In the boiler flue duct system and desulfurization flue of power plant, the expansion joint is the key component to withstand thermal displacement and vibration reduction. However, expansion joints are prone to leakage problems due to long-term operation in harsh environments with alternating action of high temperature, corrosion, vibration and negative pressure. Once the flue expansion joint of power plant leaks, it will not only cause heat loss and increase the energy consumption of induced draft fan, but more seriously, it may lead to the leakage of dust and sulfur dioxide in flue gas, resulting in environmental protection exceeding the standard accident. Therefore, it is the core skill that power plant equipment maintenance personnel must master to locate the cause of leakage quickly and accurately and take effective treatment measures. This paper will start with the analysis of leakage types and causes, systematically introduce various treatment schemes such as leakage plugging with pressure, shutdown replacement, non-metal skin repair, and give preventive suggestions according to actual working conditions.

1. Common types and hazards of expansion joint leakage

To effectively handle flue expansion joint leakage in power plants, it is necessary to first identify the specific type of leakage. According to the leakage location and manifestation form, it can be divided into the following three categories:

1. Cracking and leaking of metal bellows

Bellows with metal expansion joints may generate penetrating cracks under long-term thermal fatigue or stress concentration. This leak usually appears as a thin line or mesh slit, and the leaked smoke is white or pale gray. Because the wall thickness of bellows is thin (usually 1-3mm), the crack propagation rate is fast, and if not treated in time, it may evolve into large area tear.

2. Damage and leakage of non-metallic skin

The skin of non-metallic expansion joints is composed of multi-layered fabric composite materials, which can appear pinholes, delamination or large-area tearing under high-temperature aging, mechanical damage or chemical corrosion. The leakage form of skin damage is usually diffuse leakage, and smoke is visible from the damaged place. This type of leakage is particularly common in desulfurization flues.

3. Flange or interface seal failure leakage

Aging of the sealing gasket at the joint between the expansion joint and the flue flange, slack bolts, or deformed flanges can lead to leakage at the interface. This type of leakage usually occurs around bolt holes or at flange surface gaps, and the amount of leakage varies with fluctuations in flue pressure.

The harm of flue expansion joint leakage in power plant cannot be ignored: First, the current of induced draft fan rises and the power consumption increases; Second, the flue negative pressure drops, which affects the combustion stability of the boiler; Third, sulfur-containing flue gas leakage corrodes peripheral equipment and steel structure; Fourth, the concentration of environmental protection emissions exceeds the standard and faces the risk of punishment.

2. Systematic analysis of leakage causes

In order to solve the leakage problem thoroughly, the cause of leakage of flue expansion joint in power plant must be analyzed from the root cause. Common causes include:

1. Improper design selection

• Insufficient compensation: The actual thermal displacement exceeds the designed compensation capacity of the expansion joint, resulting in excessive stretching or compression of the bellows and cracking
• Material mismatch: ordinary 304 stainless steel is selected in the high-temperature section, which leads to high-temperature creep cracking; Corrosion resistant material is not selected for corrosion section
• Missing or unreasonable design of guide tube: dust directly washes the bellows, accelerating wear and thinning

2. Installation quality problems

• Installation without pre-tensioning or pre-compression causing the expansion joint to be in a stressed state under cold conditions
• The guide bracket is set too far or missing, causing the bellows to bear lateral forces
• Transport fixtures are not removed, limiting normal deformation of expansion joints
• Welding spatter burns the bellows surface, causing local stress concentration

3. Abnormal operating conditions

• The boiler starts and stops frequently, and the number of thermal cycles far exceeds the design fatigue life
• Smoke temperature exceeds limit operation, exceeding the design temperature resistance grade of expansion joint
• The flue gas of desulfurization system carries slurry backward and corrodes the non-metallic skin

4. Missing maintenance

• Failure to inspect regularly, minor leakage could not be discovered in time
• Non-metallic skin overdue service, material aging failure
• Blocked drain hole, condensate accumulation and corrosion

Emergency measures and repair schemes for leakage treatment

According to the severity of leakage and shutdown conditions, the leakage treatment of flue expansion joint in power plant can be divided into two levels: emergency treatment and permanent repair.

1. Leak plugging with pressure (no stop treatment)

For small area leakage, the following temporary measures can be adopted without affecting the environmental protection standards:

• Metal bellows crack: Apply high-temperature cement or ceramic fiber filler to the crack surface, and can be temporarily blocked after solidification. Stainless steel belt clamp with high temperature resistant sealing gasket can also be used for outer hoop treatment.
• Non-metallic skin pinhole: Coated with high temperature resistant silicone or fluororubber sealant, which forms an elastic sealing layer after curing.
• Flange interface leakage: Inject high-temperature sealant into the leakage part, or gradually tighten the flange bolt (it needs to be operated after shutdown, and it is strictly prohibited to tighten substantially during operation).

