Long Weld Neck Standard
Class 400 Long Weld Neck Flanges
MSS SP-97 long weld neck flanges for Class 400 service
Class 400 Long Weld Neck flanges provide an intermediate pressure option between Class 300 and Class 600 for vessel nozzle applications. The extended hub at Class 400 features increased wall thickness compared to Class 300, providing additional reinforcement for the higher pressure loading.
These flanges are specified when the operating pressure slightly exceeds Class 300 limits but the full weight and cost of Class 600 components are not justified. The Class 400 LWN design is particularly useful for vessel nozzles where the piping stress analysis shows that a heavier nozzle would reduce stress at the shell-to-nozzle junction.
Availability of Class 400 LWN flanges may require longer lead times than Class 150 or 300, as they are less commonly stocked. For projects with multiple Class 400 nozzles, early engagement with the flange manufacturer is recommended to ensure timely delivery.
