In the new dry process cement production line and various industrial kiln systems, the role of tertiary duct expansion joint is often underestimated by the on-site operation and maintenance personnel, and it is not regretted until cracking, leakage or even pipeline deformation occur. As an important channel connecting the decomposition furnace and the kiln head, the tertiary air duct is circulated by high-temperature dusty flue gas of 850℃ – 1100℃, and its thermal expansion can reach more than 200mm per 100m. If the expansion joint is not effectively compensated, the huge thermal stress will directly act on the pipeline support, connecting flange and even the kiln tail structure, causing equipment damage, production shutdown accident and even safety accident. This paper will systematically expound the function of expansion joint of tertiary air duct, from the four core functions of thermal compensation, vibration reduction, sealing and protection equipment, to help readers comprehensively understand the engineering value of this key component.
1. Core function: absorbing thermal expansion and eliminating thermal stress
The most fundamental function of the expansion joint of the tertiary air duct is to absorb the thermal expansion of the pipeline under high temperature working conditions. The tertiary duct length is typically between 30 meters and 80 meters, and the carbon steel or heat resistant steel duct elongates by approximately 1.2 – 1.5 mm per meter when heated from normal temperature (approximately 20 °C) to operating temperature (approximately 950 °C). The total expansion can reach 60 – 75 mm in a 50-meter-long air duct.
Without expansion joints to absorb this deformation, huge thermal stresses will occur inside the pipe. Calculated according to Hooke's law, only 1 mm of elongation is hindered, and tens of MPa of compressive stress can be generated within the tube wall. When the stress exceeds the yield strength of the material, the pipe can bend, arch, tear the weld, and even tear the adjacent decomposition furnace interface or kiln head hood. The function of the expansion joint of the tertiary air duct is to absorb the axial, transverse and angular displacements orderly through the elastic deformation of its bellows, so that the pipeline system is in a "flexible connection" state, thus protecting the integrity of the whole kiln tail structure.
2. Vibration and noise reduction: Improving system stability
When the industrial kiln is running, the internal wind speed of the tertiary air duct is as high as 25 – 35m/s. The impact of high-concentration dust particles on the pipe wall and the airflow pulsation generated by the fan will cause the continuous vibration of the pipe. This vibration, if not isolated, is transmitted to the decomposition furnace, preheater, and kiln head through the rigid connection, accelerating fatigue cracking at the connection of adjacent equipment.
The function of the expansion joint of the tertiary air duct is embodied here as a "damping buffer". The metal bellows itself has a certain stiffness and elasticity, which can absorb high frequency and low amplitude vibration energy. Studies have shown that the vibration amplitude transmitted from the tertiary air duct to the bearing can be reduced by 40% – 60% after installation of a reasonably designed expansion joint. At the same time, the guide tube structure inside the expansion joint can also guide the smooth transition of airflow, reduce the aerodynamic noise caused by turbulence, and improve the field operation environment.
Sealing and leak prevention: Ensuring thermal efficiency and environmental compliance
The interior of the tertiary air duct is in a negative pressure or slightly negative pressure state (usually-50Pa to-200Pa). Once the expansion joint leaks, external cold air will be sucked into the duct. This will bring three serious consequences: lower the temperature of the tertiary air, which will affect the combustion efficiency of the fuel in the decomposition furnace; Increase system heat consumption; It destroys the reducing atmosphere in the kiln and affects the quality of clinker. Therefore, the function of the expansion joint of the tertiary air duct also includes the realization of dynamic sealing under high temperature working conditions.
The high-quality expansion joint adopts multi-layer bellows structure (such as two layers of 1.2mm stainless steel plate composite) with end sealing ring, which can withstand ±50mm axial displacement while maintaining airtightness. Compared with the traditional packing box compensator, the metal bellows expansion joint has no sliding sealing surface, and there is no leakage point caused by long-term wear. This is particularly important in the context of increasingly stringent environmental regulations – leaks not only mean a waste of heat energy, but can also be identified by environmental authorities as a source of unorganised emissions.
4. Protect key equipment: prolong the life of kiln tail system
The function of the expansion joint of the tertiary air duct should not be limited to the pipe itself, but should also be amplified to the protection of the whole kiln tail system. The two ends of the tertiary air duct are respectively connected with the decomposition furnace (stationary equipment) and the kiln head cover (rotating synchronously with the rotary kiln). Without the expansion joints to absorb thermal displacement and minor installation deviations, the decomposition furnace outlet interface will be subjected to huge additional loads, which may lead to refractory liner detachment, steel structure deformation and even furnace body cracking.
Additionally, expansion joints compensate for foundation settlement and installation errors. In practical engineering, the uneven settlement between the kiln tail frame foundation and the kiln head foundation is difficult to avoid completely. The role of tertiary duct expansion joints includes absorbing these slowly occurring vertical and horizontal displacements, preventing the duct from becoming a "force lever" that transfers loads to brittle refractories or precision flanged joints.
V. Differentiation of Different Types of Expansion Joints
According to different structures and materials, the function emphasis of tertiary duct expansion joints is also different:
| Expansion joint type | Primary role and focus | Applicable working conditions |
|---|---|---|
| Single axial type | Absorbs axial displacement, compact structure | Straight pipe section with small compensation |
| Complex hinge type | Absorb lateral and angular displacements | Pipe turns or space-constrained areas |
| Pressure balance type | Eliminate internal pressure thrust and protect support | Large diameter pipe or weak support |
| Lined castable type | Resistance to high temperature dust scour, prolonged life | Temperature> 900℃ and high dust content |
In actual selection, the function of tertiary duct expansion joint is often a combination of many aspects. For example, in the tertiary air duct of cement production line, the composite structure of "metal bellows + guide tube + lining with wear-resistant heat insulation layer" is usually selected, which meets the four requirements of thermal compensation, sealing, erosion resistance and shell temperature reduction.
6. Typical consequences of ignoring the action of expansion joints
A cement enterprise with an annual output of 1.5 million tons cancelled the original expansion joint design and used rigid connection instead in order to save costs in the third air duct renovation. Only three months after the production, it was found that the steel structure at the outlet of the decomposition furnace had penetrating cracks, and the concrete foundation of the middle support of the tertiary air duct was pushed and cracked. It was detected that the pipe of only 50 meters long produced an axial expansion of about 70 mm at 950 °C, with all the thermal stresses acting on the weak nodes. In the end, the kiln had to be shut down urgently and the expansion joint was reinstalled, resulting in a direct loss of more than 800,000 yuan. This case clearly shows that underestimating the role of tertiary duct expansion joints is far more costly than purchasing expansion joints themselves.
7. Call to Action: Let a professional team evaluate the current status of expansion joints for you
Is it clear to you if the current tertiary duct expansion joint is still functioning effectively? Are there the following hidden dangers: visible cracks on the bellows surface, hot air escaping around the expansion joint, offset or cracking of the support, and forcibly compressed to the limit of the expansion joint? If these problems are not addressed in a timely manner, they may turn into unplanned downtime at any time.