Flange Rating: Class 150 to 2500

Quick Reference

Flange Rating (Class) = maximum pressure a flange can handle at a given temperature. Standard classes: 150, 300, 400, 600, 900, 1500, 2500. As temperature increases, allowable pressure decreases. The class number is dimensionless-it does not represent a specific psi or bar value.

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Need flange dimension charts? Download the ASME B16.5 Flange Chart for dimensions by class, or the ASME B16.5 Bolt Chart for bolting requirements.

Flange Pressure Rating

What Is Flange Pressure Rating?

The flange pressure rating (Class) defines the maximum pressure (psi or bars) a flange can safely handle at specified temperatures. These ratings are standardized in ASME B16.5/B16.34 and apply to all flange types.

Term	Meaning
Class, #, Lb	Interchangeable terms for pressure rating
Class 150	Approx. 285 psi at 100°F (for A105)
Class 2500	Approx. 6170 psi at 100°F (for A105)

Key Principle

Pressure decreases as temperature increases. Always check P/T rating tables for your specific material grade and operating temperature.

Practical Example

Comparing 6” A105 carbon steel flanges at 600°F:

Class	Pressure @ 600°F	Relative Size
150	140 psi	Smallest
300	570 psi	4× stronger
2500	4730 psi	34× stronger

Key takeaways:

• Higher class = larger, heavier, stronger flange (same bore size)
• Stainless steel flanges handle more pressure than carbon steel at the same class
• Material grade matters: A182 F316 > A105 at equivalent class

Common Mistake

Assuming Class 150 means 150 psi. It doesn’t - the class number is dimensionless and comes from historical conventions. A Class 150 A105 flange is actually rated at 285 psi at ambient temperature. Always check the tables.

Flange Rating Charts (ASME B16.5/B16.34)

Carbon Steel

Carbon steel flange pressure-temperature rating ASME

ASTM A105, A350 Gr. LF2/LF6 Class 1 (High-Temp. CS)

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures (Celsius or Fahrenheit) - **in PSI. Note: **For intermediate temperatures, linear interpolation of the pressure value is allowed.

Temperature (in F°)	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	285	740	990	1480	2220	3705	6170
200	260	675	900	1350	2025	3375	5625
300	230	655	875	1315	1970	3280	5470
400	200	635	845	1270	1900	3170	5280
500	170	600	800	1200	1795	2995	4990
600	140	550	730	1095	1640	2735	4560
650	125	535	715	1075	1610	2685	4475
700	110	535	710	1065	1600	2665	4440
750	95	505	670	1010	1510	2520	4200
800	80	410	550	825	1235	2060	3430
850	65	270	355	535	805	1340	2230
900	50	170	230	345	515	860	1430
950	35	105	140	205	310	515	860
1000	20	50	70	105	155	260	430

Alloy steel flange pressure-temperature rating ASME

ASTM A182 Gr. F1 (Chrome-Moly)

The flange rating table below shows the maximum pressure for flanges of classes 150 to 2500 at increasing temperatures (Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	265	695	925	1390	2085	3470	5785
200	260	680	905	1360	2035	3395	5660
300	230	655	870	1305	1955	3260	5435
400	200	640	855	1280	1920	3200	5330
500	170	620	830	1245	1865	3105	5180
600	140	605	805	1210	1815	3025	5040
650	125	590	785	1175	1765	2940	4905
700	110	570	755	1135	1705	2840	4730
750	95	530	710	1065	1595	2660	4430
800	80	510	675	1015	1525	2540	4230
850	65	485	650	975	1460	2435	4060
900	50	450	600	900	1350	2245	3745
950	35	280	375	560	845	1405	2345
1000	20	165	220	330	495	825	1370

ASTM A182 Gr. F5 (Chrome-Moly)

The flange rating table below shows the maximum pressure for flanges of classes 150 to 2500 at increasing temperatures (Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	265	695	925	1390	2085	3470	5785
200	260	680	905	1360	2035	3395	5660
300	230	655	870	1305	1955	3260	5435
400	200	640	855	1280	1920	3200	5330
500	170	620	830	1245	1865	3105	5180
600	140	605	805	1210	1815	3025	5040
650	125	590	785	1175	1765	2940	4905
700	110	570	755	1135	1705	2840	4730
750	95	530	710	1065	1595	2660	4430
800	80	510	675	1015	1525	2540	4230
850	65	485	650	975	1460	2435	4060
900	50	450	600	900	1350	2245	3745
950	35	280	375	560	845	1405	2345
1000	20	165	220	330	495	825	1370

