1 SCOPE
1.1 General
This Standard covers pressure–temperature ratings, materials, dimensions, tolerances, marking, and testing for pipe flanges in sizes NPS 26 through NPS 60. Included are flanges with rating class designations 75, 150, 300, 400, 600, and 900 with requirements given in both SI (Metric) and U.S. Customary units, with diameter of bolts and flange bolt holes expressed in inch units.
This Standard is limited to
(a) flanges made from cast or forged materials
(b) blind flanges made from cast, forged, or plate materials (see Tables 1 and 2)
Also included in this Standard are requirements and recommendations regarding flange bolting, flange gaskets, and flange joints.
1.2 Flange Series
This Standard provides two series of flange dimensions. Series A specifies flange dimensions for general use flanges. Series B specifies flange dimensions for compact flanges that, in most cases, have smaller bolt circle diameters than Series A flanges.
These two series of flanges are, in general, not interchangeable. The user should recognize that some flanged valves, equipment bolted between flanges, and flanged equipment may be compatible with only one series of these flanges.
1.3 References
Codes, standards, and specifications, containing provisions to the extent referenced herein, constitute requirements of this Standard. These references are listed in Mandatory Appendix III.
1.4 Time of Purchase, Manufacture, or Installation
The pressure–temperature ratings in this Standard are applicable upon its publication to all flanges within its scope that otherwise meet its requirement. For unused flanges maintained in inventory, the manufacturer of the flange may certify conformance to this edition, provided that it can be demonstrated that all requirements of this edition have been met.
Where such components were installed in accordance with the pressure–temperature ratings of an earlier edition of this Standard, those ratings are applicable, except as may be governed by the applicable code or regulation.
1.5 User Accountability
This Standard cites responsibilities that are to be assumed by the flange user in the areas of, for example
(a)Application
(b) installation
(c) system pressure testing
(d) operation
(e) material selection
1.6 Quality Systems
Requirements relating to the product manufacturer’s quality system program are described in Mandatory Appendix II.
1.7 Relevant Units
This Standard states values in both SI (Metric) and U.S. Customary units. As an exception, diameter of bolts and flange bolt holes are expressed in inch units only. These systems of units are to be regarded separately as standard. Within the text, the U.S. Customary units are shown in parentheses or in separate tables that appear in Mandatory Appendix I.
The values stated in each system are not exact equivalents; therefore, it is required that each system of units be used independently of the other. Except for diameter of bolts and flange bolt holes, combining values from the two systems constitutes nonconformance with the Standard.
1.8 Selection of Materials
Criteria for selection of materials suitable for particular fluid service are not within the scope of this Standard.
1.9 Convention
For determining conformance with this Standard, the convention for fixing significant digits where limits (maximum and minimum values) are specified shall be as defined in ASTM Practice E29. This requires that an observed or calculated value be rounded off to the nearest unit in the last right-hand digit used for expressing the limit. Decimal values and tolerances do not imply a particular method of measurement.
1.10 Denotation
1.10.1 Pressure Rating Designation.
Class, followed by a dimensionless number, is the designation for pressure–temperature ratings (i.e., Class 75, Class 150, Class 300, Class 400, Class 600, Class 900).
1.10.2 Size.
NPS, followed by a dimensionless number, is the designation for nominal flange size. NPS is related to the reference nominal diameter, DN, used in international and other standards. For the sizes covered in this Standard, the relationship is DN p 25  NPS.
2 PRESSURE–TEMPERATURE RATINGS
2.1 General
Pressure–temperature ratings are maximum allowable working gage pressures, in bar units, at the temperatures in degrees Celsius shown in Tables 3 through 28 for the applicable material and class designation. Tables I-1 through I-26 of Mandatory Appendix I lists pressure–temperature ratings using pounds per square inch (psi) units for pressure at the temperature in degrees Fahrenheit. For intermediate temperatures, linear interpolation is permitted. Interpolation between class designations is not permitted.
2.2 Flanged Joints
A flanged joint is composed of separate and independent, although interrelated, components: the flanges, the gasket, and the bolting, which are assembled by another influence, the assembler. Proper controls must be exercised in the selection and application for all of these elements to attain a joint that has acceptable leak tightness. Assembly and tightening techniques, such as controlled bolt tightening, are described in ASME PCC-1.
