In the wet flue gas desulfurization system, the expansion joint on the flue gas heat exchanger is the key component to connect the heat exchanger body with the inlet and outlet flue, absorb the heat displacement and prevent the leakage of corrosive gas. The flue gas heat exchanger (GGH) heats the clean flue gas by using the heat of the raw flue gas. The temperature difference between the inlet and outlet flue is large, and the medium is corrosive, which puts forward extremely strict requirements on the selection, material and installation of the expansion joint。 Once the expansion joint on the flue gas heat exchanger fails, it will lead to flue gas leakage and heat exchange efficiency decrease, or even the system shutdown in the worst case. This paper will systematically explain the professional technical knowledge of the expansion joint on the flue gas heat exchanger from the characteristics of working conditions, key points of type selection, installation specifications to fault treatment.

1. Special requirements of flue gas heat exchanger working conditions for expansion joints

1. Large temperature difference and significant thermal displacement

When the flue gas heat exchanger is running, the inlet temperature of the original flue gas side can reach 120-180℃, while the outlet temperature of the clean flue gas side usually needs to be heated to above 80℃。 The temperature difference between the inlet and outlet flue causes a significant difference in thermal expansion between the equipment body and the connecting pipe, and the expansion joint on the flue gas heat exchanger must be able to absorb this thermal displacement.

2. Corrosive Media Environment

The raw flue gas contains SO₂, SO₃, HCl and a large amount of water vapor, and the clean flue gas is in a saturated state and low temperature, which is easy to condense to form strong corrosive condensate. According to relevant research, the expansion joint is prone to dew point corrosion at low temperature flue gas pipeline, forming acid liquid, which causes early damage and failure。

3. Frequent start-stop conditions

The flue gas heat exchanger may start and stop with the boiler or desulfurization system, and the expansion joint needs to bear repeated thermal stress impact. The temperature difference in the start-stop stage changes drastically, which puts forward higher requirements for the fatigue life of the expansion joint.

2. Main types of expansion joints for flue gas heat exchangers

1. Non-metallic fabric expansion joint (original flue side)

For the original flue (GGH inlet to absorption tower inlet), the flue gas temperature is moderate (about 120-180℃) and the dust content is high. In this area, medium temperature dustproof non-metallic expansion joints are recommended for expansion joints on flue gas heat exchangers。

Structural composition: skin, insulation layer, stainless steel wire mesh, frame, baffle, dust bag.

• Skin: Multi-layer composite of fluororubber or teflon and glass fiber, excellent acid and alkali resistance
• Insulation layer: aluminum silicate fiber cotton to prevent high temperature gas from accelerating skin aging
• Dust bag: Prevent dust from accumulating in the expansion joint cavity and causing skin to burst

2. Non-metallic anti-corrosion expansion joint (net flue side)

For the clean flue (absorption tower outlet to GGH, GGH outlet to chimney), flue gas humidity is high, temperature is low, and condensate is extremely corrosive. The expansion joint on the flue gas heat exchanger in this area must adopt a low temperature anti-corrosion type structure。

Key Features:

• The skin is made of fluororubber + polytetrafluoroethylene multilayer composite
• The inner side is coated with special anticorrosive materials such as VEGF flake cement, with a thickness of ≥2mm
• A hydrophobic hole is set at the bottom of the horizontal section to discharge the condensed acid liquid in time

3. Metal expansion joint (steam heater)

In the steam heater (terminal equipment for flue gas reheating), 0.5MPa steam is used to exchange heat with flue gas, which needs to bear a certain pressure. The expansion joint on the flue gas heat exchanger here can choose metal expansion joint. For example, the inlet of the steam heater in the flue gas whitening system of a factory uses metal expansion joint, which has been damaged and leaked during operation, and will be restored to normal after isolation and maintenance。

3. Selection of Expansion Joints for Flue Gas Heat Exchangers

The correct selection of the expansion joint on the flue gas heat exchanger requires clarification of the following parameters:

Core principles of selection:

• Original flue: focus on dust prevention and heat insulation to prevent dust accumulation causing expansion joint to burst
• Clean Flue: Focus on anti-corrosion and drainage to prevent acid penetration and frame corrosion

