Valve Pressure-Temperature (P-T) Ratings Explained

ted wang
Jun 4
2 min read

What Are Pressure-Temperature Ratings

Pressure-temperature ratings define the maximum allowable working pressure (MAWP) at a given temperature for a valve body constructed of a specified material. As temperature increases, the yield and tensile strength of metals decreases, reducing the safe operating pressure. P-T rating tables allow engineers to verify that valve selection is appropriate for any operating condition within the design envelope.

ASME B16.34 Pressure Classes

Class 150: approximate 285 psi (19.6 bar) at ambient temperature for carbon steel
Class 300: approximately 740 psi (51 bar) at ambient temperature for carbon steel
Class 600, 900, 1500, 2500: progressively higher pressure ratings
Each class designation represents a specific body wall thickness and flange face geometry
P-T rating tables in ASME B16.34 Appendix A cover Group 1.1 through 3.3 material groups

How Temperature Affects Ratings

At higher temperatures, metal yield strength drops, reducing the allowable stress and hence the permissible pressure. For carbon steel (ASTM A216 WCB), the Class 150 rating drops from 285 psi at -29°C to 38°C down to 170 psi at 425°C. Alloy steels (1.25Cr, 2.25Cr, 5Cr) and stainless steels (304, 316) have different temperature-pressure profiles and must be evaluated against the appropriate material group table.

Impact of Non-Standard Materials

Exotic alloys (Inconel, Hastelloy, Duplex SS) may require special rated pressure calculation
ASME B16.34 Appendix B provides alternative alloy P-T ratings for selected materials
Manufacturer ratings must not exceed ASME B16.34 without engineering justification
Split-body or multi-piece valves: the weakest pressure boundary component determines the assembly rating
Temperature of fluid vs. ambient: design must consider both to avoid cold impact and hot derating

Using P-T Ratings in Practice

When specifying valves, check that the rated pressure at the maximum operating temperature meets or exceeds the design pressure. Do not specify a higher pressure class than needed without justification, as costs increase sharply with class. For low-temperature service, verify that the material group has been impact-tested per ASME requirements for the minimum design temperature to avoid brittle fracture.