2.3 Ratings of Flanged Joints
2.3.1 Basis.
Pressure–temperature ratings apply to flanged joints that conform to the limitations on bolting in para. 5.3 and on gaskets in para. 5.4, and that are made up in accordance with good practice for alignment and assembly (see para. 2.2). Use of these ratings for flanged joints not conforming to these limitations is the responsibility of the user.
2.3.2 Mixed Flanged Joints.
If the two flanges in a flanged joint do not have the same pressure–temperature rating, the rating of the joint at any temperature is the lower of the two flange ratings at that temperature.
2.4 Rating Temperature
The temperature shown for a corresponding pressure rating is the temperature of the pressure containing shell of the component. In general, this temperature is the same as that of the contained fluid. Use of a pressure rating corresponding to a temperature other than that 2 of the contained fluid is the responsibility of the user, subject to the requirements of applicable codes and regulations. For any temperature below −29°C (−20°F) the rating shall be no greater than the rating shown for −29°C (−20°F). See also para. 2.5.3.
2.5 Temperature Considerations
2.5.1 General.
Use of flanges at either high or low temperatures shall take into consideration the risk of joint leakage due to forces and moments developed in the connected piping or equipment. Provisions in paras. 2.5.2 and 2.5.3 are included as advisory with the aim of lessening these risks.
2.5.2 High Temperature.
Application at temperatures in the creep range will result in decreasing bolt loads as relaxation of flanges, bolts, and gaskets takes pla
ce. Flanged joints subjected to thermal gradients may likewise be subject to decreasing bolt loads. Decreased bolt loads diminish the capacity of the flanged joint to sustain loads effectively without leakage. At temperatures above 200°C (400°F) for Classes 75 and 150, and above 400°C (750°F) for other class designations, flanged joints may develop leakage problems unless care is taken to avoid imposing severe external loads and/or severe thermal gradients.
2.5.3 Low Temperature.
Some of the materials listed in Tables 1 and 2, notably some carbon steels, may undergo a decrease in ductility when used at low temperatures to such an extent as to be unable to safely resist shock loading, sudden changes of stress, or high stress concentration. Some codes or regulations may require impact testing for applications even where temperatures are higher than −29°C (−20°F). When such requirements apply, it is the responsibility of the user to ensure these requirements are communicated to the manufacturer prior to the time of purchase.
2.6 System Pressure Testing
Flanged joints may be subjected to system pressure tests at a pressure of 1.5 times the 38°C (100°F) rating rounded off to the next higher 1 bar (25 psi) increment. Testing at any higher pressure is the responsibility of the user, taking into account the requirements of the applicable code or regulation.
2.7 Welding Neck Flanges
2.7.1 Maximum Bore Size.
Ratings forwelding neck flanges covered by this Standard are based upon their hubs at thewelding end having a thickness at least equal to that calculated for pipe having a 276 MPa (40,000 psi) specified minimum yield strength. To ensure adequate flange hub thickness for flange sizes NPS 26 and larger, the bore of awelding neck flange, dimension B as shown in Figs. 1 and 2.
2.7.2 Components of Unequal Strength.
The tabulated ratings for welding neck flanges are independent of components of unequal strength or unequal wall thickness to which they may be attached. For all attachments, the pressure rating of the flange shall not be exceeded.
2.7.3 Attachment Welds.
Attachment welds should be made in accordance with the applicable code or regulation. See para. 6.4 and Fig. 3 for weld end dimensional requirements.
2.8 Multiple Material Grades
Materials for flanges may meet the requirements of more than one specification or the requirements of more than one grade of a specification listed in Table 1. In either case, the pressure–temperature ratings for any of these specifications or grades may be used provided that the material is marked in accordance with para. 4.2.8.
3 COMPONENT SIZE: NOMINAL PIPE SIZE
As applied in this Standard, the use of the phrase “nominal pipe size,” or the designation NPS followed by a dimensionless number, is for the purpose of pipe or flange end connection size identification. The number is not the same as the flange inside diameter.
