In the wet desulfurization system of coal-fired power plant, water leakage of flue expansion joint body or connecting flange is one of the frequent failures in operation and maintenance. The "water leakage" mentioned here is essentially that the water vapor in the flue gas condenses into liquid water in the low-temperature area, or the slurry of the desulfurization spray layer flows back and seeps out through the damaged part of the expansion joint. The difficulty of water leakage treatment of flue expansion joint in power plant lies in that the leakage point is often not a single position, and the acidic condensate is strongly corrosive. Long-term leakage will accelerate the failure of expansion joint, corrode the surrounding steel structure, and even affect the water balance of desulfurization system. This paper starts with the fault phenomenon, systematically analyzes the root cause of water leakage, introduces the on-site emergency leakage plugging method and permanent repair scheme, and provides a set of operational disposal guidelines for power plant maintenance personnel.

Three Typical Manifestations of Water Leakage

Before dealing with the leakage treatment of flue expansion joint of power plant, the leakage type should be accurately identified first, so as to judge the root cause of the fault.

1. Flange interface leakage

It is manifested as dripping or streaming water seepage at the connection between the expansion joint and the flue flange, and the water quality is acidic (pH value 2-4), and the color is often yellowish-brown (containing rust). The root causes are insufficient or uneven bolt pre-tightening force, aging failure of sealant, or corrosion of flange surface leading to the decrease of seal specific pressure.

2. Skin body leakage (non-metallic expansion joint)

It is manifested as wet plaques or fine water droplets exudating on the skin surface of non-metallic expansion joints, which are mostly located at the low points or folds of the skin. The reason is that the PTFE anti-corrosion layer of the skin has been damaged, and acidic condensate water seeps out through the glass fiber layer. At this time, the overall strength of the skin has decreased, and there is a risk of sudden bursting.

3. Metal bellows weld leakage

It is manifested as linear leakage at the trough or circumferential weld of the bellows of the metal expansion joint. This is a typical stress corrosion cracking-316L stainless steel produces micro-cracks under the combined action of chloride ions and residual stress, and the cracks leak after penetrating the wall thickness.

Three-layer analysis of the causes of water leakage

The leakage of flue expansion joint of power plant is not accidental, and its underlying reasons can be summarized into three levels:

Level 1: Condensate accumulation and poor discharge
After wet desulfurization, the net flue gas temperature is about 45-55℃, and the water vapor content is saturated. When the temperature of the inner wall of the flue or expansion joint is lower than the acid dew point of the flue gas, a large amount of condensed water will be produced. If no drainage port is set at the lowest point of the expansion joint or the drainage pipe is blocked, the condensed water will soak the skin or flange joint for a long time, which will accelerate the seal failure.

Layer 2: Sealing Structure and Material Aging
The aging period of non-metallic expansion joint flange sealant in acidic wet flue gas environment is generally 2-3 years, after which it will harden and crack. Metal expansion joint flange gaskets, such as flexible graphite or metal wound pads, may also come loose under alternating temperatures.

Layer 3: Improper operation
During the start-up and stop process of the unit, the flue temperature rises/cools too fast, which causes the expansion joint to bear abnormal displacement and crack the seal structure. In addition, when the slurry bubbles in the spray layer of the desulfurization tower or the demister fails, the slurry may flow back into the flue and corrode the expansion joint from the inside.

On-site emergency leak plugging method

When water leakage occurs and the furnace cannot be stopped immediately, the water leakage treatment of flue expansion joint of power plant can be carried out according to the following scheme:

Scheme 1: Flange interface leakage

• When conditions permit, heat tighten the flange bolts-one by one in diagonal order to the specified torque (40-50N·m for M12).
• If the bolt has been corroded and cannot be tightened, you can use a "leak plugging with pressure" jig: a custom-made Huff semicircular jig wraps the outside of the flange, and the inside is filled with acid-resistant sealant (fluorosilicone rubber), and the jig is pressed and sealed.
• Temporary measures: Apply acid-resistant high-temperature sealant (such as silicone rubber, temperature resistance ≥200℃) on the outside of the leakage point, and it can be maintained for 1-2 weeks after curing.

Scheme 2: Leakage of pinhole in skin body

• Small area leakage (diameter
• Large area leakage: Customized external reinforcing patch is required-cut into patch with skin material of the same material, bonded with acid-resistant high temperature adhesive and mechanically pressed. However, this is only a temporary response, and the whole circle of skin should be replaced as soon as possible.

Scheme 3: Crack leakage of metal bellows

• Metal expansion joint water leaks usually cannot be repaired on-site with pressure. It can be used with stainless steel special repair agent (such as Belzona 1311) to fill the crack and cover with glass fiber cloth for reinforcement, which is barely available only at low pressure and temperature. The fundamental solution is to shut down and replace the whole piece.

Permanent Restoration Protocol

During the scheduled shutdown, permanent repairs should be implemented for the source of the leak:

1. Flange surface treatment
After disassembly, the flange surface is thoroughly cleaned for corrosion, and when the flatness exceeds 3mm/m, machining and dressing or replacing the flange. Replace all bolts with 304 or 316 stainless steel. The double sealing structure of acid-resistant sealant + flexible graphite composite gasket is adopted during installation.

2. Non-metallic expansion joint skin replacement
Leaky pinholes in the skin mean that the corrosion protection layer has failed and must be replaced in full circle. At the same time, check and dredge the drainage pipe at the lowest point of the expansion joint to ensure that the condensed water is discharged in time.

3. Metal expansion joint treatment
Minor cracks can be repaired (certified welders need to operate according to the evaluated process), but trough cracks usually cannot be repaired reliably, so it is recommended to replace the whole piece. At the same time, upgrade to higher grade materials (such as 254SMo or C-276 alloy).

4. System modification
If water leaks repeatedly, consider adding a water baffle or changing the angle of the deflector upstream of the expansion joint to prevent the desulfurization slurry from flowing back and impacting the expansion joint. In addition, an insulating layer can be installed outside the expansion joint to raise the flue wall temperature to above the acid dew point, and fundamentally reduce the generation of condensed water.

Preventive maintenance recommendations

To avoid the leakage treatment of flue expansion joints in power plants becoming a recurrent work, a graded maintenance system should be established:

• Weekly inspection: check the appearance of the expansion joint and record the signs of leakage; unblocking drainage pipes; Measure whether the temperature of the outer wall of the expansion joint is below the acid dew point.
• Quarterly inspection: Use ultrasonic thickness gauge to detect the remaining thickness or skin aging degree of metal bellows; Infrared thermography examines cryogenic areas.
• Annual inspection: Open the inspection hole of the deflector when the machine is shut down to clean up the dust and corrosion products inside; Re-tighten all flange bolts.
• Every 5 years: Preventive replacement of non-metallic expansion joint skin, leaking or not.

epilogue

The treatment of flue expansion joint leakage in power plant cannot stop at temporary plugging, but must trace the root cause of leakage-is it condensate accumulation, seal aging or metal corrosion? Only by system governance from the four dimensions of design, installation, operation and maintenance can the problem be completely solved. Industry experience shows that the combined use of three measures: adding drainage points + regularly replacing seals + controlling flue wall temperature can reduce the water leakage failure rate by more than 80%.