Installation Specifications and Key Control Points

1. Installation direction confirmation

When installing the expansion joint on the flue gas heat exchanger, the flow direction mark must be strictly confirmed. An expansion joint with a guide tube, the small end of the guide tube must be facing the direction of flue gas flow; The design of the inner liner plate shall not prevent the free expansion of the expansion joint。

2. Bolt tightening requirements

The skin of the non-metallic expansion joint is fixed by pressure plate bolts, and the sealing performance is strengthened by applying high-strength glue that is resistant to high temperature and corrosion before tightening. Bolt spacing does not exceed 100mm。 Due to the large length of the pressure plate, the distal bolt will loosen due to skin compression deformation after one-time tightening, so the operation should be repeated by "diagonal tightening and fractional tightening".

3. On-site assembly

Expansion joints exceeding 4m need to be shipped to the site for assembly. Whether manufactured in the factory or assembled on site, fixed angle steel and plane support rods should be temporarily installed to prevent deformation during transportation and installation。

4. Hydrophobic hole setting

The horizontally installed expansion joint on the side of the clean flue must be provided with a hydrophobic hole (diameter ≤100mm) at the lowest point of the frame to discharge the condensed acid. The material should be alloy steel or stainless steel。

V. Common Faults and Treatment Measures

1. Damage and leakage of expansion joint

Case: The metal expansion joint at the inlet of module B of No.1 steam heater of flue gas whitening in a factory was damaged, and a large amount of steam leaked during operation. In the absence of spare expansion joints, the branch plant adopts blind plate to isolate Module B, and puts other modules into operation to maintain operation。

Treatment measures:

• Emergency treatment: Isolate faulty modules with blind plates
• Permanent fix: Replace expansion joint after shutdown

2. Dew point corrosion failure

Reason: The flue gas of the net flue has high humidity and low temperature, and the condensate formed by condensation is strongly corrosive to the expansion joint material。

Treatment measures:

• The inner side is painted with special anticorrosive materials such as VEGF scale cement
• Add drainage holes and drain water in time
• Select skin made of acid-resistant materials such as fluororubber

3. Powder accumulation in the cavity leads to swelling and

Cause: The original flue has high dust content, and the dust accumulates in the expansion joint cavity, resulting in skin swelling and bursting。

Treatment measures:

• Expansion joint with dustproof bag and bidirectional baffle structure is selected
• Regularly check and clean up dust accumulation

4. Thermal stress deformation

Reason: The temperature difference exceeds the allowable range or the structural design is unreasonable when starting and stopping the furnace。

Treatment measures:

• Control the temperature rise rate in the start-stop stage to be ≤3℃/min
• Optimize the connection structure between tube sheet and shell, and compensate displacement with ω-type expansion joint

VI. Maintenance and overhaul suggestions

1. Regular inspection: Check the appearance of the expansion joint every month for water leakage, air leakage and bulging; Check joint tightness
2. Temperature and Pressure Monitoring: Continuously monitor the operating temperature and pressure to ensure that the expansion joint operates within a safe range
3. Drainage inspection: Clean drainage holes monthly to prevent blockages
4. Replacement cycle: Non-metallic skin is usually replaced once every 3-5 years; If the metal frame is severely corroded, it should be replaced as a whole
5. Ledger management: record the installation date, replacement record and inspection status of each expansion joint

sum up

The expansion joint on the flue gas heat exchanger is the key component to ensure the safe and stable operation of GGH system. Because its working environment has the characteristics of large temperature difference, strong corrosiveness and high dust content, the following core principles must be followed in selection and maintenance:

• Zone selection: medium temperature dust-proof type (anti-dust accumulation) is selected for the original flue side, and low temperature corrosion-proof type (anti-corrosion) is selected for the clean flue side
• Structural optimization: water-phobic holes must be set on the side of the clean flue, and scales must be made to prevent corrosion on the inner side; Dust-proof bags and two-way baffles are arranged on the side of the original flue
• Installation specification: Install strictly according to the flow direction mark. Expansion joints exceeding 4m shall be assembled in the field specification, and bolts shall be tightened in stages
• Regular maintenance: Establish inspection ledger, monitor temperature and pressure changes, and deal with abnormalities in time

An expansion joint for flue gas heat exchanger with reasonable design and standard installation can effectively avoid leakage and corrosion problems, and ensure the long-term stable operation of desulfurization system.