Class 900 vs Class 1500 Flanges
Two high-pressure classes that are dimensionally identical in small bore and diverge sharply above it.
- Sizes in both classes
- 19
- Class 900 at 100 °F
- 2220 psig
- Class 1500 at 100 °F
- 3705 psig
- Pressure gain
- ×1.67
Short answer. Class 1500 is rated 3705 psig at 100 °F against Class 900's 2220 — a factor of 1.67. Through NPS 2 1/2 the two classes share the same flange, so the choice only becomes real at NPS 3 and above, where Class 1500 adds about 7% in diameter and 31% in thickness.
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 900 as “900 psi” is wrong at every other temperature.
At ambient, Group 1.1 carbon steel is rated 2220 psig in Class 900 and 3705 psig in Class 1500 — 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 900 · Gp 1.1 | Cl 1500 · Gp 1.1 | Gain | Cl 900 · Gp 2.1 | Cl 1500 · Gp 2.1 |
|---|---|---|---|---|---|
| 100 °F | 2220 psig | 3705 psig | +67% | 2160 psig | 3600 psig |
| 200 °F | 2035 psig | 3395 psig | +67% | 1800 psig | 3000 psig |
| 300 °F | 1965 psig | 3270 psig | +66% | 1615 psig | 2690 psig |
| 400 °F | 1900 psig | 3170 psig | +67% | 1490 psig | 2485 psig |
| 500 °F | 1810 psig | 3015 psig | +67% | 1395 psig | 2330 psig |
| 600 °F | 1705 psig | 2840 psig | +67% | 1325 psig | 2210 psig |
| 650 °F | 1650 psig | 2745 psig | +66% | 1295 psig | 2160 psig |
| 700 °F | 1590 psig | 2655 psig | +67% | 1265 psig | 2110 psig |
| 750 °F | 1520 psig | 2535 psig | +67% | 1240 psig | 2065 psig |
| 800 °F | 1235 psig | 2055 psig | +66% | 1215 psig | 2030 psig |
| 850 °F | 955 psig | 1595 psig | +67% | 1190 psig | 1980 psig |
| 900 °F | 690 psig | 1150 psig | +67% | 1165 psig | 1945 psig |
| 950 °F | 410 psig | 685 psig | +67% | 1145 psig | 1910 psig |
| 1000 °F | 255 psig | 430 psig | +69% | 1065 psig | 1770 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 900 to Class 1500 grows the flange outside diameter by about 7% and the thickness by about 31% on average across the 19 shared sizes. The bolt count rises in 0 of those sizes and the bolt diameter in 12. 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 900 OD | Cl 1500 OD | Cl 900 thk | Cl 1500 thk | Cl 900 BC | Cl 1500 BC | Cl 900 bolting | Cl 1500 bolting |
|---|---|---|---|---|---|---|---|---|
| NPS 1/2 | 4.75120.65 mm | 4.75120.65 mm | 0.8822.35 mm | 0.8822.35 mm | 3.2582.55 mm | 3.2582.55 mm | 4 × 3/4" | 4 × 3/4" |
| NPS 3/4 | 5.12130.05 mm | 5.12130.05 mm | 1.0025.40 mm | 1.0025.40 mm | 3.5088.90 mm | 3.5088.90 mm | 4 × 3/4" | 4 × 3/4" |
| NPS 1 | 5.88149.35 mm | 5.88149.35 mm | 1.1228.45 mm | 1.1228.45 mm | 4.00101.60 mm | 4.00101.60 mm | 4 × 7/8" | 4 × 7/8" |
| NPS 1 1/4 | 6.25158.75 mm | 6.25158.75 mm | 1.1228.45 mm | 1.1228.45 mm | 4.38111.25 mm | 4.38111.25 mm | 4 × 7/8" | 4 × 7/8" |
| NPS 1 1/2 | 7.00177.80 mm | 7.00177.80 mm | 1.2531.75 mm | 1.2531.75 mm | 4.88123.95 mm | 4.88123.95 mm | 4 × 1" | 4 × 1" |
| NPS 2 | 8.50215.90 mm | 8.50215.90 mm | 1.5038.10 mm | 1.5038.10 mm | 6.50165.10 mm | 6.50165.10 mm | 8 × 7/8" | 8 × 7/8" |
| NPS 2 1/2 | 9.62244.35 mm | 9.62244.35 mm | 1.6241.15 mm | 1.6241.15 mm | 7.50190.50 mm | 7.50190.50 mm | 8 × 1" | 8 × 1" |
| NPS 3 | 9.50241.30 mm | 10.50266.70 mm | 1.5038.10 mm | 1.8847.75 mm | 7.50190.50 mm | 8.00203.20 mm | 8 × 7/8" | 8 × 1 1/8" |
