1. Without it, the flue system will not survive a maintenance cycle at all-thermal expansion and contraction are no trivial matter
Have you ever seen a scene where the flue starts leaking after two or three months of operation? I've seen it. As long as the temperature of desulfurization flue and cement kiln tail pipeline of power plant goes up, the pipeline will bulge out like steamed buns. What's the problem? Many people think that the basic physical principle of thermal expansion and contraction is enough to calculate it accurately, but the actual working conditions are not so simple.
The flue gas temperature fluctuates by dozens or even hundreds of degrees in the flue, and it takes only one or two hours to start up from normal temperature to above 300℃. If the pipeline is connected with steel, the elongation per meter can reach 3-4 mm, and the 100-meter-long flue can eject a displacement of 30 or 40 centimeters. If the fixed bearing can't hold it, it will be a matter of time before the weld cracks and the flange leaks. WhileFunction of built-in expansion joint in flueIt is specifically designed to swallow this part of the displacement-it is like a soft joint that can be compressed and stretched, making the pipe "come alive" and the stress resolves itself.
Before, there was a steel mill, which did not install expansion joints, and directly hard connected them. The maintenance cycle was less than half a year, and the elbow collapsed. Replace it with oursCorrugated expansion joint for power station industryAfter that, I haven't touched the weld once in three years. Thermal expansion and cold contraction is not a small matter, it is the vital gate of flue life.
2. Three housekeeping skills of built-in expansion joints in flue: suction displacement, sealing and vibration prevention
Then how exactly does this expansion joint carry life? To put it bluntly, three things.
The first is suction displacement.Whether it is axial tension compression or lateral deflection, the crest and trough structure of bellows is inherently used for deformation. Our commonUniversal corrugated expansion jointThe single-wave compensation amount can reach tens of millimeters, and when multiple waves are combined, the displacement of the flue can be easily eaten.
The second is to keep the seal.What goes in the flue is not air, but high-temperature flue gas containing sulfur and dust. A leak is an environmental accident. The expansion joint itself adopts a multi-layer corrugated structure, with an outer layer of stainless steel and an inner layer lined with PTFE or non-metallic fabric, such asNon-metallic expansion joint (fabric fiber expansion joint)Which is temperature-resistant and corrosion-resistant and has excellent sealing performance. Moreover, the design of the guide tube can prevent the air flow from directly washing the ripples and minimize the wear.
The third is anti-vibration.The flue system inevitably has fan vibration and airflow pulsation. The rigid pipe will transmit vibration to the whole support, and the expansion joint is equivalent to a flexible vibration isolator, which can cut off the vibration transmission path. Especially in the import and export of air-cooled islands and desulfurization towers, we useDouble hinge expansion joint for air-cooled island vacuum pipelineSpecialized in dealing with the combined working conditions of large displacement plus vibration.
You see, these three skills are indispensable. There is no suction displacement, and the pipeline stress is concentrated; There is no seal and air leaks everywhere; No vibration damping, shortened equipment life. So don't underestimate this tin guy, it's a lot heavier than you think.
III. Metal rectangular expansion joint or non-metallic expansion joint? Is it enough to look only at temperature and corrosiveness in model selection?
When it comes to model selection, many engineers come up and ask, "What's the temperature? What is the corrosive medium?" Then they can make a decision. But is it really that simple?
Let's startMetal rectangular expansion jointIt is commonly used in rectangular flues, such as the inlet and outlet of desulfurization towers. The temperature can resist above 600℃, and the pressure bearing capacity is also strong. However, the problem is that once the flue gas is moist and acid, the metal bellows is easily pitted. That's when we have to considerPTFE-lined hoseOrRubber PTFE compensatorIt can handle strong corrosion under the premise of low temperature resistance.
AgainNon-metallic expansion joint(also called non-metallic compensator), the main body is fabric fiber plus rubber layer, and the upper limit of temperature resistance is generally about 250℃, but the cost is low, the corrosion resistance is good, and the compensation amount is large. Moreover, it does not generate thrust and has low requirements for fixed supports. But if your flue gas temperature exceeds 300℃ and you have to go on non-metal, it is to dig a hole for yourself. We ran through several sites, and the non-metallic burns were all due to overtemperature use.
Therefore, the selection should not only look at the temperature gauge and corrosion analysis table, but also the details of actual working conditions: is there any fly ash wear? Are there frequent starts and stops? Are there any displacement direction restrictions? For example, the rotary compensator is suitable for large rotation angles, and the straight tube pressure balance expansion joint is suitable for high pressure systems. Evaluate comprehensively, don't try to save trouble.
