- ASME B16.34 pressure classes (150, 300, 600, 900, 1500, 2500) are not direct pressure ratings — they are class designations whose allowable pressure depends on both the material group and the operating temperature, and ignoring the temperature de-rating is one of the most common and costly errors in valve specification.
- The PN (Pressure Nominal) rating system used in European standards defines the pressure rating at 20°C only, and the rated pressure at elevated temperatures is defined by separate pressure-temperature tables that differ significantly from the ASME de-rating curves — making direct equivalence between ASME Classes and PN ratings fundamentally incorrect.
- Correctly reading and applying pressure-temperature rating tables from ASME B16.34 requires understanding material groups, interpolation between tabulated temperatures, and the distinction between the cold working pressure (CWP) and the rated pressure at operating temperature — all essential for reliable valve specification.
Pressure class ratings are foundational to valve specification, and yet they’re among the most frequently misunderstood aspects of the whole discipline. I’ve reviewed specifications where a Class 150 valve was selected for a service that exceeded the allowable pressure at operating temperature — not because the engineer didn’t know Class 150 existed, but because they didn’t check the P-T tables and didn’t know the temperature de-rating was significant. This guide will make sure that error never appears in your work.
ASME B16.34: The Foundation Standard
ASME B16.34 — “Valves — Flanged, Threaded and Welding End” — is the governing standard for steel valves in North American and international process plant service. It defines six pressure classes: 150, 300, 600, 900, 1500, and 2500. These class numbers are group designations, not pressure ratings in any unit. The actual allowable pressure for each class is found in the pressure-temperature (P-T) rating tables in the standard, which give the rated pressure for each combination of class and material group at temperatures from -29°C to the maximum allowable temperature for that material.
ASME B16.34 organises valve body materials into groups based on their high-temperature strength characteristics. Group 1.1 covers standard carbon steel (A216 WCB, A105) — the most common valve material for general service. Group 2.1 covers standard austenitic stainless steels (A351 CF8M for 316 equivalent). Group 3.1 covers standard chrome-moly alloys. Higher group numbers generally indicate better high-temperature strength and therefore higher P-T ratings at elevated temperatures for the same class.
The P-T rating for Group 1.1 (carbon steel) at Class 150:
At -29°C to 38°C: 19.6 bar (285 psi)
At 100°C: 17.7 bar
At 200°C: 15.1 bar
At 300°C: 13.4 bar
At 400°C: 12.1 bar
At 450°C: 11.2 bar
The cold working pressure (CWP) — the pressure at ambient temperature — for Class 150 carbon steel is 19.6 bar. But at 300°C, the allowable pressure has dropped to 13.4 bar. A process operating at 15 bar and 300°C would exceed Class 150 carbon steel’s rated pressure at operating temperature — Class 300 (rated 51.1 bar at 38°C, 46.6 bar at 300°C) is the correct selection.
The Temperature De-Rating Principle
Temperature de-rating occurs because steel loses strength at elevated temperatures. The yield strength and tensile strength of carbon steel at 400°C are significantly lower than at ambient temperature. The pressure-temperature tables in B16.34 are calculated from the allowable stress values in ASME BPVC Section II Part D — they directly reflect the loss of material strength with increasing temperature.
For austenitic stainless steels (Group 2.1 — 316L, 304L), the de-rating is more pronounced than for carbon steel because austenitic stainless has better high-temperature properties than carbon steel at moderate temperatures but loses strength more rapidly above 400°C. The Class 150 stainless P-T rating drops from 19.6 bar at ambient to only 10.2 bar at 500°C — a much steeper decline than carbon steel.
Chrome-moly alloys (P11, P22, P91) in Groups 3.1 and higher maintain their strength to higher temperatures than carbon or stainless steel, which is why they’re specified for high-temperature steam and process service where standard materials would require impractically high pressure classes to meet the design pressure at operating temperature.
How to Read and Apply the P-T Tables
The correct procedure for valve pressure class selection using ASME B16.34:
1. Establish the design pressure and design temperature for the valve service. Design conditions are the maximum operating conditions with appropriate margins — typically the maximum allowable operating pressure (MAOP) and the maximum expected process temperature, not just normal operating conditions.
2. Identify the valve body material (or the material group from Table 1 in B16.34 for the specific material).
3. Look up the P-T table for the identified material group. Find the temperature row at or above your design temperature. Read across to find the required pressure class: select the lowest class where the rated pressure at design temperature equals or exceeds the design pressure.
4. For temperatures between tabulated values, interpolate linearly between the adjacent temperature rows. Do not extrapolate beyond the maximum tabulated temperature.
5. Verify that the selected class is also adequate at the maximum cold pressure condition (hydrostatic test pressure, relief valve set pressure before lifting, blocked-in conditions). The cold working pressure at ambient temperature for the selected class must accommodate these maximum pressure scenarios.
PN Ratings in EN Standards
European valve standards use the PN (Pressure Nominal) designation system, defined in EN 1333. The PN designation indicates the approximate gauge pressure in bar at 20°C for which the component is rated. PN16 is rated for 16 bar at 20°C; PN40 for 40 bar at 20°C.
The key point: PN is the rating at 20°C. The rated pressure at elevated temperatures for EN valves is found in EN 1092-1 (for steel flanges) and EN 13709 (for steel industrial valves), which publish pressure-temperature tables similar in structure to ASME B16.34 but with different values — because EN and ASME standards use different allowable stress bases.
The common assertion that “PN16 ≈ Class 150” or “PN40 ≈ Class 300” is a rough approximation at ambient temperature only. At elevated temperatures, the P-T curves for EN and ASME standards diverge, and the apparent equivalence breaks down. For any elevated-temperature application, verify the actual rated pressure at design temperature using the appropriate standard’s P-T tables — do not rely on ambient-temperature nominal equivalences.
Pressure class ratings are the foundation of safe valve specification. Get them wrong and every other specification decision — material, seat type, testing, certification — is built on sand. Get them right, with proper reference to the P-T tables at design temperature, and you’ve established a technically correct basis that will survive engineering review, procurement, and ultimately, years of safe service.