| NPS 4 | 11.50292.10 mm | 12.25311.15 mm | 1.7544.45 mm | 2.1253.85 mm | 9.25234.95 mm | 9.50241.30 mm | 8 × 1 1/8" | 8 × 1 1/4" |
| NPS 5 | 13.75349.25 mm | 14.75374.65 mm | 2.0050.80 mm | 2.8873.15 mm | 11.00279.40 mm | 11.50292.10 mm | 8 × 1 1/4" | 8 × 1 1/2" |
| NPS 6 | 15.00381.00 mm | 15.50393.70 mm | 2.1955.63 mm | 3.2582.55 mm | 12.50317.50 mm | 12.50317.50 mm | 12 × 1 1/8" | 12 × 1 3/8" |
| NPS 8 | 18.50469.90 mm | 19.00482.60 mm | 2.5063.50 mm | 3.6291.95 mm | 15.50393.70 mm | 15.50393.70 mm | 12 × 1 3/8" | 12 × 1 5/8" |
| NPS 10 | 21.50546.10 mm | 23.00584.20 mm | 2.7569.85 mm | 4.25107.95 mm | 18.50469.90 mm | 19.00482.60 mm | 16 × 1 3/8" | 12 × 1 7/8" |
| NPS 12 | 24.00609.60 mm | 26.50673.10 mm | 3.1279.25 mm | 4.88123.95 mm | 21.00533.40 mm | 22.50571.50 mm | 20 × 1 3/8" | 16 × 2" |
| NPS 14 | 25.25641.35 mm | 29.50749.30 mm | 3.3885.85 mm | 5.25133.35 mm | 22.00558.80 mm | 25.00635.00 mm | 20 × 1 1/2" | 16 × 2 1/4" |
| NPS 16 | 27.75704.85 mm | 32.50825.50 mm | 3.5088.90 mm | 5.75146.05 mm | 24.25615.95 mm | 27.75704.85 mm | 20 × 1 5/8" | 16 × 2 1/2" |
| NPS 18 | 31.00787.40 mm | 36.00914.40 mm | 4.00101.60 mm | 6.38162.05 mm | 27.00685.80 mm | 30.50774.70 mm | 20 × 1 7/8" | 16 × 2 3/4" |
| NPS 20 | 33.75857.25 mm | 38.75984.25 mm | 4.25107.95 mm | 7.00177.80 mm | 29.50749.30 mm | 32.75831.85 mm | 20 × 2" | 16 × 3" |
| NPS 24 | 41.001041.40 mm | 46.001168.40 mm | 5.50139.70 mm | 8.00203.20 mm | 35.50901.70 mm | 39.00990.60 mm | 20 × 2 1/2" | 16 × 3 1/2" |
Thickness excludes the raised face. Bolting is quantity × nominal diameter in inches.
Identical below NPS 3
ASME B16.5 publishes the same outside diameter, thickness, bolt circle and bolting for Class 900 and Class 1500 in NPS 1/2 through NPS 2 1/2. In those sizes a Class 900 flange is a Class 1500 flange, and specifying the lower class buys nothing. Piping specs often go straight to Class 1500 in small bore for that reason — one fewer item to stock, at no extra material.
Practical selection
Class 900 is enough when
- Pressures within 2220 psig at temperature, NPS 3 and above
- Where flange weight and bolt tensioning access are limiting
- Existing plant already standardised on Class 900
Class 1500 is worth it when
- Small bore, where Class 1500 costs the same as Class 900
- High-pressure gas, injection and hydroprocessing service
- Where relief or upset conditions exceed the Class 900 rating
- Where a single high-pressure class simplifies the spec
Dimensions in full: Class 900 weld neck · Class 1500 weld neck · Class 900 blind · Class 1500 blind · all P-T ratings · how derating works.
Common questions
Are Class 900 and Class 1500 flanges interchangeable?
In NPS 1/2 through NPS 2 1/2 they are the same flange, so they bolt together. From NPS 3 upward the dimensions differ and they do not.
Which classes require ring type joints?
None strictly, but RTJ is the practical default at Class 900 and above, and is near-universal at Class 1500 in hydrocarbon service.
What bolting is used at Class 1500?
ASTM A193 B7 studs with A194 2H nuts in ordinary service, moving to B16 or B8M for high temperature or corrosion. Bolt tensioning rather than torquing is common above NPS 8.
Is Class 1500 the highest B16.5 class?
No — Class 2500 is, and it is published through NPS 12 only. Above NPS 12 the highest B16.5 class available is Class 1500.