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What Is a Flange Pressure Class? | Piping Engineering Glossary
What Is a Flange Pressure Class?
In piping engineering and process engineering, a flange pressure class is a standard rating designation, such as Class 150, 300, 600, 900, 1500, or 2500, that defines how much pressure a flange can safely hold at a given temperature. The classes come from standards such as ASME B16.5 for flanges up to NPS 24 and ASME B16.47 for larger sizes.
Class Is Not the Same as Pressure
A common misunderstanding is that Class 150 means 150 psi. In fact, each class has a pressure rating that depends on the flange material group and the temperature. For common carbon steel flanges, Class 150 is rated at about 19.6 bar (285 psig) at ambient temperature, and Class 300 at about 51 bar (740 psig). Both ratings fall as temperature rises, so the allowable pressure at 400°C is far lower than at ambient.
Higher classes use thicker flanges with more and larger bolts, which increases weight and cost. European flanges use a different system of PN ratings, so mixing ASME and EN flanges requires care.
Applications
Piping Specifications
Each line class in a piping specification defines the flange class to be used, based on the system’s design pressure and temperature. The same class then applies to valves and other flanged components on that line.
Component Selection
Flange types such as the weld neck flange, slip-on, and blind flange are all supplied in the class required by the line.
Bolting and Gaskets
The class determines the number and size of stud bolts and the type of gaskets suited to the joint. Higher classes generally use larger bolts and may call for spiral wound or ring joint gaskets.
Benefits
Standardized Interchangeability
Flanges of the same size and class from different manufacturers mate correctly.
Simple Design Checks
Engineers can confirm suitability by comparing design conditions with published pressure-temperature tables. This makes it easy to confirm whether an existing flange can handle new operating conditions.
Clear Specification
Using classes keeps piping specifications consistent and easy to procure against. It also simplifies spare parts and maintenance planning.
Limitations
Temperature Derating
Flange ratings drop significantly at high temperature, so a class that works at ambient may not meet the temperature rating needed in hot service.
External Loads Not Included
The pressure-temperature rating does not account for bending moments from piping, which can cause leaks even below rated pressure. Critical joints may need a separate flange leakage check.
Facing Must Match
Flanges must also match in facing type, such as a raised face flange against another raised face, not only in class.
FAQ
Can a Class 150 flange be bolted to a Class 300 flange?
No. Different classes have different bolt patterns and dimensions in most sizes, so they are not designed to mate.
How do I choose the right flange class?
Find the flange material group, then select the lowest class whose rated pressure at the design temperature equals or exceeds the design pressure.
Why is there no Class 400 in most piping specs?
Class 400 exists in ASME B16.5 but is rarely used, so most specifications step directly from Class 300 to Class 600.
Do higher classes leak less?
Not necessarily. Leak tightness depends on bolting, gaskets, and assembly quality as much as on class, though higher classes use heavier flanges that resist rotation better.
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