In the wet desulfurization system, the original flue conduit is responsible for transporting the high-temperature flue gas of the boiler to the desulfurization tower. The expansion joint installed on the flue often leaks water in actual operation, which troubles the operation and maintenance personnel of many power plants and steel mills. It is of great significance to deeply analyze the causes of water leakage in the original flue expansion joint of desulfurization for quickly locating the fault and formulating effective treatment measures. This paper will systematically sort out all kinds of factors leading to water leakage, from working condition characteristics, material selection, structure design to operation and maintenance, and provide complete diagnosis ideas and solutions for engineers and technicians.

1. Special working conditions of expansion joint of original flue of desulfurization

In order to accurately understand the reason of water leakage in the expansion joint of the original flue of desulfurization, we should first know the special working environment of this part. The original desulfurization flue is located between the outlet of the boiler air preheater and the inlet of the desulfurization tower, and its working conditions have the following remarkable characteristics:

• The flue gas temperature is relatively high: usually between 120~180 ℃, instantaneously reaching above 200 ℃
• Large dust content: The dust concentration of raw flue gas of coal-fired boiler can reach 30~50g/Nm³
• High concentration of corrosive gases: contains acidic gases such as SO₂, SO₃, HCl, HF
• There is a risk of acid dew point corrosion: when the local temperature of the flue is below the acid dew point, strong corrosive condensed acid can form

It is worth noting that the "water leakage" mentioned in the title does not refer to the leakage of clean water, but to the phenomenon of liquid leakage at the expansion joint. These liquids are typically acidic condensate, external rinse water, or insulation water. Therefore, it is necessary to comprehensively judge the causes of water leakage in the original flue expansion joint of desulfurization from many angles.

2. Classification of main causes of water leakage in expansion joint of original flue of desulfurization

According to the statistics of a large number of on-site cases, the causes of water leakage in the expansion joint of the original flue of desulfurization can be summarized into the following five categories:

1. The expansion joint body is damaged

This is the most common direct cause, which manifests itself as:

Non-metallic expansion joint breakage:

• Tear of inner anti-corrosion material: The inner layer of fluororubber or PTFE is pulled due to improper pre-stretching during installation or excessive displacement during operation
• Aging and cracking of the outer layer: Long-term exposure to ozone, ultraviolet rays or high temperature environment, the outer rubber loses elasticity, appears fine cracks, and external water seeps in
• Interlayer separation bulge: Condensate penetrates into fabric layers, vaporizes at high temperatures, causes delamination, and eventually ruptures and leaks

Metal expansion joint broken:

• Bellows fatigue crack: Frequent start-and-stop leads to metal fatigue, producing micro-cracks
• Pitting perforation: Acidic condensate accumulates at bellows valleys, causing local corrosion perforation
• Weld cracking: Weld quality defects or vibration fatigue cause cracking at the weld

2. Condensate accumulation and corrosion

This point is the most overlooked cause of water leakage in desulfurization raw flue. When the flue gas temperature is lower than the acid dew point (the dew point can reach 130~150℃ in the presence of SO₃), water vapor and SO₃ combine to form sulfuric acid mist, which condenses into dilute sulfuric acid (pH value 1~3) on the inner wall of the expansion joint.

Mechanism of action:

• The acidic condensate directly corrodes the inner wall of the expansion joint (especially metal bellows)
• The condensate accumulates at the trough or low point of the expansion joint, forming a local pool of liquid accumulation and accelerating corrosion perforation
• Long-term immersion leads to hydrolysis of fabric layer and decrease of strength of non-metallic expansion joint

Therefore, a considerable part of the causes of water leakage in the original flue expansion joint of desulfurization is actually "corrosion first and then leakage", rather than the original defect.

3. External water entry

Sometimes the seal of the expansion joint itself is intact, but external water enters the flue system:

• Flue flushing water backward flow: When the desulfurization system is shut down for flushing, the flushing water flows back to the expansion joint through the flue
• Water accumulation in insulation layer: After rain and snow weather or flue cleaning, the insulation layer absorbs water, and the water seeps into the flange gap of the expansion joint
• Fire-fighting water spraying: nearby fire-fighting sprinkler system malfunctioning or water entering during test
• Poor drainage of condensate: No drainage hole is provided at the expansion joint or the low point of the flue, or the drainage pipe is blocked, and the condensate cannot be discharged in time and accumulates

4. Installation and structural design defects

Irregular installation and design will directly lead to water leakage hazards:

• Missing or insufficient length of guide tube: Smoke gas directly washes the corrugation of the expansion joint or the inner wall of the fabric, accelerating wear and corrosion
• No drainage hole is provided: the condensate cannot be discharged, and the bottom of the expansion joint is soaked for a long time
• Uneven flange surface: Flange deformation or corrosion causes the sealing gasket to be unable to be compressed and leakage occurs
• Uneven bolt tightening: Inconsistency of bolt torque, resulting in insufficient local compression of sealing gasket
• Inaccurate pre-deformation: Installation is not pre-stretched or pre-compressed according to design values, and the expansion joint is subjected to additional tension during operation

5. Abnormal operating conditions

Certain abnormal operating conditions can increase the load on the expansion joint and lead to water leakage:

• Frequent start-stop: Daily start-stop or weekly start-stop mode, alternating hot and cold to accelerate material fatigue
• Low load operation: the smoke temperature is lower than the design value for a long time (e.g.
• Bypass flue leakage: the bypass door of the original flue is not closed tightly, and high-temperature smoke enters, resulting in local overheating
• Pressure fluctuation of desulfurization tower: The flue pressure fluctuates drastically due to unstable regulation of induced draft fan, and the expansion joint bears alternating load

3. Comparison of water leakage characteristics of different types of expansion joints

According to the different causes of water leakage of the original flue expansion joint of desulfurization, the water leakage performance of non-metal and metal expansion joints has its own characteristics:

Accurate identification of leakage characteristics is helpful to quickly judge the cause of water leakage in the original flue expansion joint of desulfurization, so as to formulate targeted treatment scheme.

