Class 150 vs Class 300 Flanges
The most common step in the B16.5 ladder — what the extra class buys in pressure, and what it costs in flange size, bolting and money.
- Sizes in both classes
- 20
- Class 150 at 100 °F
- 285 psig
- Class 300 at 100 °F
- 740 psig
- Pressure gain
- ×2.60
Short answer. Class 300 holds 740 psig at 100 °F where Class 150 holds 285 — a factor of 2.60. The flange grows about 11% in outside diameter and 37% in thickness, and the bolting steps up with it. Class 150 is the default for utility and low-pressure process; Class 300 is the first step for steam, hydrocarbons and anything that derates steeply with temperature.
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 150 as “150 psi” is wrong at every other temperature.
At ambient, Group 1.1 carbon steel is rated 285 psig in Class 150 and 740 psig in Class 300 — a factor of 2.60. That ratio holds approximately across the temperature range, because every class in a group is derived from the same allowable stress curve.
| Temperature | Cl 150 · Gp 1.1 | Cl 300 · Gp 1.1 | Gain | Cl 150 · Gp 2.1 | Cl 300 · Gp 2.1 |
|---|---|---|---|---|---|
| 100 °F | 285 psig | 740 psig | +160% | 275 psig | 720 psig |
| 200 °F | 260 psig | 680 psig | +162% | 235 psig | 620 psig |
| 300 °F | 230 psig | 655 psig | +185% | 215 psig | 560 psig |
| 400 °F | 200 psig | 635 psig | +217% | 195 psig | 515 psig |
| 500 °F | 170 psig | 605 psig | +256% | 170 psig | 480 psig |
| 600 °F | 140 psig | 570 psig | +307% | 140 psig | 450 psig |
| 650 °F | 125 psig | 550 psig | +340% | 125 psig | 445 psig |
| 700 °F | 110 psig | 530 psig | +382% | 110 psig | 430 psig |
| 750 °F | 95 psig | 505 psig | +432% | 95 psig | 425 psig |
| 800 °F | 80 psig | 410 psig | +412% | 80 psig | 415 psig |
| 850 °F | 65 psig | 270 psig | +315% | 65 psig | 405 psig |
| 900 °F | 50 psig | 170 psig | +240% | 50 psig | 395 psig |
| 950 °F | 35 psig | 105 psig | +200% | 35 psig | 385 psig |
| 1000 °F | 20 psig | 50 psig | +150% | 20 psig | 360 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 150 to Class 300 grows the flange outside diameter by about 11% and the thickness by about 37% on average across the 20 shared sizes. The bolt count rises in 12 of those sizes and the bolt diameter in 16. 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 150 OD | Cl 300 OD | Cl 150 thk | Cl 300 thk | Cl 150 BC | Cl 300 BC | Cl 150 bolting | Cl 300 bolting |
|---|---|---|---|---|---|---|---|---|
| NPS 1/2 | 3.5088.90 mm | 3.7595.25 mm | 0.4411.18 mm | 0.5614.22 mm | 2.3860.45 mm | 2.6266.55 mm | 4 × 1/2" | 4 × 1/2" |
| NPS 3/4 | 3.8898.55 mm | 4.62117.35 mm | 0.5012.70 mm | 0.6215.75 mm | 2.7569.85 mm | 3.2582.55 mm | 4 × 1/2" | 4 × 5/8" |
| NPS 1 | 4.25107.95 mm | 4.88123.95 mm | 0.5614.22 mm | 0.6917.53 mm | 3.1279.25 mm | 3.5088.90 mm | 4 × 1/2" | 4 × 5/8" |
| NPS 1 1/4 | 4.62117.35 mm | 5.25133.35 mm | 0.6215.75 mm | 0.7519.05 mm | 3.5088.90 mm | 3.8898.55 mm | 4 × 1/2" | 4 × 5/8" |
| NPS 1 1/2 | 5.00127.00 mm | 6.12155.45 mm | 0.6917.53 mm | 0.8120.57 mm | 3.8898.55 mm | 4.50114.30 mm | 4 × 1/2" | 4 × 3/4" |
| NPS 2 | 6.00152.40 mm | 6.50165.10 mm | 0.7519.05 mm | 0.8822.35 mm | 4.75120.65 mm | 5.00127.00 mm | 4 × 5/8" | 8 × 5/8" |
| NPS 2 1/2 | 7.00177.80 mm | 7.50190.50 mm | 0.8822.35 mm | 1.0025.40 mm | 5.50139.70 mm | 5.88149.35 mm | 4 × 5/8" | 8 × 3/4" |
| NPS 3 | 7.50190.50 mm | 8.25209.55 mm | 0.9423.88 mm | 1.1228.45 mm | 6.00152.40 mm | 6.62168.15 mm | 4 × 5/8" | 8 × 3/4" |
| NPS 3 1/2 | 8.50215.90 mm | 9.00228.60 mm | 0.9423.88 mm | 1.1930.23 mm | 7.00177.80 mm | 7.25184.15 mm | 8 × 5/8" | 8 × 3/4" |