4 MARKING
4.1 General
Except as modified herein, flanges shall be marked as required in MSS SP-25, except as noted in para. 4.2.
4.2 Identification Markings
4.2.1 Name.
The manufacturer’s name or trademark shall be applied. 3
4.2.2 Materials.
Materials shall be identified in the following ways:
(a)Cast flanges shall be marked with the ASTM specification, 1 grade identification symbol (letters and numbers), and the melt number or melt identification.
(b) Plate flanges and forged flanges shall be marked with the ASTM specification1 number and grade identification symbol.
(c) Amanufacturer may supplement these mandatory material markings with his trade designation for the material grade, but confusion of symbols shall be avoided.
(d) For flanges manufactured from material that meets the requirements for more than one specification or grade of a specification listed in Table 1, see para. 4.2.8.
4.2.3 Rating Designation.
The flange shall be marked with the number that corresponds to its pressure rating class designation (i.e., 75, 150, 300, 400, 600, or 900).
4.2.4 Conformance.
The designation B16 or B16.47 shall be applied to the flange, preferably located adjacent to the class designation, to indicate conformance to this Standard. The use of the prefix “ASME” is optional.
4.2.5 Temperature.
Temperature markings are not required on flanges. However, if marked, the temperature shall be shown with its corresponding tabulated pressure rating for the material.
4.2.6 Size.
The NPS identification number shall be marked on flanges.
4.2.7 Ring-Joint Flange.
The edge (periphery) of each ring-joint flange shall be marked with the letter R and the corresponding ring-groove number.
4.2.8 Multiple Material Marking.
Material for components that meet the requirements for more than one ASTM specification number or grade of a specification listed in Table 1 may, at the manufacturer’s option, be marked with more than one of the applicable specification numbers or grade symbols. These identification markings shall be placed so as to avoid confusion in identification. The multiple marking shall be in accordance with the guidelines set out in ASME Boiler and Pressure Vessel Code, Section II, Part D, Appendix 7.
5 MATERIALS
5.1 General
(a)Materials required for flanges are listed in Table 1 with the restriction that plate materials shall be used only for blind flanges. Flanges shall be manufactured as one piece in accordance with the applicable material specification. Assembly of multiple pieces into the finished product by welding or other means is not permitted by this Standard.
(b) Recommended bolting materials are listed in Table 2 (see para. 5.3).
(c) Corresponding materials listed in the ASME Boiler and Pressure Vessel Code, Section II may be used provided that the requirements of the ASME specification are identical or more stringent than the ASTM specification for the Grade, Class, or type of material.
5.1.1 Application.
Criteria for the selection of materials are not within the scope of this Standard. The possibility of material deterioration in service should be considered by the user. Carbide phase conversion to graphite and excessive oxidation of ferritic materials, susceptibility to intergranular corrosion of austenitic materials, or grain boundary attack of nickel based alloys are among those items requiring attention.
A detailed discussion of precautionary considerations can be found in
(a)ASME B31.3, Appendix F
(b) ASME Boiler and Pressure Vessel Code Section II, Part D, Appendix 6
(c) ASME Boiler and Pressure Vessel Code Section III, Division 1, Appendix W
5.1.2 Responsibility.
When service conditions dictate the implementation of special material requirements [e.g., using a Group 2 material above 538°C (1,000°F)], it is the user’s responsibility to so specify to the manufacturer to ensure compliance with metallurgical requirements listed in the notes in Tables 3 through 28 (Tables I-1 through I-26).
5.1.3 Cast Surfaces.
Cast surfaces of flange pressure boundaries shall be in accordance with MSS SP-55, except that all Type I defects are unacceptable, and defects in excess of Plates “a” and “b” for Type II through Type XII are unacceptable.
5.2 Mechanical Properties
Mechanical properties shall be obtained from test specimens that represent the final heat-treated condition of the material required by the material specification.
5.3 Bolting
5.3.1 General.
Bolting listed
in Table 2 is recommended for use with flanges covered by this Standard. Bolting of other material may be used if permitted by the applicable code or government regulation. Bolting materials are subject to the limitations given in paras. 5.3.2 through 5.3.5.
