Class 150 Does Not Equal 150 psig

Pressure-Temperature Ratings for Valves

When it comes to valves, there are guidelines in place to protect workers and the environment from valve failures and leakage. In this example, we are talking about how much operating pressure a valve can withstand at a specific temperature. This is known as a Pressure-Temperature Rating.

Based on the material(s) of construction, the Pressure-Temperature ratings for each Valve Class are tabulated to provide the Maximum Allowable Working Pressures, expressed as gauge pressures, at a specific temperature.

For example, see the different Maximum Allowable Working Pressures for a Carbon Steel Valve (Group 1.1 Materials) vs. a Stainless-Steel Valve (Group 2.2 Materials) at a temperature of -20 to 100 °C according to the specifications set forth in ASME B16.34.

Valve ClassCarbon Steel ValveStainless Steel Valve
Class 150285 psig275 psig
Class 300740 psig720 psig
Class 6001480 psig1440 psig
Class 9002200 psig2160 psig
Class 15003705 psig3600 psig

Did you note how Class 150 does not equal 150 psig?

That is because Valve Class is a label, not a Pressure-Temperature rating.