Pressure-Temperature Ratings

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

Standard: ASME B16.5
Material groups: 4
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	270	170	105	50
Class 400	990	900	875	845	805	755	730	710	675	550	355	230	140	70
Class 600	1480	1350	1315	1270	1205	1135	1100	1065	1015	825	535	345	205	105
Class 900	2220	2025	1970	1900	1810	1705	1650	1600	1520	1235	805	515	310	155
Class 1500	3705	3375	3280	3170	3015	2840	2745	2665	2535	2055	1340	860	515	260
Class 2500	6170	5625	5470	5280	5025	4730	4575	4440	4230	3430	2230	1430	860	430

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

Why stainless crosses over carbon steel

Carbon steel starts higher and falls off a cliff. Austenitic stainless starts lower — a Class 300 F316 flange is 720 psig at ambient against carbon steel's 740 — but at 900 °F the stainless is still holding 395 psig while the carbon steel has dropped to 170. Above roughly 650 °F 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, A216 WCB, A515 Gr 70, A516 Gr 70, A350 LF2 (as forged)…7 classes Group 1.2A350 LF2 Cl 1, A203 Gr B, A516 Gr 65, A537 Cl 1…7 classes Group 2.1A182 F304, A182 F316, A312 TP304, A312 TP316, A240 304/316…7 classes Group 2.2A182 F304L, A182 F316L, A182 F317L, A312 TP304L, A312 TP316L…7 classes

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 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.