| NPS 4 | 9.00228.60 mm | 10.00254.00 mm | 0.9423.88 mm | 1.2531.75 mm | 7.50190.50 mm | 7.88200.15 mm | 8 × 5/8" | 8 × 3/4" |
| NPS 5 | 10.00254.00 mm | 11.00279.40 mm | 0.9423.88 mm | 1.3835.05 mm | 8.50215.90 mm | 9.25234.95 mm | 8 × 3/4" | 8 × 3/4" |
| NPS 6 | 11.00279.40 mm | 12.50317.50 mm | 1.0025.40 mm | 1.4436.58 mm | 9.50241.30 mm | 10.62269.75 mm | 8 × 3/4" | 12 × 3/4" |
| NPS 8 | 13.50342.90 mm | 15.00381.00 mm | 1.1228.45 mm | 1.6241.15 mm | 11.75298.45 mm | 13.00330.20 mm | 8 × 3/4" | 12 × 7/8" |
| NPS 10 | 16.00406.40 mm | 17.50444.50 mm | 1.1930.23 mm | 1.8847.75 mm | 14.25361.95 mm | 15.25387.35 mm | 12 × 7/8" | 16 × 1" |
| NPS 12 | 19.00482.60 mm | 20.50520.70 mm | 1.2531.75 mm | 2.0050.80 mm | 17.00431.80 mm | 17.75450.85 mm | 12 × 7/8" | 16 × 1 1/8" |
| NPS 14 | 21.00533.40 mm | 23.00584.20 mm | 1.3835.05 mm | 2.1253.85 mm | 18.75476.25 mm | 20.25514.35 mm | 12 × 1" | 20 × 1 1/8" |
| NPS 16 | 23.50596.90 mm | 25.50647.70 mm | 1.4436.58 mm | 2.2557.15 mm | 21.25539.75 mm | 22.50571.50 mm | 16 × 1" | 20 × 1 1/4" |
| NPS 18 | 25.00635.00 mm | 28.00711.20 mm | 1.5639.62 mm | 2.3860.45 mm | 22.75577.85 mm | 24.75628.65 mm | 16 × 1 1/8" | 24 × 1 1/4" |
| NPS 20 | 27.50698.50 mm | 30.50774.70 mm | 1.6942.93 mm | 2.5063.50 mm | 25.00635.00 mm | 27.00685.80 mm | 20 × 1 1/8" | 24 × 1 1/4" |
| NPS 24 | 32.00812.80 mm | 36.00914.40 mm | 1.8847.75 mm | 2.7569.85 mm | 29.50749.30 mm | 32.00812.80 mm | 20 × 1 1/4" | 24 × 1 1/2" |
Thickness excludes the raised face. Bolting is quantity × nominal diameter in inches.
The temperature trap in Class 150
Class 150 derates far more sharply than the higher classes. Group 1.1 carbon steel loses more than half its Class 150 rating by 600 °F — from 285 psig to 140 psig — while Class 300 falls from 740 to 570 psig over the same span. A line that is comfortable in Class 150 cold can be under-rated hot, which is why steam service starts at Class 300 far more often than the ambient pressure alone would suggest.
Practical selection
Class 150 is enough when
- Water, air, and low-pressure utility service
- Ambient or near-ambient temperature
- Design pressure comfortably under 285 psig at temperature
- Cost and weight matter and the duty is undemanding
- Mating to equipment nozzles that are themselves Class 150
Class 300 is worth it when
- Steam service of any pressure worth the name
- Hydrocarbons, or anything where a leak is a safety event
- Elevated temperature, where Class 150 derates below the duty
- Cyclic or vibrating service needing a stiffer joint
- Where the piping spec already calls Class 300 for consistency
Dimensions in full: Class 150 weld neck · Class 300 weld neck · Class 150 blind · Class 300 blind · all P-T ratings · how derating works.
Common questions
Can a Class 150 flange bolt to a Class 300 flange?
No. The bolt circles and bolt patterns differ in every size, so the holes do not line up. Joining the two needs a transition spool with one flange of each class.
Is a Class 150 flange rated for 150 psi?
Not exactly. A Class 150 carbon steel flange is rated 285 psig at 100 °F and falls below 150 psig only above about 550 °F. The class number is a series label, not the working pressure.
How much more does Class 300 cost?
Typically 1.5 to 2.5 times the flange cost, plus larger gaskets and more or bigger bolts. The installed difference is usually larger than the material difference because the bolting labour rises too.
Do Class 150 and Class 300 use the same gasket?
No. The raised face outside diameter is common to Classes 150 through 600 in a given size, but the gasket inside diameter, thickness and style are selected for the class and the bolt load available.