What a pressure class actually means, and why the number is not a pressure
Class 150, Class 300, Class 600: the numbers look like pressures, and almost everyone reads them that way once. They are labels for a family of rating curves, not the rating itself. Under ASME B16.34, a class identifies a wall-thickness and rating series; the allowable working pressure that goes with it is read from a table using two further inputs: the material group of the body, and the temperature the body will actually reach. Miss either input and the number you quote is somebody else's. The valve selection guide places this step in the wider sequence; this article stays inside the table.
Material groups: how carbon steel, chrome-moly and austenitic stainless get different curves
The rating tables are built from allowable stresses, and allowable stress is a property of the alloy, not of the valve. B16.34 therefore sorts materials into groups, each with its own set of curves. Group 1.1 is the familiar carbon-steel family. It is the reference group most buyers meet first and the one used throughout this article. Chrome-moly grades hold pressure further up the temperature scale and appear in the higher-numbered groups. Austenitic stainless behaves differently again: lower allowable stress at ambient than carbon steel, so a lower rating at 38 °C, but a much flatter curve that keeps useful pressure far above where carbon steel has given up. Two valves of identical class in different groups are not interchangeable. The glossary defines class, material group and trim as the standards use those terms.
Exhibit: Group 1.1 allowable working pressure by class and temperature
The excerpt below shows the shape of the problem for the carbon-steel reference group, in bar gauge. It is an illustrative excerpt, not the table. The full table runs to Class 2500 and far higher temperatures.
| Temperature | Class 150 | Class 300 | Class 600 | Class 900 | Class 1500 |
|---|---|---|---|---|---|
| 38 °C | 19.8 bar g | 51.7 bar g | 103.4 bar g | 155.1 bar g | 258.5 bar g |
| 200 °C | 13.8 bar g | 48.6 bar g | 97.2 bar g | 145.8 bar g | 243.0 bar g |
| 400 °C | 6.5 bar g | 34.7 bar g | 69.4 bar g | 104.1 bar g | 173.5 bar g |
Reference values. Confirm against the current edition of the standard and the manufacturer's rating table.
Reading the table: class, group, temperature → allowable working pressure
The lookup runs in one direction only. Fix the material group from the body specification. Enter at the design temperature, interpolating between listed temperatures rather than rounding down to the friendlier row. Read across to the class under consideration. The figure returned is the allowable working pressure of the valve body; the design pressure of the line must sit below it with the margin the project requires. Working the table backwards (choosing a class first and then hoping the temperature cooperates) is how a Class 150 valve arrives on a line whose design temperature has quietly eaten a third of its rating. Notice in the excerpt that Class 150 loses far more of its ambient rating by 400 °C than Class 1500 does.
Standard class versus special class, and what the higher rating costs you in NDE
B16.34 recognises more than one path to a rating. Valves qualified to the standard-class requirements are the everyday case. Special-class valves earn a higher allowable pressure for the same nominal class, and they earn it through tighter material requirements and additional non-destructive examination of the pressure-retaining parts. The extra rating is real, and so is the extra cost, the extra documentation and the longer manufacturing route. Specifying special class because it sounds safer, on a duty that never approaches the standard-class curve, buys paperwork rather than protection. Specifying it when the design genuinely sits close to the limit is exactly what it exists for.
The class number is the label on the box. The rating is what the table returns once you have named the material group and the temperature. It is almost never the number on the box.
Reading a rating table
Temperature derating: why a valve fine on chilled water is not fine on superheated steam
Derating is the whole point of the table. Metal loses strength as it heats, so the allowable pressure has to fall with it. On a chilled-water system running at 6 °C, the ambient row is effectively the working row, and a modest class covers a surprisingly high pressure. On saturated steam the temperature is not a free variable at all (it is fixed by the pressure), and on superheated steam it climbs well beyond that. The same casting that was comfortable in a plant room is now sitting at a fraction of its ambient rating, which is why steam and thermal-oil lines carry higher classes than their pressures alone would suggest. Selection by medium is set out in which valve does this service actually need.
Worked example: the same body across three temperatures, and the class it needs at each
Take a carbon-steel body in group 1.1 on a line whose design pressure is 12 bar g.
- At 38 °C: Class 150
The excerpt shows Class 150 clearing that comfortably at 19.8 bar g, and Class 150 is the sensible answer.
