Class 150, 300, 600: Reading the Pressure-Temperature Table

If a valve is marked “Class 300”, that marking does not tell you the pressure it may be used at. A Class is a class designation, not a value. What pressure is allowed is read from the standard's pressure-temperature table, together with the body material and the operating temperature.

What a Class fixes, and what it does not

A pressure class fixes geometry and a strength family: wall thicknesses, flange thickness, bolt circle and bolt sizes all follow from it. Two parts of the same class bolt together; parts of different classes will not, even when their outside diameters look similar.

What it does not fix is a single allowable pressure. The pressure a body can hold depends on the strength of its material at temperature, and material weakens as it heats. That is why the standard gives no single number per class but a table indexed by material group and temperature.

The order in which to read it

Reading in the right order removes the source of most mistakes:

1. Establish the body material. Carbon steel, stainless steel and cast materials fall into separate material groups in the standard, each with its own curve.
2. Establish the operating temperature. What matters is not the fluid's normal temperature but the highest metal temperature that can occur.
3. Read the cell where that material and temperature meet. That is the allowable pressure.
4. Compare it with the line's design pressure. If it falls short, the answer is not to ignore temperature but to move up a class or to a stronger material.

Why it falls as temperature rises

Yield and tensile strength decrease with temperature. A wall that carries a given stress at room temperature does not carry it with the same margin when hot, and under sustained load at high temperature the material slowly deforms as well. The standard's table accounts for both effects and expresses the result as an allowable pressure that declines with temperature.

In the field this means a valve of a given class can be entirely comfortable on cold water and inadequate on a hot medium. The class has not changed; the duty has.

Choosing between the three

One more consequence follows from this. Because the permitted pressure depends on the body material, two valves of the same Class made from different materials are not equivalent — the higher Class of a weaker material can sit below the lower Class of a stronger one at working temperature. Comparing Class numbers alone, without reading the material line beside them, is how an apparently safer specification ends up being the weaker one.

Moving up a class thickens the wall and the flange. The cost is not only price: the part gets heavier, face-to-face dimension and bolting grow, support loads increase and the spares inventory multiplies. A class is therefore chosen by calculation, not “to be on the safe side”.

Three questions are enough. What are the line's design pressure and design temperature? What will the body material be? And are those two values exceeded during commissioning, testing or off-design operation? The third is the one most often skipped, yet hydrostatic testing and abnormal operation can be more demanding than normal service and can decide the class on their own.

Finally, the table can be read backwards. If you already have a valve, its class and material marking tell you what pressure it may hold up to what temperature. When operating conditions change in an existing plant, that is the first place to look.