Round Flanges BF

BF Circular Flanges

The BF is a wider circular flange, with two sealing lips, entering deeper into the duct for more flexibility.
The duct ends do not need to be precisely cut.
Can act as a telesope fitting.
Assembly thanks to SR Joining Clips.
Manufactured in Germany.

  1. Duct wall
  2. Sealing lips
  3. The notches prevent the flange from being pulled out. Drill screws and blind rivets can also be used.
  4. The angles of the sealing lips are smaller and the sealing lips are longer than the ones on the AF Circular Flange, thus making installation easier.
  5. When SR Joining Clips with gasket are used, no gasket application on the flange face is necessary. This prevents the gasket from being damaged during installation and speeds up the assembly.


Suitable Ducts

For inner-duct diameters from 200 up to 3000mm. In-between diameters are also available. IMPORTANT: Please indicate if flanges are intended for fittings, in which case their Ø is made 3 mm smaller.

Suitable duct types: spirally wound ducts, longitudinal seam ducts and fittings.

Not suitable for ducts and fittings with turned-out edges and ducts with stiffening ribs.

Fixation Methods

Fastening in the duct through notches (see Notching Tools) and blind-rivets.

Screws are less recommended, as they protrude inside the duct, hindering the air-flow and the cleaning operations, and could be the cause of leakages. Spot-welding should not be used.

Spot welding not suitable



The metal lip of the flange automatically seals the connection. When correctly installed the air-tightness class C (DIN EN 12237) or ATC 3 (DIN EN 16798-3) are reached. The two sealing lips do not necessarily mean that the BF flange is air-tighter. The BF flange has comparable leakage rates as the AF flange. See also ZI-702.

To reach the air-tightness class C no additional measures, such as the use of adhesive tapes, sealants or mastic, are necessary. The necessary SR Joining Clips must be ordered with gasket.

The air-tightness class D or ATC 2 can also be reached when mastic is applied (refer to the instructions). Liquid-tightness see ZI-703.

The flanges stiffen the ducts and help maintaining their round shape. They also improve the resistance of the ducts under negative pressures. Ducts should nevertheless be strong enough by themselves and resist the required pressures (+ or -) without taking the flanges into account.

The flange is automatically centered in the duct. No gaps are formed between the two ends of the BF flange, since these are welded together.

Only available in galvanized steel.
Please refer to the information ZI-101



More expensive than the other flange types.

Unsuitable for fittings with short ends.

The flanges can be installed on ducts with stiffening ribs. In these cases, notches cannot be used. It is recommended to flatten the stiffening beads at the ends of the ducts and to fix the flanges with rivets and/or screws. Sealing measures (sealants, mastic) are required to reach the air-tightness class C.


Which are the advantages of flanges?

Installation Instructions

Screws or Rivets?



STP Documents

AF and BF Air-tightness


Equipotential Bonding