ASME B16.34 Pressure-Temperature Ratings for Industrial Valves: Explained

ASME B16.34 (Valves — Flanged, Threaded, and Welding End) is the American Society of Mechanical Engineers standard that establishes pressure-temperature ratings, dimensional requirements, materials, non-destructive examination, testing, and marking for industrial valves. It is the single most important standard for engineers specifying valves in pressure piping systems.

What are Pressure Classes?

ASME B16.34 defines six standard pressure classes for industrial valves: Class 150, 300, 600, 900, 1500, and 2500. These class numbers are not pressure values — they are dimensionless designators that correspond to a specific pressure-temperature rating that varies with body material and operating temperature. The higher the class number, the higher the allowable pressure.

How to Read the Pressure-Temperature Tables

The P-T rating tables in ASME B16.34 are organised by material group. Each material group has a table showing the maximum allowable working pressure at various temperatures for each pressure class. To find the correct rating for a valve:

1. 1Identify the valve body material ASTM designation (e.g., A216 WCB, A351 CF8M)
2. 2Find the material group for that material in Table 1 of ASME B16.34
3. 3Look up the P-T table for that material group
4. 4Find the row for the operating temperature and the column for the pressure class
5. 5The value in that cell is the maximum allowable working pressure at that temperature

Pressure Ratings for Common Materials at 38°C (100°F)

Note that SS316 (CF8M) has lower pressure ratings than carbon steel at the same class — this is because stainless steel has lower allowable stress values at elevated temperatures. This is one of the most common errors in valve specification.

Effect of Temperature on Pressure Rating

The allowable pressure decreases significantly as temperature rises. For example, an ASTM A216 WCB Class 600 valve rated at 102.1 bar at 38°C is only rated at 84.6 bar at 300°C and 70.0 bar at 400°C. This de-rating effect must always be checked when specifying valves for elevated temperature service. Many engineering mistakes occur when the pressure class is selected based on ambient temperature ratings without considering the actual operating temperature.

ASME B16.34 vs ANSI vs PN Designations

The ANSI pressure classes (150, 300, 600, 900, 1500, 2500) are equivalent to ASME B16.34 classes. The European PN (Pressure Nominal) rating system is different and uses PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160, PN250, and PN420 designators. PN ratings correspond to the maximum allowable pressure in bar at 20°C. ANSI and PN classes are NOT directly interchangeable — the dimensional standards (bolt circle, bolt size, flange thickness) are different.

Special Service Requirements

ASME B16.34 defines three service categories:

• Standard class: General process service — this is the default category for most industrial valves
• Special class: Higher pressure rating achieved by additional non-destructive testing (10% RT or UT of all pressure-retaining welds), used in critical or high-risk service
• Limited class: Lower pressure-temperature ratings for low-risk, non-hazardous service (e.g., water at ambient temperature)

Common Errors in Pressure Class Selection

• Not accounting for temperature de-rating — selecting Class 300 based on room-temperature ratings for a 300°C steam service
• Using ANSI Class ratings for stainless steel without checking the lower CF8M P-T table
• Confusing PN ratings with ANSI Class ratings (PN16 ≈ Class 150, but flanges are NOT interchangeable)
• Using the body material rating without checking that trim material meets the same requirements
• Not accounting for positive pressure rating above 38°C for cryogenic service (ratings increase at sub-ambient temperatures)