Flange Derating Factors

Flange derating factors quantify the reduction in allowable pressure as the service temperature increases above ambient. The derating factor is the ratio of the pressure rating at the elevated temperature to the pressure rating at ambient temperature. This factor is useful for quick assessments during preliminary piping design and for comparing the temperature sensitivity of different flange materials.

Flange Derating Factors Table

The derating factor is calculated as: DF = P(T) / P(ambient), where P(T) is the rated pressure at temperature T and P(ambient) is the rated pressure at -20 to 100°F.

Temperature	A105 (Group 1.1)	F304 (Group 2.1)	F316 (Group 2.3)	F11 (Group 1.9)
Ambient	1.000	1.000	1.000	1.000
200°F (93°C)	0.912	0.861	0.861	1.000
300°F (149°C)	0.885	0.819	0.833	0.980
400°F (204°C)	0.858	0.708	0.736	0.960
500°F (260°C)	0.811	0.646	0.701	0.940
600°F (316°C)	0.743	0.597	0.667	0.913
700°F (371°C)	0.723	0.556	0.632	0.893
800°F (427°C)	0.554	0.507	0.597	0.867
900°F (482°C)	0.365	0.465	0.549	0.773
1,000°F (538°C)	0.230	0.431	0.479	0.600

Derating factors are class-independent. The same factor applies to Class 150, 300, 600, or any other class for a given material group.

Calculation Method

To determine the derated pressure at any temperature:

1. Look up the ambient-temperature rating for the desired class and material group from ASME B16.5.
2. Find the derating factor for the service temperature (from the table above or by calculating from the PT table).
3. Multiply: Derated Pressure = Ambient Rating x Derating Factor.

Example: Class 600, A105, at 600°F:

• Ambient rating: 1,480 psig
• Derating factor at 600°F: 0.743
• Derated pressure: 1,480 x 0.743 = 1,100 psig (actual table value: 1,095 psig; the small difference is due to rounding)

Material Comparison at Key Temperatures

Temperature	Best Material	Worst Material	Difference
400°F	F11 (DF = 0.960)	F304 (DF = 0.708)	36%
600°F	F11 (DF = 0.913)	F304 (DF = 0.597)	53%
800°F	F11 (DF = 0.867)	A105 (DF = 0.554)	57%
1,000°F	F11 (DF = 0.600)	A105 (DF = 0.230)	161%

Alloy steel F11 consistently retains the highest proportion of its ambient rating across all elevated temperatures, making it the most temperature-resistant common flange material.

Pro Tip

Derating factors are useful for preliminary class selection during the front-end engineering phase. If the process design calls for temperatures above 600°F with carbon steel flanges, the derating factor drops below 0.75, meaning the flange retains less than 75% of its cold rating. This is often the threshold where alloy steel should be evaluated as an alternative to upgrading the pressure class.

For complete PT rating tables and material group assignments, see the ASME flange rating guide.

Read the full guide to flange ratings