ASME B16.34 / Pressure–Temperature Ratings for Valves
A valve has no pressure rating on its own. It has a rating at a temperature, in a material, under ASME B16.34 / and the two halves of that sentence are inseparable.
ASME B16.34 / Pressure–Temperature Ratings for Valves
ASME B16.34 is the reason a class number is not a pressure. A Class 300 valve in carbon steel is rated at one pressure when it is cold and a considerably lower pressure at high temperature, and a valve in a different material in the same class will have a different curve again.
The standard arranges materials into groups and publishes the allowable stress basis for each. The rating tables that follow are what a factory actually uses to decide whether a valve can be sold for a given line condition.
Why the rating falls with temperature
As metal gets hotter it loses strength. A rating table is a reflection of that loss, which is why the same valve that holds a high pressure at ambient may be limited to a fraction of it at an elevated design temperature.
The consequence in practice is that a specification stating only "Class 300" is incomplete. The design temperature has to travel with it, or the rating cannot be confirmed for either the body or the trim.
Material groups matter as much as class
Carbon steel, the chromium alloys used for elevated temperature, the austenitic stainless grades and the duplex grades each sit in their own group with their own curve. Two valves marked Class 300 in different materials are not interchangeable on a hot line.
This is where the marking matters. A valve body marked with the grade and the class carries the two pieces of information needed to look up the rating, which is why MSS SP-25 marking practice exists.
Wall thickness and the standard class
B16.34 also provides the minimum wall thickness basis for standard-class valves. That is the reason a valve built to a recognised specification is heavier than an unbranded look-alike of the same nominal size and class.
For a buyer, the useful test is to ask for the design basis in writing: which material group, which rating table, and what the maximum allowable pressure is at the design temperature. A supplier who can answer that in one reply is working from the standard; one who answers with the class number alone is not.
The Three Inputs To A Rating
| Item | Detail | Note |
|---|---|---|
| Class | 150 / 300 / 600 / 900 / 1500 / 2500 | Sets the reference curve, not the pressure |
| Material group | Carbon steel, Cr-Mo, austenitic, duplex and so on | Each group has its own allowable stress basis |
| Temperature | The design temperature, not the ambient | Rating falls as the metal gets hotter |
| Result | Maximum allowable pressure for that combination | Stated in writing on the datasheet |
From the works and the projects
Questions this page answers
No. The number is a class, not a pressure. The actual allowable pressure depends on the material and on the design temperature, and it is read from the rating table for that combination.
Because a valve built to a recognised specification has a minimum wall thickness requirement. Comparing weight against the standard for the size and class is the quickest way to see the difference.
Size, class, body material and the design temperature at minimum. With those four, the rating can be confirmed from the table without a second round of questions.