Flange Class / Rating

ASME and ANSI: Most flanges are supplied with flanges conforming to dimensional standards developed by the American Society of Mechanical Engineers (ASME) and accredited by the American National Standards Institute (ANSI). Various committees within ASME are charged with developing, issuing and maintaining individual standards. The ASME B16 committees have this responsibility for valves, flanges and flanged fittings. Standardized dimensions for

Cast iron flanges and flanged fittings are provided in ANSI/ASME B16.1 standard
Steel and stainless steel in the ANSI/ASME B16.5 standard
Ductile iron in the ANSI/ASME B16.42 standard.

Compliance with the applicable standardized flange and flanged fitting dimensions is voluntary but ensures uniformity so that flanged valves from one manufacturer will bolt to flanges from another.
Flange Classes: There are four common pressure classes within the ANSI/ASME standards listed above. Class 125 and Class 250 are cast iron flanges and flanged fittings while Class 150 and Class 300 are ductile iron, steel or stainless steel. Hence, ANSI/ASME B16.1 Class 125 flanges are made from cast iron, ANSI/ASME B16.5 Class 150 can be made from steel or stainless and ANSI/ASME B16.42 Class 150 is made from ductile iron.
However, the bolting pattern is the same for both Class 125 and 150 so, regardless of material, they will bolt together. This is also the case for Class 250 and 300. Cast iron flanges are usually flat faced, while ductile iron and steel are usually raised face. When bolting a cast iron flange to a steel or ductile iron flange, the raised face of the mating flange should be removed to avoid breaking the cast iron flange when tightening the bolts.
Pressure and Temperature: The ANSI/ASME standards also provide pressure ratings for various classes of flanges and flanged fittings. The pressure ratings change based on the material from which it is made and/or the temperature at which it operates. The nominal flange Class (e.g., Class 125) generally represents the maximum working pressure at the temperature of saturated steam at that pressure. For example, an ANSI/ASME B16.1 Class 125 flange is rated for 125 PSI at 353^oF.
But when used at colder temperatures, such as that of water, the ANSI/ASME maximum working pressure is higher than the nominal class. ANSI/ASME B16.1 rates Class 125 flanges and flanged fittings made from high strength cast iron for 200 PSI at temperatures up to 150^o F. ANSI/ASME B16.42 rates Class 150 ductile iron flanges and fittings for 250 PSI at temperatures up to 100^oF. Complete flange and flanged fitting pressure-temperature ratings can be found in tables within the applicable ANSI/ASME B16.x standard.
AWWA: The American Water Works Association (AWWA) also has committees charged with addressing the needs of the water works industry. The AWWA C110 committee considered the ANSI/ASME pressure ratings too conservative when used on water. Through burst testing they determined flanges and fittings made from high strength cast iron and with dimensions conforming to ANSI/ASME B16.1 Class 125 could be rated for a working pressure of 250 PSI with at least a 3.0 safety factor. This allows Class 125 cast iron flanges and flanged fittings to have the same pressure rating as Pressure Class 250 ductile iron pipe. AWWA C110 does not address ANSI/ASME B16.1 Class 250 cast iron flanges or flanged fittings that have a 300 to 500 PSI or higher pressure rating and a totally different bolt pattern than Class 125. Mistakes sometimes occur because cast iron or ductile iron flanged fittings have 250 cast in them (indicating a 250 PSI pressure rating) but the flange dimensions and bolt pattern conform to ANSI/ASME B16.1 Class 125 (or ANSI/ASME B16.42 Class 150). These will not bolt to an ANSI/ASME B16.1 Class 250 flange.
AWWA Valve Pressure Ratings: ANSI/ASME B16 and AWWA C110 pressure ratings are based on flanges and flanged fittings conforming to standardized geometry and dimensions, including wall thickness. By comparison, valves are available in countless types, shapes, configurations and geometries that vary from one manufacturer to the other. While the valve’s connecting flanges may conform to ANSI/ASME B16.x dimensions, there are many reasons why its pressure rating may not be the same as a flange or flanged fitting made from identical material as the valve. It is the responsibility of the valve manufacturer to establish the pressure ratings for their products. The design professional should consult the manufacturer’s published information and not assume it is the same as the flange or fitting being bolted to it. However, the pressure ratings of many ANSI/ASME B16.x flanged valves used in waterworks service are defined in AWWA standards applicable to that type of valve. The pressure rating (or Pressure Class) listed in the AWWA valve standard may or may not conform to the ANSI/ASME B16.x or the AWWA C110 pressure rating, leading to even more confusion.
The AWWA valve standards which define a Pressure Class or pressure rating are too numerous to detail, but include C500 gate valves, C504 butterfly valves, C507 ball valves, C508 check valves and C517 eccentric plug valves. The design professional should become familiar with the applicable AWWA valve standard and select the pressure class that is adequate for the working pressure and be aware that the valve’s pressure class and connection flange class may not correspond.
Every component of a piping system should be able to withstand the highest pressure that could be realized at the location where it is installed. It’s hoped the above sheds some light on the pressure ratings for flanges and flanged valves.

