Weld Neck vs Blind Flange
One flange carries the line onward, the other stops it. Identical bolting, opposite jobs, and a very different stress state in the disc.
- Weld neck
- Continues the line
- Blind
- Closes the line
- Shared bolting
- Yes — identical per class and size
- Highest bending stress
- Blind, of all six types
Short answer. These two flanges do opposite jobs. A weld neck carries the line onward through a butt weld to the next pipe; a blind is a solid disc that closes the opening. They share outside diameter, thickness, bolt circle and bolting within a class, so a blind bolts straight onto a weld neck — which is exactly how a line is blanked for a hydrotest or a tie-in.
ASME B16.5 tabulates one set of outside diameters, thicknesses, bolt circles and bolt patterns per class per size, and every flange type in that class uses them. A weld neck and a blind of the same class and size have identical bolting and are fully interchangeable at the joint — the difference is entirely in how each attaches to the pipe.
A blind carries the full end load
Nothing supports the middle of a blind flange. The full internal pressure acts across the whole gasket-diameter disc, and the flange has to take that load in bending. It is the highest stressed of the six B16.5 types at a given class, which is why blinds are thick and, in large sizes, genuinely heavy.
End load F = P × π/4 × d² · d = raised face outside diameter
The table below works that out for every size at the ambient rating of each class. A NPS 24 Class 300 blind holds back more than four hundred thousand pounds of force — the number is worth seeing before anyone treats a blind as a convenient temporary cover.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Raised face OD | Disc area | Cl 150 thickness | Cl 300 thickness | Cl 150 end load | Cl 300 end load |
|---|---|---|---|---|---|---|
| NPS 1/2 | 1.3835.05 mm | 1.5 in² | 0.4411.18 mm | 0.5614.22 mm | 0.4 kip | 1.1 kip |
| NPS 3/4 | 1.6942.93 mm | 2.2 in² | 0.5012.70 mm | 0.6215.75 mm | 0.6 kip | 1.7 kip |
| NPS 1 | 2.0050.80 mm | 3.1 in² | 0.5614.22 mm | 0.6917.53 mm | 0.9 kip | 2.3 kip |
| NPS 1 1/4 | 2.5063.50 mm | 4.9 in² | 0.6215.75 mm | 0.7519.05 mm | 1.4 kip | 3.6 kip |
| NPS 1 1/2 | 2.8873.15 mm | 6.5 in² | 0.6917.53 mm | 0.8120.57 mm | 1.9 kip | 4.8 kip |
| NPS 2 | 3.6291.95 mm | 10.3 in² | 0.7519.05 mm | 0.8822.35 mm | 2.9 kip | 7.6 kip |
| NPS 2 1/2 | 4.12104.65 mm | 13.3 in² | 0.8822.35 mm | 1.0025.40 mm | 3.8 kip | 9.9 kip |
| NPS 3 | 5.00127.00 mm | 19.6 in² | 0.9423.88 mm | 1.1228.45 mm | 5.6 kip | 14.5 kip |
| NPS 3 1/2 | 5.50139.70 mm | 23.8 in² | 0.9423.88 mm | 1.1930.23 mm | 6.8 kip | 17.6 kip |
| NPS 4 | 6.19157.23 mm | 30.1 in² | 0.9423.88 mm | 1.2531.75 mm | 8.6 kip | 22.3 kip |
| NPS 5 | 7.31185.67 mm | 42.0 in² | 0.9423.88 mm | 1.3835.05 mm | 12.0 kip | 31.1 kip |
| NPS 6 | 8.50215.90 mm | 56.7 in² | 1.0025.40 mm | 1.4436.58 mm | 16.2 kip | 42.0 kip |
| NPS 8 | 10.62269.75 mm | 88.6 in² | 1.1228.45 mm | 1.6241.15 mm | 25.2 kip | 65.5 kip |
| NPS 10 | 12.75323.85 mm | 127.7 in² | 1.1930.23 mm | 1.8847.75 mm | 36.4 kip | 94.5 kip |
| NPS 12 | 15.00381.00 mm | 176.7 in² | 1.2531.75 mm | 2.0050.80 mm | 50.4 kip | 130.8 kip |
| NPS 14 | 16.25412.75 mm | 207.4 in² | 1.3835.05 mm | 2.1253.85 mm | 59.1 kip | 153.5 kip |
| NPS 16 | 18.50469.90 mm | 268.8 in² | 1.4436.58 mm | 2.2557.15 mm | 76.6 kip | 198.9 kip |
| NPS 18 | 21.00533.40 mm | 346.4 in² | 1.5639.62 mm | 2.3860.45 mm | 98.7 kip | 256.3 kip |
| NPS 20 | 23.00584.20 mm | 415.5 in² | 1.6942.93 mm | 2.5063.50 mm | 118.4 kip | 307.5 kip |
| NPS 24 | 27.25692.15 mm | 583.2 in² | 1.8847.75 mm | 2.7569.85 mm | 166.2 kip | 431.6 kip |
End load computed at 285 psig for Class 150 and 740 psig for Class 300 — the Group 1.1 ratings at 100 °F. 1 kip = 1000 lbf.
Weld neck
- Bore matches the pipe, so no turbulence or crevice at the joint
- Full-penetration butt weld is radiographable
- Best fatigue and thermal-shock performance of any B16.5 flange
- High pressure, high temperature, cyclic loading, hydrogen and lethal service.
Blind
- Closes a line while leaving it easy to reopen for inspection or tie-in
- Can be drilled and tapped later for a future branch
- Vessel and line closures, future tie-in points, pressure-test blanks.
Blanking a line safely
A blind flange bolted to a live joint is a pressure boundary like any other and must be rated for the service. A spade or spectacle blind is a different component — a plate inserted between two existing flanges, sized by calculation for the pressure and the span, and not covered by B16.5 at all. The two are not interchangeable.
Dimensions: weld neck flanges · blind flanges · hydrostatic test pressure · bolt torque.
Common questions
Can a blind flange bolt onto a weld neck flange?
Yes. Within a class and size the two share bolt circle, bolt count and bolt diameter, so they mate directly with the appropriate gasket.
Why are blind flanges thicker than other flanges?
Because the disc is unsupported in the middle. B16.5 gives blinds the same tabulated thickness as other types in most sizes, but the stress state is bending across the full disc rather than a ring load, which is what makes it the most highly stressed type.
What is the difference between a blind flange and a spade?
A blind flange bolts onto an open flanged connection and is a B16.5 component. A spade is a plate inserted between two mated flanges, is not covered by B16.5, and must be sized by calculation.
Can a blind flange be drilled later for a branch?
Yes — that is one of the reasons blinds are left on future tie-in points. The drilled and tapped result is no longer a standard B16.5 item and needs to be assessed for the service.