Particularity of dense working conditions under blast furnace-why can't ordinary expansion joints bear it?
In the dense area of the blast furnace, the temperature is often seven to eight hundred degrees Celsius, the pressure is 0.5 to 1 MPa, and a large amount of coke powder and mineral powder are sandwiched in the medium. If you think about it, ordinary stainless steel bellows will corrode and crack in less than a few months in this environment. Two days ago, I met a customer, saying that the average life of the expansion joint on the lower dense pipeline in their factory is less than half a year, and it will have to be stopped for half a day if it is changed once. This is not a compensator, it is clearly a consumable. What's the problem? One is high-temperature oxidation, the other is dust erosion wear, and the third is frequent axial displacement and vibration. Ordinary general-purpose corrugated expansion joints simply can't bear this triple blow.
Core Structure Design: How to cope with the selection of materials and bellows?
Since ordinary goods can't be used, the special fund for blast furnace secrets has to be used in materials and structures. The corrugated pipe material should be Inconel 625 or 321 stainless steel, which is several grades stronger than 304 in high-temperature creep and intergranular corrosion resistance. The number of corrugated layers is usually 2, and the wall thickness is thickened to more than 2mm. Refer to the idea of "large-diameter thick-walled expansion joint" in the product. There is also a guide tube, which must be lined with wear-resistant alloy or ceramic fiber to prevent high-speed dusty air flow from directly washing the wave peak. In addition, in order to absorb the axial displacement caused by the thermal expansion of the pipeline, it is recommended to choose "high-temperature axial expansion joint" or "straight pipe pressure balance expansion joint"-the former can withstand the axial compression of large stroke, and the latter can eliminate the blind plate force and avoid excessive stress on the pipeline support. When designing, you should also consider the cold tightness. Generally, the pre-compression amount is calculated according to 50% of the thermal displacement, and the specific value should be calculated according to the actual working temperature and your pipeline materials.
Installation Points: Avoid 80% of Early Failures
No matter how good the equipment is, it will be in vain if it is not installed correctly. In the installation of dense expansion joint under blast furnace, there are several pits that are particularly easy to step on. The first is pre-stretching/pre-compression: when the ambient temperature is low during installation, the pipe has not heated up yet, and the expansion joint has to be pre-compressed to the working position as designed. And guess what? Many on-site workers found it troublesome, so they installed it directly. As a result, the bellows was pulled over and scrapped directly as soon as the temperature rose. The second is the guide bracket: guide brackets must be set on both sides of the expansion joint to ensure that the pipeline only displaces along the axial direction and does not produce lateral offset. Otherwise, the bellows will be twisted, and the life of the bellows will drop by a cliff. The third is the adjustment of the tie rod nut: the tie rod for transportation should be loosened after installation, but a certain limit position should be kept to prevent the bellows from over-stretching. How exactly to adjust? You can refer to the question and answer "How to adjust the tie rod nut of the expansion joint" on our station. In short, look at the drawings before installation, don't rely on experience.
Common Failure Modes and Preventive Maintenance
Even if the model is selected correctly and installed well, routine maintenance has to keep up. 90% of the failure of dense expansion joint in blast furnace is concentrated in three aspects: First, stress corrosion cracking-the combined action of chloride ions and high temperature, grain boundary cracking, and the crack extends outward from the trough. The prevention method is to regularly inspect the bellows surface, and if micro-cracks are found, use penetration detection to inspect, or simply set the cycle to replace them. The second is fatigue fracture-the furnace top frequently breaks and supplies air, the pipeline expands and contracts back and forth hundreds of thousands of times, and the stress concentration area at the root of the bellows is the most prone to cracking. In design, multi-layer thin-walled bellows can be used to disperse stress, or reinforcing rings can be added. Third, the inner and outer walls are worn-dust accumulates at the trough and hard grinds out pits. The countermeasure is to install purge holes or clean dust with compressed air regularly. In addition, don't forget to check the wear of the guide tube every six months, and change it when it is worn through. Otherwise, the bellows will be directly exposed to the dusty airflow, and its life will be cut in half. To put it bluntly, this "lifeblood" has to be treated as a wear part, and don't wait for an air leak before repairing it.
Case: Comparison of Blast Furnace Lower Dense Pipeline Reconstruction in a Steel Plant
Last year, I helped a steel mill in East China do the renovation. They originally used ordinary stainless steel corrugated expansion joints, with an average life of 4.5 months. The spare parts cost plus the loss of shutdown burned nearly 300,000 yuan every year. We replaced it with a "large-diameter thick-walled expansion joint" customized for the dense working conditions of blast furnace. The corrugated pipe material was upgraded to Inconel 625, lined with wear-resistant ceramic guide tube, and designed according to the high-temperature axial type. During the installation, it cooperated with the cold tightening operation of a professional team, and as a result, it took 14 months to run before it was replaced for the first time. Converted, the annual comprehensive cost has dropped by more than 60%. This does not count the reduced number of unplanned shutdowns-for continuous production equipment such as blast furnaces, an unexpected shutdown costs at least hundreds of thousands. So you said, is it more cost-effective to spend more cost in the early stage to choose a special type than to change parts frequently in the later stage?