Class 900 Weld Neck Flange Dimensions
ASME B16.5 dimensions and bolting for the Class 900 weld neck flange, NPS 1/2 through NPS 24, with what the class is rated for and where this flange type is used in it.
- Standard
- ASME B16.5
- Flange type
- Weld Neck Flange
- Pressure class
- Class 900
- Size range
- NPS 1/2 – NPS 24
- Raised face height
- 0.25 in (6.3 mm)
- Rating at 100 °F (A105)
- 2220 psig (153.1 bar)
Where a Class 900 weld neck flange is used
Class 900 belongs to high-pressure process, injection and power piping. From this class upward the ring-type joint facing becomes a common alternative to the raised face.
At this class the weld neck is, for practical purposes, the only flange used to join welded pipe. The full-penetration butt weld and the tapered hub are what let the joint match the strength of the pipe.
The number 900 is a designation and not a pressure. A Class 900 flange in A105 carbon steel is rated 2220 psig (153.1 bar) at 100 °F, 1900 psig at 400 °F, 1705 psig at 600 °F and 1235 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 2160 psig and still holds 1215 psig at 800 °F, so the material and the metal temperature have to be fixed before the class can be.
Dimensions in every size
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Size | Flange OD | Thickness | Bolt circle | Bolts | Bolt dia. | Raised face OD |
|---|---|---|---|---|---|---|
| NPS 1/2 | 4.75120.65 mm | 0.8822.35 mm | 3.2582.55 mm | 4 | 3/4 in | 1.3835.05 mm |
| NPS 3/4 | 5.12130.05 mm | 1.0025.40 mm | 3.5088.90 mm | 4 | 3/4 in | 1.6942.93 mm |
| NPS 1 | 5.88149.35 mm | 1.1228.45 mm | 4.00101.60 mm | 4 | 7/8 in | 2.0050.80 mm |
| NPS 1 1/4 | 6.25158.75 mm | 1.1228.45 mm | 4.38111.25 mm | 4 | 7/8 in | 2.5063.50 mm |
| NPS 1 1/2 | 7.00177.80 mm | 1.2531.75 mm | 4.88123.95 mm | 4 | 1 in | 2.8873.15 mm |
| NPS 2 | 8.50215.90 mm | 1.5038.10 mm | 6.50165.10 mm | 8 | 7/8 in | 3.6291.95 mm |
| NPS 2 1/2 | 9.62244.35 mm | 1.6241.15 mm | 7.50190.50 mm | 8 | 1 in | 4.12104.65 mm |
| NPS 3 | 9.50241.30 mm | 1.5038.10 mm | 7.50190.50 mm | 8 | 7/8 in | 5.00127.00 mm |
| NPS 4 | 11.50292.10 mm | 1.7544.45 mm | 9.25234.95 mm | 8 | 1 1/8 in | 6.19157.23 mm |
| NPS 5 | 13.75349.25 mm | 2.0050.80 mm | 11.00279.40 mm | 8 | 1 1/4 in | 7.31185.67 mm |
| NPS 6 | 15.00381.00 mm | 2.1955.63 mm | 12.50317.50 mm | 12 | 1 1/8 in | 8.50215.90 mm |
| NPS 8 | 18.50469.90 mm | 2.5063.50 mm | 15.50393.70 mm | 12 | 1 3/8 in | 10.62269.75 mm |
| NPS 10 | 21.50546.10 mm | 2.7569.85 mm | 18.50469.90 mm | 16 | 1 3/8 in | 12.75323.85 mm |
| NPS 12 | 24.00609.60 mm | 3.1279.25 mm | 21.00533.40 mm | 20 | 1 3/8 in | 15.00381.00 mm |
| NPS 14 | 25.25641.35 mm | 3.3885.85 mm | 22.00558.80 mm | 20 | 1 1/2 in | 16.25412.75 mm |
| NPS 16 | 27.75704.85 mm | 3.5088.90 mm | 24.25615.95 mm | 20 | 1 5/8 in | 18.50469.90 mm |
| NPS 18 | 31.00787.40 mm | 4.00101.60 mm | 27.00685.80 mm | 20 | 1 7/8 in | 21.00533.40 mm |
| NPS 20 | 33.75857.25 mm | 4.25107.95 mm | 29.50749.30 mm | 20 | 2 in | 23.00584.20 mm |
| NPS 24 | 41.001041.40 mm | 5.50139.70 mm | 35.50901.70 mm | 20 | 2 1/2 in | 27.25692.15 mm |
Flange thickness excludes the raised face. Bolt diameter is nominal; stud bolts are normally ASTM A193 B7 with A194 2H nuts.
