Every piping system has a component that does nothing until the plant shuts down - and then does everything. The blind flange is the solid disc bolted to the end of a line, with no centre opening: it closes the pipe for pressure testing, isolates equipment for maintenance, seals dead legs, and reserves future connections at a fraction of the cost of a valve.
Buyers order them by the dozens on every project, yet the ordering sheet is usually wrong somewhere - the rating, the facing, the material group, or the bolt circle that must match the mating flange. This article covers the blind flange completely: what it is, the standards, the ratings, the materials, the facings, and the six fields every purchase order needs.
What a blind flange actually is
A blind flange is a flange with a solid plate instead of a bore - a flat, machined disc with bolt holes drilled to the pattern of the standard flange it mates with. Because it has no opening, its design loads are different from a regular flange: it carries pressure as a flat plate under bending, which is why its thickness is set by the pressure rating and the material, not by any pipe bore.
That flat-plate behaviour also means the blind flange can take higher end loads and moments than an equivalent bolted connection, which is why it is used not only to close lines but to support the ends of piping where expansion and vibration loads are high.
The standards that govern it
| Standard | Scope | Size range |
| ASME B16.5 | Pipe flanges and flanged fittings, Class 150 - 2500 | 1/2 in - 24 in |
| ASME B16.47 | Large-diameter steel flanges, Series A and Series B | 26 in - 60 in |
| ASME B16.36 | Orifice flanges (blind pattern with orifice taps) | 1 in - 24 in |
| API 6A | Wellhead and Christmas-tree flanges (ring-joint type) | Drilling and production duty |
For process piping, ASME B16.5 covers virtually every blind flange the plant will need; above 24 in the B16.47 Series A (heavy) and Series B (MSS SP-44 pattern) take over, and the two series are not interchangeable - the bolt circle and facing differ, so the series must be named on the order.
Ratings: class is the pressure envelope
Blind flanges are rated by ASME class - 150, 300, 600, 900, 1500, 2500 - and each class carries a pressure-temperature curve set by the material group. The class is not a fixed pressure: a Class 150 carbon-steel blind flange is rated about 20 bar at 38°C but derates as temperature rises, while a Class 2500 flange at the same size is a solid forging rated ten times higher. The two numbers that matter on the enquiry are the class and the material group, because the allowable pressure comes from their combination in the B16.5 tables.
Materials: matching the line, not just the rating
| Material | Standard | Typical service |
| Carbon steel A105 | ASTM A105 | General industrial, water, steam, hydrocarbons |
| Alloy steel F11 / F22 | ASTM A182 | High-temperature steam and process |
| 304 / 304L | ASTM A182 F304/F304L | Corrosive and food service |
| 316 / 316L | ASTM A182 F316/F316L | Chloride and marine service |
| Duplex and super-duplex | ASTM A182 F51 / F53 / F55 | Seawater, offshore, chloride-laden duty |
| Nickel alloys (625, C276, Monel, Inconel) | ASTM B564 | Aggressive chemical and acid service |
The blind flange material should match the piping material group - a 316L line closed with a carbon-steel blind flange becomes a galvanic and corrosion hazard at the end of the system. For stainless and alloy lines the flange is normally forged to A182 (or B564 for nickel alloys), not cut from plate, because the forging direction and testing give the through-thickness integrity a flat plate under bending needs.
Facings: the sealing surface decides the gasket
| Facing | Surface | Typical gasket |
| Raised face (RF) | Concentric serrated finish | Sheet, spiral-wound, metal-jacketed |
| Flat face (FF) | Full face contact | Full-face sheet gasket - used with cast-iron or fragile equipment |
| Ring joint (RTJ) | Groove for octagonal ring | Metal ring (oval or octagonal) |
| RTJ + RF | Dual facing | Allows either gasket at installation |
The facing of the blind flange must match the mating flange: a raised-face blind flange bolts to a raised-face companion, and an RTJ blind flange to an RTJ companion - mixing them either leaks or over-stresses the joint. This is the most common field error on shutdown jobs, where a spare blind of the right class but the wrong facing is bolted to a line that cannot wait.
Where the blind flange earns its keep
- Pressure testing - the blind closes the end of the line for hydrostatic test, and the flat plate carries the test load
- Maintenance isolation - equipment and vessels are blinded off for safe work entry instead of relying on a valve that may not hold
- Future connections - a blanked flange is a ready-made tie-in point; cap the line once, connect later without cutting
- Dead-leg and drain closure - sealing unused branches with a blind instead of a valve saves cost and removes the failure mode
- Support at line ends - the solid disc handles end thrust and moment loads where a pipe would need anchors
The six fields every order needs
| Field | Example | Why it matters |
| Standard | ASME B16.5 (or B16.47 Series A/B) | Sets dimensions, bolt circle, facing |
| Nominal size | 6 in | Must match the mating flange bore |
| Class | Class 300 | Sets the pressure-temperature envelope |
| Material + standard | A182 F316L | Matches the line and the corrosion duty |
| Facing | RF (serrated finish) | Must match the companion flange |
| Quantity and marking | 4 pcs, stamped per B16.5 | Traceability and inspection |
A blind flange written up as "6-inch blind" without class, material and facing is an enquiry waiting to be wrong. The full six-field description takes ten seconds to write and saves a shutdown-day argument.
Inspection points on receipt
- Thickness and OD against the B16.5 table for the class and size
- Bolt circle and hole count against the mating flange
- Facing finish - serrated, free of damage, and the correct type
- Stamping: standard, class, material grade, heat number
- Certificate (EN 10204 3.1) with chemistry and, for alloy grades, the heat-treat condition
FAQ
Q: Is a blind flange the same as a blank flange?
A: Yes - "blind flange" and "blank flange" are two names for the same solid flange without a bore. The blind is the bolted closure; the term "blank" is also used for the temporary plate in a spectacle/line-blind arrangement.
Q: Can a blind flange be used as a temporary test plate?
A: Yes, and it is the standard component for closing a line end during hydrostatic test - the class must cover the test pressure, which is commonly higher than the design pressure.
Q: Do I need a blind flange with the same rating as the valves on the line?
A: Yes - the blind must be rated for the same class, and matched to the same pressure-temperature condition, as the line and its components.
Q: Why does the facing type have to match the other flange?
A: Because the gasket and the sealing geometry depend on it. An RF blind on an FF companion, or an RTJ blind on an RF flange, cannot seal properly and can over-stress the bolts.
