Pressure-Temperature Ratings

What a flange class is actually rated for, at your material and your metal temperature. 8 ASME B16.5 material groups, seven pressure classes, -20 °F to 1000 °F.

Standard: ASME B16.5
Material groups: 8
Pressure classes: 7
Temperature range: -20 °F to 1000 °F
Units: psig

A class is not a pressure

The single most persistent misconception in flange specification: a Class 150 flange is not rated for 150 psi. In A105 carbon steel it is rated 285 psig at 100 °F, and 140 psig at 600 °F. The class number is a designation carried over from the historic saturated-steam rating of the flange, and the real allowable pressure comes from the table for your material group at your metal temperature.

Carbon steel — Group 1.1 (A105)

The most-used table on this page: standard carbon steel flanges at every class and temperature. Pressures are psig.

ASME B16.5 pressure-temperature ratings for Group 1.1 materials (A105, A216 WCB), in psig.
Class	100 °F	200 °F	300 °F	400 °F	500 °F	600 °F	650 °F	700 °F	750 °F	800 °F	850 °F	900 °F	950 °F	1000 °F
Class 150	285	260	230	200	170	140	125	110	95	80	65	50	35	20
Class 300	740	680	655	635	605	570	550	530	505	410	320	230	135	85
Class 400	985	905	870	845	805	755	730	710	675	550	425	305	185	115
Class 600	1480	1360	1310	1265	1205	1135	1100	1060	1015	825	640	460	275	170
Class 900	2220	2035	1965	1900	1810	1705	1650	1590	1520	1235	955	690	410	255
Class 1500	3705	3395	3270	3170	3015	2840	2745	2655	2535	2055	1595	1150	685	430
Class 2500	6170	5655	5450	5280	5025	4730	4575	4425	4230	3430	2655	1915	1145	715

Interpolation between listed temperatures is permitted. Extrapolation beyond the table is not.

Why stainless crosses over carbon steel

Carbon steel starts higher and falls away steeply once it is hot. Austenitic stainless starts lower — a Class 300 flange in Type 316 is 720 psig at ambient against carbon steel's 740 — but at 900 °F the stainless is still holding 415 psig while the carbon steel has dropped to 230. From 800 °F upward the stainless is the stronger flange, which is why high-temperature service specifies it even where corrosion is not the concern.

All material groups

Group 1.1A105, A350 LF2, A350 LF6 Cl. 1, A350 LF3, A216 WCB, A515 Gr.…7 classes Group 1.2A350 LF6 Cl. 2, A216 WCC, A352 LCC, A352 LC2, A352 LC3, A203…7 classes Group 2.1A182 F304, A182 F304H, A351 CF3, A351 CF8, A240 304, A240 30…7 classes Group 2.2A182 F316, A182 F316H, A182 F317, A351 CF3M, A351 CF8M, A351…7 classes Group 2.3A182 F304L, A182 F316L, A182 F317L, A240 304L, A240 316L…7 classes Group 1.9A182 F11 Cl. 2, A217 WC6, A387 Gr. 11 Cl. 2…7 classes Group 1.10A182 F22 Cl. 3, A217 WC9, A387 Gr. 22 Cl. 2…7 classes Group 1.15A182 F91, A217 C12A, A387 Gr. 91 Cl. 2…7 classes

Guides and comparisons for this page

Pressure-temperature deratingReading a P-T table and interpolating between temperatures.Hydrostatic test pressureWhat a line is tested at, and what its flanges allow.Class 150 vs Class 300Dimensions, bolting and ratings of the two commonest classes.Class 300 vs Class 600What doubling the class does to the flange.

Common questions

What pressure is a Class 150 flange rated for?

285 psig at 100 °F in A105 carbon steel, falling to 140 psig at 600 °F and 20 psig at 1000 °F. In Type 316 stainless it is 275 psig at 100 °F. The class number itself is not a pressure.

Can I interpolate between temperatures in the table?

Yes. ASME B16.5 explicitly permits linear interpolation between listed temperatures. It does not permit extrapolation beyond the ends of the table.

Which temperature do I use — the fluid or the metal?

The metal temperature of the pressure-retaining part. For uninsulated pipe carrying a hot fluid these are close enough that the fluid temperature is used, but where they differ meaningfully the flange is rated on its own metal.

Do these ratings apply to the pipe as well as the flange?

No. B16.5 rates flanges and flanged fittings. Pipe pressure capability is calculated from wall thickness and allowable stress under the applicable code, usually ASME B31.3 or B31.1, and is often the higher of the two — leaving the flange as the limiting component.