ASTM A182 Gr. F9 (Chrome-Moly)

The flange rating table below shows the maximum pressure for flanges of classes 150 to 2500 at increasing temperatures (Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	275	720	960	1440	2160	3600	6000
200	230	600	800	1200	1800	3000	5000
300	205	540	720	1080	1620	2700	4500
400	190	495	660	995	1490	2485	4140
500	170	465	620	930	1395	2330	3880
600	140	435	580	875	1310	2185	3640
650	125	430	575	860	1290	2150	3580
700	110	425	565	850	1275	2125	3540
750	95	415	555	830	1245	2075	3460
800	80	405	540	805	1210	2015	3360
850	65	395	530	790	1190	1980	3300
900	50	390	520	780	1165	1945	3240
950	35	380	510	765	1145	1910	3180
1000	20	320	430	640	965	1605	2675
1050	20	310	410	615	925	1545	2570
1100	20	255	345	515	770	1285	2145
1150	20	200	265	400	595	995	1655
1200	20	155	205	310	465	770	1285
1250	20	115	150	225	340	565	945
1300	20	85	115	170	255	430	715
1350	20	60	80	125	185	310	515
1400	20	50	65	95	145	240	400
1450	15	35	45	70	105	170	285
1500	10	25	35	55	80	135	230

ASTM A182 Gr. F11 (Chrome-Moly)

The flange rating table below shows the maximum pressure for flanges of classes 150 to 2500 at increasing temperatures (Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	275	720	960	1440	2160	3600	6000
200	230	600	800	1200	1800	3000	5000
300	205	540	720	1080	1620	2700	4500
400	190	495	660	995	1490	2485	4140
500	170	465	620	930	1395	2330	3880
600	140	435	580	875	1310	2185	3640
650	125	430	575	860	1290	2150	3580
700	110	425	565	850	1275	2125	3540
750	95	415	555	830	1245	2075	3460
800	80	405	540	805	1210	2015	3360
850	65	395	530	790	1190	1980	3300
900	50	390	520	780	1165	1945	3240
950	35	380	510	765	1145	1910	3180
1000	20	320	430	640	965	1605	2675
1050	20	310	410	615	925	1545	2570
1100	20	255	345	515	770	1285	2145
1150	20	200	265	400	595	995	1655
1200	20	155	205	310	465	770	1285
1250	20	115	150	225	340	565	945
1300	20	85	115	170	255	430	715
1350	20	60	80	125	185	310	515
1400	20	50	65	95	145	240	400
1450	15	35	45	70	105	170	285
1500	10	25	35	55	80	135	230

ASTM A182 Gr. F22 (Chrome-Moly)

The flange rating table below shows the maximum pressure for flanges of classes 150 to 2500 at increasing temperatures (Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	265	695	925	1390	2085	3470	5785
200	260	680	905	1360	2035	3395	5660
300	230	655	870	1305	1955	3260	5435
400	200	640	855	1280	1920	3200	5330
500	170	620	830	1245	1865	3105	5180
600	140	605	805	1210	1815	3025	5040
650	125	590	785	1175	1765	2940	4905
700	110	570	755	1135	1705	2840	4730
750	95	530	710	1065	1595	2660	4430
800	80	510	675	1015	1525	2540	4230
850	65	485	650	975	1460	2435	4060
900	50	450	600	900	1350	2245	3745
950	35	280	375	560	845	1405	2345
1000	20	165	220	330	495	825	1370

Stainless Steel / Duplex

Stainless steel flange pressure-temperature rating ASME

ASTM A182 Gr. F304/304L

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures (Celsius or Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	275	720	960	1440	2160	3600	6000
200	230	600	800	1200	1800	3000	5000
300	205	540	720	1080	1620	2700	4500
400	190	495	660	995	1490	2485	4140
500	170	465	620	930	1395	2330	3880
600	140	435	580	875	1310	2185	3640
650	125	430	575	860	1290	2150	3580
700	110	425	565	850	1275	2125	3540
750	95	415	555	830	1245	2075	3460
800	80	405	540	805	1210	2015	3360
850	65	395	530	790	1190	1980	3300
900	50	390	520	780	1165	1945	3240
950	35	380	510	765	1145	1910	3180
1000	20	320	430	640	965	1605	2675
1050	20	310	410	615	925	1545	2570
1100	20	255	345	515	770	1285	2145
1150	20	200	265	400	595	995	1655
1200	20	155	205	310	465	770	1285
1250	20	115	150	225	340	565	945
1300	20	85	115	170	255	430	715
1350	20	60	80	125	185	310	515
1400	20	50	65	95	145	240	400
1450	15	35	45	70	105	170	285
1500	10	25	35	55	80	135	230

