- Pressure class ratings for valves — whether ASME Class 150 through 2500 or PN6 through PN400 — are not direct pressure limits but designations that, combined with material and temperature, define the allowable operating pressure via standard pressure-temperature rating tables.
- The most common and consequential pressure class specification error is failing to de-rate for temperature — a Class 150 carbon steel valve rated at 19.6 bar at ambient temperature is only rated at 9.3 bar at 450°C, a difference that can determine whether a valve is fit for its intended service.
- Understanding how ASME material groups, PN ratings, and the relationships between different standards systems work is fundamental engineering knowledge for anyone purchasing, specifying, or managing industrial valve inventories.
Pressure class ratings are probably the most frequently misunderstood aspect of valve specification among engineers who work with valves but don’t specialise in them. The assumption that “Class 150 means 150 psi” is wrong. The assumption that “PN16 and Class 150 are the same” is wrong. The assumption that a higher class number always means a better valve for any application is wrong. Let me give you the correct picture, clearly.
The ASME Class System
ASME B16.5 defines six pressure classes for pipe flanges and flanged fittings from NPS ½ through NPS 24: Class 150, 300, 600, 900, 1500, and 2500. These numbers are NOT pressures in psi — they are class designations that define the allowable working pressure through reference to pressure-temperature rating tables in the standard.
The actual rated pressure for any Class and material combination is found in Table 2 of ASME B16.5, organised by material group and temperature. Material groups range from Group 1.1 (standard carbon steel, ASTM A105/A216 WCB) through Group 3.3 (high-alloy nickel steels). Higher group numbers do not inherently mean higher pressure ratings — the groups reflect alloy compositions, and rating comparisons must be made at specific temperature conditions.
For Class 150, Group 1.1 (carbon steel) — the most common combination in process plant: at -29°C to 38°C (ambient): 19.6 bar (285 psi). At 100°C: 17.7 bar (257 psi). At 200°C: 15.1 bar (219 psi). At 300°C: 12.8 bar (186 psi). At 400°C: 10.2 bar (148 psi). At 450°C: 9.3 bar (135 psi). The de-rating with temperature is significant and cannot be ignored. Always look up the rated pressure at the actual design temperature, not just ambient conditions.
Selecting the Right Class for Your Application
Class selection begins with design pressure and design temperature — not operating pressure and operating temperature. Design conditions include the appropriate pressure and temperature margins above normal operating conditions, plus any upset or emergency conditions the valve must withstand. A system operating at 10 bar and 300°C but subject to upset pressures of 14 bar must be rated for the 14 bar upset condition at 300°C, not for the 10 bar normal operating condition.
For the Class 150, Group 1.1 example: at 300°C, the rated pressure is 12.8 bar. This is adequate for the 14 bar upset condition — barely. A prudent engineer might select Class 300 (rated at 51.1 bar at 300°C for Group 1.1) to provide adequate margin. This is the kind of calculation that should appear in the valve specification basis document — not just “Class 150” as a final answer.
PN Ratings: The European System
PN (Pression Nominale) ratings used in EN 1092-1 and associated European standards work differently from ASME Classes. The PN number IS approximately the rated pressure in bar — but only at the reference temperature of 20°C for the standard reference material (typically P250GH carbon steel or equivalent). At elevated temperatures, PN-rated valves and flanges are de-rated according to the relevant EN standard’s pressure-temperature tables, similar to the ASME system.
Common PN ratings and their approximate ambient-temperature pressure ratings: PN6 = 6 bar, PN10 = 10 bar, PN16 = 16 bar, PN25 = 25 bar, PN40 = 40 bar, PN63 = 63 bar, PN100 = 100 bar. The PN ratings continue up to PN400 for very high pressure applications.
The important caveat on PN-to-ASME equivalence: PN16 and Class 150 have similar pressure ratings at ambient temperature (16 bar vs 19.6 bar) but are NOT dimensionally interchangeable. The bolt circle, number of bolts, face dimensions, and face-to-face dimensions are all different. You cannot bolt a PN16 EN 1092-1 flange to a Class 150 ASME B16.5 flange and achieve a sealed joint — the geometries don’t match. Establish a clear standard boundary at any interface between EN and ASME systems, and use transition flanges at that boundary.
High-Pressure API 6A Classes
For wellhead and christmas tree equipment in oil and gas production, ASME B16.5 Class 2500 (the maximum at 420 bar at ambient for Group 1.1) is often insufficient. API 6A defines pressure ratings up to 20,000 psi (1380 bar) using a different class system: 2000 psi, 3000 psi, 5000 psi, 10,000 psi, 15,000 psi, and 20,000 psi. These valves are designed and tested to very different standards from ASME B16.5 Class valves and are not interchangeable.
API 6A valves carry the pressure rating stamped directly on the body as a psi rating, and the dimensional standards for flanges (API 6A ring joint flanges) are specific to the API pressure class. An API 6A 10,000 psi flange does not match an ASME B16.5 Class 2500 flange dimensionally. Interface planning between ASME B16.5 piping systems and API 6A wellhead equipment must account for this non-interchangeability at the connection point.
Master pressure class ratings and you’ll never be caught out by a temperature de-rating surprise, a standard interface mismatch, or an incorrectly specified valve. These are foundational concepts that underpin every valve and piping specification decision — get them right from the start of a project and they’ll never cause a problem. Get them wrong and they cause expensive problems at exactly the worst possible time.