Large Diameter Flange

What is a Large Diameter Flange?

A large diameter flange is a heavy circular flange manufactured for pipe sizes NPS 26 and above (DN650 and larger). Used in low-pressure large-bore piping such as cooling water, ventilation, storage tank connections, and ducting, these flanges are typically fabricated from rolled and welded plate or forged in one piece for higher pressure classes. Their large size requires specialized rolling, welding, and machining equipment.

Standards and Specifications

Large diameter flanges conform to ASME B16.47 Series A and Series B, EN 1092, and DIN standards. Sizes range from NPS 26 to NPS 60 (DN650 to DN1500) and larger. Pressure classes include Class 150, 300, 400, 600, and 900. Faces can be raised face (RF), flat face (FF), or ring type joint (RTJ). diameters up to 4 meters or more can be custom manufactured per project drawings.

Materials and Grades

Carbon steel Q235B, Q345R, A105, and ASTM A516 Gr 70 are standard materials. Stainless steel 304/304L and 316/316L provide corrosion resistance for water and chemical service. Large diameter flanges are often fabricated from rolled plate sections welded into a ring, then stress-relieved and machined. Forged one-piece flanges are available for higher pressure and critical service.

Pressure and Manufacturing Considerations

Large diameter flanges are most commonly used at Class 150 and Class 300, where large-bore piping carries low-pressure fluids. Fabricated ring flanges are rolled from plate, butt-welded, stress-relieved, and then faced and drilled on large CNC lathes and radial drill machines. Dimensional control of bolt circle and face flatness is critical to ensure proper gasket sealing across the large diameter.

Applications

Large diameter flanges connect cooling water lines, fire water systems, ventilation ducts, storage tank nozzles, scrubber towers, gas clean systems, and power plant circulating water piping. They are also used in water treatment plants, desalination facilities, and large industrial ducting where bolted connections are needed for equipment access.

Advantages

Large diameter flanges provide a bolted joint that can be disassembled for equipment installation, inspection, and maintenance on large-bore piping. Fabricated construction allows economical production of diameters that cannot be forged in one piece. Standard B16.47 dimensions ensure interchangeability between mating flanges from different manufacturers.

Fabrication and Installation

Rolled plate rings are welded, ultrasonically tested, stress-relieved, and then machined to final dimensions. Bolt holes are drilled on pitch circles per the standard. On installation, use a full-face or spiral-wound gasket appropriate for the large diameter. Torque bolts in a staggered cross-pattern in multiple passes to compress the gasket uniformly across the large flange face.

Quality Assurance

Each large diameter flange is identified with size, class, material, heat number, and manufacturer's stamp. Inspection includes dimensional verification, UT of the weld seam, face flatness check, and visual examination. Material test certificates and welding procedure records are provided. Flanges are coated with rust-preventive paint and shipped in sections or fully assembled depending on size.