In the industrial boiler system, the metal expansion joint of the tail flue of the high-frequency boiler is the key flexible compensation component connecting the air preheater, dust collector and other equipment. Different from ordinary boilers, high-frequency boilers start and stop frequently, and the temperature changes violently, which puts forward extremely high requirements for the fatigue resistance and thermal compensation ability of metal expansion joints. However, in actual operation, the tail flue metal expansion joint frequently appears crack, deformation, weld crack and other problems, which seriously threaten the safe operation of the boiler. This paper will start with the analysis of failure causes, and systematically explain the technical points and improvement scheme of the metal expansion joint in the tail flue of high frequency boiler.

1. Typical failure mode of metal expansion joint in tail flue of high frequency boiler

The metal expansion joint of the tail flue of high-frequency boiler is facing serious challenges in practical operation. The expansion joint of the furnace wall at the inlet of the high-temperature air preheater in the tail flue of four boilers in a factory is designed and installed by the manufacturer as a double-wave metal expansion joint. Since it was put into operation, the corrugated compensator has some problems, such as lateral tension cracking, deformation and weld cracking. Although the boiler is overhauled every time, the effect is not obvious, and the boiler will still crack when it starts and stops。

According to the on-site inspection data, there are 12 cracks in No.2 furnace, which are mainly distributed on the arc arch of the corrugated plate of the expansion joint, and arranged horizontally. The cracks are about 160mm long and 1.5-10mm wide. The longitudinal middle part of the corrugated expansion joint of No.1 furnace is arched outward about 20mm, and the welding joint at the corresponding position is cracked, and a large amount of hot flue gas leaks outward。

Due to the problem of the expansion joint, the ambient temperature is very high, especially in summer, the operation and maintenance personnel cannot stay here for a long time, and even affects the normal operation of the oxygen meter located nearby。 This case fully demonstrates that the special working conditions of high-frequency boilers put forward higher requirements for the reliability of metal expansion joints.

2. Analysis of failure causes: high temperature, alternating stress and structural defects

There are many reasons for the failure of the metal expansion joint of the tail flue of a high-frequency boiler:

1. Poor temperature resistance and material aging

The metal corrugated compensator consists of an external corrugated plate and an inner baffle, and the corrugated plate and baffle are made of A3 steel plate with a material thickness of 3mm. From the analysis of material and structure, the material used in the baffle is low temperature resistance, and it is easy to age and deform at 400℃ for a long time。 For high-frequency boilers, this aging process is accelerated due to frequent start-and-stop.

2. Baffle deformation causes flue gas to directly scour the corrugated plate

The size of the baffle is large and very thin (about 7-9.9m long and 0.4m wide), which is easy to be heated unevenly. With the newly installed expansion joint baffle intact, the amount of expansion along the circumference of the baffle will be greater than the amount of expansion of the corrugated plate. Therefore, it is very easy to deform when the boiler is often started and stopped, and the high-temperature flue gas in the flue after the baffle deformation will directly affect the corrugated plate。

3. Poor heat preservation effect and large heat loss

From the structural analysis of the metal bellows compensator, about 57% of the area on the expansion joint is directly dissipated by steel plate without heat preservation; Another about 43% of the area has a certain heat preservation effect through the inner arc surface of the corrugated plate, and there are no other heat preservation measures besides this. When the flue gas side baffle is deformed, the heat insulation performance will deteriorate more。

4. Insufficient circumferential expansion compensation ability

Judging from the position of the expansion joint, there is a large temperature difference between the lateral middle and both sides of the expansion joint when the boiler is running: the corrugated plate in the middle has high temperature and large expansion, and both sides are fixed by the iron guard plate outside the furnace wall with relatively small expansion. There is no compensation ability along the circumferential direction. When the boiler starts and stops, transverse cracks will occur on the corrugated plate or the weld joint of the fixed expansion joint will be pulled open。

3. Selection scheme of expansion joint suitable for high-frequency boiler

When selecting the metal expansion joint of the tail flue of high frequency boiler, the structural form with strong fatigue resistance, high temperature resistance and good heat insulation performance should be given priority.