5.3.2 High Strength Bolting.
Bolting materials having allowable stresses not less than those for ASTM A193 4 Gr. B7 are listed as high strength in Table 2. These and other materials of comparable strength may be used in any flanged joint.
5.3.3 Intermediate Strength Bolting.
When bolting materials listed as intermediate strength in Table 2 or other bolting of comparable strength are used in a flanged joint, it is recommended that the user verify the ability of the selected bolting to seat the selected gasket and maintain a leak-tight joint under expected operating condition.
5.3.4 Low Strength Bolting.

Bolting materials having no more than 206 MPa (30,000 psi) specified minimum yield strength are listed as low strength in Table 2. These materials and others of comparable strength are to be used only in Classes 75, 150, and 300 flanged joints, and only with gaskets described in para. 5.4.2. Flanged assemblies using low strength carbon steel bolts should not be used above 200°C (400°F) or below −29°C (−20°F).
5.3.5 Bolting to Gray Iron Flanges.
The following recommendations are made in recognition of the low ductility of gray iron.
(a)Alignment of flange faces is essential, along with control of assembly bolt torque, so as not to overstress gray iron flanges. Care must also be exercised to ensure that piping loads transmitted to gray iron flanges are controlled, taking into account its lack of ductility, and recognizing that gray iron flanges should not be used where suddenly applied loads such as rapid pressure fluctuation may occur.
(b) Where Class 150 steel flanges are bolted to Class 125 gray iron flanges, the gaskets should be made of Nonmandatory Appendix B, Group No. Ia materials, both flanges should have flat faces, and either of the following:
(1)Low strength bolting within the limitations of para. 5.3.4 should be used with ring gaskets extending to the bolt holes.
(2)Bolting of low (para. 5.3.4), intermediate (para. 5.3.3), or high (para. 5.3.2) strength may be used with full face gaskets extending to the outside diameters of the flanges.
(c) Where Class 300 steel flanges are bolted to Class 250 gray iron flanges, the gaskets should be made of Nonmandatory Appendix B, Group No. Ia materials and either of the following:
(1)Low strength bolting within the limitations of para. 5.3.4 should be used with gaskets extending to the bolt holes and with flanges having either raised or flat faces.
(2)Bolting of low (para. 5.3.4), intermediate (para. 5.3.3), or high (para. 5.3.2) strength may be used with full face gaskets extending to the outside diameters of the flanges and with both the Class 300 steel and Class 250 gray iron flanges having flat faces.
5.4 Gaskets

5.4.1 General.
Ring-joint gasket materials shall conform to ASME B16.20. Materials for other gaskets are described in Nonmandatory Appendix B. The user is responsible for the selection of gasket materials that will withstand the expected bolt loading without injurious crushing, and that are suitable for the service conditions.
5.4.2 Gaskets for Low Strength Bolting.
If bolting listed as low strength in Table 2 is used, gaskets shown in Nonmandatory Appendix B, Table B-1, Group No. Ia are recommended.
5.4.3 Gaskets for Class 150 Flanged Joints.
It is recommended that only Nonmandatory Appendix B, Table B-1, Group No. Ia or Ib gaskets be used for Class 150 flanged joints.
5.4.4 Gaskets for Class 75 Flanged Joints.
It is recommended that only Group No. Ia gaskets, with a gasket factor mp2 or less, and minimum design seating stress y p 1,600 psi or less, be used. The m and y factors are those given in Appendix 2 of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1.
6 DIMENSIONS
6.1 Flange Facings
6.1.1 General.
Classes 75, 150, and 300 flanges are regularly furnished with a 2 mm (0.06 in.) raised face. Classes 400, 600, and 900 flanges are regularly furnished with a 7 mm (0.25 in.) raised face. The thickness of the raised face in all cases is in addition to the minimum flange thickness, tf. It is recommended that the Class 75 flanges be used only with a 2 mm (0.06 in.) raised face or flat face. Dimensions for ring-joint facing are given in Table 29 (Table I-27) for Series A flanges.
6.1.1.1 Raised Face Flanges.
The raised face thickness shall be in addition to the minimum flange thickness, tf.
