In the wet flue gas desulfurization system, the expansion joint of the flue gas desulfurization tower is the core component that connects the desulfurization tower with the inlet and outlet flue, absorbs the heat displacement and prevents the leakage of corrosive gas. Because the flue gas in the desulfurization tower has the characteristics of low temperature, high humidity and strong corrosion, the working environment of the expansion joint of flue gas desulfurization tower is extremely harsh. Once it fails, it will lead to acidic liquid leakage, aggravated equipment corrosion, exceeding environmental protection standards and even system shutdown. This paper will systematically explain the professional technical knowledge of expansion joint of flue gas desulfurization tower from working condition analysis, key points of type selection to installation and maintenance.

1. Special requirements for expansion joints under working conditions of desulfurization tower

In the wet desulfurization process, the original flue gas is sprayed and washed by the desulfurization tower and turned into clean flue gas and discharged. In this process, the expansion joint of the flue gas desulfurization tower faces the following severe working conditions:

• Strongly corrosive environment: The flue gas contains SO₂, SO₃, HCl, HF and a large amount of water vapor. The pH value of the condensate can be as low as 1-3, which is strongly corrosive to both metal and non-metal materials
• Low temperature and high humidity: The flue gas temperature in the desulfurization tower usually drops to 45-55℃, which is below the acid dew point, and the inner wall is continuously in a "wet" environment
• Dusty slurry scour: The desulfurization circulation slurry contains gypsum particles, which causes scour wear on the inner wall of the expansion joint
• Large displacement compensation requirement: The body of the desulfurization tower will generate significant deformation under temperature change and fluctuation of flue gas pressure, and the inlet and outlet flues need to absorb three-way displacement
• Negative pressure operation: Most desulfurization systems have induced draft fans in the rear, and the expansion joint needs to bear negative pressure and maintain good seal

Based on the above characteristics, the expansion joint of flue gas desulfurization tower must have excellent corrosion resistance, good sealing performance, sufficient three-way displacement compensation ability and reasonable price economy.

2. Main types of expansion joints of flue gas desulfurization tower

1. Non-metallic fabric expansion joint

This is the most commonly used type in desulfurization tower systems, accounting for more than 90% of the application share. Constructed of multilayer composite materials, typical structures include:

• Inner layer (dielectric contact layer): fluororubber or EPDM, acid, temperature and oxidation resistant
• Intermediate layer: fiberglass cloth, providing strength and temperature resistance
• Insulation: Aluminum silicate fiber cotton to reduce exterior surface temperature
• Outer layer: silicone rubber coated glass fiber cloth for protection
• Frame: Carbon or stainless steel for holding skin

The outstanding advantages of non-metallic expansion joints: they can absorb three-way displacement at the same time; Good vibration isolation and noise reduction effect; Excellent corrosion resistance; Lightweight and easy to install; Prices are relatively economical. Its temperature resistance generally does not exceed 250℃, which completely meets the working conditions of desulfurization tower (≤80℃).

2. Metal expansion joint (special material)

In the high-temperature section of some desulfurization tower inlet (such as bypass flue or original flue gas inlet), the flue gas temperature may exceed 180℃, so metal expansion joints should be selected at this time. However, ordinary stainless steel (304, 316L) is prone to pitting and stress corrosion cracking in wet flue gas environment, so the materials must be upgraded:

• 254SMO Super Austenitic Stainless Steel: Excellent Cl-Pitting Resistance
• Pure titanium (TA2): almost non-corrosive in wet chloride ion environment, service life can reach more than 10 years
• Hastelloy C-276: Suitable for extreme corrosion conditions

The initial investment of metal expansion joints is high, but the life cycle cost is not necessarily higher than that of frequently replaced non-metallic products.

3. Rubber expansion joint

A small amount is used in the circulating slurry pipeline of desulfurization tower (instead of the flue gas side), which is not resistant to high temperatures and is not recommended for flue gas system.

