Series B Class 900 Flanges

Series B Class 900 flanges are the highest pressure class available in the lighter API 605 pattern for large-diameter piping. The size range at Class 900 is more restricted than lower classes, typically covering NPS 26 through NPS 36, as the combination of extreme pressure and large diameter pushes the limits of practical forging and machining capabilities.

At Class 900, the distinction between Series A and Series B becomes critically important during engineering design. While both series achieve the same pressure-temperature rating, the lighter Series B pattern provides less margin against flange rotation and requires more rigorous analysis of bolt load, gasket stress, and thermal expansion effects per ASME PCC-1 guidelines.

Selection of Series B over Series A at Class 900 should be based on thorough engineering analysis, particularly for cyclic services or applications with large temperature gradients across the flange face. For new construction, many owner-operators specify Series A at Class 900 as a matter of corporate standard, reserving Series B for specific applications where weight optimization has been validated by detailed flange analysis.