Class 2500 Blind Flange Dimensions
ASME B16.5 dimensions and bolting for the Class 2500 blind flange, NPS 1/2 through NPS 12, with what the class is rated for and where this flange type is used in it.
- Standard
- ASME B16.5
- Flange type
- Blind Flange
- Pressure class
- Class 2500
- Size range
- NPS 1/2 – NPS 12
- Raised face height
- 0.25 in (6.3 mm)
- Rating at 100 °F (A105)
- 6170 psig (425.4 bar)
Where a Class 2500 blind flange is used
Class 2500 is the heaviest class in ASME B16.5. It is used where pressure leaves no alternative, and at this class many designs avoid flanged joints altogether in favour of welded ones.
A blind flange closes the end of a line or a nozzle. At this class the pressure load on the plate is large, and both the weight of the blind and the load on its bolts need to be allowed for in handling and in the joint design.
The number 2500 is a designation and not a pressure. A Class 2500 flange in A105 carbon steel is rated 6170 psig (425.4 bar) at 100 °F, 5280 psig at 400 °F, 4730 psig at 600 °F and 3430 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 6000 psig and still holds 3380 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 | 5.25133.35 mm | 1.1930.23 mm | 3.5088.90 mm | 4 | 3/4 in | 1.3835.05 mm |
| NPS 3/4 | 5.50139.70 mm | 1.2531.75 mm | 3.7595.25 mm | 4 | 3/4 in | 1.6942.93 mm |
| NPS 1 | 6.25158.75 mm | 1.3835.05 mm | 4.25107.95 mm | 4 | 7/8 in | 2.0050.80 mm |
| NPS 1 1/4 | 7.25184.15 mm | 1.5038.10 mm | 5.12130.05 mm | 4 | 1 in | 2.5063.50 mm |
| NPS 1 1/2 | 8.00203.20 mm | 1.7544.45 mm | 5.75146.05 mm | 4 | 1 1/8 in | 2.8873.15 mm |
| NPS 2 | 9.25234.95 mm | 2.0050.80 mm | 6.75171.45 mm | 8 | 1 in | 3.6291.95 mm |
| NPS 2 1/2 | 10.50266.70 mm | 2.2557.15 mm | 7.75196.85 mm | 8 | 1 1/8 in | 4.12104.65 mm |
| NPS 3 | 12.00304.80 mm | 2.6266.55 mm | 9.00228.60 mm | 8 | 1 1/4 in | 5.00127.00 mm |
| NPS 4 | 14.00355.60 mm | 3.0076.20 mm | 10.75273.05 mm | 8 | 1 1/2 in | 6.19157.23 mm |
| NPS 5 | 16.50419.10 mm | 3.6291.95 mm | 12.75323.85 mm | 8 | 1 3/4 in | 7.31185.67 mm |
| NPS 6 | 19.00482.60 mm | 4.25107.95 mm | 14.50368.30 mm | 8 | 2 in | 8.50215.90 mm |
| NPS 8 | 21.75552.45 mm | 5.00127.00 mm | 17.25438.15 mm | 12 | 2 in | 10.62269.75 mm |
| NPS 10 | 26.50673.10 mm | 6.50165.10 mm | 21.25539.75 mm | 12 | 2 1/2 in | 12.75323.85 mm |
| NPS 12 | 30.00762.00 mm | 7.25184.15 mm | 24.38619.25 mm | 12 | 2 3/4 in | 15.00381.00 mm |
Flange thickness excludes the raised face. Bolt diameter is nominal; stud bolts are normally ASTM A193 B7 with A194 2H nuts.
Across the 14 sizes published in Class 2500, the flange grows from 5.25 in (133.3 mm) across and 1.19 in (30.2 mm) thick at NPS 1/2 to 30.00 in (762.0 mm) across and 7.25 in (184.1 mm) thick at NPS 12. The bolt count goes 4 from NPS 1/2, 8 from NPS 2 and 12 from NPS 8, always in multiples of four so that the holes can straddle the centrelines. The largest bolt in the class is 2 3/4 in, first needed at NPS 12. Against Class 150, an NPS 6 flange in Class 2500 is 8.00 in larger in diameter and 4.5 times as thick.
A blind flange has no bore and no hub, so the flange outside diameter, thickness, bolt circle and bolting are the whole of its dimensional definition. Thickness shown is the minimum required by B16.5 — many mills supply heavier.
ASME B16.5 publishes Class 2500 flanges in NPS 1/2 through NPS 12 only. Above NPS 12 there is no Class 2500 flange in the standard.