ASTM A182 Gr. F316/316L

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures (Celsius or Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	275	720	960	1440	2160	3600	6000
200	235	620	825	1240	1860	3095	5160
300	215	560	745	1120	1680	2795	4660
400	195	515	685	1025	1540	2570	4280
500	170	480	635	955	1435	2390	3980
600	140	450	600	900	1355	2255	3760
650	125	445	590	890	1330	2220	3700
700	110	430	580	870	1305	2170	3620
750	95	425	570	855	1280	2135	3560
800	80	420	565	845	1265	2110	3520
850	65	420	555	835	1255	2090	3480
900	50	415	555	830	1245	2075	3460
950	35	385	515	775	1160	1930	3220
1000	20	350	465	700	1050	1750	2915
1050	20	345	460	685	1030	1720	2865
1100	20	305	405	610	915	1525	2545
1150	20	235	315	475	710	1185	1970
1200	20	185	245	370	555	925	1545
1250	20	145	195	295	440	735	1230
1300	20	115	155	235	350	585	970
1350	20	95	130	190	290	480	800
1400	20	75	100	150	225	380	630
1450	20	60	80	115	175	290	485
1500	20	40	55	85	125	205	345

ASTM A182 Gr. F321/321L

The flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures (Celsius or Fahrenheit) - in PSI

Temperature In °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	275	720	960	1440	2160	3600	6000
200	230	600	800	1200	1800	3000	5000
300	205	540	720	1080	1620	2700	4500
400	190	495	660	995	1490	2485	4140
500	170	465	620	930	1395	2330	3880
600	140	435	580	875	1310	2185	3640
650	125	430	575	860	1290	2150	3580
700	110	425	565	850	1275	2125	3540
750	95	415	555	830	1245	2075	3460
800	80	405	540	805	1210	2015	3360
850	65	395	530	790	1190	1980	3300
900	50	390	520	780	1165	1945	3240
950	35	380	510	765	1145	1910	3180
1000	20	320	430	640	965	1605	2675
1050	20	310	410	615	925	1545	2570
1100	20	255	345	515	770	1285	2145
1150	20	200	265	400	595	995	1655
1200	20	155	205	310	465	770	1285
1250	20	115	150	225	340	565	945
1300	20	85	115	170	255	430	715
1350	20	60	80	125	185	310	515
1400	20	50	65	95	145	240	400
1450	15	35	45	70	105	170	285
1500	10	25	35	55	80	135	230

ASTM A182 Gr. F347/347L

flange rating table shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures (Celsius or Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	275	720	960	1440	2160	3600	6000
200	230	600	800	1200	1800	3000	5000
300	205	540	720	1080	1620	2700	4500
400	190	495	660	995	1490	2485	4140
500	170	465	620	930	1395	2330	3880
600	140	435	580	875	1310	2185	3640
650	125	430	575	860	1290	2150	3580
700	110	425	565	850	1275	2125	3540
750	95	415	555	830	1245	2075	3460
800	80	405	540	805	1210	2015	3360
850	65	395	530	790	1190	1980	3300
900	50	390	520	780	1165	1945	3240
950	35	380	510	765	1145	1910	3180
1000	20	320	430	640	965	1605	2675
1050	20	310	410	615	925	1545	2570
1100	20	255	345	515	770	1285	2145
1150	20	200	265	400	595	995	1655
1200	20	155	205	310	465	770	1285
1250	20	115	150	225	340	565	945
1300	20	85	115	170	255	430	715
1350	20	60	80	125	185	310	515
1400	20	50	65	95	145	240	400
1450	15	35	45	70	105	170	285
1500	10	25	35	55	80	135	230

ASTM A182 Gr. F51 (Duplex Stainless Steel)

The flange rating table below shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures (Celsius) - in bars

Temp, ℃	150#	300#	400#	600#	900#	1500#	2500#
-29 to 38	20.0	51.7	68.9	103.4	155.1	258.6	430.9
50	19.5	51.7	68.9	103.4	155.1	258.6	430.9
100	17.7	50.7	67.5	101.3	152.0	253.3	422.2
150	15.8	45.9	61.2	91.9	137.8	229.6	382.7
200	13.8	42.7	56.9	85.3	128.0	213.3	355.4
250	12.1	40.5	53.9	80.9	121.4	202.3	337.2
300	10.2	38.9	51.8	77.7	116.6	194.3	323.8

ASTM A182 Gr. F513/F55 (SuperDuplex Stainless Steel)

