Class 300 Lap Joint Flange Dimensions
ASME B16.5 dimensions and bolting for the Class 300 lap joint 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
- Lap Joint Flange
- Pressure class
- Class 300
- Size range
- NPS 1/2 – NPS 24
- Raised face height
- 0.06 in (1.5 mm)
- Rating at 100 °F (A105)
- 740 psig (51.0 bar)
Where a Class 300 lap joint flange is used
Class 300 is the usual step up when Class 150 runs out of pressure or temperature. It is common on hydrocarbon process lines, steam and hot oil, and it is often chosen over Class 150 for a hazardous fluid at modest pressure simply for the heavier joint.
A lap joint flange is loose on the pipe and bears against a stub end, so it can be turned to line up the bolt holes. At this class it is mostly used to save money on alloy lines, since only the stub end needs to be in the expensive material.
The number 300 is a designation and not a pressure. A Class 300 flange in A105 carbon steel is rated 740 psig (51.0 bar) at 100 °F, 635 psig at 400 °F, 570 psig at 600 °F and 410 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 720 psig and still holds 405 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 | Length thru hub |
|---|---|---|---|---|---|---|---|
| NPS 1/2 | 3.7595.25 mm | 0.5012.70 mm | 2.6266.55 mm | 4 | 1/2 in | 1.3835.05 mm | 0.8822.35 mm |
| NPS 3/4 | 4.62117.35 mm | 0.5614.22 mm | 3.2582.55 mm | 4 | 5/8 in | 1.6942.93 mm | 1.0025.40 mm |
| NPS 1 | 4.88123.95 mm | 0.6215.75 mm | 3.5088.90 mm | 4 | 5/8 in | 2.0050.80 mm | 1.0626.92 mm |
| NPS 1 1/4 | 5.25133.35 mm | 0.6917.53 mm | 3.8898.55 mm | 4 | 5/8 in | 2.5063.50 mm | 1.0626.92 mm |
| NPS 1 1/2 | 6.12155.45 mm | 0.7519.05 mm | 4.50114.30 mm | 4 | 3/4 in | 2.8873.15 mm | 1.1930.23 mm |
| NPS 2 | 6.50165.10 mm | 0.8120.57 mm | 5.00127.00 mm | 8 | 5/8 in | 3.6291.95 mm | 1.3133.27 mm |
| NPS 2 1/2 | 7.50190.50 mm | 0.9423.88 mm | 5.88149.35 mm | 8 | 3/4 in | 4.12104.65 mm | 1.5038.10 mm |
| NPS 3 | 8.25209.55 mm | 1.0626.92 mm | 6.62168.15 mm | 8 | 3/4 in | 5.00127.00 mm | 1.6942.93 mm |
| NPS 3 1/2 | 9.00228.60 mm | 1.1228.45 mm | 7.25184.15 mm | 8 | 3/4 in | 5.50139.70 mm | 1.7544.45 mm |
| NPS 4 | 10.00254.00 mm | 1.1930.23 mm | 7.88200.15 mm | 8 | 3/4 in | 6.19157.23 mm | 1.8847.75 mm |
| NPS 5 | 11.00279.40 mm | 1.3133.27 mm | 9.25234.95 mm | 8 | 3/4 in | 7.31185.67 mm | 2.0050.80 mm |
| NPS 6 | 12.50317.50 mm | 1.3835.05 mm | 10.62269.75 mm | 12 | 3/4 in | 8.50215.90 mm | 2.0652.32 mm |
| NPS 8 | 15.00381.00 mm | 1.5639.62 mm | 13.00330.20 mm | 12 | 7/8 in | 10.62269.75 mm | 2.4461.98 mm |
| NPS 10 | 17.50444.50 mm | 1.8145.97 mm | 15.25387.35 mm | 16 | 1 in | 12.75323.85 mm | 3.7595.25 mm |
| NPS 12 | 20.50520.70 mm | 1.9449.28 mm | 17.75450.85 mm | 16 | 1 1/8 in | 15.00381.00 mm | 4.00101.60 mm |
| NPS 14 | 23.00584.20 mm | 2.0652.32 mm | 20.25514.35 mm | 20 | 1 1/8 in | 16.25412.75 mm | 4.38111.25 mm |
| NPS 16 | 25.50647.70 mm | 2.1955.63 mm | 22.50571.50 mm | 20 | 1 1/4 in | 18.50469.90 mm | 4.75120.65 mm |
| NPS 18 | 28.00711.20 mm | 2.3158.67 mm | 24.75628.65 mm | 24 | 1 1/4 in | 21.00533.40 mm | 5.12130.05 mm |
| NPS 20 | 30.50774.70 mm | 2.4461.98 mm | 27.00685.80 mm | 24 | 1 1/4 in | 23.00584.20 mm | 5.50139.70 mm |
| NPS 24 | 36.00914.40 mm | 2.6968.33 mm | 32.00812.80 mm | 24 | 1 1/2 in | 27.25692.15 mm | 6.00152.40 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 300, the flange grows from 3.75 in (95.2 mm) across and 0.50 in (12.7 mm) thick at NPS 1/2 to 36.00 in (914.4 mm) across and 2.69 in (68.3 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 1/2 in, first needed at NPS 24. Against Class 150, an NPS 6 flange in Class 300 is 1.50 in larger in diameter and 1.5 times as thick.
The hub length shown is the lap joint figure, which ASME B16.5 tabulates separately from the slip-on hub and which is longer than it in the large sizes. Some makers supply lap joint flanges to the slip-on hub length, so confirm which you are getting. In Classes 150 and 300 a lap joint flange is also made 1/16 in thicker than the thickness tabulated here; that figure comes from a single source.
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 × 3/4 in | 0.87522.22 mm | 2.8572.32 mm | 167 lb-ft |
| NPS 4 | 8 × 3/4 in | 0.87522.22 mm | 3.0978.60 mm | 167 lb-ft |
| NPS 5 | 8 × 3/4 in | 0.87522.22 mm | 3.6392.26 mm | 167 lb-ft |
| NPS 6 | 12 × 3/4 in | 0.87522.22 mm | 2.7870.62 mm | 167 lb-ft |
| NPS 8 | 12 × 7/8 in | 1.00025.40 mm | 3.4086.45 mm | 269 lb-ft |
| NPS 10 | 16 × 1 in | 1.12528.57 mm | 2.9976.06 mm | 404 lb-ft |
| NPS 12 | 16 × 1 1/8 in | 1.25031.75 mm | 3.4988.52 mm | 593 lb-ft |
| NPS 14 | 20 × 1 1/8 in | 1.25031.75 mm | 3.1880.79 mm | 593 lb-ft |
| NPS 16 | 20 × 1 1/4 in | 1.37534.92 mm | 3.5389.77 mm | 833 lb-ft |
| NPS 18 | 24 × 1 1/4 in | 1.37534.92 mm | 3.2482.29 mm | 833 lb-ft |
| NPS 20 | 24 × 1 1/4 in | 1.37534.92 mm | 3.5389.77 mm | 833 lb-ft |
| NPS 24 | 24 × 1 1/2 in | 1.62541.27 mm | 4.19106.40 mm | 1,492 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 300 lap joint 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 300 flange rated for?
Class is a designation, not a pressure. A Class 300 flange in A105 carbon steel is rated 740 psig (51.0 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 300 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.