Layered Construction and Material Properties
Unlike metal bellows which rely on the flexibility of thin-gauge stainless steel convolutions, fabric expansion joints are composite structures. In a flue gas environment, the bellows must withstand the 'acid dew point'—the temperature at which sulphuric or nitric acids condense out of the exhaust stream. Consequently, the core of a high-performance fabric joint typically features a PTFE (Polytetrafluoroethylene) foil or coated fabric. This chemical barrier ensures that the structural glass-cloth layers are protected from corrosive attack.
The thermal insulation layer, often comprised of ceramic or mineral wool, reduces the temperature reaching the outer 'weather' layer. This allows the external surface of the joint to remain relatively cool, protecting personnel and ensuring the outer layer maintains its structural integrity. For biomass or coal-fired applications, an additional abrasion-resistant layer may be required to prevent erosion from particulate matter moving at high velocities through the ducting.
- Single-layer: Suitable for clean air and low temperatures where chemical resistance is not primary.
- Multi-layer: The industry standard for flue gas, incorporating an outer cover, thermal insulation layer, and a chemical barrier (usually PTFE).
- Reinforcement: Steel mesh or high-tensile fabrics to maintain the joint's profile under pressure or vacuum.
Frequently asked questions
Why use fabric expansion joints instead of rubber bellows for flue gas?
- Fabric bellows use layers of PTFE and technical glass fabrics to withstand up to 1000°C, whereas rubber bellows are typically limited to 90°C-115°C. Fabric joints also handle much larger lateral offsets in shorter face-to-face lengths.
Can fabric joints be used in condensing boiler flues?
- Yes, provided a high-grade PTFE chemical barrier is integrated into the multi-layer construction to prevent acid dew point corrosion of the structural fabric layers.
When is an internal sleeve required for a duct expansion joint?
- Internal stainless steel sleeves (baffles) should be specified if gas velocities exceed 10m/s or if the medium contains abrasive particulate matter (e.g., biomass combustion).
How do I identify a failure in a multi-layer fabric joint?
- A drop in flue draught or visible 'ballooning' often indicates a failure of the internal pressure-bearing layer. Immediate inspection is required to prevent heat damage to adjacent plant room equipment.
