Metal expansion joints are widely used in boiler flue, metallurgical high temperature flue gas pipeline and chemical tail gas conveying system of power plants because of their characteristics of high temperature resistance, strong pressure bearing capacity and high compensation accuracy. However, any technical solution has its applicable boundary-the disadvantages of flue metal expansion joints may be transformed into serious operating hazards under certain working conditions, resulting in leakage, jamming and even overall failure. Compared with non-metal expansion joints, metal expansion joints have some shortcomings in corrosion resistance, vibration isolation, dust accumulation prevention and cost control. Based on engineering practice, this paper objectively analyzes the main shortcomings, failure mechanism and avoidance strategy of metal expansion joint in flue application, and helps technicians to make more reasonable compensator selection decision.
Inherent deficiencies in corrosion resistance
The most prominent of the drawbacks of flue metal expansion joints is their limited corrosion resistance. Bellows of metal expansion joints are usually manufactured using 304, 316L or stainless steel alloys. In the clean flue gas section after wet desulfurization (temperature 45-55℃, containing residual SO₂, SO₃ and chloride ions), even 316L stainless steel is difficult to resist the corrosion of dilute sulfuric acid condensate for a long time. In an actual case, the 316L metal expansion joint of the desulfurization clean flue of a coal-fired power plant was put into operation for only 18 months, and multi-point stress corrosion cracks appeared at the trough of the bellows, with a leakage of more than 5%.
In contrast, the PTFE layer on the inner side of the skin of the non-metallic expansion joint has excellent acid corrosion resistance and can last up to 3-5 years under the same working conditions. When the flue gas contains a high concentration of chloride ions (such as burning high-chlorine coal or treating chlorine-containing organic waste gas), the corrosion rate of metal expansion joints will be further accelerated. This is one of the core reasons why design institutes are increasingly inclined to choose non-metallic expansion joints in highly corrosive flue systems.
Weak vibration isolation and noise reduction ability
The corrugated pipe of metal expansion joint is formed by stainless steel sheet by hydraulic pressure or rolling, and its own stiffness is much higher than that of non-metal skin. For the medium and high frequency mechanical vibration generated by induced draft fan and booster fan, the isolation effect of metal expansion joint is poor-the vibration energy will be directly transmitted to the flue and bracket along the metal structure of bellows, causing the overall structure to resonate. Long-term vibration will accelerate the fatigue cracking of bellows welds.
The disadvantage of flue metal expansion joint is that it has little damping and vibration damping ability. However, the multi-layer composite structure of non-metal expansion joint (rubber + fiberglass cloth + insulation layer) has natural elasticity and high damping characteristics, which can effectively absorb more than 80% of mechanical vibration. In the position of strong vibration source such as fan inlet and outlet, the vibration reduction advantage of non-metallic expansion joint is extremely obvious.
Single type of compensated displacement
Metal expansion joints are divided into axial type, transverse type, angular type and universal type according to their structure. However, in actual flue arrangements, the thermal displacement tends to be a combination of axial, transverse and angular displacements. It is difficult for a single type of metal expansion joint to absorb multi-directional displacement at the same time. If the universal hinge type metal expansion joint is adopted, its cost will increase sharply (about 3-5 times that of the axial type), and the structure will be complicated and the fault points will increase.
However, the shortcomings of flue metal expansion joint are exposed under complicated displacement conditions. Due to the overall flexible structure, the non-metallic expansion joint can absorb the combined axial, lateral and angular displacement simultaneously, without the need for complicated hinge or tie rod mechanism, and is more adaptable in the flue with limited space and complicated displacement direction.
Ash accumulation and diversion problems are prominent
There are obvious fluctuations in the peaks and valleys of metal expansion joint bellows, and dust accumulation is easy to occur in the flue gas with high dust content (such as the original flue in front of dust collector and steel sintering flue gas). The gradual hardening of the ash layer will limit the normal expansion and contraction of the bellows-this is the so-called "stuck" phenomenon. Once jamming occurs, the expansion joint loses its ability to compensate, and the adjacent fixed bracket will bear abnormal thrust, which can lead to deformation of the bracket or flue tear in severe cases.
The inner wall skin of non-metallic expansion joints is relatively smooth (although there are corrugated folds, they can be effectively covered by built-in deflectors), and the tendency to accumulate dust is much lower than that of metal bellows. In addition, the deflector installation requirements for metal expansion joints are extremely stringent: wrong orientation or improper clearance can accelerate bellows wear. In actual maintenance, it is not uncommon for the metal bellows to be worn out due to the deflector falling off.
Demanding installation and alignment
Metal expansion joints require extremely high accuracy of pipe alignment. During installation, the parallelism deviation of the flanges on both sides shall be ≤3mm, and the coaxiality deviation shall be ≤5mm. If the flue exceeds the standard due to foundation settlement or installation error, the metal expansion joint will bear additional bending moment during operation, which will accelerate the fatigue cracking of the bellows. The adjustment of centering often requires cutting and rewelding flue, which has a long construction period and high cost.
The disadvantages of flue metal expansion joints are shown here as "strong rigidity and poor fault tolerance". Non-metallic expansion joints are particularly suitable for old unit retrofit projects because of their flexible structure that can compensate for installation deviations of ±10mm and have much higher tolerance to foundation settlement and installation errors.
Comprehensive consideration of cost and maintenance
From the initial procurement cost, small metal expansion joints (below DN500) are lower than non-metal expansion joints. However, the price of metal expansion joints with large diameter (DN2000 or above) or special materials is high-the price of a DN3000 316L metal expansion joint can reach 80,000-120,000 yuan, while the non-metal expansion joint with the same diameter only costs 30,000-50,000 yuan.
In the aspect of whole life cycle maintenance, the shortcomings of flue metal expansion joint are more obvious: once the metal bellows is corroded, cracked or fatigued, it can only be replaced as a whole and cannot be repaired locally; However, the non-metal expansion joint only needs to replace the skin (the cost is about 30%-40% of the whole set of expansion joints), and the replacement operation can be completed within 48 hours. Considering the 10-year operating cycle, the total cost of ownership of non-metallic expansion joints is typically 30%-50% lower than that of metallic solutions.
epilogue
The shortcomings of flue metal expansion joints mainly focus on five aspects: insufficient corrosion resistance, weak vibration isolation ability, dust accumulation and sticking, high requirement for centering and high maintenance cost. However, this does not mean that metal expansion joints should be completely negated-metal expansion joints are still an irreplaceable choice in high temperature (> 400℃), high pressure (> 0.1MPa) or clean flue gas pipes containing strong oxidizing media. The correct selection strategy is to maximize strengths and avoid weaknesses according to specific working conditions: metal is selected for high-temperature clean flue gas, non-metal is selected for low-temperature corrosive flue gas, and non-metal is given priority to parts with strong vibration.