3. Selection of core parameters of expansion joint of flue gas desulfurization tower

Correct selection of expansion joint of flue gas desulfurization tower requires clarification of the following parameters:

Key points of selection:

1. Skin material: Fluorine rubber (FKM/Viton) must be used as the medium contact layer in the wet flue gas environment of desulfurization tower. Ordinary silicone rubber will age and crack rapidly in acidic wet flue gas, and its life usually does not exceed 1 year. Fluororubber has excellent acid resistance, oil resistance and high temperature resistance, and its service life can reach 3-5 years.
2. Anti-corrosion coating: Metal frames must be coated with heavy anti-corrosion coating. It is recommended to use glass flake cement or vinyl ester resin coating with a thickness ≥2mm. Ordinary paint peels off in an acidic environment for weeks.
3. Drainage design: Horizontally installed non-metallic expansion joints must be provided with drainage holes (usually DN50-DN100) at the lowest point to drain condensed acid in time and prevent skin penetration caused by water accumulation in grooves.
4. Anti-scour structure: For the working conditions with high dust content in the inlet flue, wear-resistant guide tubes or wear-resistant ceramics should be installed.

Installation Specifications and Key Precautions

1. Installation direction confirmation

The flow direction arrow on the expansion joint must coincide with the direction of flue gas flow. The small end of the guide tube is directed toward the flue gas incoming flow, and the large end is directed toward the outlet. Installation in reverse direction will directly lead to skin erosion and wear.

2. Welding and Protection

• During welding, protective measures must be taken to prevent the non-metallic skin from being damaged by welding slag splashing
• Cutting or welding on the skin is strictly prohibited
• The flanged expansion joint and sealing gasket should be acid-resistant (such as EPDM or PTFE)

3. Bolt tightening requirements

The skin of the non-metallic expansion joint is secured by a platen bolt. Due to the large length of the pressure plate, the distal bolt will loosen due to compression deformation of the skin after one-time tightening. During installation, the operation should be repeatedly operated by "diagonal fastening and fractional tightening". It is recommended to use a torque wrench, and the tightening torque should be carried out according to the manufacturer's requirements (usually 50-80N·m).

4. Drain pipe setup

Horizontally installed flue gas desulfurization tower expansion joints must be provided with drainage short pipes at the lowest point of their frame and connected to the desulfurization tower trench or collection system. Drains should be kept unobstructed to prevent blockages.

5. Removal of transport tie rod

After installation, the transport protection tie rod must be removed so that the expansion joint can be expanded and contracted freely. Note: The positioning limit tie rod must not be removed, its function is to limit the excessive deformation of the expansion joint.

V. Common faults and preventive measures

1. Skin acid penetration and water leakage

Phenomenon: Acid water leakage occurs at the bottom of the expansion joint, accompanied by white crystals

Reason: Water accumulation in skin grooves, long-term soaking of acid solution leads to penetration of fabric layer; Or damage to the fluororubber in the inner layer of skin

Prevention: Set drainage holes; Select qualified fluororubber skin; Check grooves regularly for dust accumulation

2. Bulging or ruptured skin

Phenomenon: Local bulging or tearing of skin

Reason: The inner insulation layer collapses or falls off, causing hot flue gas to contact the outer layer directly; Or abnormally elevated system pressure

Prevention: choose expansion joints with anti-collapse insulation cotton; Set Pressure Alarm

3. Frame corrosion perforation

Phenomenon: Rust holes appear in the metal frame and acid seeps out

Reason: poor quality of anti-corrosion coating or inadequate construction; Long-term exposure to acidic humid environment

Prevention: adopt glass flake heavy anti-corrosion coating with a thickness of ≥2mm; Check coating integrity periodically

4. Loose or broken bolts

Phenomenon: Gaps appear on the edge of the skin, air and water leakage

Reason: Failure to tighten the bolts multiple times as specified; Bolt material is not corrosion resistant

Prevention: Use stainless steel bolts (304 or 316L); Tightening once in 1 month and once in 3 months after commissioning

VI. Maintenance and overhaul suggestions

1. Daily inspection: check the appearance of the expansion joint every week for water leakage, air leakage, bulging and abnormal vibration
2. Periodic tightening: Full tightening of platen bolts once a quarter
3. Drainage inspection: Clean drainage holes monthly to prevent blockage
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

Call to Action

Are you bothered by frequent water leakage, severe corrosion or confusion in the selection of the expansion joint of the desulfurization tower? Contact our technical team today to get professional FGD tower expansion joint selection assessment and fault diagnosis services.