Leakage Prevention for Non-Metallic Expansion Joints in Wet Flue Gas Desulfurization

Abstract

Non-metallic expansion joints offer excellent corrosion resistance, high tensile strength, and cost-effectiveness, making them widely used in wet flue gas desulfurization systems. However, due to the corrosive nature of flue gases and construction-related factors, expansion joints are prone to leakage of both water and flue gases. In severe cases, leakage may necessitate shutting down the system for repairs. Additionally, harsh on-site conditions and high temperatures present significant safety risks to both personnel and equipment during leakage repairs. This paper presents an online sealing method utilizing polymer full-sealing technology, which effectively prevents direct contact between flue gases and expansion joints while ensuring the continuous operation of the flue gas desulfurization system.

Introduction

Currently, most coal-fired power plants use limestone-gypsum wet flue gas desulfurization. Variable operating conditions lead to uneven deformation and varying levels of corrosion in different sections of the flue. To prevent cracking caused by deformation, a compensator is installed. Due to their excellent corrosion resistance, high tensile strength, and cost-effectiveness, non-metallic expansion joints are extensively used in wet flue gas desulfurization systems. These expansion joints consist of a glass fiber base fabric, which is either calendered or impregnated with silicone rubber. Due to the corrosive nature of flue gases in the desulfurization system, the expansion joints typically consist of seven layers of material, including fluorine rubber cloth, tetrafluoroethylene cloth, polytetrafluoroethylene film, three-proof cloth, and alkali-free glass fiber cloth. In recent years, stricter flue gas emission regulations and the removal of the desulfurization bypass baffle have required a 100% success rate in commissioning desulfurization system equipment. As a result, power plants have increasingly focused on addressing leakage issues in non-metallic expansion joints.

1. Leakage Causes

The type of leakage medium in non-metallic expansion joints varies depending on the position within the flue system. Leakage at the inlet of the desulfurization absorption tower causes the emission of high-temperature, sulfur-laden flue gas, with temperatures exceeding 150°C and a high concentration of toxic gases. Leakage at the outlets of the primary and secondary absorption towers occurs post-desulfurization, significantly reducing sulfur content. However, due to the lower flue gas temperature and high moisture content, leakage of water is frequent and can easily freeze during winter.

In summary, leakage at any of these locations can result in environmental pollution and pose significant safety risks. This paper identifies three primary causes of expansion joint leakage.

(1) Leakage Due to Design Defects

A key drawback of non-metallic expansion joints in flues is the presence of grooves. During operation, significant amounts of acidic water accumulate in these grooves. The acidic water gradually seeps through the expansion joint fabric, reaching the pressure plate screws that secure the joint. This leads to loosening, corrosion, breakage, and damage to the fabric layers. Acidic water from the flue leaks through broken screw holes and damaged skin, leading to a chain reaction that exacerbates structural damage and environmental pollution. In standard operating conditions, expansion joints typically corrode and fail within one to two years.

(2) Leakage Due to Failure of Anti-Corrosion Materials

Typically, the structural frame of the expansion joint is protected with rigid glass flake coatings or advanced polymer-based silicon carbide materials for corrosion resistance. However, stress concentrations at welds can cause cracks in the protective coating, leading to leakage. Some plants use 2205 duplex stainless steel for the frame material, but failure can occur when weld joints are not sufficiently protected from corrosion. In such cases, leaks often develop at the welds, especially in high-corrosion environments like coal-fired plants.

(3) Leakage Due to Installation and Operational Issues

The outer skin of non-metallic expansion joints is made from flexible materials. Screws are used not only to secure the skin but also to create a tight seal. However, during installation, the pressure plate used to fasten the skin is typically 4–6 meters long. When screws at the rear are tightened, those at the front may loosen, allowing acidic water to penetrate and corrode the screws. Therefore, repeated tightening is necessary to ensure a secure fit. Additionally, during operation, fluctuations in flue gas pressure cause repeated expansion and contraction of the joint. This continuous flexing leads to fatigue, aging, and tearing, especially near the pressure plate. Over time, this degradation further contributes to leakage.

