1. Necessity and difficulty of replacing metal expansion joint in flue of power plant

In power plant boiler and desulfurization and denitrification system, the metal expansion joint has been subjected to the joint action of high temperature flue gas erosion, alternating thermal stress and corrosive medium for a long time, and gradually problems such as bellows cracking, guide tube falling off and seal failure appear. When the leakage exceeds the standard or the structural damage affects the safe operation, it is necessary to face the practical problem of how to replace the metal expansion joint in the flue of power plant. Compared with new installation construction, replacement operation is often limited by various unfavorable factors such as shutdown time window, limited space, deformation of old structure, etc., which puts forward higher requirements for construction scheme and on-site operation.

Preparation before replacement: from furnace shutdown conditions to technical data

2.1 Furnace shutdown conditions and safety isolation

To answer how to replace the metal expansion joint of the flue of the power plant, first of all, the safety premise should be clarified. The flue where the expansion joint to be replaced is located must be randomly shut down, and the following isolation measures must be completed:

• Cool to below 50℃: Forced ventilation or natural cooling to prevent scalding and thermal deformation
• Media cut-off: Close baffle door, plug-in valve, and install blind plate if necessary
• Gas replacement: The flue gas system shall be subjected to air replacement, and the oxygen content and toxic and harmful gases (CO, SO₂, etc.) shall be detected
• Lock and tag: execute energy isolation procedure, prohibit misoperation

2.2 Site Mapping and Condition Assessment of Old Expansion Joints

Due to long-term high-temperature operation, the original installation dimensions may have changed, and it is strictly prohibited to purchase replacement parts directly based on as-built drawings. On-site measurements shall be made:

• Actual spacing of flue flanges or interfaces (3 measuring points each upstream and downstream)
• Axial and transverse offset of bellows
• Connection mode (flanged, direct welded or plug-in)
• Wear and dust accumulation of guide tube

According to the measured data, determine the length, wavenumber and interface size of the replacement parts to ensure "new and old".

2.3 Replacement Plan Preparation and Acceptance Criteria

Before replacement, a construction plan shall be formulated, including at least: dismantling and hoisting methods, welding process requirements, counterpart accuracy standards, non-destructive testing proportions, pressure and leakage test plans. The acceptance criteria are usually DL/T 1121 Technical Specifications for Metal Expansion Joints for Flue Gas Dust Removal of Boilers in Coal-fired Power Plants or GB/T 12777.

3. Demolition operation: How to safely and completely remove the old expansion joint

3.1 Sequence of removal of expansion joint

Demolition is the key first step to answer how to replace the flue metal expansion joint of a power plant. It is recommended to operate in the following order:

1. Cleaning up dust accumulation: Remove dust accumulation at the bottom of wave trough, guide tube and flue to reduce hoisting weight
2. Release Tie Rod/Limit Device: Record factory pre-tension or cold tightening position, loosen nut or remove tie rod
3. Cutting welds or removing flanges: If it is a welded structure, leave 5-10mm margin for cutting along the flue side to avoid damage to the main flue base metal
4. Sectional disassembly: For large-size rectangular expansion joints, one-sided corrugated components can be disassembled first and hoisted out in blocks
5. Groove trimming: Remove residual welds and reprepare grooves according to welding process

3.2 Cutting Method and Flue Protection

It is recommended to use plasma cutting or mechanical cutting, and use gas cutting with caution (large heat influence and easy splash). When cutting, adjacent equipment and cables are shielded from fire, and fire extinguishing equipment is equipped in the flue.

3.3 Hoisting and temporary support

Immediately after the old expansion joint is removed, temporary supports or temporary blind plates shall be set on the disconnected surface of the flue to prevent the pipe from sinking or foreign objects falling into the system. Measure the staggered notch and non-parallelism of two sections simultaneously, as the adjustment basis of the new expansion joint in place.

4. Installation of new expansion joint: countermouth, welding and limit adjustment

4.1 Pipe Matching and Pre-Compression/Pre-Stretching

When installing a new metal expansion joint, it is necessary to determine whether pre-displacement is required according to the design displacement direction and the actual temperature at the time of installation:

• Installation at normal temperature (≤20℃): Pre-stretch or pre-compress according to the cold tightness given by the design institute
• Hot state (> 100℃) emergency repair installation: Calculate the thermal elongation corresponding to the actual shell temperature and adjust the installation length

Allowable deviation of counterpoint: axial deviation ≤3mm, radial misalignment ≤1.5mm (based on the thin-walled side), strong counterpoint is strictly prohibited.

4.2 Welding Process and Weld Quality

Welding shall be performed in accordance with the qualified WPS as assessed. For how to replace the metal expansion joint of the flue of the power plant, it is recommended that:

• Base layer: TIG welding to ensure root penetration and no oxidation on the back
• Filling and cover layer: SMAW or semi-automatic welding, strictly control the interlayer temperature ≤150℃
• Welding sequence: welding in segments symmetrically along the circumference to reduce shrinkage deformation

For different steel welding between austenitic stainless steel expansion joint and carbon steel flue, transition layer welding material (such as E309 series) should be selected, and small parameter and rapid welding process should be adopted to reduce hot crack and dilution rate.

4.3 Resetting of pull rod and limit device

After welding and cooling, reset tie rod nuts according to design requirements:

• Structure for transmitting thrust: The pull rod shall be in a stressed state, and the nut shall be properly tightened
• Structures for transportation protection only: The tie rod should be completely loosened or removed to avoid restricting the normal operation of the expansion joint

For structures with hinges or gimbal rings, confirm that the rotation is flexible and not stuck.

V. Inspection and acceptance: the key link to ensure the quality of replacement

5.1 Non-destructive testing of welds

• Surface test: 100% penetration test (PT) or magnetic particle test (MT) for all pressure-bearing or sealed welds, Class I qualified
• Internal inspection: Ray inspection (RT) for butt welds on a proportional basis, not less than Grade II

5.2 Tightness Test

The tightness test shall be carried out after the metal expansion joint is replaced. Common methods:

• Negative pressure flue: Start the induced draft fan to establish negative pressure, and use a candle or smoke generator to check the weld seam leakage
• Positive pressure flue: under the premise of safety and control, 0.05-0.1MPa compressed air pressure is maintained, and leak detection liquid is applied

5.3 Hot state review and operation monitoring

After the start-up of the unit, infrared thermal imaging and displacement observation of the expansion joint shall be carried out at 30%, 60% and 100% load stages:

• There is no abnormal overtemperature or local overheat on the bellows surface
• The actual displacement direction is consistent with the design
• No abnormal vibration or noise

VI. Frequently Asked Problems and Suggestions

VII. SUMMARY

How to replace the metal expansion joint of the flue of the power plant is not a simple "dismantling the old and installing the new", but a set of systematic engineering including furnace shutdown isolation, surveying and mapping customization, cutting and dismantling, accurate matching, welding control and tightness verification. The key to successful replacement is to guide the manufacture of new parts by measured data, control the deformation by symmetrical welding, and ensure the seal by specification inspection. For power plant maintenance personnel, selecting a construction team with similar coal-fired or gas-fired unit replacement experience, operating in strict accordance with the welding process regulations, and allowing sufficient hot state review time after replacement can significantly reduce the risk of secondary leakage and the probability of non-shutdown of units. Through the standardized replacement operation, not only the sealing and compensation function of the flue system can be restored, but also the whole life cycle of the expansion joint can be prolonged, which provides reliable guarantee for the safe and stable operation of the unit.