In coal-fired power plant boiler and desulfurization system maintenance, flue expansion joint replacement or installation is a key operation. Because the construction environment is complex (high temperature, high altitude, confined space), and involves hot work and lifting, it is the prerequisite to make a scientific and feasible construction scheme of flue expansion joint in power plant to ensure construction safety and quality. This paper will systematically explain the main points of the construction scheme of flue expansion joint in power plant from the aspects of construction preparation, dismantling and installation technology, quality control and safety measures, so as to provide professional reference for field technicians.

I. Overview of construction scheme of flue expansion joint in power plant

The construction scheme of flue expansion joint of power plant is a technical guidance document prepared for the installation and replacement of non-metal or metal expansion joint in flue duct system of power plant. The plan should cover the whole process requirements from construction preparation, removal of old expansion joints, installation of new expansion joints to final acceptance, and clarify the quality control points and safety risk prevention and control measures of each process.

The construction plan shall follow the following principles:

• Safety first: special prevention and control is required for hot fire, high altitude and lifting operations
• Quality-oriented: Ensure reliable sealing and free compensation after installation
• Protection priority: It is strictly prohibited to damage bellows or skin during construction

II. Requirements for construction preparation stage

2.1 Technical preparation

Before the commencement of the project, the construction plan shall specify the following technical preparations:

• Joint review of drawings: check that the model, installation position and flow direction mark of expansion joint are consistent with the design
• Prepare detailed safety technical measures and make written explanations to all construction personnel
• Calculate cold tightening or pre-stretching (if required by design)
• Confirm the setting scheme of expansion joint bracket and limiting device

2.2 Materials and Tools Preparation

Main construction tools:

• Lifting equipment: 25t truck crane or on-site running
• Welding equipment: AC/DC welding machine
• Cutting equipment: plasma cutting machine, angle grinder
• Testing tools: Level, theodolite, level ruler, steel tape measure

Main material:

• New expansion joint (passed the unpacking inspection)
• Welding material (electrode/wire matched to base metal)
• Sealing packing (high temperature sealant, ceramic fiber rope, etc.)
• Temporary reinforcement support

2.3 Confirmation of operating conditions

The following conditions should be confirmed before construction:

• The flue system has been shut down and cooled down, and there is no residual flue gas or coal powder in it
• Construction water, electricity and gas have been connected to the operation point
• The scaffold is erected and accepted
• Set up enclosures and warning signs in the construction area

III. Construction process of flue expansion joint

The following is the core construction process of the flue expansion joint construction scheme of power plant, taking the replacement of non-metallic expansion joints as an example:

3.1 Removal of old expansion joint

Step 1: Setting up isolation protection
An isolation protective layer is set above the bellows or skin to prevent cutting sparks from damaging the flexible components.

Step 2: Remove the connection accessories
Use a wrench or gas cutting to remove connectors such as platen bolts and positioning screws. For bolts with serious corrosion, gas cutting can be used, and attention should be paid to protecting the metal frame when cutting.

Step 3: Hoisting and unloading the old expansion joint
Set a lifting point above the expansion joint, hang a hand-pulled hoist or lifting belt, and lift the old expansion joint as a whole. Pay attention to balance during hoisting to prevent collision with peripheral equipment.

3.2 Flue interface inspection and treatment

After the expansion joint is removed, the flue interface needs to be inspected and treated:

1. Clean Surfaces: Remove old gaskets, sealants, corrosion and debris
2. Interface alignment: Check the deviation of the center of the two interface flanges or pipes. The deviation of spacing is within ±5mm and the deviation of parallelism is ≤3mm
3. Defect repair: repair or repair welding and polishing the deformed or missing flange surface

3.3 New Expansion Joint Installation

Step 1: Pre-check

• Check that the specification, model and pressure level of the new expansion joint are consistent with the design
• Check that the appearance is not damaged or deformed, and the flow direction sign is correct
• Verify that the direction of the guide tube is consistent with the direction of flue gas flow

Step 2: Lift into place
Lifting equipment is used to lift the new expansion joint smoothly to the installation position and slowly approach the flue interface. It is strictly prohibited to push or pull or tap the expansion joint body strongly.