Across the 19 sizes published in Class 900, the flange grows from 4.75 in (120.6 mm) across and 0.88 in (22.4 mm) thick at NPS 1/2 to 41.00 in (1041.4 mm) across and 5.50 in (139.7 mm) thick at NPS 24. The bolt count goes 4 from NPS 1/2, 8 from NPS 2, 12 from NPS 6, 16 from NPS 10 and 20 from NPS 12, always in multiples of four so that the holes can straddle the centrelines. The largest bolt in the class is 2 1/2 in, first needed at NPS 24. Against Class 150, an NPS 6 flange in Class 900 is 4.00 in larger in diameter and 2.3 times as thick.
In NPS 1/2 through NPS 2 1/2 the Class 900 flange is dimensionally identical to the Class 1500 flange.
Bore by pipe schedule
A weld neck flange is bored to the inside diameter of the pipe it is butt-welded to. Everything in the table above is the same for every Class 900 weld neck of a given size; the bore is not. It follows the schedule of the pipe, so one size and class covers several different part numbers, and a requisition that leaves the schedule off is incomplete. A bore that does not match leaves a step inside the joint, which disturbs the flow and concentrates stress at the root of the weld.
| Size | Pipe OD | STD bore | Sch 40 bore | XS bore | Sch 80 bore | Sch 160 bore | XXS bore |
|---|---|---|---|---|---|---|---|
| NPS 1/2 | 0.84021.34 mm | 0.62215.80 mm | 0.62215.80 mm | 0.54613.87 mm | 0.54613.87 mm | 0.46411.79 mm | 0.2526.40 mm |
| NPS 3/4 | 1.05026.67 mm | 0.82420.93 mm | 0.82420.93 mm | 0.74218.85 mm | 0.74218.85 mm | 0.61215.54 mm | 0.43411.02 mm |
| NPS 1 | 1.31533.40 mm | 1.04926.64 mm | 1.04926.64 mm | 0.95724.31 mm | 0.95724.31 mm | 0.81520.70 mm | 0.59915.21 mm |
| NPS 1 1/4 | 1.66042.16 mm | 1.38035.05 mm | 1.38035.05 mm | 1.27832.46 mm | 1.27832.46 mm | 1.16029.46 mm | 0.89622.76 mm |
| NPS 1 1/2 | 1.90048.26 mm | 1.61040.89 mm | 1.61040.89 mm | 1.50038.10 mm | 1.50038.10 mm | 1.33833.99 mm | 1.10027.94 mm |
| NPS 2 | 2.37560.32 mm | 2.06752.50 mm | 2.06752.50 mm | 1.93949.25 mm | 1.93949.25 mm | 1.68742.85 mm | 1.50338.18 mm |
| NPS 2 1/2 | 2.87573.02 mm | 2.46962.71 mm | 2.46962.71 mm | 2.32359.00 mm | 2.32359.00 mm | 2.12553.97 mm | 1.77144.98 mm |
| NPS 3 | 3.50088.90 mm | 3.06877.93 mm | 3.06877.93 mm | 2.90073.66 mm | 2.90073.66 mm | 2.62466.65 mm | 2.30058.42 mm |
| NPS 4 | 4.500114.30 mm | 4.026102.26 mm | 4.026102.26 mm | 3.82697.18 mm | 3.82697.18 mm | 3.43887.33 mm | 3.15280.06 mm |
| NPS 5 | 5.563141.30 mm | 5.047128.19 mm | 5.047128.19 mm | 4.813122.25 mm | 4.813122.25 mm | 4.313109.55 mm | 4.063103.20 mm |
| NPS 6 | 6.625168.27 mm | 6.065154.05 mm | 6.065154.05 mm | 5.761146.33 mm | 5.761146.33 mm | 5.187131.75 mm | 4.897124.38 mm |
| NPS 8 | 8.625219.07 mm | 7.981202.72 mm | 7.981202.72 mm | 7.625193.67 mm | 7.625193.67 mm | 6.813173.05 mm | 6.875174.62 mm |
| NPS 10 | 10.750273.05 mm | 10.020254.51 mm | 10.020254.51 mm | 9.750247.65 mm | 9.562242.87 mm | 8.500215.90 mm | 8.750222.25 mm |
| NPS 12 | 12.750323.85 mm | 12.000304.80 mm | 11.938303.23 mm | 11.750298.45 mm | 11.374288.90 mm | 10.126257.20 mm | 10.750273.05 mm |
| NPS 14 | 14.000355.60 mm | 13.250336.55 mm | 13.124333.35 mm | 13.000330.20 mm | 12.500317.50 mm | 11.188284.18 mm | — |
| NPS 16 | 16.000406.40 mm | 15.250387.35 mm | 15.000381.00 mm | 15.000381.00 mm | 14.312363.52 mm | 12.812325.42 mm | — |
| NPS 18 | 18.000457.20 mm | 17.250438.15 mm | 16.876428.65 mm | 17.000431.80 mm | 16.124409.55 mm | 14.438366.73 mm | — |
| NPS 20 | 20.000508.00 mm | 19.250488.95 mm | 18.812477.82 mm | 19.000482.60 mm | 17.938455.63 mm | 16.062407.97 mm | — |
| NPS 24 | 24.000609.60 mm | 23.250590.55 mm | 22.624574.65 mm | 23.000584.20 mm | 21.562547.67 mm | 19.312490.52 mm | — |
Bore is the pipe inside diameter, OD − 2t. A dash means B36.10M publishes no such schedule in that size. Each size links to its pipe page, which lists every schedule.
