In coal-fired power plants, iron and steel sintering machines and chemical tail gas desulfurization systems, skin rupture of desulfurization flue expansion joint is one of the most common and serious equipment failures. Once the skin of non-metallic expansion joint (also known as flexible band) breaks, it will directly lead to corrosive smoke leakage, environmental protection data exceeding the standard, energy consumption of induced draft fan increasing, and even force the unit to reduce load or unplanned shutdown in severe cases. This paper systematically analyzes the typical patterns, deep causes, emergency treatment methods and preventive maintenance strategies of skin rupture, and provides a set of practical guidelines that can be implemented and implemented for equipment managers.

Three typical modes of skin rupture

When dealing with the skin rupture of the expansion joint of the desulfurization flue on site, the rupture pattern should be identified accurately first, so as to quickly judge the failure mechanism. Common patterns include:

1. Tear-type rupture

Linear or zigzag cracks appear in the skin along the direction of fabric reinforcement layer, which mostly occur in the parts with large angular displacement of expansion joints. The root cause is that the compensator design compensation amount is insufficient, or the actual displacement is seriously exceeded due to the sinking of flue support and the failure of limiting device. Such ruptures typically develop rapidly, often taking only a few days from the appearance of a tiny crack to a complete tear.

2. Corrosion penetration

There are many fine holes on the surface of the skin or local thinning and powdering, and the edge is characterized by brittle failure. This is a typical acid corrosion result-after wet desulfurization, the net flue gas temperature is 45-55℃, containing residual SO₂ and SO₃, and dilute sulfuric acid condensate is formed under the condition of lower than the acid dew point, which continuously erodes the skin rubber layer and PTFE film. The leakage is small in the initial stage of corrosion penetration, but it will rapidly develop into large-scale leakage.

3. Interlayer delamination

The composite layers of the skin (fluororubber, fiberglass cloth, thermal insulation layer, etc.) are separated and bulged, and the overall strength is lost. The main reason is that the bonding process is unqualified in the manufacturing process, or the long-term high-frequency and low-amplitude vibration leads to fatigue delamination. Although there is no obvious perforation in the delaminated skin, the pressure bearing capacity drops sharply, and it is easy to burst suddenly in the process of starting and stopping the machine.

Four layers of root causes of skin rupture

No matter what kind of working environment, the deep reasons for skin rupture of desulfurization flue expansion joint can be summarized into four levels:

• Design selection error: the appropriate number of skin layers is not selected according to the actual flue gas temperature, acid dew point and displacement (the net flue gas section of wet desulfurization requires at least 6 layers of structure: PTFE anticorrosive layer + sealing rubber layer + glass fiber reinforced layer + stainless steel wire mesh + insulation layer + outer protective layer); Insufficient safety margin of angular compensation amount (recommended not less than 1.5 times of measured displacement); The influence of start-stop frequency on fatigue life is not considered.
• Installation quality defects: the skin is unevenly tensioned on the flange surface, resulting in local stress concentration; The bolts are not tightened in diagonal order, resulting in twisted wrinkles of the skin; The deflector is installed misaligned or missing, and the high-speed dusty smoke directly washes the inner surface of the skin; The corrosion of the flange surface was not cleaned, resulting in condensate seeping into the interlaminates after seal failure.
• Deviation of operating conditions: the flue gas temperature at the inlet of desulfurization system exceeds the limit (> 180℃), which accelerates the aging and hardening of rubber; Frequent start-and-stop of units and deep peak shaving lead to a sharp increase in the number of cyclic displacements (from 1000 times/year in design to 3000-5000 times/year); The dust content of flue gas exceeds the standard, and the anti-corrosion layer on the skin surface is worn.
• Maintenance inspection absence: failure to check the skin surface state and bolt preload force quarterly; Slight cracks or small holes are not repaired in time and allowed to expand after being found; The deflector of the expansion joint and the dust accumulation inside were not cleaned regularly, resulting in abnormal pushing force on the skin.

Emergency treatment and permanent repair steps

When the skin of the expansion joint of the desulfurization flue is found to be cracked, it should be disposed of quickly according to the following procedure:

Stage 1: Emergency leak plugging (non-stop furnace scheme)

Small holes or short cracks (10cm) shall be mechanically pressed and sealed with a special clamp and acid-resistant rubber plate, and the width of the clamp shall exceed the damaged edge by at least 5cm. Emergency treatment can be maintained in operation for 7-15 days to buy time for planned shutdown.

Phase 2: Shutdown and complete replacement

Thoroughly replace the whole circle of skin, and it is strictly forbidden to repair only partially. Before replacement, the actual displacement of flue in cold and hot states must be measured, and the original design parameters must be checked whether they meet the present situation. After removing the old skin, thoroughly clean the flange surface for corrosion, residual adhesion and old bolts, polish until metallic luster is exposed, and apply anti-rust primer.

Phase 3: Installation of critical controls

After the new skin is tiled on the flange, the bolts are symmetrically preloaded from the middle to both ends, and the final torque is reached in three times according to the requirements of the manufacturer; The skin cannot appear any wrinkles or twists, allowing the tension elongation to be controlled within 2%-4%; The installation direction of the deflector must be consistent with the direction of flue gas flow, and the overlap amount shall not be less than 50mm.

Phase 4: Commissioning Verification

After replacement, carry out 500Pa micro-positive pressure airtightness test, and check all flange interfaces and skin body with soapy water; After the load is heated to the working temperature, when the expansion joint reaches the thermal equilibrium state, heat tightening the bolt to the specified torque again; After 24 hours of continuous operation, re-measure the displacement and confirm that the compensation amount is within the design range.

Preventive maintenance system recommendations

To avoid repeated rupture of the skin of the expansion joint of the desulfurization flue, a graded maintenance system should be established:

• Daily inspection (weekly): Visually check the skin surface for abnormal bulging, cracking and wrinkles; Whistling or visible smoke leakage; Whether the bolts are loose or falling off.
• Professional testing (quarterly): Use ultrasonic thickness gauge to detect the remaining thickness of each layer of skin; Infrared thermal imaging scanning to find tiny leakage hotspots; Check the wear of the deflector and the thickness of dust accumulation inside.
• Comprehensive evaluation (annually): measure the cold and hot displacement of the flue, and compare it with the design value. If the deviation exceeds 15%, the bracket should be adjusted or the expansion joint selection should be recalculated; Replacing aged bolts and gaskets; Samples were taken to detect the aging degree of skin materials.

epilogue

Although the skin rupture of the expansion joint of desulfurization flue cannot be completely eliminated, the failure frequency and maintenance cost can be greatly reduced by accurately identifying the rupture mode, digging deep into the root cause, and implementing the maintenance strategy combining emergency response and prevention. Industry practice shows that by combining design review, standardized installation and graded maintenance, the average service life of skin can be extended from 12-18 months to 3-5 years, and the comprehensive operation and maintenance cost can be reduced by more than 40%.