Effective Use of ASME B16.5 Standards for Flange Pressure Ratings
Engineers rely on ASME/ANSI standards and codes throughout the design and construction of piping systems to ensure they meet prescribed pressure-integrity requirements.
These codes and standards provide the criteria needed to determine pressure integrity characteristics and simplify design rules.
Understanding the difference between standards and codes helps designers with detail design and specifications for components.
Piping codes provide design criteria and rules for a piping system. Frequently encountered criteria and rules include:
Permissible construction materials
Allowable working stresses
Minimum wall thickness
Structural behaviour due to the effects of internal pressure, deadweight, seismic loads, live loads, thermal expansion, and other imposed internal or external loads
Piping Codes do not provide design rules for standard in-line components such as valves, flanges, and standard fittings.
Instead, they define the design requirements for these classes of components through reference to industry standards.
Piping Standards provide specific design criteria and rules for individual components or classes of components such as valves, flanges, and fittings. There are two general types of standards:
Dimensional standards: These standards provide configuration control parameters for components. The primary purpose of dimensional standards is to ensure that similar parts manufactured by different suppliers will be physically interchangeable.
Conformity to a specific dimensional standard during the manufacture of a product does not imply that all such similarly configured products will provide equal performance.
Pressure-Integrity standards: These standards provide uniform minimum-performance criteria. Components designed and manufactured to the same standards will function equivalently.
However, pressure integrity is not synonymous with leakage integrity. During test and operation, pressure integrity is only an assurance of a leak-tight condition and does not account for added stresses or the pressure boundary’s structural stability.
Several different ASME standards for piping components provide pressure-temperature ratings in their included “Reference Codes and Standards” sections.
In accordance with ASME B16.5, 2017, there are:
8 carbon and low-alloy steel material groups
10 high-alloy steel material groups
16 nonferrous metal groups.
Within each of the 34 material groups is a subgroup listing of ASTM materials specifications for forgings, castings and plates.
Partial listings of the various material groups and subgroups are shown in Table A1.1.
Also, pressure ratings on Table U1.1, U2.1, and U2.2 are provided for material groups 1.1, 2.1, and 2.2.
See the ASME B16.5, 2017, Standard: “Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard” for complete lists and tables.
All flanges made from any material in the same group, which carry the same ASME flange pressure class, have the same pressure-temperature rating for any single material group.
ASME B16.5 provides seven pressure classes for flanges.
They are Classes 150, 300, 400, 600, 900, 1500, and 2500.
The pressure-temperature ratings for flanges representing all material groups are organized within 44 tables, one table included in ASME B16.5, 2017, for each material group or sub-group.
Table U2.1 is adapted from ASME Standard B16.5 and is typical of the 34 flange rating tables, providing the pressure-temperature ratings for flanges in material group 2.1.
The table is organized with the pressure classes listed across the top and the maximum working temperatures along the left-hand border.
The table’s body provides the pressure ratings for flanges from each pressure class at the given temperature.
In practice, the use of ASME B16.5 to determine a flange rating is quite simple. The recommend three-step process is outlined below:
Determine the maximum operating pressure and temperature for the required flange.
Select a flange material and, therefore, a material group from one of the 44 listed material groups. Be aware that qualifying notes concerning maximum operating temperatures for various materials may influence the final material selection.
Consult the appropriate material group table, starting at the temperature listed one increment higher than the desired maximum operating temperature. Start with the Class 150 column and proceed to the right until you find a pressure rating for the desired temperature which equals or exceeds the required operating pressure.
The column that satisfies the requirements will dictate the required pressure class and specify the flange’s actual pressure-temperature rating.
For example, Assume that an ASTM A182-F316 stainless steel flange is required to satisfy a pressure rating of 850 psig (5860 kPa gage, 58.6bar) at 650°F (343°C).
ASTM A182 is a material group 2.2 material.
Consulting Table U2.2 at a temperature of 650°F (343°C), a Class 600 flange is found to have a rating of 890 psig (6136 kPa gage) at 650°F (343°C).
Therefore, a Class 600 ASTM A182 flange is suitable for the stated conditions.