A common spreadsheet error is to place “Class 150” beside design pressure and stop there. The useful question is whether the complete valve is rated for the stated design pressure at the stated design temperature, using the applicable material group and construction rules.
01 / Language
Class, PN and working pressure are different systems.
ASME class designations such as 150, 300 and 600 identify rating families used with defined materials, temperatures and construction requirements. PN designations belong to another standardized system. Neither should be treated as a direct unit conversion.
A catalogue “maximum working pressure” may apply only to a limited temperature, size range, seat material or product series. Preserve the source and conditions whenever that number enters a project data sheet.
| Entry | What it tells you | What is still missing |
|---|---|---|
| ASME Class | The applicable rating family. | Material group, temperature, size and product construction. |
| PN designation | A standardized nominal-pressure designation. | Applicable EN/ISO product and flange rules. |
| WOG/CWP marking | A manufacturer or product-standard working-pressure expression. | Temperature range, medium restrictions and certification basis. |
| Test pressure | A production test condition. | The allowable continuous service rating. |
02 / Temperature
Design temperature changes the available pressure rating.
Material strength and design rules change with temperature. The selected pressure boundary may therefore carry a lower allowable pressure at elevated temperature than at ambient conditions. The correct check starts with the design temperature used by the piping specification, not the normal operating temperature alone.
Low temperature needs equal care. Toughness, impact-test requirements, bolting, seals and operating torque may control even when the pressure value looks modest.
03 / Assembly
The complete valve is limited by its least suitable element.
A pressure-boundary rating does not automatically qualify polymer seats, packing, stem seals, body joints, bolting or an actuator for the same temperature. Soft-seated ball valves are a familiar example: the body material may tolerate a temperature at which the seat system is no longer suitable.
End connections also matter. Thread form, socket-weld preparation, butt-weld end details, flange standard and flange facing must match the piping design. A body marked with a class cannot correct an incompatible mating joint.
Body, bonnet/cover, wall thickness, material group and design standard.
Seat, seal, differential pressure, direction and allowable leakage.
Gasket, fastener material, assembly and temperature limits.
Packing friction, actuator output, cycle rate and ambient conditions.
04 / Review
A rating check should leave an auditable trail.
Record the standard and edition, material specification and grade, pressure class or PN designation, design pressure, design temperature, size and any derating applied by the manufacturer. Keep the catalogue or calculation revision used for the decision.
Where the manufacturer claims a rating outside a recognized product-standard route, ask for the design basis and verification method. “Equivalent to Class 300” is not an engineering record.
- Design basisPressure, temperature, fluid and governing piping code.
- Material basisBody/bonnet material specification, grade and rating group.
- Product basisValve design standard, construction and size range.
- Non-metallic limitsSeats, seals, packing and gasket limits with media compatibility.
- EvidenceControlled datasheet, drawing, calculation or certificate tied to the offered model.
05 / RFQ
Give the supplier a design point, not a class label.
State both design and operating conditions, including upset or cleaning temperatures when they belong in the design envelope. Ask the supplier to confirm the allowable rating of the complete offered valve at the design temperature and list any component that sets a lower limit.
If several materials are acceptable, require each alternative to be identified rather than allowing a generic class statement to cover unlike material groups.
Valve shall be suitable for a design pressure of […] at a design temperature of […]. Supplier shall identify the governing rating standard and edition, material group, complete-valve temperature limit and any seat, seal, packing, bolting or end-connection restriction.
Sources / editorial control
Source basis
- ASME B16.34-2025 — valves, flanged, threaded and welding end. Official ASME scope and current edition page.
- ASME B16.5-2025 — pipe flanges and flanged fittings. Official scope includes ratings, materials, dimensions and flange-joint requirements.
Review limit: Exact rating values are copyrighted standard content and are intentionally not reproduced. Verify the project edition and the offered manufacturer’s controlled data before selection.