6.1.1.2 Ring-Joint Flanges.
The thickness required to form the ring-joint groove (dimension E in Table 29 and Table I-27) shall be in addition to the minimum flange thickness, tf.
6.1.2 Blind Flanges.
Blind flanges need not be faced in the center if, when this center part is raised, its diameter is at least 25 mm (1 in.) smaller than the mating pipe inside diameter. When the center part is depressed, its diameter shall not be greater than the inside diameter of the mating flange. Machining of the depressed center is not required.
6.1.3 Flat Face Flanges.
The raised face may be removed by the user from a raised face flange to convert it to a flat face flange.
6.1.4 Flange Facing Finish.
Flange facing finishes shall be in accordance with paras. 6.1.4.1 and 6.1.4.2, except that other finishes may be furnished by agreement between the user and the manufacturer. The finish of the gasket contact faces shall be judged by visual comparison with Ra standards (see ASME B46.1) and not by instruments having stylus tracers and electronic amplification.
6.1.4.1 Ring-Joint.
The side wall surface finish of the gasket groove shall not exceed 1.6 m (63 in.) roughness.
6.1.4.2 Other Flange Facings.
Either a serrated concentric or serrated spiral finish, having a resultant surface finish from 3.2 mto 6.3 m(125 in. to 250 in.) average roughness shall be furnished. The cutting tool employed should have an approximate 1.5 mm (0.06 in.) or larger radius, and there should be from 1.8 grooves/mm through 2.2 grooves/mm (45 grooves/in. through 55 grooves/in.).
6.1.5 Flange Facing Finish Imperfections.
Imperfections in the flange facing finish shall not exceed the dimensions shown in Table 30 (Table I-28). Adjacent imperfections shall be separated by a distance of at least 4 times the maximum radial projection. A radial projection shall be measured by the difference between an outer radius and an inner radius encompassing the imperfection where the radii are struck from the centerline of the bore. Imperfections less than half the depth of the serrations shall not be considered cause for rejection. Protrusions above the serrations are not permitted.
6.2 Flange Bolt Holes
Bolt holes are in multiples of four. Bolt holes shall be equally spaced.
6.3 Bolting Bearing Surfaces
Flanges shall have bearing surfaces for bolting that are parallel to the flange face within 1 deg. Back facing or spot facing shall not reduce the flange thickness below the dimensions tf given in Tables 31 through 41 (Tables I-29 through I-39). Spot facing or back facing shall be in accordance with MSS SP-9.
6.4 Welding End Preparation for Welding Neck Flanges
6.4.1 Illustrations.
Welding ends are illustrated in Figs. 1 through 3 (Figs. I-1 through I-3).
6.4.2 Bores.
Cylindrical bores shown in Fig. 1 (Fig. I-1) are standard unless specifica
lly ordered to suit the special conditions illustrated in Figs. 2 and 3 (Figs. I-2 and I-3). See para. 2.7 for maximum bore sizes.
6.5 Flange Bolting Dimensions
6.5.1 Dimensional Standards.
Stud-bolts, threaded at both ends or threaded full length, or bolts may be used in flange joints. Dimensional recommendations for bolts, stud-bolts, and nuts are shown in Table 42. See para. 5.3 for bolting material recommendations.
6.5.2 Bolting Recommendations.
For flange joints, stud bolts with a nut at each end are recommended for all applications.
6.6 Gaskets
6.6.1 Ring-Joint Gaskets.
Ring-joint gasket dimensions should conform to ASME B16.20.
6.6.2 Nonmetallic Gaskets.
Nonmetallic gasket dimensions should conform to ASME B16.21.
6.6.3 Spiral Wound and Double-Jacketed Gaskets.
Spiral wound and double-jacketed corrugated metal gaskets should conform to ASME B16.20.
6.7 Hub Dimensions
Any modification to hub dimensions shown in Tables 31 through 41 (Tables I-29 through I-39) shall be by agreement between the purchaser and manufacturer and shall be confirmed by calculations in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Appendix 2. Flanges so modified shall be marked with the material designation of the pipe to which they are to be welded in addition to the marking per para. 4.2.2.