The flange rating table below shows the maximum pressure for flanges of classes 150/300/400/600/900/1500/2500 at increasing temperatures (Fahrenheit) - in PSI

Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	275	720	960	1440	2160	3600	6000
200	230	600	800	1200	1800	3000	5000
300	205	540	720	1080	1620	2700	4500
400	190	495	660	995	1490	2485	4140
500	170	465	620	930	1395	2330	3880
600	140	435	580	875	1310	2185	3640
650	125	430	575	860	1290	2150	3580
700	110	425	565	850	1275	2125	3540
750	95	415	555	830	1245	2075	3460
800	80	405	540	805	1210	2015	3360
850	65	395	530	790	1190	1980	3300
900	50	390	520	780	1165	1945	3240
950	35	380	510	765	1145	1910	3180
1000	20	320	430	640	965	1605	2675
1050	20	310	410	615	925	1545	2570
1100	20	255	345	515	770	1285	2145
1150	20	200	265	400	595	995	1655
1200	20	155	205	310	465	770	1285
1250	20	115	150	225	340	565	945
1300	20	85	115	170	255	430	715
1350	20	60	80	125	185	310	515
1400	20	50	65	95	145	240	400
1450	15	35	45	70	105	170	285
1500	10	25	35	55	80	135	230

Flange Rating Selection

Selection Steps

Step	Action
1. Define requirements	Max operating pressure, max temperature, fluid type/corrosiveness
2. Select material	Choose from ASME B16.5 material groups (44 options) based on P/T and corrosion needs
3. Check P/T tables	Use tables below to verify class handles your conditions
4. Apply safety margin	Select class exceeding max anticipated P/T for unexpected spikes
5. Plan for future	Account for potential system upgrades

ASME B16.5 Material Groups

Group 2-1.1 (Carbon Steel)

Designation	Forgings	Castings	Plates
C-Si	A105	A216 WCB	A515 Gr.70
C Mn Si	A350 LF2	-	A516 Gr.70
3½Ni	A350 LF3	-	-

Group 2-2.3 (Stainless Steel)

Designation	Forgings	Plates
16Cr 12Ni 2Mo	A182 F316L	A240 316L
18Cr 8Ni	A182 F304L	A240 304L

Critical Selection Factors

Material choice is the most important decision-it determines the P/T capability. Different materials at the same class have different allowable pressures. Always verify against the P/T tables for your specific grade.

Pro Tip

When selecting flange class, always check the P/T rating at your maximum operating temperature, not ambient. A Class 150 flange rated at 285 psi at 100°F drops to 140 psi at 600°F - a 50% reduction that catches many engineers off guard.

Proper flange connections also require correctly specified gaskets and stud bolts.

Frequently Asked Questions

How do flange pressure ratings change with temperature?

Flange pressure ratings decrease as temperature increases. At ambient temperature (38°C / 100°F), a Class 150 carbon steel (A105) flange handles 285 psi. At 250°C (482°F), this drops to about 185 psi. At 400°C (752°F), it drops to approximately 100 psi. The exact derating depends on the flange material group ; stainless steels and alloy steels retain higher pressures at elevated temperatures than carbon steel.

How do I convert ASME flange class to PN rating?

The approximate equivalences are: Class 150 ≈ PN 20, Class 300 ≈ PN 50, Class 600 ≈ PN 100, Class 900 ≈ PN 150, Class 1500 ≈ PN 250, Class 2500 ≈ PN 420. Note: these are not exact equivalents ; ASME and EN standards use different P-T rating methodologies, materials, and dimensional tolerances. Never assume a PN-rated flange is interchangeable with an ASME-rated flange without verifying dimensions, bolt holes, and pressure-temperature compatibility.

What are ASME B16.5 material groups and why do they matter?

ASME B16.5 organizes flange materials into groups (1.1, 1.2, 1.3, etc.) based on their mechanical properties at elevated temperatures. Each group has its own pressure-temperature rating table. For example, A105 (carbon steel) falls in Group 1.1, while A182 F304 (stainless steel) falls in Group 2.1. The material group determines the allowable pressure at every temperature point ; choosing the wrong group can lead to under-rated or over-rated flanges.

What happens if I use a flange above its rated temperature?

Operating a flange above its rated pressure-temperature envelope risks creep deformation, bolt relaxation, gasket failure, and ultimately a flange leak or catastrophic rupture. Even a few degrees above the rated temperature can reduce the allowable pressure below the operating pressure. Always design with a safety margin and account for process upsets, startup conditions, and blocked-in scenarios where temperature or pressure can spike.