In industrial pipeline system, the flue expansion joint is an important part to compensate the thermal displacement and absorb the vibration of the pipeline. Its sealing state directly affects the operation efficiency and safety of the whole system. However, flue expansion joint leakage is one of the common equipment failures in many power plants, chemical plants and steel enterprises. Once air leakage occurs, it will not only cause energy waste, but also lead to environmental penalties and even safety accidents. This paper will systematically analyze the main causes and typical hazards of flue expansion joint leakage, and provide effective detection and repair strategies to help enterprises solve the problems quickly and restore the stable operation of the system.
1. Common causes of gas leakage in flue expansion joint
To solve the problem of flue expansion joint leakage, it is first necessary to accurately locate its root source. According to on-site maintenance experience, air leakage is usually caused by the following types of factors:
- Fatigue cracking of bellows
Fatigue cracks may occur in corrugated pipe material under the long-term alternating temperature and pressure of metal expansion joint. Such cracks are small and difficult to detect at the initial stage, but they will gradually expand over time, eventually leading to obvious air leakage. - Flange sealing surface failure
Aging and corrosion of the sealing gasket at the connecting flange or uneven bolt preloading force during installation will cause gaps on the joint surface of the flange, which will lead to air leakage of the flue expansion joint. Especially in the high temperature flue gas environment, the common rubber gasket is prone to carbonization and failure. - Improper structural design or selection
If the compensation of the expansion joint is insufficient, the stiffness is not matched or the corrosion resistance grade of the material is low, excessive additional stress will be generated during the system operation, which will accelerate the structural damage and induce air leakage. - External mechanical injury
Bumps during installation, vibration collisions of adjacent equipment or artificial trampling during maintenance may also cause deformation or perforation of the expansion joint shell.
2. Main hazards caused by air leakage
Flue expansion joint air leakage is by no means a minor problem, and its cascading effects often exceed expectations:
- Increased energy consumption: Air leakage leads to pressure imbalance of flue gas negative pressure system, and the induced draft fan needs to increase the power to maintain negative pressure, which significantly increases the power consumption.
- Environmental protection exceeds the standard: the flue gas containing sulfur or dust leaks out, which directly causes the emission index to exceed the standard, and faces high fines or even stops production for rectification.
- Equipment corrosion acceleration: after high temperature wet flue gas leaks, it condenses, forms corrosive acid, and erodes the surrounding steel structure and instrument circuit.
- Potential safety hazards: If the smoke contains toxic gases (such as CO) or flammable components, air leakage may cause poisoning, fire or explosion accidents.
3. How to accurately judge the leakage location
Because flue systems are typically bulky and have high surface temperatures, routine visual inspections make it difficult to detect subtle leaks. The following three highly efficient detection methods are recommended:
- Ultrasonic leak detector
Ultrasonic waves will be generated when gas passes through small gaps. Using a handheld ultrasonic leak detector, the leakage point of flue expansion joint can be accurately located in the background noise. - Soap water spraying method
Spray high-temperature resistant leak detection solution (or high-concentration soapy water) on the suspected leakage. If bubbles continue to emerge, it is the leakage point. The method is suitable for shutdown maintenance or low temperature working conditions. - Infrared thermography
The leaked high-temperature flue gas will make the surface temperature of the leaked area rise abnormally. The whole expansion joint can be quickly scanned by infrared thermal imaging camera, and the abnormal hot spots can be found intuitively.
Fourth, repair scheme by scene
For different degrees of flue expansion joint leakage, appropriate repair technology should be selected, taking into account both efficiency and reliability.
Scenario 1: Air leakage from flange sealing surface
Applicable scheme: Replace high temperature resistant gasket + tightening torque calibration
After shutdown, remove flange bolts, remove old gasket residue, and replace it with flexible graphite composite gasket or toothed pad. When reassembling, tighten the bolts in three steps in cross-over sequence, and use a torque wrench to ensure that the preload force of each bolt is consistent.
Scenario 2: Large area of corrugated pipe cracking or damage
Applicable scheme: Covered bellows compensator
If the main body of the original expansion joint is seriously damaged, the on-site coating scheme can be adopted-a half-type bellows shell with the same specification is installed externally, and both ends are welded or fixed with the original structure by sealing rings. This method does not need to cut the flue, and the construction period can be controlled within 8 hours.
Scenario 3: Severe aging needs to be replaced as a whole
Recommendation: When flue expansion joint air leakage occurs repeatedly and the repair cost is close to 50% of the new part, it should be replaced directly with a higher grade expansion joint. It is recommended to choose the design of 316L stainless steel bellows + multi-layer heat insulation lining, and add a guide tube to reduce the damage of high-speed flue gas erosion.
V. Daily maintenance suggestions to prevent air leakage
In order to reduce the risk of flue expansion joint leakage from the source, it is recommended to establish the following maintenance mechanisms:
- Quarterly infrared inspection: record the surface temperature distribution of the expansion joint, and compare the historical data to find early cracks.
- Record the number of starts and stops: The fatigue life of bellows is directly related to the number of thermal cycles, and should be replaced planned before approaching the design life.
- Check the integrity of the guide tube: The fall of the guide tube will cause the flue gas to directly impact the bellows, greatly increasing the probability of air leakage.
- Keep the drainage smooth: The expansion joint in the low temperature section should regularly drain condensed water to avoid liquid accumulation and corrosion.
PROFESSIONAL ACTION CALL
Air leakage of flue expansion joint not only affects production efficiency, but also poses safety and environmental protection risks. If you are facing an air leak or wish to establish a preventive maintenance program, please contact our engineering technical team today.