Class 150 vs Class 300 Valve — ASME Pressure Class Selection
ASME Class 150 and Class 300 are the most common flanged valve ratings in process plants. They are not interchangeable: flange dimensions, pressure-temperature capability, and often face-to-face differ. Under-classing a steam or hydrocarbon line is a safety and compliance issue.
Quick selection
Class 150 for low-to-moderate pressure water, utility, and many process lines near ambient. Class 300 when design pressure/temperature exceeds Class 150 limits (steam headers, higher DP hydrocarbons) or when the piping class mandates 300# flanges.
Comparison table
| Factor | Class 150 | Class 300 |
|---|---|---|
| Flange standard | ASME B16.5 Class 150 | ASME B16.5 Class 300 |
| Relative rating | Lower P-T envelope | Higher P-T envelope (roughly ~2× at ambient) |
| Flange / bolting | Lighter flanges, fewer/smaller bolts | Thicker flanges, more bolting capacity |
| Typical services | CW, utilities, low-pressure process | Steam, higher DP oil & gas, pressurized process |
| Valve mass / cost | Lower | Higher for same NPS |
| Interchangeable? | No — bolt circle and thickness differ | No — do not mate 150# to 300# flanges |
Pressure-temperature reality check
Working pressure falls as temperature rises. A Class 150 WCB valve that is adequate on cold water may be undersized for saturated steam. Always check ASME B16.34 / B16.5 P-T tables for the body material at design temperature — do not equate “150#” with a fixed bar rating.
Class 150 vs PN16
PN16 (EN) and Class 150 (ASME) are often discussed together but are not identical. Bolt circles and P-T curves differ. See also PN16 vs Class 150 flanges. Match the project flange standard exactly.
RFQ checklist
- State Class 150 or Class 300 (or PN) on every line — never omit rating.
- Design pressure, design temperature, and medium.
- Face type (RF/RTJ), MOC, and valve type (gate/globe/ball/check).
- Gear operator if NPS and class make handwheel torque high.
FAQ
Can I bolt a Class 150 valve to a Class 300 flange?
No. ASME flange classes are not mating-compatible. Match valve class to the piping flange class.
Is Class 300 always required for steam?
Not always — low-pressure steam may remain Class 150 if P-T tables allow. Many steam headers still specify Class 300. Follow the piping class.
Does higher class change face-to-face?
Often yes for a given valve standard. Confirm ASME B16.10 / API face-to-face before replacing valves.
Class 150 vs Class 800?
Class 800 is typically forged small-bore SW/threaded service (API 602), not a flanged alternative to 150/300.
Related RFQs and hubs
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