"Class 150" does not mean 150 psi. That single misunderstanding causes wrong quotations, wrong deliveries and, in the worst cases, unsafe installations. Understanding flange pressure class ratings lets you read enquiries correctly and guide buyers who are unsure what they need.
What a pressure class means
A pressure class is a dimensionless number that groups flanges by strength. Under ASME B16.5 (NPS 1/2" to 24") and ASME B16.47 (NPS 26" to 60"), the classes are:
150, 300, 400, 600, 900, 1500 and 2500
Each class has its own dimensions — outside diameter, thickness, bolt circle, number and size of bolts. Higher class means a thicker, heavier flange with more or larger bolts.
The actual allowable pressure depends on two other things: the material and the operating temperature.
Pressure–temperature ratings
ASME B16.5 groups materials (for example Group 1.1 for carbon steel such as ASTM A105, Group 2.2 for 316/316L) and gives a table of maximum working pressure for each class at each temperature.
Approximate ratings for Group 1.1 carbon steel (A105) at ambient temperature (–29 to 38°C):
| Class | Max pressure (bar) | Max pressure (psi) |
|---|---|---|
| 150 | 19.6 | 285 |
| 300 | 51.1 | 740 |
| 400 | 68.1 | 990 |
| 600 | 102.1 | 1,480 |
| 900 | 153.2 | 2,220 |
| 1500 | 255.3 | 3,705 |
| 2500 | 425.5 | 6,170 |
As temperature rises, allowable pressure falls. For the same A105 Class 150 flange, the limit drops to roughly 13.8 bar at 200°C and about 6.5 bar at 400°C. Stainless and alloy groups have their own curves. Always refer to the current edition of ASME B16.5 tables for design — the numbers above are for understanding only.
Why class matters in quotations
When an enquiry says "4" WNRF Class 300 A105", the class fixes:
- flange outside diameter and thickness,
- bolt circle and number of bolt holes,
- stud bolt size and length,
- gasket dimensions,
- weight and therefore price and freight.
Mixing classes is impossible — a Class 150 flange will not bolt to a Class 300 flange of the same size. If a buyer sends "4 inch flange, 20 bar", ask for the class before quoting. Our flange types guide covers the type and facing details that complete the specification.
Which class is used where?
- Class 150: water, cooling water, low-pressure utilities, fire water, many chemical lines.
- Class 300: process lines, steam, hydrocarbons at moderate pressure.
- Class 600: high-pressure steam, gas transmission, refinery units.
- Class 900 and 1500: wellhead, high-pressure gas, hydrocracker and power plant lines.
- Class 2500: extreme pressure — wellheads, high-pressure injection, special services.
Class 400 is rarely used; most designs jump from 300 to 600.
ASME class vs EN PN rating
European buyers often use EN 1092-1 with PN ratings: PN 6, 10, 16, 25, 40, 63, 100 and above. PN is roughly the pressure in bar at room temperature for the reference material.
Approximate equivalents used in trade:
| ASME class | Closest PN |
|---|---|
| 150 | PN 20 (often PN 16 or PN 25 used) |
| 300 | PN 50 (often PN 40) |
| 600 | PN 100 |
| 900 | PN 150 |
| 1500 | PN 250 |
| 2500 | PN 420 |
These are not interchangeable dimensionally. A PN 16 flange will not bolt to a Class 150 flange of the same nominal size because bolt circles differ. If a European buyer wants ASME flanges or the reverse, confirm in writing. Our ASTM vs EN vs IS standards guide explains more of these cross-standard traps.
Facing and class
Raised-face height also depends on class. Under ASME B16.5, Class 150 and 300 flanges have a 1.6 mm (1/16") raised face, while Class 400 and above have a 6.4 mm (1/4") raised face. RTJ facing is common at Class 600 and above. When weights are quoted, check whether the raised face is included in the thickness — practices vary between catalogues.
Bolting for each class
Higher classes need stronger, larger bolts. Typical pairs are ASTM A193 B7 studs with A194 2H nuts for carbon steel service, and B8M/8M for stainless. See stud bolt grades explained for the full picture, and flange gasket types for choosing the right gasket for each class.
Frequently asked questions
Does Class 150 mean 150 psi?
No. A carbon steel Class 150 flange is rated about 285 psi (19.6 bar) at ambient temperature, falling as temperature increases.
Can I use a higher class flange than required?
Technically yes, but it must match the mating flange, gasket and bolting. It also costs more, so only upgrade if the buyer asks.
What standard covers flanges above 24 inches?
ASME B16.47 Series A (MSS SP-44) and Series B (API 605) cover NPS 26" to 60". The two series have different dimensions, so always confirm which one is needed.
Is PN 16 equal to Class 150?
They are close in pressure but not identical, and they are dimensionally different. They are not interchangeable.
Key takeaways
- Class is a strength group, not a pressure in psi.
- Allowable pressure depends on class, material group and temperature.
- ASME classes and EN PN ratings are not dimensionally interchangeable.
- Always confirm class, type, facing, material and standard before quoting.
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