In wet flue gas desulfurization (WFGD) systems, metal expansion joints are commonly used in the circular flue of the original flue gas section or the clean flue gas section. However, a failure mode known as "inversion" — i.e., bellows trough sagging inward, local overturning or even overall collapse — has been frequent in several units in recent years. The inversion of the metal expansion joint in the desulfurization flue not only causes the loss of compensation ability, but also causes the weld cracking, the guide tube falling off and even the flue tearing. This paper will deeply analyze the mechanical mechanism, common inducements and prevention measures of varus, and provide systematic solutions for operation and maintenance personnel.

1. What is metallic expansion joint varus

Metal expansion joints are usually composed of multi-layered stainless steel bellows, end tubes and guide tubes. During normal operation, bellows will undergo elastic deformation under the action of axial pressure or negative pressure. The so-called "inversion" means that the crest and trough of the bellows protrude in the opposite direction, that is, the originally outwardly convex crest collapses inward, or the single wave of the whole bellows is plastically unstable, showing a "inversion" shape.

Once the inversion of the metal expansion joint of the desulfurization flue occurs, it will have the following consequences:

• The effective compensation distance decreases sharply, and the flue thermal stress increases sharply
• The trough at the inversion forms a stress concentration point and quickly develops into a fatigue crack
• The varus site can scratch or destroy the guide tube, or even fall off into the downstream equipment
• The overall stiffness of the expansion joint is lost, and the vibration of the induced draft fan is intensified

Therefore, identifying the early signs of inversion and intervening in time is the key to ensure the safety of desulfurization flue.

2. Mechanical mechanism and common triggers of varus

From the mechanical point of view, it is a condition that the waveform of metal bellows remains stable when it is subjected to axial compression or tension. Inversion occurs when the actual load exceeds the critical instability load of the bellows. The main causes of inversion of metal expansion joint in desulfurization flue include:

2.1 Excessive negative pressure (most common cause)

When the desulfurization system is in operation, the clean flue behind the induced draft fan and the outlet section of the absorption tower are usually in negative pressure state. If the flue resistance increases (such as clogging of the demister, dust accumulation of the GGH heat exchanger element), or the inlet baffle of the induced draft fan is misclosed, the negative pressure may plummet from the normal-2kPa to-8kPa or even-15kPa. For metal expansion joints with a design negative pressure of only-5kPa, exceeding the limit negative pressure will make the bellows bear axial tensile force, and when the tensile displacement exceeds the stability limit of the bellows, the wave crest flips inward.

Typical phenomenon: inversion mostly occurs at the net flue expansion joint between the outlet of the absorption tower and the chimney, and turns in the opposite direction of the flue gas flow direction.

2.2 Improper cold pre-tension/pre-pressing during installation

When metal expansion joints are installed, they need to be pre-compressed or pre-stretched in cold state according to the design temperature. For example, when the design temperature is 300℃ and the installation temperature is 20℃, it should be pre-compressed by about 1~2mm (depending on the specific length). If the installer mistakenly pre-stretches or forcibly welds the transportation positioning rod without removing it, the expansion joint will be in an abnormal stress state when it is cold, and it is easy to turn in under the combined action of heat load and pressure after it is put into operation.

2.3 The guide tube is stuck or falling off

The guide tube is a protective piece lined with the inner wall of the bellows, one end of which is fixed and the other end is free. If the free end of the guide tube is stuck at the bellows trough due to dust accumulation, corrosion or installation eccentricity, when the flue thermally expands, the guide tube will push the bellows to generate additional bending moment, causing the local trough to flip inward. More seriously, the guide tube falls off and directly hits the inner wall of the bellows, causing multiple varus.

2.4 Plastic instability after fatigue accumulation

After many years of operation (usually more than 8~10 years), stainless steel corrugated pipe material will undergo cyclic hardening or softening. When the accumulated plastic deformation reaches a certain degree, the stability margin of the bellows decreases, and it may suddenly turn in under normal design load. This is an end-of-life failure mode.

3. Rapid identification and diagnosis of varus

On-site operation and maintenance personnel can quickly judge whether there is inversion of metal expansion joint in desulfurization flue by the following methods:

1. External visual inspection: Observe the crest profile of the bellows on the outside of the expansion joint. Normal should be uniform arc shape; After varus, local troughs can be seen bulging outward (because the crest collapses inward), or obvious depressed pits appear.
2. Displacement measurement: Make marks on the flanges at both ends of the expansion joint to measure the actual displacement of cold and hot states. If the measured displacement is much smaller than the design value (for example, the designed axial compensation is 40mm, but the measured axial compensation is only 10mm), and the flue stuck factor is excluded, it can be basically judged that the compensation function is lost due to inversion.
3. Tapping listening method: Tapping different nodes of bellows with a wooden hammer. Normal expansion joints emit crisp echoes; The varus area produces a dull, short "click" sound due to metallic deformation.
4. Endoscopy: Insert an industrial endoscope from the manhole door after shutdown to directly observe the inner wall of the bellows. Varus manifests as an inward bulge of the trough, sometimes accompanied by friction marks or cracks.

