Understanding Flange Pressure Ratings: How to Pick the Right Class

A flange’s pressure rating (or class) is the maximum pressure it can safely handle at a given temperature, expressed in psi or bars. Higher‑class flanges are designed to withstand greater pressures, making them stronger and more durable than lower‑class equivalents.

ANSI / ASME B16.5 Specification

Under the ANSI / ASME B16.5 standard, flange pressure classes are defined as the highest permissible pressure a flange can support at elevated temperatures—often called the Maximum Allowable Working Pressure (MAWP). The standard specifies seven common classes: 150, 300, 400, 600, 900, 1500, and 2500.

Terms such as “pressure rating,” “class,” “#,” “Lb,” or “Lbs” all refer to the same concept: the flange’s pressure‑temperature performance. For example, a Class 150 flange can handle 150 psi at its specified temperature.

• Class 150# valve
• 150 lb valve
• 150 psi valve
• 150 class valve

Illustrative Example

Consider two 3‑inch flanges made from the same material (e.g., SS 04) but with different pressure classes: one is Class 150 and the other Class 600. The Class 150 flange will be smaller, lighter, and less robust than the Class 600 flange, as shown below.

Choosing the Correct Flange Rating

1. Consult the material‑specific rating table for your flange (flanges of different grades have distinct pressure ratings).
2. Determine the maximum operating temperature in your piping system.
3. Select a class that supports the maximum pressure expected at that temperature.

Following these steps ensures your piping system remains safe, reliable, and compliant with industry standards.