Flue expansion joints are used to absorb heat displacement in power plant boiler flue, metallurgical high temperature flue gas pipeline and chemical tail gas treatment system. However, if the thermal insulation of the expansion joint is improperly performed, it will lead to increased heat dissipation loss, risk of scalding due to excessive outer wall temperature, and even accelerated corrosion due to local condensation and dew condensation. A scientific and standardized thermal insulation construction scheme of flue expansion joint needs to take into account the thermal insulation effect, structural safety and free compensation function of expansion joint-the thermal insulation layer cannot limit the normal displacement of expansion joint. Starting from engineering practice, this paper systematically explains the construction preparation, material selection, construction technology, quality control and safety precautions of flue expansion joint insulation, and provides an operable technical guide for construction technicians and project managers.

Particularity of Expansion Joint Insulation

Different from ordinary straight pipe section insulation, flue expansion joint insulation construction faces three special challenges:

• Displacement compensation requirements: The insulation structure must be able to follow the axial expansion and contraction, lateral offset or angular rotation of the expansion joint, and must not produce cracking or extrusion damage.
• Special-shaped structure: The expansion joint body (bellows or skin) and the connecting flange are irregular in shape, and it is difficult to lay the insulation layer.
• Temperature gradient: There is a large difference in the linear expansion coefficient between the metal parts of the expansion joint and the insulation material, and peeling or gaps may occur at high temperatures.

Therefore, the insulation construction scheme of flue expansion joint must be specially designed according to the specific type of expansion joint (metal or non-metal), working temperature, compensation mode and space condition, and the insulation technology of straight pipe section should not be directly applied.

Requirements for selection of thermal insulation materials

Reasonable material selection is the basis of heat insulation construction scheme of flue expansion joint. The recommended materials and parameters are as follows:

Key Requirements:

• Insulation materials must be Class A non-flammable and provide type inspection report.
• When the surface temperature of non-metallic expansion joint skin does not exceed 200℃, the composite structure of high temperature glass wool + rock wool can be directly adopted.
• Chlorine-free products with low chloride ion content (Cl⁻≤ 25ppm) should be selected as insulation material for the area of corrugated pipe of metal expansion joint to prevent accelerated stress corrosion cracking of stainless steel.
• It is strictly forbidden to use insulation materials with high water absorption (such as ordinary rock wool) to avoid aggravated corrosion after moisture absorption under acid dew point conditions.

Construction preparation

Before implementing the flue expansion joint insulation construction scheme, the following preparatory work should be completed:

1. Technical disclosure: Construction personnel should be familiar with the structural drawings, insulation thickness and fixing method of expansion joints, and clarify the allowable displacement direction and maximum displacement amount of expansion joints.
2. Surface treatment: Remove oil, floating rust and dust on the surface of the expansion joint and adjacent flue, and paint high-temperature anti-rust paint on the metal surface (temperature resistance ≥400℃).
3. Material inspection: the appearance of insulation material is not damaged, and the negative deviation of thickness does not exceed 5%; The galvanized iron sheet has no rust spots on the surface.
4. Safety measures: The surface temperature of the flue should be reduced below 60℃ before operation; Setting up qualified scaffolding for high operation; Approval for hot work.

Construction process steps

The standard flue expansion joint insulation construction scheme shall follow the following process flow:

Step 1: Bellows/skin area treatment

• Metal expansion joint: The bellows shall not be filled with hard insulation materials in the trough, and shall be filled by hand with flexible aluminum silicate fiber blanket to ensure the free expansion and contraction of the bellows. The filling density is controlled at 120-140kg/m³. If it is too tight, it will limit the displacement, while if it is too loose, it will produce convection heat transfer.
• Non-metallic expansion joint: It is strictly forbidden to directly lay insulation layer on the skin surface. It is necessary to preset steel mesh skeleton support, and maintain a gap of 30-50mm with the skin to prevent the deformation of the skin due to gravity pressure of the insulation material.

Step 2: Laying the insulation layer

• Layered staggered seams: When double-layer or multi-layer insulation is used, the layers should be laid in staggered seams, and the staggered distance of the seams should not be less than 100mm to avoid forming a straight thermal bridge.
• Fixation method: Stainless steel belt or heat-resistant steel wire bundling, spacing 150-200mm. The bellows area of the metal expansion joint shall not use welded fixtures (the solder joint will generate stress concentration), but shall be fixed by hoop type.
• Thickness control: The total thickness of the insulation layer is performed according to the design, and the allowable deviation is +10mm/ -5mm. The insulation thickness of the expansion joint is usually 20%-30% larger than that of the straight pipe section to compensate for the increase in heat dissipation caused by the shape.

Step 3: Expansion joint setting

This is the most critical link of expansion joint insulation. Expansion joints must be set along the axis of the expansion joint:

• An annular expansion joint is arranged at the connection between the two ends of the expansion joint and the flue, and the width of the joint is 20-30mm.
• The expansion joint shall be filled with flexible aluminum silicate fiber rope, and no hard material shall be filled.
• The expansion joint is covered with a retractable metal corrugated protective cover plate, one end of the cover plate is fixed and the other end slides freely.

Step 4: Protective Layer Installation

• Use galvanized iron sheet or aluminum sheet (thickness 0.5-0.7mm), circular overlap ≥50mm, longitudinal overlap ≥100mm.
• The protective layer must be disconnected at the expansion joint-the upper and lower two independent shells, overlapping with each other but not fixed, the overlapping amount is 50-60mm, and relative sliding is allowed.
• The fixing screws are made of stainless steel self-tapping nails with a spacing of 200-250mm. It is strictly prohibited that the screw penetrates the insulation layer and touches the expansion joint body.
• The surface of the protective layer is kept smooth, and an obvious warning sign of "Expansion joint area, no trampling" is attached outside.

Key points of quality control

The quality acceptance of the flue expansion joint insulation construction scheme shall include:

1. Insulation thickness: Measure one section every 10 meters, measure 4 points in each section, and the qualified rate is ≥95%.
2. Tight seam: Visual inspection and gauge inspection, the gap between the seams of the insulation layer is ≤5mm, and the overlapping direction of the protective layer is along the water (pressing up and down).
3. Expansion joint function: the clearance of expansion joint in cold state conforms to the design; During hot inspection, the sliding end should be able to move freely without jamming.
4. Appearance: The protective layer is flat and smooth, the rivets are evenly spaced, and there is no warping or falling off.

Safety and Environmental Precautions

• A dust mask should be worn when cutting the insulation material to avoid inhaling fiber dust.
• Working at heights must wear seat belts and hang at reliable anchoring points.
• Clean the surrounding flammable materials before hot work (such as protective layer cutting) and equip fire extinguishers.
• Waste insulation materials are disposed of according to solid waste classification, and it is strictly prohibited to discard them at will.

epilogue

The construction scheme of heat insulation of flue expansion joint is not a simple copy of heat insulation of straight pipe section. Only according to the displacement characteristics, structural shape and working temperature of the expansion joint, reasonable material selection, standardized construction and reserved expansion joint can achieve both heat insulation effect and compensation function. A qualified expansion joint insulation project can reduce the heat dissipation loss by more than 80%, and at the same time avoid the early failure of expansion joint caused by condensation corrosion.