Class 1500 vs Class 2500 Flanges
The top two classes in ASME B16.5 — and the size limit that decides between them more often than pressure does.
- Sizes in both classes
- 14
- Class 1500 at 100 °F
- 3705 psig
- Class 2500 at 100 °F
- 6170 psig
- Pressure gain
- ×1.67
Short answer. Class 2500 holds 6170 psig at 100 °F against Class 1500's 3705, a factor of 1.67. The catch is size: B16.5 publishes Class 2500 only through NPS 12, so above that diameter Class 1500 is the highest class available and higher pressures need a different standard.
What the class number means
A flange class is not a pressure. It is a rating series: the pressure a flange in that class may hold depends on the material group and the metal temperature, and it is read out of ASME B16.5 Table 2. The class number happens to equal the approximate working pressure in psig for Group 1.1 carbon steel at about 500 °F, which is where the numbers came from historically — but treating Class 1500 as “1500 psi” is wrong at every other temperature.
At ambient, Group 1.1 carbon steel is rated 3705 psig in Class 1500 and 6170 psig in Class 2500 — a factor of 1.67. That ratio holds approximately across the temperature range, because every class in a group is derived from the same allowable stress curve.
| Temperature | Cl 1500 · Gp 1.1 | Cl 2500 · Gp 1.1 | Gain | Cl 1500 · Gp 2.1 | Cl 2500 · Gp 2.1 |
|---|---|---|---|---|---|
| 100 °F | 3705 psig | 6170 psig | +67% | 3600 psig | 6000 psig |
| 200 °F | 3395 psig | 5655 psig | +67% | 3000 psig | 5000 psig |
| 300 °F | 3270 psig | 5450 psig | +67% | 2690 psig | 4480 psig |
| 400 °F | 3170 psig | 5280 psig | +67% | 2485 psig | 4140 psig |
| 500 °F | 3015 psig | 5025 psig | +67% | 2330 psig | 3880 psig |
| 600 °F | 2840 psig | 4730 psig | +67% | 2210 psig | 3680 psig |
| 650 °F | 2745 psig | 4575 psig | +67% | 2160 psig | 3600 psig |
| 700 °F | 2655 psig | 4425 psig | +67% | 2110 psig | 3520 psig |
| 750 °F | 2535 psig | 4230 psig | +67% | 2065 psig | 3440 psig |
| 800 °F | 2055 psig | 3430 psig | +67% | 2030 psig | 3380 psig |
| 850 °F | 1595 psig | 2655 psig | +66% | 1980 psig | 3300 psig |
| 900 °F | 1150 psig | 1915 psig | +67% | 1945 psig | 3240 psig |
| 950 °F | 685 psig | 1145 psig | +67% | 1910 psig | 3180 psig |
| 1000 °F | 430 psig | 715 psig | +66% | 1770 psig | 2950 psig |
Group 1.1 is A105 carbon steel; Group 2.1 is Type 304 stainless. Ratings are maximum allowable working gauge pressure at the metal temperature.
The flange gets bigger, thicker and more heavily bolted
Moving from Class 1500 to Class 2500 grows the flange outside diameter by about 13% and the thickness by about 36% on average across the 14 shared sizes. The bolt count rises in 0 of those sizes and the bolt diameter in 11. None of that is free: the flange costs more, the gasket costs more, the bolting costs more, and the extra bolt load has to be delivered with a torque wrench that can reach it.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Cl 1500 OD | Cl 2500 OD | Cl 1500 thk | Cl 2500 thk | Cl 1500 BC | Cl 2500 BC | Cl 1500 bolting | Cl 2500 bolting |
|---|---|---|---|---|---|---|---|---|
| NPS 1/2 | 4.75120.65 mm | 5.25133.35 mm | 0.8822.35 mm | 1.1930.23 mm | 3.2582.55 mm | 3.5088.90 mm | 4 × 3/4" | 4 × 3/4" |
| NPS 3/4 | 5.12130.05 mm | 5.50139.70 mm | 1.0025.40 mm | 1.2531.75 mm | 3.5088.90 mm | 3.7595.25 mm | 4 × 3/4" | 4 × 3/4" |
