Pressure-Temperature Derating

Ratings are

Maximum allowable working gauge pressure

Basis

Metal temperature, not fluid temperature

Interpolation

Permitted between listed temperatures

Extrapolation

Not permitted

Reading a rating in four steps

1. Find the material group. A105 carbon steel is Group 1.1; F304/F316 stainless is Group 2.1; the L grades are Group 2.2. The group, not the class, determines the shape of the curve.
2. Use the metal temperature. B16.5 rates on the temperature of the flange metal. For uninsulated flanges that is usually taken as the fluid temperature; where they differ and it can be justified, the actual metal temperature may be used.
3. Read across to the class. That gives the maximum allowable working gauge pressure.
4. Interpolate if needed. Linear interpolation between two listed temperatures is permitted.

Worked example: interpolation

A Class 300 A105 flange operating at 450 °F. The table lists 635 psig at 400 °F and 605 psig at 500 °F.

P = 635 + (605 − 635) × (450 − 400) / (500 − 400) = 620 psig

So the flange is rated 620 psig at 450 °F. Note that this is a linear interpolation between two tabulated points, not a formula — B16.5 permits it precisely because the underlying allowable stress curve is close to linear over a 100 °F interval.

Group 1.1 carbon steel, every class

Group 1.1 covers A105, A216 WCB, A515/A516 Gr 70 and A350 LF2 as forged. See the full rating tables for the other groups.

How much rating is lost, and where

The two material families behave completely differently. Carbon steel starts higher and falls away steadily. Austenitic stainless starts lower and then flattens — it holds most of its rating right through to 1000 °F.

Percentages are relative to each material's own 100 °F rating, so they show shape rather than absolute capacity.

Where derating catches people out

• Steam service in Class 150. A Class 150 carbon steel flange holds 285 psig cold but only 140 psig at 600 °F. Saturated steam at 150 psig is around 366 °F, which is within rating — but a small superheat or an upset can put the joint outside it.
• Steam-out and regeneration. Lines are often steamed out at temperatures far above their normal operating point. The rating at the steam-out temperature governs, not the rating at the process temperature.
• Mixed material groups in one joint. A stainless flange bolted to a carbon steel flange is limited by whichever has the lower rating at temperature — and which one that is changes with temperature.
• Valves and fittings are not all B16.5. A B16.34 valve, a B16.47 large flange and a B16.5 flange in the same line have different rating tables. The lowest-rated component governs the line.
• Bolting material matters. The tabulated ratings assume bolting of at least the strength B16.5 specifies. Lower-strength bolting derates the joint further.

The −20 °F to 100 °F column

The first column covers everything from −20 °F up to 100 °F with a single value. Below −20 °F, B16.5 does not extend the rating — the limit becomes the material's toughness, and impact testing under the applicable code decides whether the material may be used at all. That is a material qualification question, not a pressure rating one.