Class 600 vs Class 900 Flanges
The point where flanges stop being incidental hardware: Class 900 changes the flange size, the gasket type and the way the joint is assembled.
- Sizes in both classes
- 19
- Class 600 at 100 °F
- 1480 psig
- Class 900 at 100 °F
- 2220 psig
- Pressure gain
- ×1.50
Short answer. Class 900 holds 2220 psig at 100 °F against Class 600's 1480 — a factor of 1.50. Unlike the 300-to-600 step, this one grows the flange itself: about 13% more outside diameter and 31% more thickness, with ring type joints becoming the normal gasket choice.
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 600 as “600 psi” is wrong at every other temperature.
At ambient, Group 1.1 carbon steel is rated 1480 psig in Class 600 and 2220 psig in Class 900 — a factor of 1.50. That ratio holds approximately across the temperature range, because every class in a group is derived from the same allowable stress curve.
| Temperature | Cl 600 · Gp 1.1 | Cl 900 · Gp 1.1 | Gain | Cl 600 · Gp 2.1 | Cl 900 · Gp 2.1 |
|---|---|---|---|---|---|
| 100 °F | 1480 psig | 2220 psig | +50% | 1440 psig | 2160 psig |
| 200 °F | 1350 psig | 2025 psig | +50% | 1240 psig | 1860 psig |
| 300 °F | 1315 psig | 1970 psig | +50% | 1120 psig | 1680 psig |
| 400 °F | 1270 psig | 1900 psig | +50% | 1025 psig | 1540 psig |
| 500 °F | 1205 psig | 1810 psig | +50% | 955 psig | 1430 psig |
| 600 °F | 1135 psig | 1705 psig | +50% | 900 psig | 1350 psig |
| 650 °F | 1100 psig | 1650 psig | +50% | 885 psig | 1330 psig |
| 700 °F | 1065 psig | 1600 psig | +50% | 865 psig | 1295 psig |
| 750 °F | 1015 psig | 1520 psig | +50% | 845 psig | 1270 psig |
| 800 °F | 825 psig | 1235 psig | +50% | 830 psig | 1245 psig |
| 850 °F | 535 psig | 805 psig | +50% | 810 psig | 1215 psig |
| 900 °F | 345 psig | 515 psig | +49% | 790 psig | 1180 psig |
| 950 °F | 205 psig | 310 psig | +51% | 770 psig | 1155 psig |
| 1000 °F | 105 psig | 155 psig | +48% | 720 psig | 1080 psig |
Group 1.1 is A105 carbon steel; Group 2.1 is 304/316 stainless. Ratings are maximum allowable working gauge pressure at the metal temperature.
The flange gets bigger, thicker and more heavily bolted
Moving from Class 600 to Class 900 grows the flange outside diameter by about 13% 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 19. 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 600 OD | Cl 900 OD | Cl 600 thk | Cl 900 thk | Cl 600 BC | Cl 900 BC | Cl 600 bolting | Cl 900 bolting |
|---|---|---|---|---|---|---|---|---|
| NPS 1/2 | 3.7595.25 mm | 4.75120.65 mm | 0.5614.22 mm | 0.8822.35 mm | 2.6266.55 mm | 3.2582.55 mm | 4 × 1/2" | 4 × 3/4" |
| NPS 3/4 | 4.62117.35 mm | 5.12130.05 mm | 0.6215.75 mm | 1.0025.40 mm | 3.2582.55 mm | 3.5088.90 mm | 4 × 5/8" | 4 × 3/4" |
| NPS 1 | 4.88123.95 mm | 5.88149.35 mm | 0.6917.53 mm | 1.1228.45 mm | 3.5088.90 mm | 4.00101.60 mm | 4 × 5/8" | 4 × 7/8" |
| NPS 1 1/4 | 5.25133.35 mm | 6.25158.75 mm | 0.8120.57 mm | 1.1228.45 mm | 3.8898.55 mm | 4.38111.25 mm | 4 × 5/8" | 4 × 7/8" |
| NPS 1 1/2 | 6.12155.45 mm | 7.00177.80 mm | 0.8822.35 mm | 1.2531.75 mm | 4.50114.30 mm | 4.88123.95 mm | 4 × 3/4" | 4 × 1" |
| NPS 2 | 6.50165.10 mm | 8.50215.90 mm | 1.0025.40 mm | 1.5038.10 mm | 5.00127.00 mm | 6.50165.10 mm | 8 × 5/8" | 8 × 7/8" |
| NPS 2 1/2 | 7.50190.50 mm | 9.62244.35 mm | 1.1228.45 mm | 1.6241.15 mm | 5.88149.35 mm | 7.50190.50 mm | 8 × 3/4" | 8 × 1" |
| NPS 3 | 8.25209.55 mm | 9.50241.30 mm | 1.2531.75 mm | 1.5038.10 mm | 6.62168.15 mm | 7.50190.50 mm | 8 × 3/4" | 8 × 7/8" |
