NPS 20 Flange Dimensions
Outside diameter, thickness, bolt circle and bolting for every ASME B16.5 pressure class published in NPS 20 — Class 150 through Class 1500.
- Standard
- ASME B16.5
- Nominal size
- NPS 20
- Classes published
- 6
- Class 150 flange OD
- 27.50 in (698.5 mm)
- Class 150 bolting
- 20 × 1 1/8 in
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Class | Flange OD | Thickness | Bolt circle | Bolting | Raised face OD | Rating @ 100 °F |
|---|---|---|---|---|---|---|
| Class 150 | 27.50698.50 mm | 1.6942.93 mm | 25.00635.00 mm | 20 × 1 1/8 in | 23.00584.20 mm | 285 psig |
| Class 300 | 30.50774.70 mm | 2.5063.50 mm | 27.00685.80 mm | 24 × 1 1/4 in | 23.00584.20 mm | 740 psig |
| Class 400 | 30.50774.70 mm | 2.7569.85 mm | 27.00685.80 mm | 24 × 1 1/2 in | 23.00584.20 mm | 990 psig |
| Class 600 | 32.00812.80 mm | 3.5088.90 mm | 28.50723.90 mm | 24 × 1 5/8 in | 23.00584.20 mm | 1480 psig |
| Class 900 | 33.75857.25 mm | 4.25107.95 mm | 29.50749.30 mm | 20 × 2 in | 23.00584.20 mm | 2220 psig |
| Class 1500 | 38.75984.25 mm | 7.00177.80 mm | 32.75831.85 mm | 16 × 3 in | 23.00584.20 mm | 3705 psig |
Ratings are for A105 carbon steel at 100 °F and fall with temperature. Thickness excludes the raised face.
Length through hub
How far the flange stands off the joint, which is what a spool drawing needs. B16.5 carries hub length for Classes 150 and 300 only; above that the hub follows the bore and varies with the schedule the flange is ordered to.
| Class | Weld neck | Slip-on / threaded | Raised face height |
|---|---|---|---|
| Class 150 | 5.69144.53 mm | 4.06103.12 mm | 0.06 in (1.5 mm) |
| Class 300 | 6.38162.05 mm | 3.7595.25 mm | 0.06 in (1.5 mm) |
The pipe these flanges bolt to
NPS 20 pipe is 20.000 in (508.0 mm) outside diameter in every schedule, which is why one flange OD serves the whole range. Standard weight is a 0.375 in (9.5 mm) wall, giving a 19.250 in (488.9 mm) bore — the figure a weld neck flange in this size is normally bored to.
NPS 20 pipe dimensions in every schedule →
Flange types in this size
Common questions
What is the bolt circle of an NPS 20 flange?
Class 150 is 25.00 in with 20 × 1 1/8 in bolts; Class 300 is 27.00 in with 24 × 1 1/4 in bolts; Class 400 is 27.00 in with 24 × 1 1/2 in bolts; Class 600 is 28.50 in with 24 × 1 5/8 in bolts. The bolt circle grows with the class, which is why an NPS 20 flange of one class will not bolt to another.
Do all six flange types share these NPS 20 dimensions?
Yes. Flange outside diameter, thickness, bolt circle, bolt count and bolt size are set by NPS and class alone — a weld neck, slip-on, threaded, lap joint, socket weld and blind of the same size and class are dimensionally interchangeable at the bolted joint. What differs is how each attaches to the pipe.
How much bigger is a Class 1500 flange than a Class 150 in NPS 20?
38.75 in outside diameter against 27.50 in, and 7.00 in thick against 1.69 in — 4.1× the metal in the flange ring, plus heavier bolting on a wider bolt circle.