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In industrial furnaces and boilers, refractory or ceramic fibre linings play a vital role in ensuring heat is contained within this extremely high temperature equipment, conserving heat and protecting the surrounding equipment and structures as well. However, they can fail for various reasons, causing operational issues and costly repairs. Let’s explore why this happens and how to prevent it.
Refractory failures often occur due to corrosion of the anchors holding the refractory or ceramic modules in place. This corrosion can be caused by high temperatures or corrosive elements in flue gases. This is exacerbated in high humidity environments, where the water content in the combustion air is high. When high water content flue gasses seep through cracks in the refractory or ceramic modules, the water can condense when it comes into contact with the cooler casing temperatures on which the anchors are welded on. This combined with corrosive elements such as Sulphur or salts from coastal facilities, forms ideal conditions for aggressive corrosion to take place, weakening the anchors and potentially leading to refractory lining failure or perforations/pinholes in the casing.
Figure 1: Flue gas condensing on cold roof plate through gaps in refractory lining and corroding the anchors.
Figure 2: Yellowing Sulphurous deposits between the refractory and furnace casing plate caused by condensation of flue gasses that seeped through the cracks.
Figure 3: Cracked insulation with the anchor corroded and detached from the casing. Image taken after cutting the external furnace plate during inspection.
Poor installation practices contribute to refractory failures. Gaps or improperly secured modules and blankets create openings for hot flue gases to penetrate the refractory lining, accelerating corrosion (see impact above). Proper installation techniques ensuring all gaps are sealed and high-quality anchoring systems are essential to prevent such issues. Particular attention should be taken to ensure interfaces between transition points are adequately filled with compressed blankets or fillers.
One common aspect overlooked is that ceramic fibre modules and vacuum formed ceramic systems (e.g. peepholes) actually shrink when heated up the first time. Which is why, even when the systems are inspected after initial installation and all gaps seem adequately filled, as soon as the furnace is started up and reaches operating temperatures, the shrinkage occurs and gaps form leading to hotspots on these casings. A good practice is to ensure vacuum formed components are pre-shrunk by the manufacturer before being installed in the system to avoid unexpected and unaccounted for shrinkage
Cementitious and other water based refractory materials are commonly applied in industry. However great care needs to be taken to ensure that the refractory is properly dried out when initially starting up. This is because if there are pockets of water trapped within the lattice structure of the refractory and it is heated up too quickly, the water rapidly evaporates into steam and mini steam explosions occur in the refractory lining causing them to fracture and in extreme cases collapse. Most experienced applicators will share a recommended curing curve that will guide operators on how fast they should be heating up their units to allow adequate time to dry the refractory out during the initial startup. It is important to note though that when furnaces have been shut down for prolonged durations and/or have been exposed to high humidity environments, it has to be assumed that some amount of moisture would have seeped into the existing refractory lining, and some time to dry out the refractory should also be allocated in the restart procedure.
Mechanical failures, like malfunctioning or improperly secured cup locks in blanket-type installations, can cause significant failures of the lining. Once the cuplocks have given way, the exposed anchors start to corrode rapidly due to high temperature exposure and the layers of blankets start to dislodge and sag. The sagging blankets, coupled with flue gas velocities add additional stresses on the adjacent cuplocks and anchors leading to a propogation of the failures. Regular inspection and maintenance of anchoring systems are crucial to prevent such failures and should form and integral part of turnaround and maintenance inspection programmes.
Flame impingement from damaged or poorly tuned burners can cause localized overheating or “licking” and damage the refractory material. Similarly, high-velocity flue gas impingement at transition points within the furnace which cause a high degree of turbulence, can lead to mechanical wear and erosion of the lining and can cause hotspots to appear on the other side of the furnace casing. Proper burner maintenance and furnace design can minimize these risks. Alternatively, in very high velocity or turbulent areas, a protective lining (insert link to 3M Nextel lining systems) may be applied.
Aside from the common causes mentioned, thermal cycling or thermal shocks due to emergency unplanned trips or shutdowns, prolonged exposure to extreme temperatures, and the presence of abrasive particulates in flue gases can also contribute to refractory degradation. Moreover, inadequate material selection, insufficient refractory thickness, and operational deviations from design parameters can worsen the situation.
If you are in a situation where you have already encountered a failure in your refractory and do not have the opportunity to shut down the unit to repair without large economic consequences, an Online Hotspot Repair would be the best solution to fix the failed lining without any shutdown or slowdown. One company that does these sorts of repairs quite effectively is KLAY EnerSol who have carried out numerous such repairs over the years saving their clients huge sums of money from unplanned shutdowns due to refractory failures.
The repair methodology generally involves the cutting of the external casing adjacent to the failure areas, insertion of a custom made high temperature backing system and the cavity is then filled with a propriety compound designed for pumpability and curing in fully running furnace conditions. This allows the repairs to be done while the plant continues to run at full capacity without requiring a slowdown or shutdown. Click for more information on how these repairs are carried out and to see some interesting case studies. It has to be noted though that these are considered emergency and temporary repairs only and are not a substitute to good refractory installation and maintenance practices.
In conclusion, refractory failures in furnaces and boilers can stem from various factors. Addressing these challenges requires proactive measures such as robust material selection, meticulous installation practices, regular maintenance, and adherence to operational best practices. By mitigating the risks associated with refractory failures, industries can ensure operational reliability and efficiency.