Flange rating: what it is, examples, and reference standards

In industrial plant design and operations, jointing components play a key role in connecting piping and equipment: they must ensure a safe, effective seal, especially under high pressure and elevated temperatures.

For this reason, flange rating gives engineers a clear indication of how resistant flange joints are under operating conditions such as pressure and temperature.

In this guide, we explain what flange rating means and which standards govern its use, to support the selection of the most suitable flanges for each industrial application.

What is flange rating

Flange rating, or flange class, is a classification that indicates the level of resistance of flanges to the pressure and temperature loads they will be subjected to. This classification helps identify the most appropriate components for a specific system, based on expected operating conditions.

Flanges with higher ratings are designed to withstand higher pressures, while lower ratings are suitable for less demanding pressure conditions (at the same temperature). The pressure–temperature performance of each component also varies depending on the piping material selected.

Flange rating is generally expressed as a numeric value that represents the maximum pressure levels a piping joint component can withstand at specific temperatures.

In practice, engineers often use a flange rating calculator approach—checking ASME pressure–temperature tables—to confirm the allowable pressure rating for flanges for a given material and operating temperature.

Examples of flange pressure rating

To understand flange pressure resistance, here are a few examples. A 6″ Class 150 carbon steel flange, at 600°F, can withstand a maximum pressure of 140 psi (about 10 bar). This is a typical class 150 flange pressure rating (also commonly written as 150 flange pressure rating).

If we consider a flange of the same size but with flange rating 300, again in carbon steel, resistance increases significantly, allowing it to handle up to 570 psi (about 40 bar) at the same temperature—i.e., a higher flange pressure rating.

Moving to an even higher class, a 6″ Class 2500 carbon steel flange can withstand an impressive 4730 psi (about 320 bar) at 600°F (about 315°C), more than 30 times the Class 150 value.

As mentioned, material also affects the resistance of components used to join piping. For example, a 6″ Class 150 stainless steel flange can withstand higher pressures than a carbon steel flange of the same class and size, because stainless steel has higher intrinsic strength. The same applies to advanced alloys such as Hastelloy, Monel, Inconel, and Incoloy, which deliver high performance in high-pressure, high-temperature environments.

ASME and ANSI standards for flange rating

Standards for flange rating provide uniform criteria for selecting and classifying flanges, enabling engineers to choose components that match expected operating conditions while ensuring compatibility and safety in piping systems.

ASME B16.5 is one of the most widely used and internationally recognised standards for classifying flanges used in industrial plants. The specifications in ASME flange rating B16.5 include detailed requirements for dimensions, tolerances, pressure, and temperature, ensuring safety and consistency and making selection and integration into plant piping systems easier.

ANSI (American National Standards Institute) works in synergy with ASME to develop standards that support the adoption of these jointing components in US and international markets, making it possible to use shared criteria for selection. ANSI/ASME standards help create compatible and safe piping systems that meet required operating conditions. The most widely used standards for flange classification are therefore:

• ANSI/ASME B16.5: it covers flanges and flanged fittings from NPS ½” (DN 15) up to NPS 24″ (DN 600) and specifies pressure classes from 150 to 2500. Each pressure class defines the maximum pressure a flange can be subjected to at specific temperatures (i.e., the pressure rating for flanges, based on pressure–temperature tables).
• ANSI/ASME B16.47: it governs large-diameter flanges from NPS 26″ (DN 650) to NPS 60″ (DN 1500), with pressure classes 75 to 900. This standard was introduced to meet the needs of plants requiring larger-diameter components.

When connecting two flanges with different nominal pressure ratings, the maximum pressure the assembly can withstand is determined by the flange with the lower flange rating. Therefore, selecting compatible components is essential to maintain structural and operational integrity.

To support this selection and speed up piping system design, engineers can also use advanced tools such as ESAPRO 3D Piping by ESAIN, which provides comprehensive functionality for creating industrial plant layouts in a 3D environment. The software also makes it easy to create piping classes by adding pipes, elbows, valves, and flanges, integrating the class with branch tables, weld gaps, and pipe cut lengths.

In some cases, however, non-standard flange classifications may be required, custom-developed to meet specific needs that fall outside ASME or ANSI standard parameters. These components must be carefully designed and tested against project-specific requirements and often require regulatory approval to ensure compliance with the required safety and performance rules.