Class 150, Class 300, Class 600. The numbers get treated as though they were pressures in psi, and specifications get written as if a Class 300 valve is good for 300 psi. It is not that simple, and the misunderstanding shows up as a valve that is technically compliant and practically unsuitable.
A class is a curve, not a number
ASME B16.34 assigns each class a table of allowable working pressures against temperature, and the allowable pressure falls as the metal gets hotter. A Class 300 carbon steel valve is rated well above 300 psi at ambient temperature and considerably less than its ambient rating at 400 degrees Celsius. The class number is a label for the whole curve, not a value read off it.
The practical consequence is that the design temperature matters as much as the design pressure. Two lines at the same pressure, one cold and one at steam temperature, can land in different classes. If a datasheet gives a pressure and no temperature, the class cannot be fixed from it.
The material changes the curve
Class ratings are material dependent. A Class 300 valve in ASTM A216 WCB carbon steel and a Class 300 valve in A351 CF8M stainless follow different pressure-temperature curves, and the stainless is not automatically the stronger of the two at every temperature. Austenitic stainless holds up better at high temperature but is rated lower than carbon steel through much of the mid-range. Swapping a body material without rechecking the curve is a common and quiet mistake.
What the class also fixes
Beyond the pressure rating, the class carries a set of dimensional consequences that catch people out on retrofits. Flange dimensions, bolt circle, number and size of bolts and flange thickness all come from ASME B16.5 for the class. Body wall thickness follows the class through B16.34 or, for gate valves, through API 600. And face-to-face dimensions in ASME B16.10 are listed per class, so a Class 300 valve is usually longer than the Class 150 valve of the same size and type.
That last point is why raising a class late in a project is rarely a paper change. The valve gets longer, the flanges get thicker, the bolting changes and the pipe spool either has to be recut or the valve has to be sourced in a short pattern that may not exist.
What the class does not constrain
The class says nothing about the seat. Seat tightness is a separate matter, set by the test practice in API 598 and by the seat material and design. A Class 600 valve with a worn seat leaks exactly as much as a Class 150 valve with a worn seat. Nor does the class say anything about the trim material, corrosion resistance, cycle life or fire safe construction. Those are specified separately and are frequently the reason a valve is actually rejected in service.
What to write on the enquiry
Design pressure and design temperature together, the medium, and the material you expect for the body. Then let the class fall out of the pressure-temperature table rather than choosing it first and hoping the temperature fits.