Aluminium Storm Collar should be fitted to the vent immediately above the Flashing. The Collars are a tight fit to the vent and should be sealed with the silicone sealant.
Gas Vent lengths and fittings are available with the outer casing fabricated from galvanised steel. The flue liner is aluminium. Vertical seams are rolled and securely locked. The inner and outer casing are joined at one end only and a 6mm (1/4") air gap is incorporated: the inner liner is thus free to expand as the temperature changes in the vent.