In wet flue gas desulfurization system, the condensate problem of flue outlet expansion joint is one of the most headaches for operation and maintenance personnel. The net flue gas temperature at the outlet of the desulfurization tower drops to 45-55℃, which is in a completely saturated state, carrying a large amount of water mist and acidic droplets. When these wet flue gases pass through the expansion joint at the flue outlet, they condense into strong acidic liquid when exposed to condensation, which accumulates in the groove of the expansion joint, resulting in skin corrosion, loose bolts, acid leakage, equipment corrosion and environmental pollution in the slightest case, and icing in winter in the worst case, threatening personnel safety. This paper will systematically explain the professional technical knowledge of condensate water in flue outlet expansion joint from the cause mechanism, hazard analysis to treatment scheme.

1. How is condensate produced?

The formation of condensate in the flue outlet expansion joint has its specific physicochemical mechanism. In the wet desulfurization process, after the original flue gas is sprayed and washed by limestone slurry in the absorption tower, acidic gases such as SO₂ and SO₃ are effectively removed, and the flue gas temperature drops sharply, usually to 45-55℃. At this time, the clean flue gas becomes saturated wet flue gas, carrying a large amount of water mist and tiny droplets.

When the saturated wet flue gas passes through the clean flue and flue outlet expansion joints, the water vapor in the flue gas condenses into liquid water when condensed because the temperature of the metal frame and skin of the expansion joint is usually lower than the temperature of the flue gas. At the same time, the residual acidic gases such as SO₂, SO₃, Cl⁻¹ and F⁻¹ in the flue gas are dissolved in the condensed water to form a strongly acidic condensate with a pH value as low as 2-3.

Key Features: When the non-metallic expansion joint is installed, an annular groove will naturally form between the skin and the platen. It is this structural defect that has become a "hotbed" for condensate accumulation. Once the condensate enters the groove, it cannot be discharged naturally by gravity, and it soaks the skin and bolts for a long time, gradually causing osmotic corrosion.

2. The harm of condensate should not be underestimated

The hazards of condensate in flue outlet expansion joint are many aspects, which seriously threaten the safety of equipment and the health of operators.

1. Skin penetration and leakage

Soaking the skin of the expansion joint with acid water for a long time will slowly penetrate through the multi-layer fabric layer and reach the position of the fixed screw, causing the screw to loosen and corrode and break. Finally, the acid liquid flows out from the broken screw hole and the damaged part of the skin, resulting in a serious "waterfall" phenomenon. Judging from the operation situation, the expansion joint without preventive measures will generally be corroded and damaged in one to two years.

2. Metal frame and bolt corrosion

Acidic condensate with high concentration of Cl⁻¹ is extremely corrosive to stainless steel bolts and metal frames. The results show that the average life of 316L expansion joint is not more than two years in the practical use of desulfurization wet flue, and the pitting corrosion and stress corrosion cracking caused by chloride ions are the main failure modes.

3. Potential safety hazards of icing in winter

Condensate leaks from flue outlet expansion joints can pose serious safety concerns in cold northern regions. The acidic liquid leaked from the expansion joint will freeze rapidly in winter and accumulate on the equipment platform and channel, which seriously affects the passage safety of operators and maintenance personnel, and may even cause slip and fall accidents.

4. Environmental impact and equipment corrosion

The leaked acidic condensate will seriously corrode the steel structure, platform and insulation layer of the equipment, and at the same time produce pungent acid mist, which will affect the on-site working environment.

3. Governance plan: from emergency treatment to radical repair

Aiming at the condensate problem of flue outlet expansion joint, the industry has developed a variety of mature and effective treatment schemes, which can be selected according to the actual situation.

Option 1: Groove filling technology-radical solution

To completely solve the problem of condensate leakage, the core lies in filling the water-accumulated groove of the expansion joint, so that acid water can't contact the skin and bolts.

Construction steps:

1. Shutdown cleaning: remove dust around expansion joint, remove damaged skin
2. Skeleton repair: Clean up the floating ash of the metal skeleton and weld and repair the corroded part
3. Bottom layer anti-corrosion: High elasticity tung oil gel is applied to both sides of the groove of the expansion joint
4. Filler filling: Fill with high-temperature and corrosion-resistant sponge-shaped closed-cell foamed rubber filler (compression ratio 7:1), and compact in layers
5. Surface sealing: the top layer is smoothed with high elasticity tung oil gel, and the thickness is controlled at about 5mm
6. Perimeter anti-corrosion: Use high elastic flexible glass flakes to prevent corrosion at the joints around the expansion joints, and the thickness should be more than 3mm

Technical advantages: This scheme makes the acid water in the flue not contact the skin part of the expansion joint, which does not affect the absorption of the expansion amount, and completely solves the problem of skeleton corrosion and leakage of the expansion joint.

Scheme 2: Set up drainage device

For horizontally mounted flue outlet expansion joints, a drainage system must be provided at the lowest point of the frame.

Manual drainage scheme:

• Set drainage holes at the lowest point of the expansion joint frame (at least DN150)
• Install drain stub and valve
• Drain pipe leads to gutter or acid collection system
• The valve is normally closed, and the drainage is opened regularly

Operation requirements: Start drainage for 1-2 minutes per shift, observe the properties of discharged liquid, and close the valve after drainage to prevent negative pressure operation from inhaling outside air.

Scheme 3: Structural design of water baffle

An effective structural optimization scheme is that a water retaining device is arranged inside the expansion joint, and a water retaining plate and an inner lining plate are arranged on the metal frame to respectively double-block the condensed water refluxing in the pipeline and the condensed water dripping above, thereby effectively reducing the probability of the condensed water flowing to the inner ring belt and accumulating. This design reduces the erosion of the skin by condensate from the structural source.

IV. Preventive measures and maintenance suggestions

The long-term prevention and treatment of condensate problem in flue outlet expansion joint needs to establish a perfect prevention and maintenance system.

1. Prevention during the installation phase

• When installing the horizontal flue expansion joint, ensure that the drain hole is at the lowest point
• Install strictly according to the flow direction mark, with the small end of the guide tube facing the incoming flow
• The bolts are tightened diagonally and tightened in fractional intervals, and are tightened once in 1 month and once in 3 months after operation

2. Monitoring during the operation phase

• Check the appearance of the expansion joint every week for water seepage and bulge
• Open the drain valve regularly and observe the properties of the discharged liquid
• Clear and transparent is normal condensed water; Yellow or reddish brown indicates that the frame is beginning to corrode

3. Maintenance cycle

• Full tightening of platen bolts quarterly
• Clean drain holes monthly to prevent clogging
• Check groove fill layer and anticorrosive coating integrity periodically

4. Inspection during shutdown

Each time the furnace is shut down for maintenance, the expansion joint at the flue outlet should be checked:

• Check the skin for damage and aging
• Check bolts for looseness and corrosion
• Check the groove filling layer for cracking and detachment

V. Summary

The problem of condensation in flue outlet expansion joint is a common technical problem in wet desulfurization system, which is rooted in the condensation of saturated wet flue gas when cooled and the groove structure of expansion joint. The correct prevention and control strategy should follow the principle of "combining prevention and discharge and comprehensive management":

A flue outlet expansion joint with reasonable design and proper maintenance can solve the stubborn problem of condensate leakage from the root cause, ensure the safe and stable operation of desulfurization system, and eliminate the hidden safety hazard caused by icing in winter. It is suggested that enterprises should include the condensate water problem of expansion joint into the key inspection items every time the furnace is shut down for maintenance, so as to achieve early detection and treatment.