Long Weld Neck Standard
Class 900 Long Weld Neck Flanges
MSS SP-97 long weld neck flanges for Class 900 service
Class 900 Long Weld Neck flanges serve the most demanding vessel nozzle applications where operating pressures require heavy-wall construction. The forging weight and machining complexity increase significantly at Class 900, and these flanges represent a major cost element in high-pressure vessel fabrication.
The extended hub on a Class 900 LWN flange provides substantial nozzle reinforcement, often reducing or eliminating the need for separate reinforcing pads on the vessel shell. This simplifies vessel fabrication and removes the concern about trapped moisture between the reinforcing pad and the vessel shell that can lead to corrosion.
Applications include high-pressure reactors, letdown vessels, and synthesis gas equipment in ammonia, methanol, and hydrogen production facilities. Post-weld heat treatment of the hub-to-shell weld is typically required, and the LWN forging must be ordered in the pre-PWHT condition to accommodate this field treatment.
