During the operation of cement production line, the reasonable formulation of the replacement scheme of expansion joint of tertiary air duct is directly related to the maintenance efficiency of kiln shutdown and the long-term stable operation of equipment. The expansion joint of the tertiary air duct is in the high temperature flue gas environment of 850℃-1100℃ for a long time, and it is subjected to multiple effects of thermal stress, dust erosion and mechanical vibration. Usually, problems such as bellows cracking and fatigue failure occur every 2-3 years。 This paper will systematically elaborate a set of complete and executable three-time duct expansion joint replacement scheme from five dimensions: working condition diagnosis before replacement, preparation before construction, standardized operation process, quality acceptance standard and safety control.

1. Why should we formulate a professional replacement plan?

The expansion joint of the tertiary air duct is located at the key connection between the kiln tail preheater and the decomposition furnace. The replacement operation is usually carried out at high altitude (more than 30 meters above the ground), and involves many risk factors such as high temperature, limited space and hoisting。 If the replacement scheme of the expansion joint of the tertiary air duct is not perfect, it may lead to: excessive replacement period affecting the resumption of production, unqualified welding quality causing secondary leakage, improper pre-stretching of the expansion joint to accelerate damage, and even safety accidents. A scientific scheme should cover the technical points of the whole process, such as dismantling old parts, installing new parts, repairing liners and adjusting supports。

2. Working condition diagnosis and data acquisition before replacement

Before formulating a specific three-time replacement plan for the duct expansion joint, the following diagnostic work must be completed:

1. Failure Cause Analysis: Check the failure mode of the old expansion joint-is it high temperature oxidation of the bellows, intergranular corrosion, or mechanical tension cracking caused by cylinder deformation? For example, the cylinder of the tertiary air duct in a factory is oval due to long-term operation, and the flange of the expansion joint is cracked in the circumferential direction. If the expansion joint is simply replaced without correcting the cylinder, it will fail again in a short time。
2. Field measurement: The length, diameter and support point position of each section of air duct are measured, and the design drawings are corrected in combination with thermal expansion。 Special attention should be paid to the ellipticity of the cylinder before and after the expansion joint-if the deformation is serious, the coil plate repair welding should be performed before replacement to fill the gap (usually 0-200mm)。
3. Bearing Condition Assessment: Check the pipe bearing for sinking, offset, or jamming. Bearing failure will change the stress state of the expansion joint, resulting in insufficient compensation。

3. Preparation before construction: people, machines, materials, methods and environment

A successful tertiary duct expansion joint replacement protocol must include careful preparation:

Standardized replacement operation process

The following are the core operating steps of the tertiary duct expansion joint replacement scheme verified by several cement enterprises:

Step 1: Kiln shutdown, temperature reduction and isolation

Verify that the system is completely out of service and the temperature drops to a safe range. Close the tertiary air sluice plate and install a blind plate to prevent hot air from entering.

Step 2: Remove old expansion joints and accessories

1. Removing the bellows and guide tubes before and after the old expansion joint;
2. Remove the internal castables and refractory bricks (Note: the refractory bricks of the tertiary air duct are usually 114-220mm high alumina bricks or silica moxide bricks, with large thickness and high hardness)；
3. Cut the connecting weld between the old expansion joint and the air duct, and pay attention to protecting the base metal;
4. Hoist the old expansion joint to the designated position on the ground for recovery。

Step 3: Cylindrical correction and support repair

• Carry out coil calibration or partial replacement on the deformed cylinder to ensure that the roundness of the interface of the new expansion joint reaches the standard;
• Adjust and repair damaged bearings to eliminate pipe sinking or offset。

Step 4: New Expansion Joint Installation – Critical Quality Control Points

This is the technical core of the tertiary duct expansion joint replacement scheme:

1. Pre-stretch treatment: The expansion joint is pre-stretched according to the amount of thermal expansion calculated by the design. During installation, the direction of expansion and contraction of bellows must be consistent with the direction of air flow；
2. Alignment welding:
   • The weld seam needs to be welded on both sides, and the weld seam is 2mm higher than the steel plate;
   • The surface is smooth, and pores and trachoma are not allowed;
   • It is strictly prohibited to start an arc on the wave plate of the expansion joint, and the welding splash shall not fall on the wave plate；
3. Gap treatment: If there is a gap between the new expansion joint and the pipe, it should be filled with coil plate repair welding。

Step 5: Liner Refractory Repair

After the expansion joint is replaced, the liners in the front and rear areas of the expansion joint need to be repaired simultaneously:

• It is recommended to adopt the combination scheme of low thermal conductivity mullite brick (114mm) + nano insulation board (80mm), which can reduce the shell temperature by about 55℃；
• The castable needs to be poured in a supporting mold, and the temperature can be raised only after the curing is in place;
• The pouring hole must be sealed after construction to prevent air leakage。

Step 6: Anti-corrosion and Acceptance

• Painting high temperature resistant anti-corrosion paint on pipe surface；
• Check all welds for tightness and conduct airtightness tests if necessary;
• Record the installation data and file it for future reference.

Quality Acceptance Criteria

The qualified tertiary duct expansion joint replacement plan must be matched with clear acceptance indicators:

1. Weld quality: no pores, slag inclusion, unfused, weld height up to standard;
2. Expansion joint state: the pre-stretching amount conforms to the design value, and the bellows is free of twist and scratch;
3. Sealability: There is no hot air leakage around the expansion joint after operation, and the surface temperature is normal;
4. Compensation function: Expansion energy-saving free expansion and contraction after heating, no sticking or abnormal deformation.

6. Typical cases and experience

When a cement enterprise replaced the expansion joint of air duct three times in 2021, the expansion joint cracked again only 6 months after the replacement because the deformed cylinder and support were not corrected synchronously. Subsequently, the process described in this scheme is adopted: first, the ellipticity of the cylinder is corrected, the three sinking supports are repaired, and then the new expansion joint (RA330 material + lining castable) is installed, which has been in stable operation for 26 months so far。 This case proves that the complete tertiary duct expansion joint replacement scheme must include barrel correction and bearing repair in the scope of work, rather than "only replacing the expansion joint itself".