Blocking is no trivial matter-common causes and troubles caused by clogged boiler flue expansion joints
Two days ago, I met a friend from a power plant, saying that the expansion joint of their boiler flue was blocked with smoke, the exhaust temperature soared, and the fan current also jumped. His first reaction was to replace it, but when he removed it, it was full of dust and coke. Alas, the money was wrongly spent. The boiler flue expansion joint is blocked. To put it bluntly, the ash, sulfide and water vapor in the flue gas condense into hard scale in the low-temperature area, or the sundries falling in during maintenance are stuck in the ripples. Don't underestimate this matter. After blocking, the exhaust resistance increases, and the boiler efficiency decreases. It is also easy to cause local stress concentration in the expansion joint and accelerate fatigue failure. In serious cases, the expansion joint is directly burst, the smoke leaks, and the environmental protection emissions can exceed the standard.
How to tell if it's blocked or normal aging? Some tips for on-site investigation
Many people think it is aging when they see cracks in the expansion joints, and they are anxious to change them. Not necessarily. Look at the differential pressure meter first: if the differential pressure between the inlet and outlet of the flue is more than 30% larger than usual, it is mostly blocked. Touch the temperature again: the temperature downstream of the blocked part will be obviously lower, because the smoke can't pass. There is also an indigenous way-tapping the outer wall of the expansion joint with a mallet and listening to the sound. The sound is stuffy where it is blocked, and the sound is clear where it is normal. Be careful not to knock too hard, the metal rectangular expansion joint and stainless steel bellows can not withstand the smash. If internal corrosion is suspected, you can use an endoscope to reach in through the manhole to see if there is any perforation or thinning inside the corrugation. Normal aging is generally a uniform thinning or fatigue crack, which will not only plug in one part.
Cleaning and maintenance scheme: what conditions can be repaired and what must be replaced (by the way, talk about the cooperation of baffle door and expansion joint)
If it is blocked by dust accumulation, just open the access port directly and purge it with high-pressure water gun or compressed air. If the coking is hard, spray some descaling agent to soften it before cleaning it. Remember one thing: Don't use sharp tools to scrape the corrugated surface when cleaning. Once it is scratched, the stress concentration point will come out, and the life will be greatly reduced. Then what circumstances must be changed? The corrugation has been perforated, deformed, corroded and thinned by more than 30%, or the skin of the non-metallic expansion joint (fabric fiber expansion joint) is severely damaged and cannot be repaired. Don't hesitate at this time. A mouthful of smoke baffle door by the way-it is partnered with the expansion joint. If the baffle door is not closed tightly, air leakage will accelerate dew condensation and cause the expansion joint to be blocked. So when cleaning or replacing the expansion joint, check the tightness of the flapper door by the way. For example, double-seal single-axis circular baffle door, if the sealing surface is worn out, you have to change the sealing sheet.
Preventing problems before they happen: what to look for in model selection, installation, and routine maintenance
Model selection is the first step. Where the flue temperature is low and dew condensation is easy, non-metallic expansion joints are preferred, which are corrosion-resistant and non-scaling. In areas with high temperature, such as the flue gas inlet section, high-temperature axial expansion joints or corrugated expansion joints used in power station industry should be used, and the temperature resistance grade should be matched. Don't choose the general-purpose type cheaply, but it will be blocked in less than two years. Pay attention to the flow direction mark during installation. The direction of the guide tube must be consistent with the flow direction of flue gas, otherwise ash and slag will be directly poured into the corrugation. Don't be lazy in daily maintenance. Open the manhole once a quarter to see the dust accumulation. If conditions permit, install differential pressure transmitters on both sides of the expansion joint to monitor the blockage trend online. In addition, the pipeline behind the desulfurization flue gas baffle door is the easiest to block, because the flue gas has high humidity and low temperature, and regular water spraying can effectively alleviate scaling.
If it doesn't work, change it-when should it be changed, and which expansion joint is the most appropriate
If corrosion has occurred in a large area, or the corrugated fatigue crack extends to the weld, then don't fix it, just change it. Don't blindly choose models when changing. For coal-fired boiler flue, it is recommended to use metal rectangular expansion joint with non-metal compensation section, which can not only absorb displacement but also resist wear. For the wet flue gas after desulfurization, non-metallic expansion joint (fabric fiber expansion joint) is lined with fluororubber, which is acid-resistant and fouling-resistant. Cement industry customers often give rotary kiln flue with cement industry metal corrugated expansion joints, because the dust is large, the temperature is high, and the metal corrugated life is longer. As for the baffle door, if the original manual plug-in insulation door was used, the working condition can be upgraded to an electric plug-in insulation door or a single-axis double baffle door, which has better sealing performance, less air leakage, and the expansion joint is not so easy to block. Remember one sentence: If you change to the right model, you can save the accident handling fee for the next five years.