In flue gas duct systems, expansion joints are the core components that compensate for thermal displacement, absorb vibrations, and protect equipment. However, even if the selection is correct and the quality is qualified, if the installation requirements of flue expansion joints are not implemented in place, it will also lead to leakage, deformation and even early failure. According to statistics, about 35% of expansion joint failures originate from wrong operation in the installation process. This paper systematically expounds the whole process standards of flue expansion joint installation requirements, covering the preparation before installation, positioning principles, welding/flange connection specifications, pre-stretching setting and acceptance standards, so as to help engineers do it right at one time and eliminate hidden dangers.

I. Pre-installation inspection-the first step to meet the installation requirements of flue expansion joints

Before formal hoisting, the following four inspections must be completed, which are the basic links in the installation requirements of flue expansion joints:

1. Check the model and parameters

Confirm that the following information is correct by comparing the design drawings and the physical goods arrived:

• The nominal diameter of the expansion joint matches the flue (allowable deviation ±5mm)
• Whether the design compensation amount satisfies the calculated value of the actual thermal displacement
• Corrugated pipe material grades (e.g. 316L, Inconel 625) meet the requirements of working conditions
• Guide direction marking-the arrow must coincide with the direction of flue flow

2. Appearance and transportation protection inspection

• No scratches, pits, corrosion or weld slag splashes on the bellows surface
• Flange sealing surface is smooth without radial scratches
• Transport fixing screw (if any) does not have bending deformation
• Port protective cover is intact and there is no foreign matter inside

3. Confirm the installation space and bracket status

• The length of the reserved straight pipe section on both sides of the expansion joint is ≥1.5 times the diameter of the flue
• The fixed bracket and guide bracket have been constructed according to the design drawings and the strength is up to standard
• Deviation of coincidence between stent centerline and flue centerline ≤3mm/m

If any item is not satisfied, it shall not be forcibly installed. Strictly implement the inspection link of flue expansion joint installation requirements, which can avoid 80% of installation quality problems.

2. Positioning and direction-the core installation principle

The most overlooked but critical installation requirements for flue expansion joints are positioning and directional control.

1. Determination of installation position

• The expansion joint shall be installed between two fixed brackets and as close as possible to one of the fixed brackets (usually ≤4 times the diameter of the flue)
• The centerline of the expansion joint must be coaxial with the centerline of the flue, and the coaxiality deviation shall be ≤5mm
• It is strictly prohibited to install the expansion joint within 300mm directly behind the flue elbow, reducer pipe or baffle door

2. Directionality requirements

• Expansion joint with guide tube: The open end of the guide tube must be directed in the direction from which the smoke is coming (upstream). After reverse installation, the high-speed dusty flue gas will directly wash the bellows trough, and the flue expansion joint leakage will inevitably occur within 3 months
• Single hinge expansion joint: hinge pin axis must be perpendicular to pipe displacement direction
• Complex universal hinge type: the plane of each hinge shall be consistent with the design drawing

3. Cold Draw (Pre-Draw/Pre-Compress) Settings

When the flue temperature is ≥350℃ under design conditions, the installation requirements of flue expansion joints usually require setting pre-stretching or pre-compression:

• Pre-stretching amount = theoretical thermal elongation ×0.5~0.7
• Pre-compression amount = theoretical cold shrinkage ×0.5
• Use special pull rods and jacks, and barbaric adjustment with crowbars is strictly prohibited

3. Connection methods-detailed specifications for welding and flanges

In the installation requirements of flue expansion joints, the connection process directly determines the sealing performance.

Welded connection (suitable for high temperature, high pressure or flue that is not suitable for disassembly)

Preparation before welding:

• Groove form: V-shaped or U-shaped, angle 30°-35°, blunt edge 1-2mm
• Remove oil, rust and oxide scale within 20mm of the bevel and both sides
• Preheating: When the base metal thickness is ≥20mm or the ambient temperature is

Welding process requirements:

• Adopt argon arc welding base + manual arc welding cover, and the welding electrode grade should match the base metal (for example, A022 welding electrode should be selected for 316L)
• The interlayer temperature is controlled below 150℃
• The weld excess height is ≤2mm, and no biting edges, pores or unfusing shall occur
• Small wire energy and rapid multi-pass welding should be used in the welding joint between bellows and end pipe, so as to reduce the influence of heat input on bellows

Post-weld treatment:

• Remove splash and weld slag, sand flat
• To recover the corrosion resistance of stainless steel corrugated pipe welding seam by acid pickling and passivation treatment

Flanged connection (suitable for flues requiring regular maintenance)

Gasket Selection:

• Temperature ≤400℃: Flexible graphite composite pad or metal wound pad (304/graphite)
• Temperature 400-650℃: Tooth-shaped combination pad (316L + flexible graphite layer)
• Temperature> 650°C: pure metal pad (e.g. Inconel cladding)

Bolt tightening requirements:

• The material of the bolt shall not be less than 35CrMoA (use heat-resistant alloy steel in high temperature zone)
• Cross-crossing sequence, tightening in three passes: 30% → 70% → 100% target torque
• Recommended torque value (M20 bolt): 200-250N·m in dry state, 160-200N·m in lubricated state
• After tightening, check the flange clearance, the difference between the four points on the circumference is ≤0.5mm

IV. Inspection and adjustment after installation-verify the installation requirements of flue expansion joint

Once the connection is complete, the following checks must be performed item by item:

1. Retest of concentricity and horizontality

• Check the concentricity of the flue at both ends of the expansion joint with laser centering instrument or pull wire method, and the deviation is ≤3mm
• Horizontally mounted expansion joint, the axis slope shall be consistent with the flue, the allowable deviation shall be ±2mm/m

2. Temporary fixture removal

This is the most forgotten step: temporary screws, pads or internal supports used to protect the bellows during transportation and installation must all be removed before the system can be put into operation. Otherwise, the expansion joint cannot deform freely and will tear due to excessive restraint.

3. Airtightness test

• Test pressure: 1.05 times design pressure, but not more than 1.15 times design pressure
• Hold time: ≥30 minutes
• Allowable pressure drop: ≤1%/10min
• Leak detection method: Apply leak detection liquid on the welding seam and flange joint surface. No bubbles are qualified

4. Displacement marking and initial recording

Mark the cold position reference line on the expansion joint housing and record the initial length and angle values. After the system runs to the hot state, measure the actual displacement again to verify whether it is within the design compensation range.

V. Common installation errors and their consequences