ASME B16.34 Valve Pressure-Temperature Ratings: Understanding P-T Tables

What Is ASME B16.34?

ASME B16.34 (Valves–Flanged, Threaded, and Welding End) is the primary standard governing pressure-temperature ratings and minimum wall thickness for valves. It establishes the relationship between the maximum allowable working pressure and the operating temperature for various valve body materials. Every valve engineer must be proficient in using the P-T tables to specify valves correctly.

Material Groups and Class Ratings

ASME B16.34 organizes materials into groups (1 through 3) based on metallurgy. Each material group has its own P-T rating table. The valve Class designation (e.g., Class 150, Class 300, Class 600) is the reference working pressure in psi at 100°F for standard carbon steel. The actual allowable working pressure decreases as temperature increases due to the reduction in material allowable stress.

Common Material Groups

• Group 1: carbon steel (A216 WCB, A105): up to 538°C (1000°F)

• Group 2: Cr-Mo alloys (A217 WC6, WC9): up to 593°C (1100°F)

• Group 3: austenitic stainless (A351 CF8M, A182 F316): up to 816°C (1500°F)

• Higher-alloy materials: nickel alloys, duplex stainless not in standard groups

Using the P-T Tables

To determine the maximum allowable working pressure (MAWP) for a given valve: (1) identify the valve body material and its group number; (2) find the corresponding P-T table in ASME B16.34; (3) locate the valve Class along the top row; (4) find the operating temperature in the left column; (5) the MAWP at that temperature is the intersection of the Class column and temperature row.

Special Class Valves

If a valve has heavier wall thickness and meets the 'Special Class' marking requirements of ASME B16.34, it may be rated for an intermediate pressure level between standard Classes. For example, a Special Class valve with a body designed between Class 150 and Class 300 may be rated at approximately 1.5× the Class 150 pressure at each temperature.

Implications for Valve Specification

• Always specify valves by the material grade and Class, not by MAWP at any specific temperature

• For transitional temperature conditions (startup, shutdown, steam-out), ensure the valve P-T rating covers all cases

• For fire-safe valve requirements (API 607 / 6FA), confirm the post-fire pressure containment rating

• Use higher-class flanges and bodies as needed, not as a 'safety factor'

Summary

ASME B16.34 P-T tables are the foundation of valve pressure and temperature specification. Every valve engineer and procurement specialist must understand how to read these tables and apply the correct material group and Class designation to ensure that each valve can safely contain its process fluid at all operating and transient conditions.