- At 200 °C: Class 150, thinly
Class 150 has fallen to 13.8 bar g: it still clears 12 bar g, but the margin has thinned to the point where a project engineer may reasonably want the next class up.
- At 400 °C: Class 300
Class 150 is down to 6.5 bar g and is simply out. The duty now needs Class 300, which still holds 34.7 bar g at that temperature.
One body, one design pressure, three answers, and the only thing that moved was temperature.
Reference values. Confirm against the current edition of the standard and the manufacturer's rating table.
Flange compatibility: ASME B16.5 flanges carry the same class system
A valve does not sit alone. Its end connections have to match the flanges already in the line, and ASME B16.5 uses the same class-and-material-group logic for pipe flanges and flanged fittings. That is convenient, because it means the whole joint derates together, and it is a trap, because a correctly rated valve bolted to a flange of a lower group or a different facing produces a joint rated by its weakest member. Gasket selection under ASME B16.20 follows the same class. Confirm class, group, facing and bolting as one set, not as four separate line items.
Where the table is silent: low temperatures, cyclic service and external loads
A rating table answers one question (steady internal pressure at temperature), and it is silent on several others that fail valves in practice. It does not cover impact toughness at low temperature, which is a separate material qualification. It does not cover fatigue from pressure or thermal cycling. It does not cover external loads: pipe stress, unsupported weight, a large actuator cantilevered off the bonnet, or seismic restraint. It does not cover erosion, corrosion allowance or the seat, only the pressure-retaining envelope. A valve can be perfectly rated and still be the wrong valve, which is why the rating step sits in the middle of the selection sequence rather than at the end of it.
Governing standards and the confirm-against-the-current-edition rule
Three documents underpin everything above. ASME B16.34 fixes the ratings and the standard-versus-special-class rules. ASME B16.5 carries the same classes into flanges and flanged fittings. The allowable stresses behind both come from the ASME Boiler and Pressure Vessel Code material sections. All three are revised on their own cycles, and a rating quoted from a superseded edition is a quotation from a document that no longer exists. Every number on this page is published reference data reproduced to explain the method.
Reference values. Confirm against the current edition of the standard and the manufacturer's rating table.
Buyer mistakes: too cheap buys downtime, over-engineered buys cost
Two mistakes, one root cause: reading the class instead of the table. Too cheap is Class 150 accepted because the line runs at 12 bar g, with nobody checking what 12 bar g means at the design temperature. The derating is discovered by a leaking joint rather than by a calculation. Over-engineered is Class 600 bought on a chilled-water riser to be safe. The wall thickness is paid for in price, the mass is paid for by the pipe supports, and the unusual combination is paid for in waiting. Neither mistake is a knowledge problem; both come from skipping a two-minute lookup. Where the class is genuinely borderline, the answer is the calculation, not the reflex.
Documentation to request: class and material group on the nameplate and certificate
The rating is only as good as the evidence that this valve belongs to that curve. Ask for it in writing:
- Nameplate data showing class, body material designation and the standard the valve is built to.
- The manufacturer's pressure and temperature rating table for that exact material, with the edition cited.
- Material test certificate to EN 10204, type 3.1 for pressure-retaining parts, traceable to the heat number.
- Where special class is specified, the non-destructive examination records that qualify it.
- Shell test evidence to API 598, which is pressure-tested against the rating, not against the class label.
How to read those documents, and how to spot one stretched past its scope, is covered in sourcing and documentation.
Related product lines and the category pages this guide resolves to
Ratings apply across the whole industrial valves category: ball, gate, globe, check and butterfly bodies are all rated the same way, though their practical temperature ceilings differ, as gate, ball or butterfly sets out. Higher-temperature duties usually land on pressure-reducing, safety and steam valves, while HVAC and district-cooling valves mostly live in the ambient rows. The product lines we supply span those categories; class and group availability for any given line and size are To confirm per enquiry.
Next step: send an enquiry with the service condition pre-framed
The rating question is answerable in a single exchange if the enquiry carries the inputs the table needs. State the medium, the operating and design pressure in bar g, the operating and design temperature in °C, and the size with the end connection and flange standard already in the line. Send that to us and the reply can name the class, the material group and the test and certificate scope, with the edition each figure was read from.