In the wet flue gas desulfurization system, the expansion joint on the desulfurization flue gas is the key component to connect the inlet and outlet of the desulfurization tower with the flue, absorb the heat displacement and prevent the leakage of corrosive gas. Desulfurization flue gas has the obvious characteristics of low temperature, high humidity and strong corrosion, which puts forward extremely strict requirements for the selection, material and daily maintenance of expansion joints. Once the expansion joint on the desulfurization flue gas fails, it will lead to acidic liquid leakage and equipment corrosion, or even system shutdown in the worst case. This paper will systematically explain the professional technical knowledge of expansion joint on desulfurization flue gas from the characteristics of working conditions, key points of type selection, installation specifications to fault treatment.

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

In the wet desulfurization process, the expansion joint on the desulfurization flue gas 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 the 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 used on desulfurization flue gas must have excellent corrosion resistance, good sealing performance, sufficient three-way displacement compensation ability and reasonable price economy.

2. Main types of expansion joints for desulfurization flue gas

1. Non-metallic fabric expansion joint

This is the most commonly used type of expansion joint for desulfurization flue gas, accounting for more than 90% of the application share. Constructed of multilayer composite materials, typical structures include:

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 the inlet of some desulfurization towers (such as when the original flue gas inlet does not have GGH), 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

3. Rubber expansion joint

A small amount is used for the circulating slurry pipeline of desulfurization tower (instead of the flue gas side), which is not resistant to high temperatures. It is not recommended to use rubber material for expansion joints on desulfurization flue gas.

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

Correct selection of expansion joints for desulfurization flue gas requires clarification of the following parameters:

Key points of selection:

• Skin material: Fluorine rubber (FKM) must be selected 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.
• Anti-corrosion coating: The metal frame must be coated with heavy anti-corrosion coating. It is recommended to use glass flake cement or vinyl ester resin coating with a thickness of ≥2mm.
• Drainage design: horizontally installed expansion joints must be provided with drainage holes at the lowest point to discharge condensed acid in time.
• Anti-scour structure: For the working conditions with high dust content in the inlet flue, wear-resistant guide tubes 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".

4. Drain pipe setup

Expansion joints for desulfurization flue gas installed horizontally must be provided with drainage short pipes at the lowest point of its frame and connected to the trench or collection system of desulfurization tower.

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.

V. Common Faults and Treatment 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 the skin

Treatment: setting 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

Treatment: Expansion joint with anti-collapse insulating cotton is selected; 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

Treatment: adopt glass flake heavy anticorrosive 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

Treatment: Stainless steel bolts (304 or 316L) are selected; 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 of each expansion joint

sum up

The use of expansion joints on desulfurization flue gas is the key link of flue anti-corrosion in wet desulfurization system. Because its working environment has the characteristics of low temperature and high humidity, strong corrosion and condensate accumulation, the following core principles must be followed in selection and maintenance:

• Material priority: fluororubber composite material is selected for the skin, and heavy anti-corrosion coating is made for the metal frame (glass flake ≥2mm)
• Structural optimization: the horizontal expansion joint must be provided with drainage holes to discharge the condensate in time; Adopt groove filling technology to eliminate hidden danger of water accumulation
• Installation specification: Install strictly according to the flow direction mark, tighten the bolts repeatedly, and remove the transport rod in time after installation
• Regular maintenance: Establish inspection ledger, regularly drain water, tighten bolts, and clean drainage holes

An expansion joint for desulfurization flue gas with reasonable design and standard installation can effectively avoid the phenomenon of "running, dripping and leaking" and ensure the long-term stable operation of desulfurization system.