Treatment of Boiler Flue Expansion Joint Leakage: A Complete Protocol from Diagnosis to Repair

1. Harm and Urgency of Boiler Flue Expansion Joint Leakage

In power station boiler and industrial boiler system, flue expansion joint takes on the important functions of absorbing heat displacement, absorbing shock and noise and sealing flue gas. However, long-term exposure to high temperature, corrosive media and alternating stress environment, the expansion joint will inevitably have leakage problems. Whether the leakage treatment of boiler flue expansion joint is timely and appropriate is directly related to the operating efficiency of boiler, environmental protection emission index and the safety of on-site personnel. Flue gas leakage not only leads to the increase of energy consumption of induced draft fans and the aggravation of local flue corrosion, but also may lead to fines for exceeding environmental protection standards. More seriously, high-temperature smoke (up to 150-400℃) leaks to the surrounding areas, resulting in scald risks and fire hazards. Therefore, establishing a set of scientific and rapid leakage treatment scheme is the core skill that boiler operation and maintenance personnel must master. This article will systematically introduce the complete treatment process from leak type diagnosis, temporary plugging to permanent repair.

2. Type and cause diagnosis of boiler flue expansion joint leakage

2.1 Classification by leak location

Before carrying out boiler flue expansion joint leak treatment, it is first necessary to accurately locate the leak location. Common leak sites include:

2.2 Classification by leakage mechanism

Leakage caused by different mechanisms, boiler flue expansion joint leakage treatment methods are also completely different:

• Corrosion leakage: The flue gas contains SO₂, SO₃, Cl⁻¹ and other corrosive media, which forms acidic corrosion under the action of condensate, and the wall thickness of bellows gradually decreases to perforation. It is more common in the expansion joint at the inlet or low temperature section of desulfurization system.
• Fatigue leakage: Under the action of repeated thermal displacement, fine cracks appear at the crest or trough of the bellows, which gradually expand into penetrating cracks. It is common in peak shaving units with frequent start-and-stop.
• Overload leakage: The expansion joint is subjected to displacement beyond the design value due to bracket failure or installation pre-displacement error, resulting in local tearing of the bellows.
• Scour leakage: the guide tube falls off or the guide tube is not set, and the dusty flue gas directly washes the inner wall of the bellows, resulting in abrasion and perforation.

2.3 Methods for rapid diagnosis of leaks

The following methods can be used to quickly locate the leakage point on site:

• Visual inspection: Observe soot traces after shutdown. There is usually white or black soot accumulation around the leakage point
• Soap water leak detection: Spray soap water on the suspected leakage part, and continuous air bubbles can be seen under positive pressure
• Smoke generator: The negative pressure flue can release smoke near the expansion joint, and observe whether the smoke is inhaled
• Infrared thermal imaging: the temperature at the leakage is abnormal, and there are obvious hot or cold spots on the thermal image (positive pressure leakage is a hot spot, and negative pressure leakage of cold air is a cold spot)

3. Temporary treatment measures for boiler flue expansion joint leakage

When a leak is found and the boiler cannot be shut down immediately, the following temporary measures can be taken to control the leak and buy time for the planned shutdown.

3.1 Leak plugging under pressure (positive pressure flue)

For the leakage treatment of boiler flue expansion joint in positive pressure flue, pressure plugging is the most commonly used temporary scheme:

• Small hole leakage (≤3mm): Use tapered wooden plug or metal tapered plug to knock into the leakage hole, and apply high temperature sealant (temperature resistance ≥300℃) on the outside
• Crack leakage: Drill crack stop holes (φ 3-5mm) at both ends of the crack, and then tie them with clamps or steel strips, lined with high temperature resistant gaskets
• Flange leakage: Use flange pressure plugging fixture, inject high temperature sealing injection, or add clamp on the outside

It should be noted that pressure plugging is only used as a temporary emergency measure and should not replace permanent repair.

3.2 Outer Wrap Sealing

For large area leaks that cannot be plugged at a single point:

• High temperature resistant stainless steel sheet (thickness 0.5-1.0mm) is cut into wrapping shape
• Lined with ceramic fiber blanket or high temperature sealing gasket
• Tie and fix on the outside of the expansion joint with stainless steel belt or wire rope
• Apply high-temperature sealant to the interface

This method can significantly reduce the amount of leakage without stopping the furnace, and is suitable for short-term transitions before planned maintenance (generally no more than 1 month).

