Series A Class 400 Flanges

Series A Class 400 flanges occupy an intermediate position between Class 300 and Class 600, providing a weight-optimized solution for systems that marginally exceed Class 300 pressure limits. This class is less commonly specified than Class 300 or 600 but fills an important niche in applications such as high-pressure gas transmission and offshore production.

The dimensional envelope for Class 400 shares the same outside diameters and bolt circle as Class 300 in many sizes, but with increased flange thickness and heavier bolting. This allows Class 400 flanges to be used in mixed-class piping systems where some joints require a higher rating without changing the overall piping layout.

Material and fabrication requirements for Class 400 large-diameter flanges are identical to other B16.47 classes, with forged construction being mandatory. Non-destructive examination including ultrasonic testing is typically specified for flanges above NPS 36 to ensure internal soundness of the forging.