Pressure - Temperature Ratings
For Steel Pipe Flanges and Flanged Fittings
American National Standard ANSI B16.5 - 1988

Class	150 lb.	300 lb.	400 lb.	600 lb.	900 lb.	1500 lb.	2500 lb.
Pres psig	450	1125	1500	2225	3350	5575	9275
Temp ^oF	Maximum Allowable Pres psig
-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

Ratings apply to materials.
SA-105^1.2 SA-515-70² SA-516-70² SA-181-70^1.2 SA-350-LF2 SA-537-C1.1³ SA-216-WCB²

NOTES:

1. For service temperatures above 850 F it is recommended that killed steels containing not less than 0.10% residual silicon be used.

2. Upon prolonged exposure to temperatures above 800 F, the carbide phase of carbon steel may be converted to graphite.

3. The material shall not be used in thickness above 2-1/2 in.

Flanges of ANSI B16.5 shall not be used for higher ratings except where it is justified by the design methods of the Code. Ratings are maximum allowable non-shock working pressures expressed as gauge pressure, at the tabulated temperatures and may be interpolated between temperatures shown. Temperatures are those on the inside of the pressure-containing shell of the flange. In general, it is the same as that of the contained material.
Working Pressure by Classes, bar

Class	150 lb.	300 lb.	400 lb.	600 lb.	900 lb.	1500 lb.	2500 lb.
Temp ^oC	Maximum Allowable Pressure, bar
-29 to 38	19	49.6	66.2	99.3	148.9	248.2	413.7
50	18.4	48.1	64.2	96.2	144.3	240.6	400.9
100	16.2	42.2	56.3	84.4	126.6	211.0	351.6
150	14.8	38.5	51.3	77.0	115.5	192.5	320.8
200	13.7	35.7	47.6	71.3	107.0	178.3	297.2
250	12.1	33.4	44.5	66.8	100.1	166.9	278.1
300	10.2	31.6	42.2	63.2	94.9	158.1	263.5
325	9.3	30.9	41.2	61.8	92.7	154.4	257.4
350	8.4	30.3	40.4	60.7	91.0	151.6	252.7
375	7.4	29.9	39.8	59.8	89.6	149.4	249.0
400	6.5	29.4	39.3	58.9	88.3	147.2	245.3
425	5.5	29.1	38.9	58.3	87.4	145.7	242.9
450	4.6	28.8	38.5	57.7	86.5	144.2	240.4
475	3.7	28.7	38.2	57.3	86.0	143.4	238.9
500	2.8	28.2	37.6	56.5	84.7	140.9	235.0
538	1.4	25.2	33.4	50.0	75.2	125.5	208.9

Material for Flanges Carbon Steel Flange
ASTM A105, ASTM A350 LF1, LF2 CL1/CL2, LF3 CL1/CL2
ASTM A694 F42, F46, F48, F50, F52, F56, F60, F65, F70

Stainless Steel Flange
ASTM A182 F304/304L, F316/316L, F316H, F310, F321
F44 (UNS S31254)

