ANSI B16.5 flange pressure refers to the pressure-temperature rating system for pipe flanges and flanged fittings covered by ASME B16.5. The applicable pressure rating depends on the pressure class, flange material and operating temperature—not on the Class number alone.
ASME B16.5 covers pressure-temperature ratings, materials, dimensions, tolerances, marking and testing for pipe flanges and flanged fittings. Flanges are covered in NPS 1/2 through NPS 24 for Classes 150 through 1500, while Class 2500 flanges are covered from NPS 1/2 through NPS 12.
ANSI B16.5 Flange Pressure Classes
ASME B16.5 uses standardized flange pressure classes of 150, 300, 400, 600, 900, 1500 and 2500. These Class designations are used together with pressure-temperature ratings to determine the allowable pressure for a specified flange material and operating temperature.
| Flange Class | Flange Size Range |
| Class 150 | NPS 1/2–24 |
| Class 300 | NPS 1/2–24 |
| Class 400 | NPS 1/2–24 |
| Class 600 | NPS 1/2–24 |
| Class 900 | NPS 1/2–24 |
| Class 1500 | NPS 1/2–24 |
| Class 2500 | NPS 1/2–12 |
Class 150 Does Not Mean 150 PSI
The Class number is not a direct pressure value in psi. For example, a Class 150 flange should not automatically be interpreted as a 150 psi flange, and a Class 300 flange does not automatically mean 300 psi.
The actual allowable pressure is determined from the applicable pressure-temperature rating for the flange material and operating temperature.
How Is ANSI B16.5 Flange Pressure Determined?
The pressure rating of an ANSI B16.5 flange is determined by three primary factors:
Pressure Class + Material + Temperature → Applicable Pressure-Temperature Rating
The pressure class establishes the rating category, while the flange material and operating temperature determine the applicable allowable pressure within that category.
Therefore, NPS and Class alone are not sufficient to determine the allowable working pressure of a B16.5 flange.
For engineering and procurement, the design pressure, design temperature and specified flange material should be checked against the applicable ASME B16.5 pressure-temperature rating.
ANSI B16.5 Flange Pressure Rating by Temperature
Temperature changes the allowable pressure for a given flange Class and material.
| Condition | Effect on Rating |
| Lower operating temperature | Generally permits a higher allowable pressure |
| Higher operating temperature | Generally reduces allowable pressure |
| Different material | May produce a different pressure-temperature rating |
The important point is that a flange Class does not represent one fixed pressure value across all operating temperatures.
The applicable pressure is determined at the specified design temperature—not simply by the flange Class.
For actual engineering selection, the applicable ASME B16.5 pressure-temperature rating table should be used for the selected material.
ANSI B16.5 Flange Pressure Rating by Material
The flange material also affects the applicable pressure-temperature rating.
| Same Pressure Class | Can the Rating Be Different? |
| Same Class + different material | Yes |
| Same Class + same material + different temperature | Yes |
| Same Class + same material + same temperature | Rating basis is the same |
Therefore, specifying only Class 300, for example, does not provide enough information to establish the allowable pressure.
The flange specification should identify the applicable material specification or grade together with the pressure Class and design temperature.
ANSI B16.5 Flange Pressure Rating Chart
ASME B16.5 does not assign one fixed pressure value in psi to each flange Class. The applicable pressure is determined by the pressure Class, flange material and temperature.
For example, Class 150, Class 300 and Class 600 flanges can have different allowable pressures depending on the material and operating temperature. Therefore, the correct pressure should be taken from the applicable ASME B16.5 pressure-temperature rating table rather than from the Class number alone.
Class 150 ≠ 150 psi
Class 300 ≠ 300 psi
Class 600 ≠ 600 psi
ANSI B16.5 Flange Pressure vs Pipe Pressure
Flange pressure rating and pipe pressure rating are related but are not the same specification.
A piping system may contain pipe, flanges, valves, fittings, gaskets and bolting, each with applicable design requirements. The pressure capability of the complete system should therefore be evaluated rather than assuming that the flange Class directly represents the pipe’s allowable pressure.
For example, selecting a Class 300 flange does not mean that the entire piping system is automatically rated for 300 psi.
ANSI B16.5 Flange Pressure and Dimensions
Although pressure is the primary focus of B16.5 flange selection, the standard also specifies flange dimensions and tolerances. These include flange outside diameter, thickness, bolt-hole arrangement, bore and applicable facing dimensions.
The dimensions depend on factors such as NPS, pressure Class and flange configuration.
| Parameter | Main Function |
| NPS | Defines nominal pipe size |
| Pressure Class | Defines the applicable rating category and affects flange geometry |
| Material | Determines applicable pressure-temperature rating |
| Temperature | Affects allowable pressure |
| Flange Type | Determines applicable dimensional configuration |
| Facing | Defines the applicable sealing interface |
When checking flange dimensions, NPS, Class and flange type must be considered. For pressure rating, the key factors are Class, material and temperature.
ANSI B16.5 vs ASME B16.5 for Flange Pressure
| Item | ANSI B16.5 | ASME B16.5 |
| Common usage | Common industry and search term | Current standard designation |
| Pressure classes | 150, 300, 400, 600, 900, 1500, 2500 | 150, 300, 400, 600, 900, 1500, 2500 |
| Pressure rating basis | Pressure class, material and temperature | Pressure class, material and temperature |
| Application | Pipe flanges and flanged fittings | Pipe flanges and flanged fittings |
| Technical specification | Common terminology | Preferred current designation |
ASME B16.5 is the current standard designation. “ANSI B16.5” remains a widely used industry and search term for the standard.
How to Select an ANSI B16.5 Flange Pressure Class
Selecting an ANSI B16.5 flange pressure Class should begin with the design pressure and design temperature of the piping system. The flange material is then checked against the applicable ASME B16.5 pressure-temperature rating.
The selection process can be summarized as:
Design Pressure → Design Temperature → Flange Material → NPS → Flange Type and Facing → Pressure Class
The selected pressure Class should provide an applicable pressure-temperature rating that meets or exceeds the system’s design conditions. NPS, flange type and facing should also match the connected pipe and other piping components.
For a complete piping system, the flange should be evaluated together with the pipe, valves, fittings, gasket and bolting rather than selected based on the flange Class alone.
FAQ
What is ANSI B16.5 flange pressure?
ANSI B16.5 flange pressure refers to the pressure-temperature rating of pipe flanges and flanged fittings covered by ASME B16.5. The applicable rating depends on the pressure Class, flange material and operating temperature.
Does Class 150 mean 150 psi?
No. Class 150 does not mean 150 psi. The actual allowable pressure depends on the applicable material and operating temperature.
What are the ANSI B16.5 pressure classes?
The pressure classes covered by ASME B16.5 are Class 150, 300, 400, 600, 900, 1500 and 2500. The applicable pressure for each Class must be determined from the corresponding pressure-temperature rating for the specified material and temperature.
