In coal-fired power plants, iron and steel sintering, cement building materials and chemical tail gas treatment systems, non-metallic expansion joints are the key flexible connecting parts to absorb flue thermal displacement, isolate vibration and adapt to installation errors. The skin (also known as flexible band or compensator fabric layer) in non-metallic expansion joints is a weak link that is prone to aging and damage. When the skin has corrosion penetration, delamination between layers, tearing or exceeding the designed service life, the non-metallic expansion joint skin replacement becomes an overhaul work that must be performed strictly in accordance with the process requirements. The wrong replacement process may cause the new skin to be damaged again in a short time, or even cause environmental accidents of flue gas leakage. Starting from engineering practice, this paper systematically explains the timing judgment, detailed operation steps, key technical standards and common mistake avoidance of skin replacement, and provides an operable and complete guide for equipment maintenance engineers and maintenance personnel.

When do you need to replace the non-metallic expansion joint skin?

Before replacing non-metallic expansion joint skin, you should first accurately judge whether it really needs to be replaced, so as to avoid excessive maintenance or delayed maintenance. Replacement should be decisively arranged in the following five cases:

1. Penetrating rupture: Penetrating cracks or holes visible to the naked eye appear in the skin, and the smoke has obviously leaked out.
2. Large-area corrosion thinning: the surface of the skin is powdered and cracked, and it feels obviously soft or thin when pressed by hand, and the remaining thickness is less than 50% of the original thickness.
3. Interlayer delamination bulge: The composite layers of the skin are separated, forming local bulge and losing overall strength.
4. Beyond service life: Even if there is no obvious damage, it should be replaced preventively after 5-6 years of operation-at this time, the rubber and PTFE layers have been aged to varying degrees.
5. Damage of deflector and associated damage: After the deflector falls off or is seriously worn, high-speed smoke directly washes the inner surface of the skin to cause damage.

Three necessary preparations before replacement

Non-metal expansion joint skin replacement is a key operation involving safety and environmental protection, and the preparation work directly determines the replacement quality.

1. Parameter review and customization of new skin

Before removing the old skin, the following raw data must be measured and recorded: external dimensions of the expansion joint (length × width or diameter), flange hole spacing and hole diameter, total skin thickness and thickness of each layer, design compensation amount (axial, transverse and angular), maximum flue gas temperature and acid dew point, and dust concentration in the medium. The new skin is customized according to these parameters, and it is recommended that the wet desulfurization net flue gas section adopt a six-layer structure: PTFE anti-corrosion layer (1.0-1.5mm) + sealing rubber layer + glass fiber reinforced layer (2-3 layers) + stainless steel wire mesh + thermal insulation layer (aluminum silicate fiber felt 50-80mm) + outer protective layer (weather-resistant rubber or fluorine glue coated glass fiber cloth).

2. Implement safety isolation measures

The flue where the expansion joint is located must be completely shut down for cooling, and the temperature should drop below 50℃ before operation. The upstream side baffle door is closed and locked, and the medium is evacuated and replaced with nitrogen or air. Blind plates or obvious disconnect points are installed on both sides of the working point. Prepare acid-proof clothing, gas mask and portable SO₂/H₂S alarm.

3. Prepare special tools and materials

Includes: angle grinder, pneumatic shovel, torque wrench, stainless steel bolt (specification according to original design, must be 304 or 316 material), acid-resistant sealant (silicone rubber or fluorosilicone rubber), acetone cleaning agent, brush, crowbar, hand hoist (for adjusting misaligned flue).

Nine-Step Operation Process for Skin Replacement

Standardized non-metallic expansion joint skin replacement should be performed strictly according to the following steps:

Step 1: Remove the old skin and platen
Loosen the fixing bolts one by one, and remove the old skin and pressure plate (the pressure plate should be replaced together if it is seriously deformed). Pay attention to protect the flange surface and avoid strong prying injury.

Step 2: Clean the flange surface
Use an angle grinder with a wire brush to thoroughly remove old glue, rust, burrs and residual fabric from the flange surface until the metallic luster is exposed. When the flatness deviation of the flange exceeds 3mm/m, it should be trimmed first.

Step 3: Inspect the expansion joint frame and deflector
Check whether the frame is deformed and the weld is cracked; The deflector shall be intact without falling off, and if the wear exceeds 40% of the original thickness, it shall be replaced. The gap between the deflector and the inside of the skin should be 10-20mm to avoid friction under hot state.

Step 4: Correct Flue Centering Status
Measure the actual displacement deviation of the flanges on both sides. If the deviation exceeds 80% of the designed compensation amount, the flue should be pulled to close to the cold median by hand-pulled hoist, otherwise the new skin will tear soon after installation.

Step 5: Glue coating and skin laying
Evenly apply acid-resistant sealant (thickness 2-3mm) on the flange surface and the contact surface of the pressure plate. Lay the new skin flat on the flange, ensuring that the skin width centerline coincides with the expansion joint centerline. Note that the skin should not be stretched too tightly, and a relaxation margin of 2%-3% is allowed to absorb thermal expansion.

Step 6: Install the pressure plate and pre-tightening bolts
Place the platen and install the new stainless steel bolts. Tighten in three times symmetrically from the middle to the ends: the first preload to 30% target torque, the second to 70%, and the third to 100%. The target torque is executed at M10=25-30 N·m, M12=40-50 N·m.

Step 7: Check the seal and flatness
After tightening, check that the skin surface should be flat and free of wrinkles, and the sealant between the pressure plate and the flange should be evenly extruded to be qualified. Visually, there is no distortion or bulge in the skin.

Step 8: Clean up the site and maintain
Remove excess extruded sealant and work debris. After the skin is installed, it should be left to stand for 12-24 hours (depending on the curing time of the sealant), during which no external force shall be applied to the expansion joint.

Step 9: Air tightness test and heat tightening
After the system is restored, the 500-1000Pa micro-positive pressure airtightness test is carried out first, and all interfaces are checked with soapy water. After the system is heated to a stable working temperature, heat tighten the bolts once while they are hot, and the torque reaches the specified value.

5 Common Mistakes and Avoidance

1. Error 1: Replace directly without measuring the displacement → Always measure the hot and cold displacement before customizing the skin.
2. Mistake 2: Use carbon steel bolts instead of stainless steel → the risk of corrosion fracture is extremely high, and 304 or 316 materials must be used.
3. Mistake 3: Skin tension is too tight → 2%-3% relaxation should be retained.
4. Mistake 4: One-time one-sided fastening bolts → must be tightened symmetrically in fractions.
5. Mistake 5: Hot tightening is missed after installation → Re-tightening in hot state is the key step of sealing guarantee.

epilogue

The replacement of non-metallic expansion joint skin is not simple disassembly and reinstallation, but a systematic engineering involving parameter review, safety isolation, fine construction and verification test. Strictly follow the standardized process operation, the service life of the skin can reach 3-5 years, while effectively avoiding environmental penalties and unplanned shutdown losses caused by leakage. On the contrary, rough replacement operations often lead to new skin damage again within months.