It should be emphasized that leak plugging under pressure is a temporary measure, which is only applicable to short-term maintenance in emergency situations, and should not exceed 3 months at most, after which it must be shut down for complete repair.

2. Shutdown replacement or repair (permanent treatment)

During planned downtime or service windows, appropriate permanent repair actions should be taken depending on the type of leak.

(1) Cracking treatment of metal bellows

• For local cracks and the overall condition of the bellows is good, repair welding can be used to repair them. It is necessary to select welding materials matching with the base metal, clean the crack area before welding, and carry out coloring flaw detection after welding.
• For multiple cracks or large-area aging, the expansion joint should be replaced as a whole. The design compensation amount, pressure and temperature parameters should be re-checked during replacement.

(2) Non-metallic skin damage treatment

• Local damage can be repaired by skin: cutting composite fabric with the same specification, bonding or sewing with high-temperature glue, and fixing by pressing.
• When a large area is aged or torn, the skin should be replaced as a whole. Replacement steps are as follows:
  • Removal of old skin and insulation
  • Clean the corrosion and residue on the surface of the frame
  • Installation of new insulation (aluminum silicate fiber blanket, thickness ≥50mm)
  • Lay a new skin, from the inside to the outside in order of fluorine adhesive cloth, glass fiber cloth, outer silicone cloth
  • Fixed with pressure plates and bolts, uniform torque

(3) Flange interface repair

• To replace the failed gasket, it is recommended to use high temperature resistant graphite composite gasket or flexible graphite packing
• Correction of flange flatness, field machining if necessary
• Replace corroded bolts and tighten them evenly according to the design moment diagonal

4. Treatment points of leakage of expansion joints in different parts

V. Maintenance suggestions to prevent leakage

The best strategy for dealing with flue expansion joint leakage in power plants is prevention. The following measures can effectively reduce the incidence of leakage:

1. Establish a regular inspection system: monthly appearance inspection, focusing on whether there are corrosion spots on the bellows surface, damage to the skin, and loosening of the bolts
2. Infrared temperature measurement monitoring: Infrared thermal imaging camera is used to detect the surface temperature distribution of expansion joint. Abnormal hot spots indicate the failure of heat insulation layer
3. Record the number of starts and stops: The fatigue life of metal expansion joints is usually 1000-2000 cycles, and spare parts should be prepared in advance when approaching the life
4. Drainage management: regularly dredge drainage holes to prevent long-term soaking and corrosion of condensate
5. Spare parts management: One set of spare parts with the same specification should be reserved for expansion joints in key parts, and at least one set of non-metal skin spare parts should be reserved
6. Maintenance window repair: Take advantage of the opportunity of minor or major repair of the unit, check all expansion joints one by one, and deal with any aging and minor damage in time to avoid leakage accident

VI. Evaluation criteria of treatment effect

After completing the treatment of flue expansion joint leakage in the power plant, the quality of repair should be evaluated from the following dimensions:

• Airtightness: No visible smoke leakage after repair, no bubbles in soapy water detection
• Appearance integrity: No abnormal deformation or bulging of bellows or skin
• Normal temperature: Infrared temperature measurement shows that the surface temperature of the expansion joint is uniform and there is no local overheating
• System parameter recovery: induced draft fan current returns to normal range, flue negative pressure is stable

conclusion

Leakage treatment of flue expansion joint in power plant is a systematic work involving fault diagnosis, emergency plugging, shutdown repair and preventive maintenance. This paper systematically analyzes the causes and harms of three common leakage types: cracking of metal bellows, damage of non-metal skin and failure of flange joints. According to different working conditions, the temporary measures of leakage plugging under pressure and the concrete schemes of permanent repair after shutting down are given. The core processing principle can be summarized as follows: small leakage is temporarily repaired, and large leakage must be replaced; Metal cracks can be repaired and welded, and skin aging can be replaced as a whole; The flange interface should be re-sealed and the drainage hole should be open. At the same time, the establishment of regular inspection, infrared monitoring and life management system can reduce the probability of leakage from the source. It is hoped that this paper can provide a set of complete and practical technical guidelines for leakage treatment for the operation and maintenance personnel of boiler, desulfurization and dust removal in power plants, and help the power plants to achieve the operation objectives of safety, environmental protection and economy.