The Short Answer
A valve’s pressure class (Class 150, 300, 600, 900, 1500, 2500) is a rating designation from standards like ASME B16.34 that defines the maximum allowable working pressure of the valve at a given temperature — it is not a fixed pressure limit. The class number approximately indicates the design pressure at ambient temperature (Class 150 ≈ 150 psi at 100°F for some materials, but the exact value depends on material and temperature), and the allowable pressure decreases as temperature rises. To select a valve you must read the pressure-temperature table for the specific material group at your design temperature — the class number alone is not the rating.
What the Class Number Means
The class system groups valves by their pressure capability:
| Pressure Class | Approximate Ambient Rating (carbon steel) | Typical Service |
|---|---|---|
| Class 150 | ~285 psig (19.6 bar) at 100°F | Low-pressure utilities, water |
| Class 300 | ~740 psig (51 bar) at 100°F | General process |
| Class 600 | ~1480 psig (102 bar) at 100°F | Higher-pressure process, pipelines |
| Class 900 | ~2220 psig (153 bar) at 100°F | High-pressure process |
| Class 1500 | ~3705 psig (255 bar) at 100°F | High-pressure, injection |
| Class 2500 | ~6170 psig (425 bar) at 100°F | Very high pressure, valve internals |
These are approximate carbon-steel values at ambient — always use the actual rating table for the material and temperature in question. Class numbers are not exact pressures and the ratings vary by material group.
The key idea: a class is a curve, not a number. The rating table gives the maximum pressure at every temperature, and the curve slopes downward as temperature rises because materials weaken with heat.
How a Rating Table Works
A typical ASME B16.34 rating table (simplified):
| Temperature | Class 150 (psi) | Class 300 (psi) | Class 600 (psi) |
|---|---|---|---|
| −20 to 100°F | 285 | 740 | 1480 |
| 200°F | 260 | 675 | 1350 |
| 300°F | 230 | 655 | 1315 |
| 400°F | 200 | 635 | 1270 |
| 500°F | 170 | 600 | 1200 |
| 600°F | 140 | 550 | 1095 |
| 700°F | 110 | 530 | 1060 |
| 800°F | 80 | 510 | 1015 |
(Illustrative values — always consult the current ASME B16.34 table for your exact material group.)
Read the table like this:
- Identify the material group (carbon steel, 316 SS, alloy steel — each has its own table).
- Find your design temperature (use the maximum, including transients).
- Read the maximum allowable pressure at that temperature.
- Verify it is ≥ your system’s maximum operating/shutoff pressure.
The same Class 300 valve that handles 740 psi at ambient may only be rated for 600 psi at 500°F — select by the worst case, not by the name of the class.
Why Temperature Derating Exists
Metals weaken at high temperature. The allowable stress of steel decreases as temperature rises, so a valve that can hold a given pressure at ambient cannot hold it at 800°F. Standards codify this by publishing the derated values in the pressure-temperature tables. Designers must always check the maximum design temperature, not the normal operating temperature — a transient temperature spike can exceed the rating even if the normal condition is comfortable.
Common Mistakes with Pressure Classes
- Using the class number as the pressure limit. “Class 150 = 150 psi” is wrong for most materials; the actual ambient rating for carbon steel is ~285 psi, and it derates with temperature.
- Rating at ambient instead of design temperature. The valve that passes at ambient may fail at operating temperature.
- Ignoring the material group. Carbon steel, 316 SS, and alloy steel have different tables; a Class 600 valve in one material is not the same rating as in another.
- Forgetting surge and shutoff pressures. The class must cover the maximum pressure including transients (water hammer, pump surge, shutoff differential) — see our water hammer article.
- Mixing flange class and valve class. The flanges (ASME B16.5) and the valve body must be rated for the same pressure-temperature envelope; a Class 150 valve on Class 300 flanges is still a Class 150 valve.
How to Specify a Valve by Class — Correctly
The right way to write a specification:
- Gate valve, Class 300, flanged to ASME B16.5, design conditions 500 psig @ 400°F.
- Ball valve, Class 600, API 6D, design 1200 psig @ 150°F.
- Trim and seats rated to the same pressure-temperature envelope.
Always state both the class and the design pressure/temperature so the supplier verifies the rating table covers your conditions.
The European/ISO Equivalent
In metric systems, the same concept appears as PN ratings (e.g., PN16, PN40, PN100 — nominal pressure in bar at 20°C). A PN16 valve is roughly equivalent to Class 150, PN40 to Class 300, PN100 to Class 600. The same rule applies: the PN number is a nominal rating at a reference temperature, and the actual allowable pressure derates with temperature per the standard (EN 12516). If your project mixes ASME and EN equipment, check both rating systems before pairing a valve with a line.
Conclusion
A valve pressure class is a temperature-dependent rating designation, not a fixed pressure number. Select the class by reading the material’s pressure-temperature table at your maximum design temperature — including transients — and state both class and design conditions in your specification. Understanding this single concept prevents the most common valve mis-specification in industry: the valve that looks right by class number and fails because its rating at operating temperature was never checked.