Round Flange Forming: Complete Guide to Angle Ring Flanges (2026)

Types of Round Flanges

Round flanges are used to connect sections of spiral duct, attach duct to equipment (fans, filters, grilles), and provide a rigid, leak-free joint. There are three main types used in HVAC:

Angle Ring Flanges (Most Common)

Angle ring flanges are made by rolling angle iron (L-shaped steel profile) into a circle, then welding the joint. The flange is then welded to the end of the spiral duct.

Why Angle Ring Flanges Are Preferred

• High strength — The L-profile provides rigidity in two axes
• Excellent sealing — Flat mating surface for gasket compression
• Easy to bolt — Standard hole patterns match across manufacturers
• Wide size range — Available from 80mm to 2000mm+ diameter
• Industry standard — Recognized by SMACNA, DW144, and other standards

Angle Iron Sizes

Flat Steel Flanges

Flat steel flanges are made by rolling flat steel strip into a ring. They are simpler and cheaper to produce but less rigid than angle ring flanges.

Flat flanges are suitable for:

• Low-pressure systems (under 500 Pa)
• Small diameter duct (under 400mm)
• Residential and light commercial
• Budget-sensitive projects

For medium and high-pressure systems, angle ring flanges are always specified because the L-profile provides much better resistance to bending under pressure.

Hole Patterns and Punching

Flange holes must match between mating flanges. Standard hole patterns are defined by industry standards (SMACNA, DW144, Eurovent).

Welding Flanges to Duct

Proper welding is critical for flange strength and air tightness. The flange is welded to the duct end using one of two methods:

1. Continuous Fillet Weld (Inside Only)

Weld the inside joint between the flange and duct continuously. This is the standard method and provides adequate strength for most applications.

2. Inside and Outside Weld (Heavy-Duty)

For high-pressure or large-diameter duct, weld both inside and outside. This provides maximum strength and prevents flange separation under pressure.

Welding tips:

• Use MIG welding with ER70S-6 wire for galvanized duct
• Use TIG welding with ER308L wire for stainless steel duct
• Ensure full penetration — no gaps between flange and duct
• Grind welds smooth on the mating face to ensure flat sealing surface
• Use zinc-rich paint to repair galvanized coating damaged by welding

Connection Methods

Bolted Connection (Standard)

The most common method. Two flanges are bolted together with a gasket between them. Use:

• Neoprene or EPDM gasket (3-5mm thick)
• Galvanized or stainless bolts, nuts, and washers
• Even torque sequence (tighten opposite bolts gradually)

Clamp Connection (Quick Assembly)

A ring clamp wraps around both flanges and tightens with a bolt. Faster than bolting but not suitable for high pressure. Common in residential and light commercial.

Van Stone Connection (Spigot and Ring)

The duct end is flared outward (spigot), and a loose flange ring slides over it. The spigot faces seal against each other. This allows the flange to rotate for easy bolt alignment.

Equipment for Round Flange Production

To produce angle ring flanges in-house, you need this equipment:

1. Round Flange Forming Line — Automatically punches holes and rolls angle iron into rings. This is the core machine for flange production.
2. Welding machine — MIG welder for welding the ring joint and welding flanges to duct.
3. Grinder — For smoothing welds and preparing mating surfaces.
4. Cutting saw — For cutting angle iron to length (if not using coil feed).

A round flange forming line typically produces flanges from 80mm to 1500mm diameter, with automatic punching at standard hole spacing. Production rate is 20-40 flanges per hour depending on size.

Standard Flange Specifications

Always check which standard your project specifies. Hole patterns differ between standards, and flanges from different standards may not bolt together.

Frequently Asked Questions