Weight and end load
With a blind flange the two figures most often wanted are not in B16.5 at all: what the flange weighs, for lifting and for support loads, and how much force the contained pressure puts on it. Both are calculated here from the dimensions above. The end load is what the bolts must resist in addition to the load needed to seat the gasket.
| Size | Approximate weight | End thrust at rated pressure | Thrust per bolt |
|---|---|---|---|
| NPS 1/2 | 7.3 lb3.3 kg | 9,229 lbf41.0 kN | 2,307 lbf |
| NPS 3/4 | 8.4 lb3.8 kg | 13,840 lbf61.6 kN | 3,460 lbf |
| NPS 1 | 12.0 lb5.4 kg | 19,384 lbf86.2 kN | 4,846 lbf |
| NPS 1 1/4 | 17.6 lb8.0 kg | 30,287 lbf134.7 kN | 7,572 lbf |
| NPS 1 1/2 | 24.9 lb11.3 kg | 40,194 lbf178.8 kN | 10,048 lbf |
| NPS 2 | 38.1 lb17.3 kg | 63,503 lbf282.5 kN | 7,938 lbf |
| NPS 2 1/2 | 55.3 lb25.1 kg | 82,256 lbf365.9 kN | 10,282 lbf |
| NPS 3 | 84.0 lb38.1 kg | 121,148 lbf538.9 kN | 15,143 lbf |
| NPS 4 | 131.0 lb59.4 kg | 185,676 lbf825.9 kN | 23,210 lbf |
| NPS 5 | 219.5 lb99.6 kg | 258,946 lbf1151.8 kN | 32,368 lbf |
| NPS 6 | 341.7 lb155.0 kg | 350,117 lbf1557.3 kN | 43,765 lbf |
| NPS 8 | 526.8 lb239.0 kg | 546,543 lbf2431.0 kN | 45,545 lbf |
| NPS 10 | 1016.7 lb461.2 kg | 787,763 lbf3504.0 kN | 65,647 lbf |
| NPS 12 | 1453.4 lb659.2 kg | 1,090,329 lbf4849.8 kN | 90,861 lbf |
Weight is a solid-disc estimate from the outside diameter and the minimum thickness at 0.2836 lb/in³, and ignores the raised face, so it runs slightly light against a real forging. End thrust is the 100 °F rating for A105 acting on the raised face area; it is not a bolt-load calculation, which must also cover gasket seating.
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 | 2.7569.82 mm | 167 lb-ft |
| NPS 3/4 | 4 × 3/4 in | 0.87522.22 mm | 2.9574.81 mm | 167 lb-ft |
| NPS 1 | 4 × 7/8 in | 1.00025.40 mm | 3.3484.78 mm | 269 lb-ft |
| NPS 1 1/4 | 4 × 1 in | 1.12528.57 mm | 4.02102.14 mm | 404 lb-ft |
| NPS 1 1/2 | 4 × 1 1/8 in | 1.25031.75 mm | 4.52114.71 mm | 593 lb-ft |
| NPS 2 | 8 × 1 in | 1.12528.57 mm | 2.6567.33 mm | 404 lb-ft |
| NPS 2 1/2 | 8 × 1 1/8 in | 1.25031.75 mm | 3.0477.30 mm | 593 lb-ft |
| NPS 3 | 8 × 1 1/4 in | 1.37534.92 mm | 3.5389.77 mm | 833 lb-ft |
| NPS 4 | 8 × 1 1/2 in | 1.62541.27 mm | 4.22107.23 mm | 1,492 lb-ft |
| NPS 5 | 8 × 1 3/4 in | 1.87547.62 mm | 5.01127.18 mm | 2,429 lb-ft |
| NPS 6 | 8 × 2 in | 2.12553.97 mm | 5.69144.63 mm | 3,694 lb-ft |
| NPS 8 | 12 × 2 in | 2.12553.97 mm | 4.52114.71 mm | 3,694 lb-ft |
| NPS 10 | 12 × 2 1/2 in | 2.62566.67 mm | 5.56141.31 mm | 7,404 lb-ft |
| NPS 12 | 12 × 2 3/4 in | 2.87573.02 mm | 6.38162.12 mm | 9,946 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 2500 blind 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 × 1 in; NPS 1 1/2 takes 4 × 1 1/8 in. Bolt count rises with size — see the table above for the full range.
What pressure is a Class 2500 flange rated for?
Class is a designation, not a pressure. A Class 2500 flange in A105 carbon steel is rated 6170 psig (425.4 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 2500 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.