4. Guide tube, pull rod and baffle door-don't ignore these 'supporting roles'. Most of the accidents are caused by them
The main part of the expansion joint is fine, but the accident at the scene is most likely the pot of the accessories.
guide tubeIs a liner mounted on the inner wall of the bellows, and its function is to guide the flue gas to flow smoothly and avoid the air flow directly washing the corrugations. Many people think it's just a tin tube, and it doesn't matter. However, once the guide tube falls off or is corroded and perforated, the high-speed dusty smoke directly hits the ripples, and it can be worn out in a few months. WeSpecific Function of Expansion Joint Guide TubeAs mentioned in the Q&A, it can also reduce local resistance and reduce energy consumption. Don't omit it.
tie rodWhat? Used to limit excessive stretching or compression of the expansion joint while withstanding internal pressure thrust. A customer tightened the tie rod nut too tightly, and as a result, the expansion joint completely lost its ability to compensate, and cracks were forced out. Others forget to remove the transport screw after installation-yes, the screw for temporary fixation, which must be loosened or removed during operation. Our QA specializes inDo you need to remove the screw of the expansion jointThe answer is that it must be dismantled.
baffle doorIt is also a place where moths often go wrong. Such asDesulfurization flue gas baffle doorUsed to switch smoke path or overhaul isolation. If the baffle door is not closed tightly, smoke flows back, the side of the expansion joint bears extra pressure, and the sealing surface is damaged. More seriously, improper operation causes the baffle door to get stuck, and forced opening and closing can tear the expansion joint.
So don't just stare at the bellows, these "supporting roles" are the high-incidence areas of accidents. If something goes wrong, check the guide tube, tie rod and baffle door first, and most of them can find the cause.
5. Mistakes made during installation, doubled and returned after operation-lessons about pre-stretching and fixing bearings
Installation, I want to tell a real case. Some cement factory loaded ourMetal Corrugated Expansion Joints in Cement IndustryAs a result, it leaked after less than a month of use. At the scene, no pre-stretching was done during installation. When the expansion joint leaves the factory, it is given according to the free length at the working temperature, but the ambient temperature is low when it is installed on site, and the pipe has not expanded yet. Without pre-stretching, when the temperature rises, the bellows will be directly compressed to the end. If it exceeds the design limit, it will crack.
Before installation, calculate the pre-stretching amount according to the actual temperature difference, pull it to the design value with a screw, and fix the re-welded pipe. ForCorrect installation method of expansion joint of large tie rodWe also emphasized that the tie rod nut must be adjusted so that the bellows are in a pre-tensioned state and then locked.
Another pit is the fixed mount. Many project drawings save trouble, and ordinary brackets are used instead of fixed supports. Results When the pipe is thermally expanded, the fulcrum displacement causes the expansion joint to bear additional bending moment. OurExternal pressure single axial expansion jointIt is particularly sensitive to this working condition and must be equipped with a fixed support. How to judge? Look at the type of support on the design drawing. The fixed support should be able to withstand the thrust and bending moment of the pipeline, and there should be no displacement.
Install it wrong and double the problem at runtime turns into double or even triple the loss. You don't want to climb the flue every day to fix the leak, do you? Then get the pre-stretch and the support right.
6. Maintenance is not metaphysical: taking a look every day is more effective than checking standards
Finally, let's talk about maintenance. Many people think that the expansion joint is an iron pimple, and they don't care if they put it on. But in fact, it requires the most basic daily inspection.
Take a minute a day to see if there are any of these phenomena: Are there any rust spots or cracks on the corrugated surface? Is there any deformation in the exposed part of the guide tube? Is the tie rod nut loose? Especially in high temperature and high corrosion areas, such as those imported from desulfurization towersRectangular non-metallic expansion jointOnce the fabric layer becomes brittle and discolored, it is time to consider replacing it.
Without any complicated testing instruments, most hidden dangers can be found with the naked eye. For example, if you see oily liquid seeping out of the bellows surface, nine times out of ten, the inner lining layer is broken and the flue gas condensate seeps out. Early detection and early shutdown of the furnace is not an order of magnitude compared with waiting until the air leak is locked and then repaired.
Maintenance is not metaphysical, is to be diligent. Looking at it every day works better than cramming through the standard manual. After all, the standard book won't tell you which tie rod is loose today.