In flue gas ducting systems, most technicians are familiar with expansion joints that are mounted outside the flue. However, there is an expansion joint that is installed inside the flue – it is called the "flue internal expansion joint" or "internal expansion joint" and is a critical component for internal compensation of high temperature pipes. Different from the traditional expansion joints installed on the outer wall of the flue, the expansion joints inside the flue directly bear the smoke gas scour, and the working environment is more severe, so the design and selection also have special requirements. This paper will systematically analyze the professional technical knowledge of flue internal expansion joint from structural characteristics, applicable scenarios to installation and maintenance.

1. Definition and structural characteristics of internal expansion joint of flue

The flue internal expansion joint refers to the expansion joint device installed inside the flue shell and in direct contact with the flue gas. The core difference between it and the external expansion joint is:

The internal expansion joint usually consists of a metal bellows, a castable protective layer, and an inner liner. Since the bellows is directly exposed to high-temperature dusty smoke, refractory castable must be poured on the bellows surface as a protective layer to prevent the high-temperature smoke from directly washing the bellows。

2. Typical application scenarios of flue internal expansion joints

1. High temperature flue

In high temperature areas such as boiler outlet and catalytic cracking regeneration flue gas pipeline, the internal temperature of pipeline can reach 600-1000℃. The flue internal expansion joint here needs to be installed in the flue lining layer, and the bellows surface needs to be protected by pouring refractory castable, and the thickness of castable can reach 100-360mm (determined according to working conditions).

2. SCR denitrification flue

For the expansion joint of SCR flue gas system, it is necessary to withstand axial, radial and angular displacement and absorb vibration effectively, so the non-metallic expansion joint with inner baffle and high temperature resistance should be preferred。 In practical SCR engineering, due to the high flue gas temperature, the lateral displacement and horizontal displacement of flue are large, and non-metallic expansion joints are used to compensate the thermal displacement.

3. Catalytic cracking hood inlet pipe

In the inlet pipe of the hood, the application of the internal expansion joint is crucial. An inlet pipe of a cigarette hood is provided with two balance ring expansion joints at the horizontal pipe close to the inlet of the cigarette hood, so that the connection of the inlet pipe of the cigarette hood is changed into a soft connection, so that the influence of misalignment, deflection, deformation and settlement of the inlet pipe in the transverse and vertical directions on the cigarette hood is greatly reduced。

3. Key points of design of expansion joint inside flue

1. Castable protective layer design

The metal bellows of the expansion joint inside the flue must be provided with a castable protective layer. Design requirements include:

• The thickness of castable is determined according to the working temperature. Generally, it needs 100-360mm in high temperature conditions
• Needed to set the scraper (anchor) to fix the castable to the bellows surface
• Castables should have good wear resistance and thermal shock stability

2. Inner baffle (guide tube) setting

For the expansion joints of SCR flue gas system to withstand axial, radial and angular displacements and absorb vibration effectively, the non-metallic expansion joints with inner baffles and high temperature resistance should be preferred。 The role of the inner baffle is to:

• Guide the flow direction of flue gas to avoid direct flushing of the bellows
• Protect the castable layer for extended service life

3. Drainage design (wet flue)

For expansion joints installed inside a wet flue, drainage design should be considered when the expansion joints inside the flue contact wet flue gas. The expansion joint contacting the wet flue gas and located in the horizontal flue section will drain water through the expansion joint frame, the drain hole is at least DN150 and located on the center line of the horizontal flue section, the drain fitting is made of FRP or nickel-based alloy steel material。

4. Expansion joint reservation

For the internal expansion joints where castables need to be poured, the reservation of expansion joints cannot be ignored. The reserved widths of expansion joints for different castable types are as follows:

The expansion joint should avoid the stress zone, an L-shaped expansion joint should be set when the two pouring surfaces cross to form a concave angle, and a control groove of about 200mm should be set at the convex angle.

4. Precautions for selecting the expansion joint inside the flue

1. Liner Barrel Direction

For compensators with liners, care should be taken to make the direction of the liner cylinder coincide with the direction of the medium flow (install according to the flow direction sign of the compensator)。 Installation in reverse direction will cause smoke to directly flush the bellows, accelerating failure.

2. Suitability of non-metallic expansion joints

Non-metallic expansion joints are ideal for internal expansion joints in high temperature, multi-directional displacement conditions such as SCR flues:

• Stainless steel wire mesh is required in the material of the expansion joint to withstand the flue gas pressure
• Select silicone rubber or fluorine rubber for skin material according to flue gas temperature
• Because of the high temperature of flue gas, metal expansion joints generally cannot meet the requirements of safe use, so non-metal expansion joints are used to compensate for thermal displacement

3. Installation pre-bias requirements

For high temperature flue such as SCR flue gas system, in order to ensure the uniformity of flue gas flow field, if the flue is wrong edge during operation, it will inevitably affect the flow field distribution and the use effect of ammonia spray grid. Therefore, the expansion joint inside the flue should be 100% pre-biased when installing, that is, the flue on both sides of the expansion joint is misaligned during installation, and the axis of the flue on both sides is on a straight line during operation。

5. Installation and maintenance of internal expansion joint of flue

1. Preparation before installation

• Check that the model, specification and design of the expansion joint are consistent
• Check the protective layer of castable for cracks and shedding
• Verify that the direction of the liner barrel is consistent with the flow direction of the medium
• Check that the nail weld is firm

2. Installation Points

• The marking direction of the wear-resistant bushing should be consistent with the flow direction of the pipeline medium
• For large diameter internal expansion joints, temporary supports added before leaving the factory should be removed as soon as possible after installation is completed
• Pay attention to protecting bellows and castables during welding operations

3. Overhaul and maintenance

The internal expansion joint of the flue is located inside the flue, which is difficult to overhaul. During maintenance, we should pay attention to:

• Check the surface of castable for peeling and cracking
• Check whether the inner baffle is deformed or fallen off
• Regularly dredge drainage holes (wet flue)
• Abnormalities should be dealt with in time to avoid expanding problems

6. Stability failure analysis of expansion joint

The failure of the expansion joint of high temperature flue gas pipeline is often closely related to the arrangement of pipeline support and the change of operating conditions. For the flue internal expansion joint, the selection and reasonable arrangement of pipe support is the key to ensure the safety and effective service of the expansion joint。

Common causes of failure include:

• Unreasonable arrangement of pipe supports causes additional loads to the expansion joints
• The calculation of thermal displacement is inaccurate and the selection of compensation amount is insufficient
• Internal castables fall off and bellows are directly exposed
• Corrosive media attack, bellows perforation

Precautionary measures: Combined with the calculation of pipeline thermal displacement and blind plate force, design optimization, reasonable setting of pipeline supports, and ensure that the expansion joint works within its design parameters。

sum up

The flue internal expansion joint is a key flexible connection component in high temperature flue gas pipeline system, and its structure and technical requirements are significantly different from those of ordinary external expansion joint:

An internal expansion joint of flue with reasonable design and standard installation can operate stably for a long time under the harsh working conditions of high temperature, corrosion and dust. It is suggested that the structural form of internal expansion joint, castable thickness and installation pre-bias requirements should be clarified in the design stage, so as to ensure the safety and reliability of flue gas system from the source.