Displacement of flue metal expansion joint: the number that is most likely to stumble during type selection
The medium running in the flue always moves two or three hundred degrees, and the thermal expansion and contraction of the pipe is a sure thing. To put it bluntly, the amount of displacement is the amount of expansion and contraction that the expansion joint needs to "swallow". But the reality is that many people only stare at the pressure level and caliber, and fill in the displacement as a reference value at will. As a result, within two years of installation, the bellows bulged, the weld cracked, and even the top of the adjacent bracket was deformed. How exactly is displacement defined? Why is the flue system different from the steam pipe? This has to start with three directions: axial, transverse and angular.
Let's talk about the axial direction first, that is, the amount of expansion and contraction along the centerline of the pipe; Lateral is a horizontal movement perpendicular to the centerline; The angular direction is the angular change caused by the bending of the pipe. The flue diameter is large, the wall is thin, and the temperature field is not uniform-the temperature difference between the inlet and the outlet can be several tens of degrees. More troublesome is that flues generally have multiple expansion bends, and the displacement amount is a combined vector of thermal displacements in multiple directions. Two days ago, I came across a case in a cement factory. They usedMetal Corrugated Expansion Joints in Cement IndustryIn the design, only the axial direction was calculated, but the lateral offset caused by the self-weight sag of the pipe was not calculated. As a result, the one-sided wear of the bellows was serious. What about that? It is best to run it with professional pipeline stress analysis software, or at least estimate it according to the simplified formula in the Technical Regulations for the Design of Smoke, Wind and Pulverized Coal Pipelines in Thermal Power Plants.
Calculating displacement is not just a formula
The higher the flue gas temperature, the larger the linear expansion coefficient and the larger the displacement amount. But is the flue material carbon steel or alloy steel? Are the pipes insulated? Will the external wind load and earthquake load be superimposed? There's also the layout – Z-bends and L-bends compensate completely differently. Which explains whyCorrugated expansion joint for power station industryAndUniversal corrugated expansion jointThe structural design is different: the flue of the power station is usually usedCompound hinge transverse typeOrStraight pipe pressure balance typeTo absorb multi-directional displacement, while the general-purpose type tends to only bear axial.
The flue at the inlet of the desulfurization tower of a power plantDouble-sealed single-axis circular baffle doorThe displacement of the metal expansion joint next to it does not consider the cold tightness during installation. As a result, the expansion joint is compressed to the limit in summer operation, and the plasticity of the bellows is unstable. When I removed it, I saw that the trough had cracked and the smoke leaked directly. Replacing a two-meter-diameter expansion joint, plus the downtime loss, hundreds of thousands are gone. What's more dangerous is that if the displacement is too small, the expansion joint bears tensile stress, which may suddenly break, smoke flow back, and scald the maintenance personnel. This is not alarmist. There have been lives in the industry.
How much displacement is reported to the manufacturer when selecting the model?
Actual displacement × safety factor。 The safety factor is usually 1.25 to 1.5, depending on the importance of the flue. At the same time, it is necessary to clarify the direction and distribution of displacement-is it simply axial, or with angles? For example, when the flue is turned from horizontal to vertical, it needs to be set at the elbowCompound hinge transverse expansion jointTo absorb the lateral displacement. In addition, don't forget to install pre-deviation: the expansion joint installed in winter, the displacement direction is reverse when running in summer, and it must be pre-stretched or pre-compressed according to the calculated value. This operation detail is the same as "how to adjust the expansion joint tie rod nut" mentioned in our FAQ.
For example, a 20-meter-long carbon steel flue has a working temperature of 300℃ and an ambient temperature of 20℃. The linear expansion coefficient is calculated as 12×10⁻⁶/℃, and the thermal elongation is 20×1000×280×12e-6 ≈ 67.2mm. If you count the lateral displacement of 0.5°, you have to use vector synthesis. Finally, the displacement amount reported to the manufacturer is multiplied by the safety factor of 1.3, and then the respective components of axial, transverse and angular directions are defined.
Deflector-a "pit" that can easily be overlooked
Influence of guide tube on displacement。 Flue metal expansion joints are usually equipped with guide tubes inside (refer to our product information, the expansion joints of power stations and cement industries are equipped with guide tubes). In addition to diverting flow, the guide tube has another function-it will move with the bellows when axially displaced. If the gap between the guide tube and the inner wall of the pipe is not large enough and stuck, the actual displacement will be limited. Therefore, when designing, it is necessary to calculate the stroke and clearance of the guide tube clearly, otherwise, if the expansion section is selected correctly, the guide tube will get stuck first, which will be useless.
How to calculate? Generally, the unilateral gap between the guide tube and the inner wall of the pipeline is required to be not less than 10~15mm, and the length of the guide tube should be able to cover the maximum axial displacement to avoid smoke from washing the bellows. These details are also mentioned in the FAQ article "The Specific Function of Expansion Joint Guide". In addition, forLarge diameter thick wall expansion jointOrHigh temperature axial expansion jointThe material and wall thickness of the guide tube also have to be upgraded with the temperature, otherwise the deformation is stuck, which will also cause accidents.
The displacement measure seemed simple, but there were quite a few people who stumbled over it. When selecting the model next time, don't just look at the caliber and pressure. Go through the direction, temperature distribution and hot and cold state changes of the flue, and then calculate it accurately with software or regulations to give the manufacturer a clear displacement data. Only the expansion joint installed in this way can run safely for ten or eight years.