What Is a Blind Flange?
A blind flange is a solid disc-shaped flange with bolt holes but no center opening. It is used to close the end of a piping system or pressure vessel opening. Blind flanges provide positive pressure containment at fittings, valves, and equipment nozzles. They are essential components for pressure testing assembled piping sections, providing future tie-in points for plant expansion, and isolating decommissioned equipment. Blind flanges can be installed between two flanges to isolate sections of a piping system or at the end of a pipe run for permanent or temporary closure.
Design and Dimensions
Blind flanges are dimensioned per ASME B16.5 for sizes NPS 1/2 through 24. They are manufactured as flat solid plates with bolt holes matching the standard pattern for each pressure class. The thickness must meet the pressure class requirements for the given material, and because there is no bore, the full cross-section is available to resist pressure. Blind flanges are available in all facing types: raised face, flat face, ring type joint, male and female, and tongue and groove. Large-diameter blind flanges may require lifting lugs for safe handling during installation and removal.
| NPS | Class 150 Thk (in) | Class 300 Thk (in) | Class 600 Thk (in) | Class 900 Thk (in) |
|---|---|---|---|---|
| 2 | 0.44 | 0.56 | 0.62 | 0.75 |
| 4 | 0.62 | 0.75 | 0.94 | 1.12 |
| 6 | 0.75 | 0.94 | 1.12 | 1.38 |
| 8 | 0.88 | 1.12 | 1.38 | 1.62 |
| 10 | 1.00 | 1.25 | 1.62 | 1.88 |
| 12 | 1.12 | 1.38 | 1.75 | 2.12 |
Pressure Ratings
Blind flanges are available in all ASME pressure classes from 150 through 2500. Because there is no bore, there is no pressure loss from internal thinning, and the full flange thickness resists the internal pressure. Blind flanges at the end of pipe runs see the full hydrostatic test pressure, typically 1.5 times the design pressure. Higher classes require significantly thicker plates, making large-diameter, high-class blind flanges very heavy components that require careful handling planning.
Materials
Blind flanges are available in the same material range as other ASME B16.5 flanges. ASTM A105 carbon steel is standard for general service. ASTM A350 LF2 low-temperature carbon steel is used for applications down to -50°F. ASTM A182 F304 and F316 stainless steels serve corrosive environments. Alloy grades F11, F22, and F91 are available for high-temperature service. Duplex materials are used for marine and sour gas applications per NACE MR0175.
Applications
Blind flanges serve multiple critical functions in piping systems. As end closures, they seal the termination points of piping systems and pressure vessels. For pressure testing, blind flanges are installed at test boundaries to contain the test pressure while allowing inspection of the assembled system. Future tie-in points are capped with blind flanges that can be removed when expansion is needed. Manhole covers on tanks and vessels frequently use blind flanges for access. Decommissioned equipment is isolated by installing blind flanges at its connections.
Selection Criteria
The size of a blind flange must match the adjacent piping nominal diameter. The pressure class must match or exceed the system design pressure, accounting for hydrostatic test conditions. The facing type should match the mating flange or equipment nozzle facing. Material must be compatible with the process media, temperature, and ambient conditions. For large-diameter blind flanges, weight and handling requirements should be evaluated, including the need for lifting lugs and mechanical handling equipment.
Comparison with Caps and Plugs
Blind flanges offer significant advantages over other end closure methods. Unlike pipe caps which are welded on and difficult to remove, blind flanges provide a bolted connection that can be easily removed for access. Plugs are threaded and limited to smaller sizes and lower pressure ratings. Blind flanges are preferred for pressure testing because of their easy removal after the test is complete. They also allow visual inspection of the closed end and can be reused multiple times, unlike welded caps which must be cut off and discarded.
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