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 Class||Carbon Steel Valve||Stainless Steel Valve|
|Class 150||285 psig||275 psig|
|Class 300||740 psig||720 psig|
|Class 600||1480 psig||1440 psig|
|Class 900||2200 psig||2160 psig|
|Class 1500||3705 psig||3600 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.