A flange marked Class 150 is not automatically limited to 150 psi, and a Class 300 flange is not suitable for every service below 300 psi. ASME flange classes group standardised dimensions with pressure-temperature ratings that change according to material and operating temperature. Correct selection requires more information than the class number.
Key takeaways
- A flange class is a pressure-temperature rating designation, not a direct psi value.
- Material group and operating temperature determine the allowable pressure.
- Mating flanges must be compatible in standard, class, facing and dimensions.
What the class number represents
ASME B16.5 covers pressure-temperature ratings, materials, dimensions, tolerances, marking and testing for flanges from NPS 1/2 through NPS 24. Its rating classes include 150, 300, 400, 600, 900, 1500 and 2500, subject to the size limits in the standard.
The class number identifies a defined flange system. As the class increases, the flange generally becomes thicker and uses a different bolting arrangement to carry the required load. The exact changes depend on flange type and nominal size.
Steelmor carries stainless steel flanges in ANSI Classes 150 to 2500, with standard ranges in 304L, 316L and 321H.
Why Class 150 does not mean 150 psi
Pressure-temperature tables assign an allowable pressure to a particular material group at a stated temperature. As operating temperature rises, allowable pressure usually falls because material strength changes. The same Class 150 designation can therefore have different allowable pressures for different materials and temperatures.
The piping designer must use the table for the specified flange material and design temperature. The system design also needs to account for transients, external loads, corrosion allowance and the applicable piping code. A class label on its own is not a design calculation.
What changes when the class increases
Higher-class flanges commonly have thicker hubs and rims, larger or more numerous bolts and different dimensions. These changes affect weight, cost, availability and the space needed for installation and maintenance. They can also influence welding, support loads and access for bolt tightening.
Flange type must be selected separately from class. Weld neck, slip-on, socket-weld, threaded, lap-joint and blind flanges solve different connection and maintenance requirements. A higher class does not correct an unsuitable flange type, poor gasket selection or inadequate bolting.
Flanges of different classes are generally not dimensionally interchangeable. Bolt circles, hole sizes and flange thicknesses may differ even when the nominal pipe size is the same. Connecting two flanges therefore requires verification of the standard, size, class and facing on both sides.
How large diameter flanges are treated
ASME B16.47 covers large-diameter steel flanges from NPS 26 through NPS 60. Its classes and dimensional systems differ from B16.5, so the applicable standard must be stated on drawings and purchase orders.
Custom plate flanges also require careful review. A flange made to a drawing is not automatically equivalent to a standard forged flange with the same outside diameter. Material, design calculation, facing, bolting, inspection and testing must be specified for the intended service.
What belongs in a flange specification
Include the nominal size, type, class, material grade, standard, facing, bore or schedule, quantity and certification. Add bolt and gasket requirements where the supplier’s scope includes them. For welded flanges, state the pipe wall and end preparation so the joint can be prepared correctly.
Select the complete flange joint
The flange, gasket, bolting and mating component work as one joint. Specifying only size and class leaves important compatibility and design questions unanswered. A complete requisition reduces the risk of receiving components that cannot be assembled or used at the intended temperature.
To confirm stock, sizes or made-to-order flange requirements, contact Steelmor.