Alloy Steel Flange
ASTM A182 F1, F5a, F9, F11, F12, F22, F91

Chemical Requirements (Composition, %, max)
Material Symbol	C	Mn	P	S	Si	Ni	Cr	Mo	Cu	N	V	Co
ASTM A105	0.35	0.60-1.05	0.035	0.040	0.10-0.35	0.40	0.30	0.12	0.40		0.08
ASTM A350 LF2	0.30	0.60-1.35	0.035	0.040	0.15-0.30	0.40	0.30	0.12	0.40		0.08	0.02
ASTM A182 F304	0.08	2.00	0.045	0.030	1.00	8.0-11.0	18.0-20.0
ASTM A182 F304L	0.03	2.00	0.045	0.030	1.00	8.0-13.0	18.0-20.0
ASTM A182 F316	0.08	2.00	0.045	0.030	1.00	10.0-14.0	16.0-18.0	2.00-3.00
ASTM A182 F316L	0.03	2.00	0.045	0.030	1.00	10.0-15.0	16.0-18.0	2.00-3.00
ASTM A182 F321	0.08	2.00	0.045	0.030	1.00	9.0-12.0	17.0-19.0
EN 10025 S235JR	0.20	1.40	0.045	0.045						0.009
20Mn	0.17-0.23	0.70-1.00	0.035	0.035	0.17-0.37	0.30	0.25		0.25
20	0.17-0.23	0.60-1.00	0.030	0.020	0.17-0.37	0.25	0.25		0.25
16Mn	0.13-0.19	1.20-1.60	0.030	0.020	0.20-0.60	0.30	0.30		0.25

Mechanical Requirements
Material Symbol	Tensile Strength, min, MPa	Yield point, min, Mpa	Elongation, min, %	Reduction of area, min, %	Hardness, max, HB	Min. Impact Average, J	Test Temp, °C
ASTM A105	485	250	22	30	187
ASTM A350 LF2	485-655	250	22	30		20	-46
ASTM A182 F304	515	205	30	50
ASTM A182 F304L	485	170	30	50
ASTM A182 F316	515	205	30	50
ASTM A182 F316L	485	170	30	50
ASTM A182 F321	515	205	30	50
20Mn	450	275	24	50	197
20	410	245	25	55	156

A ASTM cross reference material specification of fittings, flanges, unions and cast and forged valves can be found in the table below:

Material	Forgings	Castings	Wrought Fittings
Carbon Steel Cold Temperature Service	A105 A350-LF2	A216-WCB	A234-WPB A420-WPL6
Carbon-1/2 Molybdenum Alloy Steel High Temperature Service	A182-F1	A217-WC1 A352-LC1	A234-WP1
3-1/2 Nickel Alloy Steel Low Temperature Service	A350-LF3	A352-LC3	A420-WPL3
1/2 Cr-1/2 Mo Alloy Steel 1/2 Cr-1/2 Mo-1 Ni Alloy 3/4 Cr-1 Mo-3/4 Ni Alloy Steel 1 Cr-1/2 Mo Alloy Steel	A182-F2 A182-F12 CL2	A217-WC4 A217-WC5	A234-WP12 CL2
1-1/4 Cr-1/2 Mo Alloy Steel 2-1/4 Cr-1 Mo Alloy Steel 5 Cr-1/2 Mo Alloy Steel 5 Cr-1/2 Mo Alloy Steel 9 Cr-1 Mo Alloy Steel 13 Cr Alloy Steel	A182-F11 CL2 A182-F22 CL3 A182-F5 A182-F5a A182-F9 A182-F6	A217-WC6 A217-WC9 A217-C5 A217-C12 A743-CA15	A234-WP11 CL2 A234-WP22 CL3 A234-WP5 A234-WP9
Type 304 Stainless Steel (18 Cr-8 Ni) Standard Low Carbon High Temperature Service	A182-F304 A182-F304L A182-F304H	A351-CF3 A351-CF8	A403-WP304 A403-WP304L A403-WP304H
Type 310 Stainless Steel (25 Cr-20 Ni) Type 316 Stainless Steel (16 Cr-12 Ni-2 Mo) Standard Low Carbon High Temperature	A182-F310H A182-F316 A182-F316L A182-F316H	A351-CK20 A351-CF3M A351-CF8M	A403-WP310 A403-WP316 A403-WP316L A403-WP316H
Type 317 Stainless Steel (18 Cr-13 Ni-3 Mo) Type 321 Stainless Steel (18 Cr-10 Ni-Ti) Standard High Temperature Service	A182-F321 A182-F321H		A403-WP317 A403-WP321 A403-WP321H
Type 347 Stainless Steel (18 Cr-10 Ni-Cb) Standard High Temperature Service	A182-F347 A182-F347H	A351-CF8C	A403-WP347 A403-WP347H
Type 348 Stainless Steel (18 Cr-10 Ni-Cb) Standard High Temperature Service	A182-F348 A182-F348H		A403-WP348 A403-WP438H