Bolt holes, spacing and torque
Bolt holes in ASME B16.5 flanges are drilled 1/8 in larger than the nominal bolt and are set to straddle the vertical and horizontal centrelines, never to sit on them. Spacing is the distance between neighbouring holes measured round the bolt circle, which shows how much room there is for a wrench or a tensioner. The torque column is a starting figure for a lubricated stud, not a joint-specific value.
| Size | Bolting | Bolt hole dia. | Hole spacing | Torque for 50 ksi bolt stress |
|---|---|---|---|---|
| NPS 1/2 | 4 × 3/4 in | 0.87522.22 mm | 2.5564.83 mm | 167 lb-ft |
| NPS 3/4 | 4 × 3/4 in | 0.87522.22 mm | 2.7569.82 mm | 167 lb-ft |
| NPS 1 | 4 × 7/8 in | 1.00025.40 mm | 3.1479.80 mm | 269 lb-ft |
| NPS 1 1/4 | 4 × 7/8 in | 1.00025.40 mm | 3.4487.38 mm | 269 lb-ft |
| NPS 1 1/2 | 4 × 1 in | 1.12528.57 mm | 3.8397.35 mm | 404 lb-ft |
| NPS 2 | 8 × 7/8 in | 1.00025.40 mm | 2.5564.83 mm | 269 lb-ft |
| NPS 2 1/2 | 8 × 1 in | 1.12528.57 mm | 2.9574.81 mm | 404 lb-ft |
| NPS 3 | 8 × 7/8 in | 1.00025.40 mm | 2.9574.81 mm | 269 lb-ft |
| NPS 4 | 8 × 1 1/8 in | 1.25031.75 mm | 3.6392.26 mm | 593 lb-ft |
| NPS 5 | 8 × 1 1/4 in | 1.37534.92 mm | 4.32109.72 mm | 833 lb-ft |
| NPS 6 | 12 × 1 1/8 in | 1.25031.75 mm | 3.2783.12 mm | 593 lb-ft |
| NPS 8 | 12 × 1 3/8 in | 1.50038.10 mm | 4.06103.07 mm | 1,131 lb-ft |
| NPS 10 | 16 × 1 3/8 in | 1.50038.10 mm | 3.6392.26 mm | 1,131 lb-ft |
| NPS 12 | 20 × 1 3/8 in | 1.50038.10 mm | 3.3083.79 mm | 1,131 lb-ft |
| NPS 14 | 20 × 1 1/2 in | 1.62541.27 mm | 3.4687.78 mm | 1,492 lb-ft |
| NPS 16 | 20 × 1 5/8 in | 1.75044.45 mm | 3.8196.75 mm | 1,923 lb-ft |
| NPS 18 | 20 × 1 7/8 in | 2.00050.80 mm | 4.24107.73 mm | 3,018 lb-ft |
| NPS 20 | 20 × 2 in | 2.12553.97 mm | 4.63117.70 mm | 3,694 lb-ft |
| NPS 24 | 20 × 2 1/2 in | 2.62566.67 mm | 5.58141.64 mm | 7,404 lb-ft |
Torque is T = K·F·d with nut factor K = 0.16 and a bolt stress of 50,000 psi on the tensile stress area. The bolt torque guide gives the method and other nut factors.
Guides and comparisons for this page
Common questions
How many bolts does a Class 900 weld neck flange use?
NPS 1/2 takes 4 × 3/4 in; NPS 3/4 takes 4 × 3/4 in; NPS 1 takes 4 × 7/8 in; NPS 1 1/4 takes 4 × 7/8 in; NPS 1 1/2 takes 4 × 1 in. Bolt count rises with size — see the table above for the full range.
What pressure is a Class 900 flange rated for?
Class is a designation, not a pressure. A Class 900 flange in A105 carbon steel is rated 2220 psig (153.1 bar) at 100 °F, and that rating falls as metal temperature rises. In stainless the same class starts lower at ambient but holds up far better in the heat — see the P-T rating tables.
Will a Class 900 flange bolt to a Class 150 flange?
No. Bolt circle diameter, bolt count and bolt size all change with class, so flanges of different classes in the same NPS will not mate. The only exceptions inside B16.5 are the deliberate ones the standard calls out, such as Class 400 sharing Class 600 dimensions in NPS 1/2 through 3 1/2.