1. Function and installation difficulties of lining of high-temperature flue expansion joint

In the high temperature flue system, although the metal expansion joint can effectively absorb the heat displacement, its stainless steel bellows cannot withstand the direct erosion and high temperature corrosion of high dust flue gas for a long time. Therefore, it is necessary to set a high-temperature resistant liner inside the expansion joint, which plays the triple functions of heat insulation, wear resistance and flow diversion. However, many field technicians do not know how to install the high-temperature flue expansion joint liner to be the most reliable. Different from ordinary flue lining, the expansion joint lining not only has to bear the dynamic deformation caused by the expansion and contraction of the bellows, but also to ensure firm anchoring and sealing of gaps in a narrow space. Once the liner is improperly installed, it will fall off locally and block the flue, and in the worst case, the bellows will be overheated and cracked and the flue gas will leak. This paper will systematically answer this question from material selection, structure design to construction steps.

2. Selection of lining materials and performance requirements

2.1 Insulation material

Before answering how to install the liner of the high-temperature flue expansion joint, it is first necessary to clarify what material to use. The liner of the expansion joint usually adopts a composite structure, and the commonly used materials for the thermal insulation layer include:

• Ceramic fiber blanket: It is the most widely used thermal insulation layer material with operating temperature of 800-1260℃, low thermal conductivity and light bulk density (96-128kg/m³)
• Aluminum silicate fiber board: suitable for higher temperature or strong air flow washing parts, strength is better than fiber blanket
• Aerogel felt: Ultra-low thermal conductivity, suitable for high temperature working conditions with limited space, but high cost

2.2 Hot surface layer (wear-resistant layer)

The side that directly contacts the flue gas needs to be provided with a wear-resistant layer. The common schemes are:

• Refractory castables: such as high aluminum castables or corundum castables, with a thickness of 30-50mm and an applicable temperature of up to 1300℃
• Ceramic fiber modules: prefabricated modular construction, fast installation, medium erosion resistance
• Coating layer: Apply high temperature hardening coating to the surface of the fiber blanket to form a dense protective layer

2.3 Anchor Material

The anchor is the key component to fix the liner to the expansion joint shell. High temperature resistant stainless steel (such as 310S and Incoloy 601) must be selected, and the welded end should be cold-bent to prevent brittle fracture. Anchor forms include V-shaped, Y-shaped, and tortoise shell nets, among others.

3. Key Points of Structural Design of Expansion Joint Liner

3.1 Layered Structure Design

The standard high-temperature flue expansion joint liner features a three-layer construction:

The three layers of materials should be laid in staggered seams to avoid hot flue gas penetration caused by straight seams.

3.2 Setting of expansion joints

Because the liner material also has thermal expansion and the coefficient of expansion differs significantly from the metal shell, it is necessary to reserve expansion joints in the liner:

• Set one expansion joint every 1-1.5m along the circumferential direction of the expansion joint
• At least 2-3 expansion joints are set along the axial direction (peak and trough direction of bellows)
• Expansion joint width 3-5mm, filled with ceramic fiber paper

3.3 Cooperation between guide tube and inner liner

For the expansion joint with guide tube, the liner shall be terminated at the downstream end of the guide tube, and a sliding gap of 10-20mm shall be reserved between the guide tube and the liner to avoid squeezing and damaging the liner during thermal expansion of the guide tube.

Installation and construction steps of high-temperature flue expansion joint lining

4.1 Preparation before construction

The following tasks need to be completed before formal installation:

• Place the expansion joint in a horizontal position to clean the inner surface of the housing from oil, corrosion and weld slag
• Check the surface of the bellows for damage, and there shall be no sharp objects in the trough
• Carry out sandblasting or grinding treatment on the metal shell to meet the rust removal standard of Sa2.5
• Set out the line on the shell according to the design drawing, and mark the welding position of the anchor and the position of the expansion joint

4.2 Welding of Anchors

The welding quality of anchors directly affects the success or failure of how to install the liner of high-temperature flue expansion joint:

• The anchors shall be welded vertically to the surface of the shell with a spacing of 150-200mm in a plum blossom shape
• The weld length of each anchor is not less than 30mm, and the weld angle height is not less than 4mm
• The ends should be bent or flattened to increase the bite area with the lining material
• Remove splash after welding, and apply anti-corrosion treatment to the weld (apply high-temperature anti-rust paint)

It should be noted that the anchor is strictly forbidden to be welded directly on the crest or trough of the bellows, and should only be welded at the end flange, intermediate connector or reinforcing ring position of the expansion joint.