Step 3: Counterpart alignment

• Adjust the coaxiality between the expansion joint and the flue with the deviation ≤3mm
• Control the amount of cold tightening or pre-stretching (as required by design documents)

Step 4: Welding/Fastening

• Metal expansion joint: spot welding and positioning first, and then full welding after inspection and confirmation
• Non-metallic expansion joint: Install pressure plate and bolt in sequence, tighten evenly at diagonal angles, and the torque meets the design requirements

Step 5: Remove the transport tie rod
After the expansion joint is installed in place and the pipeline is fixed, the positioning tie rod or temporary support used for protection during transportation must be removed, and warning signs must be posted to indicate that it has been removed.

3.4 Sealing and Thermal Insulation Restoration

• Fill with sealing packing (e.g. ceramic fiber rope) and apply high temperature sealant
• Restore external insulation of expansion joint (if designed)
• Install protective cover on the outside of non-metallic expansion joint as required

4. Key Points of Quality Control

In the construction scheme of flue expansion joint of power plant, the following quality control points should be monitored:

Safety and Environmental Protection Measures

5.1 Key Safety Risks and Countermeasures

Risk 1: Falling from Height

• A firm scaffold shall be set up on the working surface, which can only be used after passing the acceptance inspection
• Workers at heights wear 100% seat belts and hang them at reliable anchoring points

Risk 2: Hot fire

• Handle hot work tickets, and provide fire extinguishers and fire blankets on site
• Clean up the combustible materials around the fire point, and set up special personnel to monitor it
• Take measures to prevent slag from splashing onto bellows or skin during welding

Risk 3: Lifting Injury

• Overloading of cranes and spreaders is strictly prohibited after passing the inspection
• It is strictly forbidden to stand under the hanging object, and the signal worker shall command it uniformly

Risk 4: Confined Space Injury

• Gas detection (O₂, CO, H₂S) before working inside the flue
• Set up special supervision to maintain smooth internal and external communication

5.2 Environmental protection measures

• Classification and recycling of dismantled old expansion joints and waste insulation materials
• Take baffle or vacuum measures for cutting and welding operations
• Construction waste shall be removed and transported in time, and shall not be stacked at will

Acceptance and Records

After the completion of the construction, the acceptance shall be organized and recorded according to the following items:

1. Appearance inspection: The expansion joint is installed smoothly, without twist, and the bolts are fully tightened
2. Seal inspection: Light leakage method or kerosene penetration method can be used to check the sealing performance of welds and joints
3. Displacement Check: Confirm that the positioning rod is removed and the expansion joint is in free condition
4. Data filing: including construction logs, concealed project acceptance records, welding records, expansion joint certificates, etc

VII. SUMMARY

Compiling and strictly implementing a complete construction scheme of flue expansion joint in power plant is the core measure to ensure construction safety and the installation quality of expansion joint. The key takeaways are summarized as follows:

1. Fully prepared: technical disclosure, materials and tools, and operating conditions are indispensable. The unpacking inspection should check the model number, flow sign and cosmetic integrity.
2. Process specification: dismantling → interface treatment → hoisting in place → alignment of counterpart → welding/fastening → dismantling of transportation rod, the sequence of each process can not be reversed.
3. Protection in place: protective measures are taken during the construction process, and welding slag is strictly prohibited from damaging the bellows/skin; Strongly centering or removing the positioning rod before installation is strictly prohibited.
4. Quality control: Focus on monitoring interface alignment (coaxiality ≤3mm), bolt tightening (uniform diagonal), weld quality and sealability.
5. Safety red line: hot operation has tickets and monitoring, high operation fastens seat belts, lifting operation is directed by a special person, and restricted space is detected first before entering.
6. Traceability: The inspection records and acceptance data of the whole construction process should be archived and saved for future maintenance and reference.

It is suggested that the construction unit organize the construction personnel to study the scheme and disclose the safety technology before entering the site, and clarify the responsibilities of each post. During the implementation process, strictly implement the process acceptance system, and only after the previous process passes the acceptance can you enter the next process. Through refined construction management, the installation quality of flue expansion joint is ensured to meet the standard at one time, which lays the foundation for the long-term safe operation of smoke and air system of power plant.