4. Diagnosis methods and troubleshooting steps of water leakage problems

When the expansion joint is found to be leaking, it can be systematically checked according to the following steps:

Step 1: Preliminary judgment of water source

• Use pH test paper to test the leakage liquid: if pH
• Observe the timing of water leakage: during operation, the leakage is flue gas condensation or body damage; The leakage after shutdown is mostly caused by external water backward flowing or water accumulation in the insulation layer

Step 2: Visual inspection

• Check the external surface of the expansion joint for any cracks, bulges and burning marks
• Check whether the flange bolts are loose or corroded, and whether the flange surface clearance is even
• Check that the drainage hole is clear

Step 3: Shutdown internal inspection (after safety measures are in place)

• Enter the flue or use an endoscope to check the condition of the inner wall of the expansion joint
• Check the guide tube for detachment, erosion and wear
• Check troughs (metal) or fabric layers (non-metal) for fluid accumulation and perforations

Step 4: Analysis of operating parameters

• View Smoke Temperature History Curve: Are There Periods Below Acid Dew Point for a Long Period
• View start-stop frequency: is it too frequent
• View desulfurization tower inlet pressure fluctuation range

Targeted Solutions and Preventive Measures

In view of the above-mentioned reasons for water leakage of the expansion joint of the original flue of desulfurization, the following treatment measures can be taken:

Against condensation corrosion

• Opening DN25~ DN50 drainage holes at the lowest part of the expansion joint, and connecting them to the water sump
• Insulate the external wall of the expansion joint and the nearby flue to prevent dew condensation on the inner wall
• Increase the smoke temperature at the flue inlet (e.g. by bypass adjustment) to ensure above 20℃ above the acid dew point

Aiming at body breakage

• Minor damage (non-metal): temporarily seal with repair tape or special glue of the same material
• Severe breakage or aging: Expansion joint must be replaced in its entirety
• Pitting corrosion of metal bellows: small area can be repaired and large area can be replaced

For installation defects

• Reassemble or extend the guide tube, and the length of the guide tube extending into the inside of the expansion joint shall be ≥ wave height
• Re-tighten the bolts in diagonal sequence, and the torque value is as required in the design drawing
• Machine repair or replacement of uneven flanges

For Operation Management

• Avoid long-term low-load operation and maintain exhaust temperature ≥ acid dew point +15℃
• Control temperature change rate ≤50℃/h during start-stop process
• Establish an expansion joint inspection ledger, and check the drainage hole and appearance once a quarter

VI. Analysis of typical cases

Case: In a 300MW unit of a coal-fired power plant, the non-metallic expansion joint of the original flue of wet desulfurization had intermittent water leakage after 2 years of operation. After shutdown inspection, it was found that:

• Phenomenon: There is yellow acidic liquid oozing from the bottom of the expansion joint, pH about 2.5
• Check: The guide tube is intact, but there is no drainage hole at the lowest part of the expansion joint; The inner layer of fluororubber has local swelling and foaming
• Conclusion: The reason for water leakage of flue expansion joint in desulfurization is that acid condensate accumulates and soaks, which leads to hydrolysis failure of fluororubber layer
• Treatment: Replace expansion joint and add DN32 drain hole. After treatment, continuous operation for 4 years without water leakage

VII. SUMMARY

The causes of water leakage in the original flue expansion joint of desulfurization are complex and diverse, which can be summarized into five categories: body damage, condensation corrosion, external water entry, installation defect and operation abnormality. Among them, corrosion perforation caused by acid condensate accumulation is the most common and easily overlooked root cause.

Core conclusions:

1. Diagnosis before treatment: Quickly judge the nature of water leakage through pH test, leakage timing and appearance inspection, and avoid blind replacement
2. The drainage hole is a life-saving design: a reliable drainage device at the lowest point of the expansion joint is the most effective and cheapest measure to prevent water leakage
3. Guide tube should not be omitted: qualified guide tube can avoid smoke directly washing the inner wall and significantly prolong the service life
4. Material matches with working conditions: Acid-resistant and temperature-resistant fluororubber or PTFE non-metallic expansion joints should be selected for the original flue of desulfurization. Ordinary metal expansion joints have shorter life under this working condition
5. Institutionalization of operation and maintenance: Keeping the smoke temperature above the acid dew point, controlling the number of starts and stops, and checking the drainage holes regularly can reduce the water leakage failure rate by more than 70%

For the operation and maintenance personnel, to thoroughly solve the water leakage problem of the original flue expansion joint of desulfurization, we can't just replace new parts, but should systematically manage it from three levels: working condition analysis, structural optimization and operation management. Only by finding out the real cause of water leakage in the original flue expansion joint of desulfurization can we prescribe the right medicine and achieve long-term stable operation.