Flange Pressure Rating Explained (and Charts)

What means ANSI flange pressure rating? How to select the right flange rating?
The term “flange rating” (or class) refers to the maximum pressure (in psi or bars) that a flange withstands at increasing temperatures. Flanges with a higher rating (class) are stronger than flanges with lower ratings, as they bear more pressure at increasing temperatures. Flanges made in different materials show different pressure-temperature performance at the same rating. In the article, you find the pressure rating charts for carbon, alloy, and stainless steel flanges and some guidance about selecting the proper rating for your piping application.

ANSI FLANGE PRESSURE RATING EXPLAINED

Pressure rating is defined as the maximum allowed pressure that a flange can withstand at increasing temperatures. According to the ANSI/ASME B16.5 specification, there are seven flange pressure ratings: 150, 300, 400, 600, 900, 1500, and 2500.

The terms “pressure rating”, “class”, “#”, “Lb” or “Lbs” are interchangeable, i.e. they all refer to the same exact concept of pressure/temperature performance of a flange (and other equipment like valves, fittings, etc).

Let’s make this clear with an example:

If two flanges have the same bore size (example 6 inches), the same material (example A105), but two different pressure ratings (one flange is class 150, the other 300), the lower rated flange (class 150) will be smaller, lighter, and less robust than the higher rated flange (class 300). This is represented in the picture:

Now, let’s look at what this means in terms of pressure rating:

• the class 150 flange withstands just 140 psi at a temperature of 600 degrees F° (as per the rating chart below)
• the class 300 flange (bigger and stronger, but with the same bore size) withstands 570 psi at the same temperature of 600 degrees F°
• finally, a class 2500 flange of the same size bears 34 times more pressure than the class 150 rated flange, reaching a whopping 4730 psi rating at 600 F°!

So, how to select the proper flange rating?

1. Find the rating table that applies to your flange scrolling below (this depends on the material of the flange, as flanges with different material grades have different pressure ratings)
2. Determine the max working temperature in your piping system (i.e. select one line in the table)
3. Select a rating based on the maximum pressure you expect at that temperature level (i.e. select one column in that line)
4. Voilà, you have the required rating!

To help you do this easily, find below the ASME B16.34 pressure rating charts for the most common flange materials (carbon, alloy, stainless).

CARBON STEEL FLANGE RATING CHART

ANSI FLANGE ASTM A105, A350 Gr. LF2/LF6 Class 1

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

ANSI/ASME B16.34	ANSI PRESSURE RATING
Temperature (in F°)	150#	300#	400#	600#	900#	1500#	2500#
< 100	285	740	985	1480	2220	3705	6170
200	260	680	905	1360	2035	3395	5655
300	230	655	870	1310	1965	3270	5450
400	200	635	845	1265	1900	3170	5280
500	170	605	805	1205	1810	3015	5025
600	140	570	755	1135	1705	2840	4730
650	125	550	730	1100	1650	2745	4575
700	110	530	710	1060	1590	2655	4425
750	95	505	675	1015	1520	2535	4230
800	80	410	550	825	1235	2055	3430
850	65	320	425	640	955	1595	2655
900	50	230	305	460	690	1150	1915
950	35	135	185	275	410	685	1145
1000	20	85	115	170	255	430	715
Hydrostatic Test Pressure (in Psig)	450	1125	1500	2225	3350	5575	9275

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 bars

ANSI/ASME B16.34	ANSI PRESSURE RATING
Temperature in C°	150#	300#	400#	600#	900#	1500#	2500#
-29 / 38	19.6	51.1	68.1	102.1	153.2	255.3	425.5
50	19.2	50.1	66.8	100.2	150.4	250.6	417.7
100	17.7	46.6	62.1	93.2	139.8	233	388.3
150	15.8	45.1	60.1	90.2	135.2	225.4	375.6
200	13.8	43.8	58.4	87.6	131.4	219	365
250	12.1	41.9	55.9	83.9	125.8	209.7	349.5
300	10.2	39.8	53.1	79.6	119.5	199.1	331.8
325	9.3	38.7	51.6	77.4	116.1	193.6	322.6
350	8.4	37.6	50.1	75.1	112.7	187.8	313
375	7.4	36.4	48.5	72.7	109.1	181.8	303.1
400	6.5	34.7	46.3	69.4	104.2	173.6	289.3
425	5.5	28.8	38.4	57.5	86.3	143.8	239.7
450	4.6	23	30.7	46	69	115	191.7
475	3.7	17.4	23.2	34.9	52.3	87.2	145.3
500	2.8	11.8	15.7	23.5	35.3	58.8	97.9

Notes:

• ASTM A105: An extended exposure to temperatures above 425°C, converts the carbide phase of steel to graphite (therefore this material is not recommended for constant temperatures above this value.
• ASTM A350 LF6: shall not be used for temperatures above 260 C°

ANSI FLANGE ASTM A350 Gr. LF3, A350 LF6, Class 2

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

ANSI/ASME B16.34	ANSI PRESSURE RATING
Temperature in °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	290	750	1000	1500	2250	3750	6250
200	260	750	1000	1500	2250	3750	6250
300	230	730	970	1455	2185	3640	6070
400	200	705	940	1410	2115	3530	5880
500	170	665	885	1330	1995	3325	5540
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	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

ANSI FLANGE ASTM A350 Gr. LF1 

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

ANSI/ASME B16.34	ANSI PRESSURE RATING
Temperature °F	150#	300#	400#	600#	900#	1500#	2500#
-20 to 100	235	620	825	1235	1850	3085	1545
200	215	560	750	1125	1685	2810	4680
300	210	550	730	1095	1640	2735	4560
400	200	530	705	1060	1585	2645	4405
500	170	500	665	995	1495	2490	4150
600	140	455	610	915	1370	2285	3805
650	125	450	600	895	1345	2245	3740
700	110	450	600	895	1345	2245	3740
750	95	445	590	885	1325	2210	3685
800	80	370	495	740	1110	1850	3085
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 RATING CHART

ANSI FLANGE ASTM A182 Gr. F1 (Chrome Moly)

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

ANSI/ASME B16.34	ANSI PRESSURE RATING
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 FLANGE RATING CHART

ANSI FLANGE 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

ANSI/ASME B16.34	ANSI PRESSURE RATING
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

ANSI FLANGE 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

ANSI/ASME B16.34	ANSI PRESSURE RATING
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

HOW TO SELECT FLANGE RATING

If you are still in doubt about how to select the flange rating, take a look at this useful video: