In the operation of industrial flue gas treatment system, the burst of flue expansion joint is one of the sudden failures that cause the most headache for operation and maintenance personnel. After a loud noise, high-temperature flue gas gushed out from the breach of the expansion joint, and the scene was forced to shut down urgently-this scene is not uncommon in power plants, chemical plants and oil refineries. The burst of flue expansion joint not only causes economic losses, but also threatens the safety of personnel on site. This paper will systematically analyze the root cause and prevention measures of flue expansion joint bursting from the failure mechanism, typical cases to emergency treatment.

1. Main manifestations of flue expansion joint burst

The typical manifestation of flue expansion joint burst is: the bellows or skin of the expansion joint suddenly ruptures during operation, and a large amount of flue gas leaks. Depending on the type of expansion joint, there are also differences in burst forms:

2. Analysis of Main Reasons for Flue Expansion Joint Burst

Through the analysis of multiple failure cases, the causes of flue expansion joint bursting can be summarized into the following four categories:

1. Material problem: "chronic poison" of high-temperature corrosion

Explanation: The material selection of the expansion joint is improper, and it deteriorates rapidly in the high-temperature corrosive environment, and eventually loses strength and bursts.

Typical case 1: For the ignition air duct expansion joint of a 410t/h boiler in a factory, the technical agreement requires that the metal material performance of the part in direct contact with the flue gas should not be lower than 0Cr25Ni20Si2 (310S), but ordinary stainless steel is actually used. In high temperature flue gas, common stainless steel corroded and deteriorated rapidly, resulting in the breakage of expansion joint。

Typical case 2: Expansion joint of flue gas pipeline of a catalytic cracking unit, working temperature 700℃, corrugated pipe material is Inconel625 nickel-based alloy. Failure analysis shows that δ phase Ni3 (Nb, Mo) precipitates at the grain boundary after corrosion, resulting in Cr-poor region at the grain boundary, which makes it difficult to form a dense oxide film, and S element corrodes the matrix. NiS and Ni formed eutectic crystals with a melting point of only 645℃ at high temperature, which penetrated between the grains after melting and intergranular corrosion occurred。

Core lesson: High-temperature resistant alloys must be selected in high-temperature sulfur-containing environments, and the materials should not be degraded. The 310S/Inconel625 is the first choice for the high temperature segment, and the ordinary 304/316 cannot withstand it for a long time.

2. Design defects: congenital deficiencies and hidden dangers

Explanation: The design parameters of the expansion joint do not match with the working conditions, or there are weak links in the structure itself.

Case: Within 8 months after the desulfurization units of two 660MW units in a power plant were put into operation, a large overall displacement occurred from GGH to the original flue of the absorption tower, which cracked the expansion joint at the entrance of the absorption tower, resulting in flue gas leakage。 After analysis, the original design did not fully consider the influence of the flue overall thermal displacement on the expansion joint.

Core lesson: The selection and design of expansion joints must fully consider the thermal displacement of actual working conditions, and leave sufficient safety margin.

3. Improper installation: field operation buried minefield

Explanation: The installation stage did not operate according to the specifications, resulting in hidden dangers in the expansion joint before it was put into operation.

Case: The expansion joint of the ignition air duct of a 410t/h boiler in a factory did not reserve an installation misalignment of 80mm according to the design requirements during installation. During the operation, the bellows of the expansion joint was seriously deformed, and the internal pouring material was damaged and fell off。

Core lesson: The installation must be carried out strictly according to the design requirements, and the pre-offset and cold tightness value should not be omitted.

4. Process operation: the operating conditions exceed the design

Explanation: The actual operating conditions are beyond the design tolerance range of the expansion joint, such as over-temperature, over-pressure, frequent start-stop, etc.

Case: During the ignition of the above 410t/h boiler, due to the excessive rated output of the burner oil gun, the low atomization air pressure, the insufficient combustion of light diesel oil, the temperature of the combustion chamber connected to the expansion joint exceeds the design temperature, and the high-temperature flue gas directly enters the bellows of the expansion joint, which finally leads to the damage of the expansion joint。

Core lesson: The process operation should be controlled within the design parameters, and the ignition heating rate should be controlled in a standardized manner.

3. Emergency treatment of flue expansion joint burst

When the flue expansion joint bursts, the following steps should be followed for emergency treatment:

1. Emergency disposal (in operation)

Reference to the treatment experience of a gas turbine power plantEmergency response measures include:

• Temporary plugging: Use high temperature resistant sealing tape, silicone or steel plate for rapid plugging
• Shield protection: Enclosure protection for nearby equipment to prevent continuous damage by high-temperature smoke
• Load Reduction: Temporarily reduce system load and leakage if conditions allow
• Apply for shutdown: Serious leakage should be applied for shutdown treatment immediately

Note: Temporary blocking is an emergency measure only and is not a substitute for permanent repair.

2. Permanent repair after downtime

• Replace skin/bellows: Replace broken skin or bellows
• Structural upgrade: Consider structural optimization and material upgrade after analyzing the cause of bursting

IV. Prevention and control measures and improvement suggestions

1. Material upgrade

2. Structural Optimization

In view of the high-risk working conditions such as catalytic cracking units, it is suggested to adopt the concept of "single layer bearing pressure, double layer design", supplemented by a bellows interlayer leakage early warning system with remote monitoring function。 Once the inner layer leaks, the monitoring system can alarm in time to avoid unplanned parking caused by sudden burst。

3. Installation quality control

• Reserve pre-bias strictly according to design
• Remove transport tie rod in time after installation
• Verify that the guide tube is oriented correctly

4. Operation and maintenance management

• Control the heating/cooling rate of the start-stop machine to avoid overheating
• Inspect the appearance and wall thickness of the expansion joint during each overhaul
• Establish expansion joint condition monitoring ledger (maintenance log)
• Protect against corrosion of dew point during shutdown maintenance

V. Summary

The burst of flue expansion joint is the result of the superposition of many factors, and its prevention and control should start with the whole chain of design, manufacture, installation and operation:

Behind a burst accident, there are often omissions in material, design, installation and operation. Only through the three-pronged approach of source checking, process control and regular maintenance can the risk of flue expansion joint bursting be minimized. It is recommended that all enterprises conduct a comprehensive inspection of the expansion joint during the maintenance period, and replace the deteriorated products in time to avoid "small problems leading to big accidents".