| NPS 1 | 5.88149.35 mm | 6.25158.75 mm | 1.1228.45 mm | 1.3835.05 mm | 4.00101.60 mm | 4.25107.95 mm | 4 × 7/8" | 4 × 7/8" |
| NPS 1 1/4 | 6.25158.75 mm | 7.25184.15 mm | 1.1228.45 mm | 1.5038.10 mm | 4.38111.25 mm | 5.12130.05 mm | 4 × 7/8" | 4 × 1" |
| NPS 1 1/2 | 7.00177.80 mm | 8.00203.20 mm | 1.2531.75 mm | 1.7544.45 mm | 4.88123.95 mm | 5.75146.05 mm | 4 × 1" | 4 × 1 1/8" |
| NPS 2 | 8.50215.90 mm | 9.25234.95 mm | 1.5038.10 mm | 2.0050.80 mm | 6.50165.10 mm | 6.75171.45 mm | 8 × 7/8" | 8 × 1" |
| NPS 2 1/2 | 9.62244.35 mm | 10.50266.70 mm | 1.6241.15 mm | 2.2557.15 mm | 7.50190.50 mm | 7.75196.85 mm | 8 × 1" | 8 × 1 1/8" |
| NPS 3 | 10.50266.70 mm | 12.00304.80 mm | 1.8847.75 mm | 2.6266.55 mm | 8.00203.20 mm | 9.00228.60 mm | 8 × 1 1/8" | 8 × 1 1/4" |
| NPS 4 | 12.25311.15 mm | 14.00355.60 mm | 2.1253.85 mm | 3.0076.20 mm | 9.50241.30 mm | 10.75273.05 mm | 8 × 1 1/4" | 8 × 1 1/2" |
| NPS 5 | 14.75374.65 mm | 16.50419.10 mm | 2.8873.15 mm | 3.6291.95 mm | 11.50292.10 mm | 12.75323.85 mm | 8 × 1 1/2" | 8 × 1 3/4" |
| NPS 6 | 15.50393.70 mm | 19.00482.60 mm | 3.2582.55 mm | 4.25107.95 mm | 12.50317.50 mm | 14.50368.30 mm | 12 × 1 3/8" | 8 × 2" |
| NPS 8 | 19.00482.60 mm | 21.75552.45 mm | 3.6291.95 mm | 5.00127.00 mm | 15.50393.70 mm | 17.25438.15 mm | 12 × 1 5/8" | 12 × 2" |
| NPS 10 | 23.00584.20 mm | 26.50673.10 mm | 4.25107.95 mm | 6.50165.10 mm | 19.00482.60 mm | 21.25539.75 mm | 12 × 1 7/8" | 12 × 2 1/2" |
| NPS 12 | 26.50673.10 mm | 30.00762.00 mm | 4.88123.95 mm | 7.25184.15 mm | 22.50571.50 mm | 24.38619.25 mm | 16 × 2" | 12 × 2 3/4" |
Thickness excludes the raised face. Bolting is quantity × nominal diameter in inches.
Class 2500 stops at NPS 12
This is the constraint that usually settles the choice. B16.5 tabulates Class 2500 for NPS 1/2 through NPS 12 and no further. A large-bore line above Class 1500 leaves B16.5 entirely — it goes to API 6A, to a manufacturer's proprietary connector, or to a design-by-analysis flange under ASME Section VIII Appendix 2. The flanges are also genuinely heavy: a Class 2500 NPS 12 flange is a crane lift, not a two-person carry.
Practical selection
Class 1500 is enough when
- Any size above NPS 12 — Class 2500 does not exist there
- Pressures within 3705 psig at temperature
- Where flange weight, handling and bolt tensioning access are limiting
Class 2500 is worth it when
- Small and medium bore above the Class 1500 rating
- High-pressure injection, test and hydraulic headers
- Where the alternative is leaving B16.5 for a proprietary connector
Dimensions in full: Class 1500 weld neck · Class 2500 weld neck · Class 1500 blind · Class 2500 blind · all P-T ratings · how derating works.
Common questions
What is the highest ASME B16.5 flange class?
Class 2500, rated 6170 psig at 100 °F in Group 1.1 carbon steel. It is published from NPS 1/2 through NPS 12 only.
What comes above Class 2500?
Nothing within B16.5. Higher pressures use API 6A flanges, proprietary clamp connectors, or a flange designed under ASME Section VIII Division 1 Appendix 2.
Are Class 2500 flanges always RTJ?
Almost always in practice. Raised face Class 2500 flanges are published, but the bolt loads involved make a metal ring seal the sensible choice.
Can I use a Class 2500 flange at low pressure?
You can, and it is occasionally done for erosion or mechanical reasons, but it is expensive, heavy and usually signals that something else in the design should change.