Specifying pressure class: what Class 150, 300 and 600 actually mean
Class 150, Class 300 and Class 600 are not working pressures. Each is a pressure-temperature envelope defined per material group in ASME B16.34, and the allowable pressure falls as temperature rises. A line's design pressure and temperature together pick the class; ambient-only thinking under-specifies hot lines. This article shows how the derating works with worked examples and what to state on the enquiry.
Class is an envelope, not a pressure
"Class 150" does not mean 150 psi, 150 bar, or 150 of anything the line will see. A pressure class is a family of pressure-temperature envelopes: for each material group, ASME B16.34 tabulates the pressure a valve of that class may hold at each temperature step, and the permitted pressure falls as the metal gets hotter. The same is true of Class 300, Class 600 and the rest of the series. All designations here are published reference data — confirm against the current edition of the standard, because your purchase order governs. The practical consequence is that a class can only be chosen from design pressure and design temperature together. A rating that looks generous at ambient can be exhausted at process temperature, which is exactly how ambient-only thinking under-specifies hot lines.
Where the tables live
The ratings come from ASME B16.34, the valve standard whose pressure-temperature tables are organised by material group: the common carbon steels gathered in one group, the chrome-moly grades in others, the stainless grades in others again, each with its own numbers. Flanges carry parallel tables in ASME B16.5, which is why valve class and flange class line up on a data sheet instead of fighting each other. Both are published standards, and the working extract this site keeps — with the same caveat attached — is on the ASME pressure class reference. Use any of these tables for orientation and checking, never as a substitute for the edition your contract actually names. The series runs beyond the three classes in this memo's title — Class 900, Class 1500 and Class 2500 continue it upward for high-pressure duty, and European practice rates by PN number instead — but the envelope logic is identical at every step, so learning to read one column is learning to read them all.
How temperature derates pressure
Read any single class column downward and the shape is always the same: the permitted pressure is highest in the cold band and steps down at every temperature increment — gently at first, steeply once the material's strength begins to fall away in earnest. The mechanism is nothing more exotic than hot metal: allowable stress drops as temperature rises, and the envelope simply follows it down. Two habits fall out of that shape. First, rate the valve at design temperature, not at the ambient figure the line spends its nights at. Second, remember the derating belongs to the material as much as the class, so the same class in a different body material lands on different numbers at the same heat.
The class is bought at ambient but spent at temperature. Rate the valve where the line runs hot, not where it stands cold.
Worked examples
The extract below repeats rows from the published Group 1.1 table for common carbon steel bodies (A105, A216 WCB, A350 LF2), in bar gauge — published reference data: confirm against the current edition of ASME B16.34, and your purchase order governs.
| °C | 150 | 300 | 600 |
|---|---|---|---|
| −29 → 38 | 19.6 | 51.1 | 102.1 |
| 200 | 13.8 | 43.8 | 87.6 |
| 300 | 10.2 | 39.8 | 79.6 |
| 400 | 6.5 | 34.7 | 69.4 |
Now work two lines against it. A line designed for 16 bar at ambient sits comfortably inside Class 150's cold rating of 19.6 bar. Carry the same 16 bar to 200 °C and Class 150 is already gone — it permits 13.8 bar there — so the line becomes a Class 300 line, with 43.8 bar in hand at that temperature. Push harder: a 40 bar line at 400 °C fails Class 300, which allows 34.7 bar at that heat, and lands in Class 600 with 69.4 bar available. Same fluid, same pipe size — the class was decided by the pair of numbers, never by pressure alone.
Material group differences
The class alone is not the rating; the material group completes it. A Class 300 valve in carbon steel and a Class 300 valve in stainless do not permit the same pressure at the same elevated temperature, because each group's allowable-stress curve falls away on its own schedule — carbon steel's ratings drop steeply toward the top of its range, which is one reason sustained hot service migrates to the chrome-moly grades whose curves hold on longer. The habit to build: quote the class and the body material together, and read the rating from that group's column, not from a half-remembered figure for another metal. Sour service adds material limits of a different kind through NACE MR0175 / ISO 15156, the subject of the sour service memo. State the fluid and the temperature plainly and let class and group be checked as a pair.
Stating the class on an enquiry
Write the class with its basis: design pressure, design temperature, and body material. "Class 300, A216 WCB, 16 bar at 200 °C" hands a supplier everything needed to check the pick against the current tables, where "Class 300" alone invites assumption. The stated basis is also what protects the order when conditions change late in a project — a class chosen against written numbers can be re-checked in a minute, while a bare class has to be reverse-engineered. The full discipline — from line conditions through materials and ends to the finished purchase order — is the subject of the specifying guide.
NF Teguh is an industrial valve stockist in Subang Jaya, Selangor. Send the pair of numbers to the counter through the ordering route and the class check comes back with the quotation; the rest of the register — fire safety, certificates, the family memos — is on insights.
Related memos
Standards named above are published reference data — confirm against the current edition; your purchase order governs. Questions: the counter.