4.3 Backing Laying

• Cut the ceramic fiber blanket to a size slightly larger than the spacing of the anchors
• Laying by "staggered seam lamination", and the overlapping width of two adjacent blankets is not less than 50mm
• Fixing the fiber blanket to the anchor with special penetrating nails or tablets
• The fiber direction of the fiber blanket shall be perpendicular to the direction of heat flow (i.e. the blanket surface shall be parallel to the shell)

4.4 Installation of Main Insulation

• Laying a ceramic fiber board or a second layer of fiber blanket over the backing layer
• Likewise use staggered seam laying, at least 100mm staggered from the seam of the backing layer
• For fiberboard, it can be attached and fixed with high temperature adhesive, with anchors passing through reserved holes
• 3-5mm thick ceramic fiber paper is embedded in the expansion joint position

4.5 Hot Surface Layer Construction (Castable Scheme)

For the expansion joint with castable as the hot surface layer, construction is the most critical:

• Template Support: Support the template according to the design thickness (usually 30-50mm), and paint the inner surface of the template with release agent
• Castable mixing: Use a forced mixer, strictly control the amount of water added (usually 5-7%) according to the manufacturer's instructions, and the mixing time is not less than 3 minutes
• Pouring construction: Use vibrating rod or flat plate vibrator to compact, and use manual tamping at the corners to avoid vibration leakage
• Reserved expansion joints: Insert 3-5mm thick fiber cardboard in predetermined position during casting, and take it out after initial setting
• Curing: After pouring, cover with plastic film for moisturizing and curing for at least 24 hours, and the ambient temperature shall not be less than 10℃

4.6 Thermal surface layer construction (module scheme)

For thermal surface layers with ceramic fiber modules:

• Install the modules one by one along the circumferential direction of the expansion joint, and fill the seams between the modules with compensation blankets
• Each module shall be fixed with at least 2 anchor bolts, and the bolt tightening torque shall meet the design requirements
• After the module is installed, flatten the hot surface layer with a special thorn roller, and apply high-temperature hardener if necessary

V. Common installation errors and avoidance measures

In practice, how to install the liner of the high-temperature flue expansion joint often has the following problems, which need to be paid attention to:

6. Drying and preparation before production

The liner is not ready for high-temperature operation immediately after installation, and must be dried (oven):

• Temperature rising curve: the temperature was raised to 110 ℃ at the rate of 25 ℃/h, and the free water was discharged at constant temperature for 24 h
• Second stage: raise the temperature to 350°C at 30°C/h and keep the temperature constant for 16h
• The third stage: raise the temperature to 600℃ at 40℃/h, and then cool naturally after constant temperature for 8h (depending on the needs of working conditions)
• During drying, slight negative pressure in flue is maintained, and water vapor is discharged in time

After drying, check the surface of the liner for cracks and peeling, and tap and listen to judge whether there is any cavity. It can only be put into operation after passing the qualification.

QUALITY INSPECTION STANDARDS

The high temperature flue expansion joint liner after installation shall meet the following quality specifications:

• Thickness deviation: Design thickness ± 5mm
• Surface crack: width ≤0.5mm and length ≤50mm, no penetrating crack
• Flatness: Check with 2m ruler, clearance ≤5mm
• Anchor exposure: The distance between the top end of the anchor and the hot surface after the hot surface layer is covered is 5-10mm

VIII. Summary

The core of how to install the liner of high-temperature flue expansion joint lies in understanding the composite structural logic and dynamic working conditions of the liner. The key to successful installation can be summarized as four key points: "reliable anchoring, delamination and staggering seams, reserved expansion and contraction, and control of water materials". In actual construction, the order of "backing layer → main insulation layer → hot surface layer" should be strictly followed. Anchors should only be fixed in rigid structural parts, and each layer of materials should be laid in staggered seams to avoid straight seams. The water consumption of castables should be strictly controlled and fully maintained. In particular, it should be noted that the drying process after installation is indispensable, and directly skipping the oven and putting it into high-temperature operation will cause the lining to burst due to rapid vaporization of moisture. Through standardized construction and inspection, the lining of high-temperature flue expansion joint can effectively protect the bellows, control the shell temperature below 300℃, significantly prolong the service life of the expansion joint, and avoid the flue blockage accident caused by the lining falling off.