The ASME B16.5 flanges standard is a globally recognized specification for pipe flanges and flanged fittings, covering sizes from ½ inch to 24 inches and various pressure classes. This standard ensures that flanges meet precise dimensions, material requirements, and pressure-temperature ratings, providing reliability and safety for critical piping systems. Choosing ASME B16.5 compliant flanges guarantees compatibility, high performance, and long-term durability in industrial applications.
ASME B16.5 Flanges Standard General
Standard Scope
ASME B16.5 flanges standard covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings. ASME B16.5 standard covers Steel Pipe Flanges and Flanged Fittings from NPS 1/2 through NPS 24 Metric/Inch in pressure class 150 to class 2500.
This Standard is limited to:
•Flanges and flanged fittings made from cast or forged materials.
•Blind flanges and certain reducing flanges made from cast, forged, or plate materials .
•requirements and recommendations regarding flange bolting, gaskets, and joints.
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.
Denotation
Pressure Rating Designation
Class, followed by a dimensionless number, is the designation for pressure-temperature ratings as follows:
Class 150, 300, 400, 600, 900, 1500, 2500
If you wish to learn the dimensions for each flange at different pressure ratings, you can click the link below.
Size
NPS, followed by a dimensionless number, is the designation for nominal flange or flange fitting size. NPS is related to the reference nominal diameter, DN, used in international standards. The relationship is, typically, as follows:
| NPS | DN |
| 1/2 | 15 |
| 3/4 | 20 |
| 1 | 25 |
| 1 1/4 | 32 |
| 1 1/2 | 40 |
| 2 | 50 |
| 2 1/2 | 65 |
| 3 | 80 |
| 4 | 100 |
General Note: For NPS ≥ 4, the related DN = 25 multiplied by the NPS numbe
Pressure–Temperature Ratings
The pressure-temperature rating is the maximum allowable working pressure (bar unit) of the material and grade used at the rated temperature (Celsius).For intermediate temperatures, linear interpolation is permitted. Interpolation between class designations is not permitted.
Flanged Joints
A flanged joint is composed of separate and independent, although interrelated, components: the flanges, gasket, and bolting, which are assembled by another influence, the assembler. Proper controls must be exercised in the selection and application for all these elements to attain a joint that has acceptable leak tightness.
Ratings of Flanged Joints
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.
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 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).
Temperature Considerations
Use of flanged joints 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.
High Temperature.
Application at temperatures in the creep range will result in decreasing bolt loads as relaxation of flanges, bolts, and gaskets takes place. At temperatures above 200°C (400°F) for Class 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 or thermal gradients.
Low Temperature.
Some materials, notably some carbon steels, may undergo a decrease in ductility when used at low temperatures. This may prevent them from safely resisting shock loading or high stress concentration. Some codes may require impact testing for applications even where temperatures are higher than -29°C (-20°F).
System Hydrostatic Testing
Flanged joints and flanged fittings may be subjected to system hydrostatic 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.
Welding Neck Flanges
Ratings for welding neck flanges are based upon their hubs at the welding end having thickness at least equal to that calculated for pipe with 276 MPa (40,000 psi) specified minimum yield strength. To ensure adequate flange hub thickness for flange sizes NPS 2 and larger, the bore of a welding neck flange shall not exceed Bmax as defined by the calculation formula provided.
Bmax=Ah(1−CoPc/50,000)
where:
- Ah = tabulated hub diameter, beginning of chamfer as listed in the dimensional tables
- Bmax = maximum permissible diameter for the bore of a welding neck flange
- Co= 14.5 when pcp_cpc is expressed in bar units or 1.0 when pcp_cpc is expressed in psi units
- Pc= ceiling pressure value at 38°C (100°F), Nonmandatory Appendix A, Tables A-1 and A-2
The resultant units for diameter BmaxB_{max}Bmax are the same as those entered for diameter AAA.
Straight Hub Welding Flanges
Hub Dimensions.
Straight hub welding flanges have hubs of uniform thickness. They shall have dimensions and tolerances of the welding neck flanges of the same size and class.
Length Through Hub.
The length through hub shall be 229 mm (9 in.) for NPS 4 and smaller and 305 mm (12 in.) for larger sizes. Other lengths may be furnished by agreement between user and manufacturer.
Bore.
The bore diameter B shall be equal to the B dimension of the welding neck flange. Other bores may be furnished by agreement.
Hub End.
The standard flange shall be provided with a square cut end. The end user may specify welding end preparation.
Multiple Material Grades
Material for flanges and flanged fittings may meet the requirements of more than one specification or grade. In either case, the pressure-temperature ratings for any of these may be used provided the material is marked accordingly.
Identification Markings
A ASME B16.5 flange shall furnished with below marking requirements. Flange and flange fittings shall be marked in accordance with MSS SP-25 except for modifications to this specification.
•Name: Manufacturer name or trademark.
•Material: Cast or Forged, material standard ASTM xxx.
•Rating Designation: Class 150 to 2500.
•Conformance: ASME B16.5 or other B16 standard.
•Temperature: the temperature shall be shown with its corre sponding tabulated pressure rating for the material.
•Size:The NPS designation shall be marked on flanges and flangedfittings.
•Ring Joint Flanges: Shall be marked with letter R and ring groove number.
•Multiple Material Marking: In case multiple material used.
•Case Number
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