Why a Duct Needs a Flexible Element
A flue gas duct is not a static piece of steel. A boiler duct running from the economiser outlet to the air preheater may grow by tens of millimetres between cold shutdown and full load, and the supporting steelwork grows at a different rate because it stays cooler. Without a flexible element somewhere in the run, that differential ends up as load on the duct walls, on the casing of the equipment at each end and on the anchor bolts. The usual symptoms are cracked casing welds, distorted fan housings and expansion joints that were never designed to take the movement they receive.
A non-metallic compensator solves the problem with a fabric element that is soft in the directions that matter and strong enough to contain the gas. Metallic bellows can absorb the same movement, but a fabric element carries far larger movements in a short face-to-face length, costs less at large duct sizes, and tolerates the misalignment that is inevitable in field-installed ductwork.
Layered Construction of a Fabric Element
A working compensator is a stack of layers, each with a job. A gas-tight membrane, usually an elastomer-coated fabric or a fluoropolymer film, holds the pressure boundary. Insulation layers behind it reduce the gas temperature reaching the outer layers. A structural fabric carries the pressure load, and an outer cover protects the assembly from weather, mechanical damage and ultraviolet exposure. Between the layers are barriers that stop hot fines from migrating towards the outer skin.
The details differ by service. In wet flue gas downstream of a scrubber, chemical resistance dominates and the membrane is selected for acid condensate rather than temperature. In dry, hot gas upstream of the air preheater, temperature capability and abrasion resistance dominate.
Movement Capability and Design Life
Movement is specified in three directions: axial compression and extension along the duct, lateral offset across it, and angular rotation. Real installations rarely move in one direction only, and the combined movement is what determines whether a given element survives. Stating each component separately, together with the number of thermal cycles expected per year and the design life in years, gives the factory enough information to size the belt and select the layer stack.
Cycle life is where specifications most often disappoint. A fabric element that lasts five years on a base-load unit may last eighteen months on a plant that cycles daily, because every start and stop reverses the movement and flexes the coated fabric. Sharing the expected operating pattern with the supplier is more useful than asking for a generic guarantee.
Temperature and Chemical Limits
Specify the normal operating temperature, the excursion temperature and the duration of excursions, since an element that survives a brief upset is very different from one rated for continuous duty at that level. Equally important is the acid dew point. If the gas side surface of the membrane sits below the dew point for part of the operating envelope, condensate will form and the material selection has to change, regardless of the nominal gas temperature.
Abrasion is the other silent killer. Fly ash travelling at duct velocity erodes the inner surfaces of the element, particularly at the frame where the belt turns. Flow liners, baffle plates and a straight duct section upstream that lets the flow settle all extend life, and they are far cheaper than replacing a failed belt during an unplanned outage.
Installation Details That Decide Service Life
Most premature failures have installation causes rather than material causes. The element must be installed at the correct cold pre-set so that it is neither over-compressed nor over-extended when the duct reaches operating temperature. Bolting has to be tightened in a controlled sequence to the torque the manufacturer states, because uneven clamping leaves a leak path at the frame. Weld spatter, sharp frame edges and misaligned flanges all cut into the belt.
Handling before installation matters too. Fabric belts are stored flat, protected from sunlight and kept away from solvents and sharp tools; dragging one across a concrete floor or leaving it folded for months damages coatings that cannot be repaired on site.
Inspection, Failure Signs and Specification Data
During routine outages, look for stiffening or cracking of the belt, staining that indicates condensate has been weeping through layers, abrasion at the frame radius, and loose or corroded clamping bars. A small external damp patch is often the first visible sign of an internal membrane breach, and it is worth investigating before it becomes a duct leak.
When requesting a quotation, provide duct dimensions, the design and excursion temperatures, gas composition including acid content and dust loading, pressure and vacuum, the three movement components, cycle frequency, frame material preference and the available face-to-face length. A responsive supplier will return a drawing showing the layer stack, the movement capability, the recommended pre-set and the expected life for the stated duty rather than a single price line.
References
Expansion Joint Manufacturers Group. (2021). Standards of the Expansion Joint Manufacturers Association (10th ed.). EJMA.
ASTM International. (2019). ASTM D751, Standard Test Methods for Coated Fabrics. ASTM.
ASTM International. (2019). ASTM D5035, Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method). ASTM.
American Society of Mechanical Engineers. (2022). ASME B31.1, Power Piping. ASME.
