.75" |
Dimensions vary by standard (ASME B16.5, EN 1092-1, JIS, etc.). Always verify exact specifications before ordering.
Pressure Ratings
Blind flanges are rated by the same pressure classes as regular flanges:
- ASME: Classes 150, 300, 400, 600, 900, 1500, 2500 (in PSI)
- EN 1092-1: PN ratings from PN6 to PN40 and above (in bar)
The pressure rating depends on the flange size, material, and class. A 2" Class 150 carbon steel blind flange is rated for 150 PSI at 100°F.
Installation Considerations
Gasket Selection
Choose gasket material based on the service fluid (water, oil, gas, steam, chemicals) and temperature. Common gaskets include:
- Spiral wound (general service)
- Compressed asbestos-free (CAF)
- PTFE-lined (chemical resistant)
- Metal ring joint (high-pressure)
Bolt Torque
Follow manufacturer's specifications for bolt torque based on flange size and class. Improper torque can lead to leaks or bolt breakage.
Flange Alignment
Ensure the mating flange (blind flange and equipment flange) are parallel and properly aligned before bolting. Misalignment creates uneven gasket compression.
FAQ
Q: What's the difference between a blind flange and a cap?
A: A blind flange has a bolted connection interface (hub and bolt holes) and must be bolted to a mating flange. A pipe cap simply screws onto the end of a pipe. For pressurized systems, blind flanges are stronger and more reliable.
Q: Can I replace a regular flange with a blind flange?
A: Yes, if you want to close off that connection. The bolt patterns must match. Dimensions (OD, BCD) should be the same or larger.
Q: What size blind flange do I need?
A: The blind flange must match the size and class of the mating flange. For example, if your system has 4" Class 300 ASME flanges, use a 4" Class 300 blind flange.
Q: Can blind flanges be made to custom specifications?
A: Yes. We can fabricate blind flanges with custom OD, thickness, bolt patterns, material, and facing (RF, FF, RTJ) to meet your exact requirements.
Q: Are dished blind flanges necessary for all applications?
A: Dished flanges are commonly used for large sizes (12" and above) to improve stress distribution under pressure. For smaller flanges, flat design is standard and adequate.
Q: What materials should I use for high-temperature service?
A: For temperatures above 400°F, use alloy steel grades (A182 F5, F9, F11, F22, F91) or nickel alloys. Carbon steel loses strength at high temperatures.
Q: How long are blind flanges good for?
A: Blind flanges have no wear mechanisms if properly installed and maintained. They will last indefinitely in service. Replace only if damaged, corroded, or system requirements change.
Q: Can I use a temporary blind flange for testing?
A: Yes. Blind flanges are often used for hydrostatic testing to isolate sections of pipe. After testing, remove the blind flange and install the final connection or spool.
Ready to Order?
Contact Custom Flanges for blind flange quotes — we'll help you specify the right size, material, and pressure class for your application.