2. Leakage Sealing Technology

Once the cause of leakage is identified, an appropriate solution must be developed to effectively resolve the expansion joint's leakage issues. The conventional approach involves tightening the skin flange externally to minimize leakage of water and air. First, water is directed into a trench, followed by maintenance activities such as complete skin replacement, anti-corrosion treatment for the flange, and the addition of drainage grooves during scheduled shutdowns. However, this approach may not fully resolve the leakage issue due to factors such as suboptimal skin materials, insufficient anti-corrosion coating, limited drainage capacity, or a lack of maintenance expertise. To overcome these limitations, we propose an online sealing method using polymer full-sealing technology, specifically the polymer alloy wing full-sealing method, which effectively prevents direct contact between flue gas and expansion joints.

2.1 Equipment Parameters

The key parameters of the equipment used in the polymer full-sealing online sealing method are presented in Tables 1–5.

Table 1: Performance Parameters of High-Elastic Tung Oil Gel

Table 2: Performance Parameters of the Polymer Alloy Plate

Table 3: Performance Parameters of the Sealing Device

Table 4: Technical Parameters of the Sealing Device

Table 5: Operating Parameters of the Sealing Device

2.2 Cost Analysis

Although replacing the expansion joint skin may lower initial costs, it does not yield long-term savings. Additionally, the primary drawback of this method is the need to erect scaffolding for high-altitude operations, which poses significant safety risks to workers. Some sections of the clean flue near the chimney can reach heights of 30–40 meters, which makes scaffolding both costly and a significant safety concern. Overall, installing a sealing device on a non-metallic expansion joint costs just 1/22 of the cost of replacing the entire expansion joint. Below is a cost comparison between replacing the skin and installing a sealing device.

(1) Cost of Skin Replacement

For a 20-meter expansion joint at the absorption tower entrance, erecting scaffolding for a height of 12–15 meters costs approximately 6,000 RMB. If the height of the clean flue reaches 30 meters, the cost of scaffolding increases to approximately 30,000 RMB. The cost to manufacture and replace a 20-meter skin is 24,000 RMB. Assuming annual replacement of the skin, the total cost over five years would amount to approximately 150,000 RMB: (6,000 + 24,000) × 5.

(2) Sealing Device Installation Cost

A 20-meter expansion joint sealing device at the absorption tower entrance requires a one-time cost of approximately 70,000 RMB (exact cost may vary depending on the expansion joint width). The sealing device is safe, reliable, and comes with a five-year warranty. After five years, it can be inspected during scheduled maintenance, and any minor issues can be addressed by reapplying sealant.

(3) Additional Risks and Costs of Skin Replacement

The quality of replacement expansion joints may not always be assured. Leakage may occur immediately after installation or due to flue deformations in winter, particularly if the replacement was performed in summer. In northern regions, winter leakage and ice buildup pose serious safety hazards. Frequent high-altitude scaffolding for skin replacement introduces additional risks, with any accidents further increasing costs.

2.3 Construction Plan and Precautions

• Remove any dust or debris from the area surrounding the expansion joint.
• After thoroughly cleaning the metal frame of the non-metallic expansion joint, perform welding and repair on any corroded areas.
• Once the surface repairs are complete, polish and prime the surface.
• Apply high-elastic tung oil gel evenly to the bottom of the expansion joint and both sides of the groove, then proceed with installing the sealing device.
• The desulfurization expansion joint should be configured with a diversion slope based on the acid discharge port location to facilitate the efficient flow of condensed acid water.
• Seal and protect all joints around the sealing device with high-elastic tung oil gel, ensuring a minimum coating thickness of 2mm.
• When sealing leaks, workers must wear appropriate protective equipment, including gas masks (such as long-tube or positive pressure respirators), heat-insulating clothing, and ventilation equipment with axial flow fans.

3. Conclusion

Field applications have demonstrated that this treatment is substantially more effective than traditional methods, reducing costs by 50% and providing a comprehensive solution to the problem. The design concept of the polymer fully sealed expansion joint adheres to the "install and forget" principle, efficiently isolating the expansion joint frame from contact with smoke. The corrosion resistance of the polymer alloy is significantly superior to that of 1.4529 stainless steel and titanium plates, effectively resolving wear and corrosion issues and ensuring the skin remains leak-proof. To fill the groove between the flanges without restricting expansion, an acid-resistant and elastic material is required: high-elastic tung oil gel. This material can compress and expand freely, resist high temperatures and corrosion, and is non-absorbent. It provides a reliable seal to prevent the expansion joint from detaching during expansion and contraction. Additionally, a high-elastic film adhesive with a tensile elongation of 450% must be applied to the surface to prevent acid water or smoke from contacting the expansion joint skin or flange.