| NPS 4 | 10.75273.05 mm | 11.50292.10 mm | 1.5038.10 mm | 1.7544.45 mm | 8.50215.90 mm | 9.25234.95 mm | 8 × 7/8" | 8 × 1 1/8" |
| NPS 5 | 13.00330.20 mm | 13.75349.25 mm | 1.7544.45 mm | 2.0050.80 mm | 10.50266.70 mm | 11.00279.40 mm | 8 × 1" | 8 × 1 1/4" |
| NPS 6 | 14.00355.60 mm | 15.00381.00 mm | 1.8847.75 mm | 2.1955.63 mm | 11.50292.10 mm | 12.50317.50 mm | 12 × 1" | 12 × 1 1/8" |
| NPS 8 | 16.50419.10 mm | 18.50469.90 mm | 2.1955.63 mm | 2.5063.50 mm | 13.75349.25 mm | 15.50393.70 mm | 12 × 1 1/8" | 12 × 1 3/8" |
| NPS 10 | 20.00508.00 mm | 21.50546.10 mm | 2.5063.50 mm | 2.7569.85 mm | 17.00431.80 mm | 18.50469.90 mm | 16 × 1 1/4" | 16 × 1 3/8" |
| NPS 12 | 22.00558.80 mm | 24.00609.60 mm | 2.6266.55 mm | 3.1279.25 mm | 19.25488.95 mm | 21.00533.40 mm | 20 × 1 1/4" | 20 × 1 3/8" |
| NPS 14 | 23.75603.25 mm | 25.25641.35 mm | 2.7569.85 mm | 3.3885.85 mm | 20.75527.05 mm | 22.00558.80 mm | 20 × 1 3/8" | 20 × 1 1/2" |
| NPS 16 | 27.00685.80 mm | 27.75704.85 mm | 3.0076.20 mm | 3.5088.90 mm | 23.75603.25 mm | 24.25615.95 mm | 20 × 1 1/2" | 20 × 1 5/8" |
| NPS 18 | 29.25742.95 mm | 31.00787.40 mm | 3.2582.55 mm | 4.00101.60 mm | 25.75654.05 mm | 27.00685.80 mm | 20 × 1 5/8" | 20 × 1 7/8" |
| NPS 20 | 32.00812.80 mm | 33.75857.25 mm | 3.5088.90 mm | 4.25107.95 mm | 28.50723.90 mm | 29.50749.30 mm | 24 × 1 5/8" | 20 × 2" |
| NPS 24 | 37.00939.80 mm | 41.001041.40 mm | 4.00101.60 mm | 5.50139.70 mm | 33.00838.20 mm | 35.50901.70 mm | 24 × 1 7/8" | 20 × 2 1/2" |
Thickness excludes the raised face. Bolting is quantity × nominal diameter in inches.
Where RTJ takes over
Raised face flanges are permitted at Class 900, but ring type joints dominate in practice from here upward. An RTJ seals by plastically deforming a solid metal ring into a machined groove, which tolerates thermal cycling and high temperature far better than a compressed sheet or spiral wound gasket. The trade is that the groove must be machined true, the ring is a consumable, and the flange faces never touch — so bolt elongation, not face contact, is what tells you the joint is made up. See RF vs RTJ.
Practical selection
Class 600 is enough when
- High-pressure process within 1480 psig at temperature
- Most refinery high-pressure service
- Where raised face joints and spiral wound gaskets are preferred
- Where flange weight and handling are already a constraint
Class 900 is worth it when
- Gas compression discharge and high-pressure separation
- Boiler and high-energy steam service above Class 600
- Wellhead-adjacent and upstream production piping
- Where thermal cycling favours a metal ring seal
Dimensions in full: Class 600 weld neck · Class 900 weld neck · Class 600 blind · Class 900 blind · all P-T ratings · how derating works.
Common questions
Is Class 900 the same as API 5000?
No, though they are often adjacent in service. API 6A designations are a separate rating system for wellhead equipment; B16.5 Class 900 is a piping flange rating. They must not be mixed without a documented transition.
Do Class 900 flanges have to be RTJ?
No. B16.5 publishes raised face Class 900 flanges. RTJ is the common specification choice rather than a code requirement.
Why does Class 900 share dimensions with Class 1500 in small sizes?
Through NPS 2 1/2, B16.5 makes Class 900 identical to Class 1500 — there was no practical reason to publish a separate lighter flange at those diameters, so the standard does not.
How much heavier is a Class 900 flange?
Substantially — commonly 1.6 to 2.2 times a Class 600 flange in the same size, which is enough to change lifting and support design.