1. Curved tube pressure balance corrugated compensator

The Type QPB Curved Pipe Pressure Balanced Corrugated Compensator (Metal Expansion Joint) is installed at the bend of the pipe or on the space pipe connected to the equipment. It can compensate axial displacement and transverse displacement without generating internal pressure thrust to pipeline system or other equipment. It is commonly used in pumps, compressors, gas turbines and other load-sensitive pipeline systems, and can absorb vibration and reduce noise to ensure the safe operation of equipment。

This type of compensator has the characteristics of good flexibility, corrosion resistance, high temperature resistance, wear resistance and long fatigue life, and can absorb axial, transverse and three-way displacement of pipeline。 It is especially suitable for high-frequency boilers with frequent start-stop and violent temperature alternating conditions.

2. Special design for high frequency vibration

Among the types of pipe compensators, there are products specifically designed for high-frequency mechanical vibration environments. QPB Curved Pipe Pressure Balance Corrugated Compensator is mainly installed in high temperature, medium temperature and low temperature hot air duct, smoke air duct and high frequency mechanical vibration duct。 Through special structural design, this kind of product can effectively absorb high-frequency vibration and avoid early failure caused by vibration fatigue.

3. Structural requirements of improved metal expansion joints

According to the working condition characteristics of high frequency boiler, the following structural improvements should be considered for the metal expansion joint of tail flue:

• Material upgrade: Upgrade A3 steel plate to high temperature resistant stainless steel (such as 304, 316L) to improve the high temperature strength and anti-aging performance of the material
• Increase insulation layer: Increase insulation layer on the outside of the expansion joint to reduce heat dissipation loss and temperature gradient
• Optimize the design of baffles: Adopt segmented baffles or add reinforcing ribs to improve the ability of baffles to resist deformation
• Increase circumferential compensation capability: select the expansion joint structure with multi-directional compensation capability to meet the circumferential thermal expansion demand

4. Key points of installation and maintenance

The installation and maintenance of the metal expansion joints in the tail flue of a high frequency boiler requires special attention:

1. Installation requirements

The small tie rod must be removed after the on-site installation of the metal extension joint is completed。 This is a critical step in ensuring that the compensator can be freely expanded and retracted, which is often overlooked in installation.

2. Regular inspections

High-frequency boiler starts and stops frequently, and the expansion joint should be checked every time it stops for maintenance:

• Check the corrugated plate for cracks and deformations
• Check the weld for cracking
• Check whether the baffle is deformed or fallen off
• Check that the insulation layer is intact

3. Timely discovery and handling

According to the field experience, the leakage area on the expansion joint of the furnace wall at the inlet of the high temperature air preheater of No.2 furnace has been about 0.96cm² (the average crack width is calculated as 5mm), and the other three furnaces also have similar air leakage and ash leakage phenomenon。 This indicates that the defects will gradually expand, and should be dealt with in time when detected early to avoid the problem from worsening.

V. Improvement Suggestions and Summary of Selection

The selection and improvement of the metal expansion joint of the tail flue of the high-frequency boiler should follow the following principles:

1. Type selection according to working condition characteristics

2. Structural improvement direction

• Material upgrade: Change A3 steel plate to 304/316L stainless steel to improve temperature resistance and aging resistance
• Add heat insulation: Insulate the exposed steel plate area to reduce the temperature gradient
• Optimization of baffle: Adopt segmented structure or add reinforcing ribs to improve deformation resistance
• Multidirectional compensation: selection of expansion joint structure with circumferential compensation capability

3. Installation and maintenance

• Remove the small tie rod after installation to ensure free expansion and contraction
• Focus on checking the condition of the expansion joint every time the machine is shut down for maintenance
• Cracks and deformations should be treated in time to avoid enlargement

sum up

The reliable operation of the metal expansion joint of the tail flue of the high frequency boiler is an important link to ensure the safe, stable and economical operation of the boiler. In the traditional design, the corrugated plate and baffle made of A3 steel have problems such as poor temperature resistance, easy deformation, poor heat insulation effect and insufficient circumferential compensation ability under high temperature of 400℃ and high frequency start-stop conditions, which lead to frequent transverse cracks and weld cracks.

The improvement direction should focus on material upgrading (using 304/316L stainless steel), structural optimization (adding insulation layer, optimizing baffle design) and adopting curved tube pressure balanced corrugated compensator with multi-directional compensation and high-frequency vibration absorption capability. During installation, the small tie rods must be strictly dismantled according to the requirements, and a regular inspection mechanism must be established to discover and deal with defects in time. A scientific selection and standardized installation can significantly improve the service life of the metal expansion joint in the tail flue of high frequency boiler, and reduce the equipment failure rate and maintenance cost.