Copper Alloy Flange Pressure-Temperature Ratings

Blog / August 11, 2026 / 5 min read

This is the single most common technical error on copper alloy flange work. Someone specifies a Class 150 flange in copper-nickel, receives a flange with a Class 150 bolt pattern, and assumes it carries the Class 150 pressure-temperature rating. It does not.

What class actually means here

ASME B16.5 is a steel flange standard. Its pressure-temperature rating tables are computed from the allowable stresses of specific steel material groups. Class 150 is shorthand for a particular combination of geometry and rated capability in those steels.

When we machine a B16.5 Class 150 pattern out of C70600, the resulting part has the geometry, the outside diameter, thickness, bolt circle and drilling, of a Class 150 flange. It does not have the material properties the rating table was computed from. Copper-nickel is considerably weaker than carbon steel.

Where the rating comes from

For copper alloy flanges on a steel pattern, the rating has to be derived, not looked up:

1. Take the allowable stress for the actual alloy at the actual design temperature, from the governing design code. For process piping in the United States that is ASME B31.3 and its allowable stress tables.
2. Apply the flange design rules the code and the owner require, which for a non-standard combination of geometry and material typically means an ASME Section VIII Division 1 Appendix 2 style flange calculation.
3. Check the gasket and bolting as part of the same calculation, because a flange rating is a joint rating. Bolt load, gasket seating stress and flange moment are one problem.
4. Have it signed by the engineer of record. This is a design calculation, not a catalog lookup.

Cast copper alloy flanges bought to ASME B16.24 are different, and easier. That standard publishes pressure-temperature ratings for its own material and classes, so the rating is a lookup. Use B16.24 ratings for B16.24 product.

How steeply copper alloys derate

Steel loses strength gradually through the moderate temperature range. Copper alloys fall away much earlier and much faster. The practical consequences:

• Most copper alloy flange service sits under 400 degrees F, and a great deal of it under 200.
• 70/30 copper-nickel holds up better than 90/10 as temperature rises, which is why the hot stages of desalination plants and condenser duty favour it.
• The leaded cast bronzes derate earlier still. They are moderate water service materials.
• A rating that is comfortable at ambient can be marginal at 300 degrees F. Always check at the design temperature, not at room temperature.

Also check the low end

Copper alloys do not have the ductile to brittle transition problem that plagues carbon steel, which makes them genuinely good at cryogenic and low temperature service. If anything the low end is where copper alloys have an advantage. Confirm the code allows the grade at the temperature, but do not expect the impact testing headaches that carbon steel brings.

What to send us

The way to avoid all of this is to quote the service, not the class. Send:

• Design pressure and design temperature, and the upset case if there is one
• The fluid, and whether it carries solids or entrained air
• The alloy, or the service if you want a recommendation
• The bolt pattern you have to mate to
• The governing design code

With that we can tell you whether the alloy and thickness carry the duty, or whether the job needs a heavier flange, a different pattern, or a different material.

The summary

See weights and dimensions for the standards that govern the geometry, and copper alloy grades for the material properties behind the calculation.