3.3 Temporary treatment of negative pressure flue

For negative pressure flue, the leakage is manifested by external cold air being sucked in. Although there is no risk of smoke leakage, it will affect the accuracy of smoke monitoring data and increase the load of induced draft fan. Provisional processing may be adopted:

• Apply high temperature resistant aluminum foil tape to the outside of the leak
• Apply high temperature repair mud
• In severe cases, it can be covered with a sealing cloth and vacuum attached

IV. Permanent leakage repair after boiler shutdown

4.1 Local repair welding

For corrosion pits or small-scale cracks, repair welding can be used to repair them. Repair welding in boiler flue expansion joint leakage treatment should strictly follow the following process:

• Clean-up: Thoroughly remove oil, rust and soot around the leakage point and polish to the natural color of metal
• Groove preparation: the crack is polished and grooved, the groove is deep to the bottom of the crack, and the angle is 60-70°
• Welding method: Tungsten Arc Welding (TIG), low current, fast welding
• Welding material selection: Match with base metal, stainless steel expansion joint with the same material welding wire; Transition welding material for dissimilar steel parts
• Post-weld treatment: polishing the weld residual height and performing 100% penetration test

IMPORTANT NOTE: The thickness of corrugated pipe substrate is usually only 1-3mm, which is easy to burn through or cause thermal deformation during repair welding. It is recommended to be operated by welders with thin plate welding qualifications, and the length of single continuous welding does not exceed 10mm, segmental jump welding.

4.2 Local replacement of bellows

When the leakage area is large (greater than 50cm²) or multiple leakage points are concentrated, local repair welding is no longer reliable, and local replacement of bellows should be performed:

• Remove the corrugated pipe from the damaged section, leaving both ends intact
• Prepare a new corrugated pipe section (the wave shape and material should be consistent with the original)
• Butt joint welding was used, and the weld seam was 100% radiographic inspection
• The compensation ability and stiffness of the expansion joint should be re-checked after replacement

4.3 Overall replacement of expansion joints

It is recommended to directly replace the expansion joint in its entirety instead of repairing it locally in the following cases:

• Multiple fatigue cracks in the bellows, approaching the design fatigue life (usually 1000 cycles)
• Improper material selection (if the original 304 is used, the actual need is 316L)
• The guide tube falls off and causes extensive wear of the bellows
• Structural defects of expansion joints (such as wave height and wave pitch not meeting the standard)

When replacing the whole, it is necessary to install in place, butt welding and limit adjustment according to the original design requirements.

V. Liner repair and seal strengthening

5.1 Liner Inspection and Repair

In the treatment of boiler flue expansion joint leakage, lining damage is often the cause of leakage. The repair steps are as follows:

1. Check the guide tube for detachment, deformation and wear
2. Check the heat insulation layer (ceramic fiber blanket) for burn and collapse
3. To repair local damage: fill in ceramic fiber cotton and apply high-temperature cement to the surface
4. If the guide tube falls off, it should be re-welded and fixed, and segment welding should be used to prevent deformation during welding

5.2 Flange Seal Replacement

For flanged connection structure leakage:

• Remove all bolts, clean up old gaskets and corrosion on flange surface
• Check the flatness of the flange surface and smooth the local high points with a grinder
• Select a suitable new gasket: Stainless steel clad graphite pad or high strength graphite composite pad is recommended
• Tighten the bolts 2-3 times in cross-symmetric order to achieve the specified torque
• After tightening, check that the flange clearance is uniform and the deviation is ≤0.3mm

Maintenance strategies to prevent leakage

6.1 Regular inspection system

The establishment of a regular inspection system for expansion joints can greatly reduce the risk of sudden leakage:

• Monthly: Visually check for soot traces and discoloration of the insulation layer
• Quarterly: Infrared thermography examination to compare temperature distribution for abnormalities
• Semi-annually: Internal inspection at shutdown to measure bellows wall thickness (ultrasonic thickness measurement)
• Annually: Check the integrity of deflectors, anchors, lining materials

6.2 Leak warning indicators

Incorporate the following indicators into the boiler operation monitoring system for early warning:

• Induced draft fan current is abnormally increased (flue leakage increases air volume demand)
• Abnormal flue oxygen (high oxygen due to negative pressure leakage and inhaled air)
• Sudden change in surface temperature of expansion joint (abnormal temperature near leakage point)

6.3 Life Management Ledger

Establish a full lifecycle profile for each expansion joint:

• Date of commissioning, design fatigue life
• Time and frequency of each start-and-stop
• Record of previous leaks and repairs
• Analysis of the trend of wall thickness thinning

When the cumulative number of cycles reaches 80% of the design life, it is included in the scheduled replacement list.

VII. Common problems and countermeasures

VIII. Summary

Boiler flue expansion joint leak treatment is a systematic work from rapid diagnosis to standard repair. The key to successfully handling leaks can be summarized as follows: "locate first, classify, plug if urgent, fill if slow, and replace if serious". For the leakage occurring in operation, first of all, the leakage location and mechanism must be accurately judged. Positive pressure flue can be temporarily controlled by pressure plugging, but it must not be relied on for a long time; After the planned furnace shutdown, repair welding, local replacement or overall replacement schemes should be selected according to the damage degree of the bellows, and the lining, guide tube and flange seal should be repaired simultaneously. In the long run, it is more economical and effective to establish a regular inspection system, life management ledger and leak warning indicators than passively handling leaks. For the boiler flue with frequent leakage, it is recommended to fundamentally review whether the expansion joint selection is reasonable, whether the bracket is set correctly, and whether the operating conditions are beyond the design range. Through standardized leakage treatment and perfect preventive measures, the risk of unplanned shutdown caused by expansion joint leakage can be minimized, and the safe, environmental protection and economic operation of boiler system can be ensured.