IV. Prevention and control measures and treatment plans

4.1 Prevention Design Phase

• Reasonable selection of design negative pressure: For the clean flue metal expansion joint located behind the induced draft fan, the design negative pressure should not be lower than-10kPa, and it is recommended to check according to-15kPa.
• Increase bellows stiffness: Improve destabilization resistance by reducing wave pitch, increasing the number of layers (from 2 to 4 layers), or reducing wave height.
• Installation of limit tie rods: Set 4 uniformly distributed limit tie rods on the outside of the expansion joint to limit the maximum stretching amount to not exceed 120% of the design value. This is the most effective structural measure to prevent inversion caused by negative pressure excess.
• Optimization of guide tube: Reserve a gap of more than 20mm between the free end of the guide tube and the bellows, and adopt a bell mouth design to prevent jamming.

4.2 Installation Phase Control

• Cold pre-compression (high-temperature pipe) or pre-stretching (low-temperature pipe) shall be carried out strictly according to the requirements of the drawings, and the deviation shall not exceed ±2mm.
• The removal of the transportation positioning rod must be carried out after all the flue system is installed in place and the bracket is fixed. It is strictly prohibited to remove it in advance.
• When installing the guide cylinder, ensure that the coaxiality error between the guide cylinder and the expansion joint is ≤3mm, and the free end is not jammed.

4.3 Operation and Maintenance Phase

• Negative pressure monitoring: Set a negative pressure measuring point in the clean flue near the expansion joint, connect to DCS and set an alarm (for example, 5 seconds delay alarm if it is less than-8kPa). Once the negative pressure exceeds the limit, check the demister, GGH pressure differential immediately and take backwash or cleaning measures.
• Regular displacement inspection: record the length change of cold state and hot state of expansion joint every six months, and establish trend archives. If it is found that the displacement decreases year by year, it should be arranged to stop the machine for internal inspection.
• Annual Endoscopic Inspection: Video inspection of the inner wall at each overhaul, focusing on troughs for bulge or friction marks.

4.4 Treatment regimen after varus

Once the occurrence of varus is confirmed, it is treated according to the following grades:

Note: It is strictly forbidden to force the inverted bellows with a jack under pressure, which may induce a burst accident.

V. Alternative considerations of metallic and non-metallic expansion joints

For net flue locations where varus occurs frequently, replacing the metal expansion joint with a circular flue non-metallic fabric expansion joint may be considered. Non-metallic expansion joints have no risk of varus and better absorb negative pressure deformation (their multilayer flexible structure shrinks uniformly under negative pressure rather than locally flipping). But note that:

• Confirm the flue gas temperature before replacement: the non-metallic expansion joint is suitable for ≤250℃ working condition; If the net flue temperature exceeds this value, the metal expansion joint still needs to be retained.
• The pressure resistance of non-metallic expansion joints is usually ≤ ± 30kPa, which fully meets the requirements of net flue negative pressure.
• The replacement involves the modification of the flange interface, and the dimensional matching needs to be calculated.

VI. Typical cases

Five years after the net flue metal expansion joint (diameter 2.2m, 4 layers 316L bellows) of the desulfurization system of a 330MW unit was put into operation, an abnormal depression was found on its surface. After inspection, it was confirmed that the negative pressure of the net flue rose from-3kPa to-11kPa due to the blockage of the demister, which exceeded the design negative pressure of the expansion joint-8kPa, resulting in inversion of the two waves. Due to the failure to treat it in time, the crack at the inversion spread after three months, and smoke leaked, and the environmental protection data exceeded the standard. Eventually replaced with non-metallic fabric expansion joints and added negative pressure alarm logic. After the transformation, the problem of varus did not occur again after continuous operation for 3 years.

VII. SUMMARY

Inversion of metal expansion joint in desulfurization flue is a failure mode of plastic instability caused by over-limit negative pressure, installation error or stuck guide tube. Its harm lies in the loss of compensation ability and weld cracking quickly. The core measures of prevention and control include:

• Design stage: Improve the negative pressure design margin, install the limit rod, and optimize the clearance of the guide tube.
• Installation stage: strictly implement cold pre-compression quantity to ensure coaxiality.
• Operation stage: real-time monitoring of flue negative pressure, regular displacement measurement and endoscopic inspection.
• Treatment stage: Mild varus can be temporarily reinforced, and severe varus must be replaced immediately.

For the occasions where the negative pressure fluctuates greatly, such as the desulfurization net flue, if the metal expansion joint is repeatedly inverted, it is recommended to evaluate and replace it with a non-metallic fabric expansion joint to fundamentally eliminate the risk of inversion. Through the above systematic prevention and control measures, the failure rate of metal expansion joint inversion can be reduced by more than 80%, and the long-term safe operation of desulfurization flue can be ensured.