Class 400 Threaded Flange Dimensions
ASME B16.5 dimensions and bolting for the Class 400 threaded 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
- Threaded Flange
- Pressure class
- Class 400
- Size range
- NPS 1/2 – NPS 24
- Raised face height
- 0.25 in (6.3 mm)
- Rating at 100 °F (A105)
- 985 psig (67.9 bar)
Where a Class 400 threaded flange is used
Class 400 is published but seldom specified. Most piping specifications step from Class 300 straight to Class 600, so Class 400 flanges are rarely held in stock and usually have to be made to order.
A threaded flange needs no welding. At this class it is used on small-bore lines, and the thread is sometimes seal-welded, which gives up the advantage of a weld-free joint in exchange for leak tightness.
The number 400 is a designation and not a pressure. A Class 400 flange in A105 carbon steel is rated 985 psig (67.9 bar) at 100 °F, 845 psig at 400 °F, 755 psig at 600 °F and 550 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 960 psig and still holds 540 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 | 3.7595.25 mm | 0.5614.22 mm | 2.6266.55 mm | 4 | 1/2 in | 1.3835.05 mm |
| NPS 3/4 | 4.62117.35 mm | 0.6215.75 mm | 3.2582.55 mm | 4 | 5/8 in | 1.6942.93 mm |
| NPS 1 | 4.88123.95 mm | 0.6917.53 mm | 3.5088.90 mm | 4 | 5/8 in | 2.0050.80 mm |
| NPS 1 1/4 | 5.25133.35 mm | 0.8120.57 mm | 3.8898.55 mm | 4 | 5/8 in | 2.5063.50 mm |
| NPS 1 1/2 | 6.12155.45 mm | 0.8822.35 mm | 4.50114.30 mm | 4 | 3/4 in | 2.8873.15 mm |
| NPS 2 | 6.50165.10 mm | 1.0025.40 mm | 5.00127.00 mm | 8 | 5/8 in | 3.6291.95 mm |
| NPS 2 1/2 | 7.50190.50 mm | 1.1228.45 mm | 5.88149.35 mm | 8 | 3/4 in | 4.12104.65 mm |
| NPS 3 | 8.25209.55 mm | 1.2531.75 mm | 6.62168.15 mm | 8 | 3/4 in | 5.00127.00 mm |
| NPS 3 1/2 | 9.00228.60 mm | 1.3835.05 mm | 7.25184.15 mm | 8 | 7/8 in | 5.50139.70 mm |
| NPS 4 | 10.00254.00 mm | 1.3835.05 mm | 7.88200.15 mm | 8 | 7/8 in | 6.19157.23 mm |
| NPS 5 | 11.00279.40 mm | 1.5038.10 mm | 9.25234.95 mm | 8 | 7/8 in | 7.31185.67 mm |
| NPS 6 | 12.50317.50 mm | 1.6241.15 mm | 10.62269.75 mm | 12 | 7/8 in | 8.50215.90 mm |
| NPS 8 | 15.00381.00 mm | 1.8847.75 mm | 13.00330.20 mm | 12 | 1 in | 10.62269.75 mm |
| NPS 10 | 17.50444.50 mm | 2.1253.85 mm | 15.25387.35 mm | 16 | 1 1/8 in | 12.75323.85 mm |
| NPS 12 | 20.50520.70 mm | 2.2557.15 mm | 17.75450.85 mm | 16 | 1 1/4 in | 15.00381.00 mm |
| NPS 14 | 23.00584.20 mm | 2.3860.45 mm | 20.25514.35 mm | 20 | 1 1/4 in | 16.25412.75 mm |
| NPS 16 | 25.50647.70 mm | 2.5063.50 mm | 22.50571.50 mm | 20 | 1 3/8 in | 18.50469.90 mm |
| NPS 18 | 28.00711.20 mm | 2.6266.55 mm | 24.75628.65 mm | 24 | 1 3/8 in | 21.00533.40 mm |
| NPS 20 | 30.50774.70 mm | 2.7569.85 mm | 27.00685.80 mm | 24 | 1 1/2 in | 23.00584.20 mm |
| NPS 24 | 36.00914.40 mm | 3.0076.20 mm | 32.00812.80 mm | 24 | 1 3/4 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 20 sizes published in Class 400, the flange grows from 3.75 in (95.2 mm) across and 0.56 in (14.2 mm) thick at NPS 1/2 to 36.00 in (914.4 mm) across and 3.00 in (76.2 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, 20 from NPS 14 and 24 from NPS 18, always in multiples of four so that the holes can straddle the centrelines. The largest bolt in the class is 1 3/4 in, first needed at NPS 24. Against Class 150, an NPS 6 flange in Class 400 is 1.50 in larger in diameter and 1.7 times as thick.
