ASME B16.34 Pressure-Temperature Ratings: How Class Becomes Allowable Pressure
Quick Answer
ASME B16.34 defines allowable working pressures for flanged and butt-weld valves by pressure class (150, 300, 600, 900, 1500, 2500) and material group. A Class number is not a pressure in psi - the allowable pressure depends on temperature and material, and it falls as temperature rises.
Class Is Not Pressure
A "Class 300" valve is not rated for 300 psi at all temperatures. The number is a designation; the actual allowable working pressure comes from the B16.34 table for the valve material group at the service temperature.
Material Groups
B16.34 groups materials (1.1, 1.2, 1.3, 1.4, 1.5, 1.10, 1.15, etc.), each with its own pressure-temperature table. A 1.1 carbon-steel valve and a 1.15 austenitic-stainless valve of the same class have different allowable pressures at the same temperature.
Temperature Dependence
As temperature rises, the allowable stress of the material drops, so the allowable working pressure drops. At the reference temperature (about 38 C / 100 F) the allowable pressure for many groups is close to the class number expressed in bar, but that is only the starting point.
Practical Use
Always read the allowable pressure from the table at the *maximum* service temperature, not at ambient. For converting PN and Class ratings see PN vs ASME Class and the rating chart. Full code map in the standards hub.
Frequently Asked Questions
Is a Class 300 valve rated for 300 psi?
No. The class number is a designation; the allowable working pressure at a given temperature comes from the ASME B16.34 table for the valve material group, and it falls as temperature rises.
Why do different materials of the same class have different pressures?
Each B16.34 material group has its own pressure-temperature table because allowable stress differs by material and temperature.
At what temperature is the class number closest to the allowable pressure?
Near the reference temperature of about 38 C (100 F); for many groups the allowable pressure in bar is then close to the class number, but it declines steadily with temperature.