In NPS 1/2 through NPS 3 1/2 the Class 400 flange is dimensionally identical to the Class 600 flange; ASME B16.5 does not publish a separate Class 400 table for those sizes.
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 × 1/2 in | 0.62515.88 mm | 2.0652.27 mm | 47 lb-ft |
| NPS 3/4 | 4 × 5/8 in | 0.75019.05 mm | 2.5564.83 mm | 94 lb-ft |
| NPS 1 | 4 × 5/8 in | 0.75019.05 mm | 2.7569.82 mm | 94 lb-ft |
| NPS 1 1/4 | 4 × 5/8 in | 0.75019.05 mm | 3.0577.40 mm | 94 lb-ft |
| NPS 1 1/2 | 4 × 3/4 in | 0.87522.22 mm | 3.5389.77 mm | 167 lb-ft |
| NPS 2 | 8 × 5/8 in | 0.75019.05 mm | 1.9649.87 mm | 94 lb-ft |
| NPS 2 1/2 | 8 × 3/4 in | 0.87522.22 mm | 2.3158.65 mm | 167 lb-ft |
| NPS 3 | 8 × 3/4 in | 0.87522.22 mm | 2.6066.03 mm | 167 lb-ft |
| NPS 3 1/2 | 8 × 7/8 in | 1.00025.40 mm | 2.8572.32 mm | 269 lb-ft |
| NPS 4 | 8 × 7/8 in | 1.00025.40 mm | 3.0978.60 mm | 269 lb-ft |
| NPS 5 | 8 × 7/8 in | 1.00025.40 mm | 3.6392.26 mm | 269 lb-ft |
| NPS 6 | 12 × 7/8 in | 1.00025.40 mm | 2.7870.62 mm | 269 lb-ft |
| NPS 8 | 12 × 1 in | 1.12528.57 mm | 3.4086.45 mm | 404 lb-ft |
| NPS 10 | 16 × 1 1/8 in | 1.25031.75 mm | 2.9976.06 mm | 593 lb-ft |
| NPS 12 | 16 × 1 1/4 in | 1.37534.92 mm | 3.4988.52 mm | 833 lb-ft |
| NPS 14 | 20 × 1 1/4 in | 1.37534.92 mm | 3.1880.79 mm | 833 lb-ft |
| NPS 16 | 20 × 1 3/8 in | 1.50038.10 mm | 3.5389.77 mm | 1,131 lb-ft |
| NPS 18 | 24 × 1 3/8 in | 1.50038.10 mm | 3.2482.29 mm | 1,131 lb-ft |
| NPS 20 | 24 × 1 1/2 in | 1.62541.27 mm | 3.5389.77 mm | 1,492 lb-ft |
| NPS 24 | 24 × 1 3/4 in | 1.87547.62 mm | 4.19106.40 mm | 2,429 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 400 threaded flange use?
NPS 1/2 takes 4 × 1/2 in; NPS 3/4 takes 4 × 5/8 in; NPS 1 takes 4 × 5/8 in; NPS 1 1/4 takes 4 × 5/8 in; NPS 1 1/2 takes 4 × 3/4 in. Bolt count rises with size — see the table above for the full range.
What pressure is a Class 400 flange rated for?
Class is a designation, not a pressure. A Class 400 flange in A105 carbon steel is rated 985 psig (67.9 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 400 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.