[Weekly Topic] How to address the frequent thinning at the elbow where the outlet pipe of the secondary economizer connects to the steam drum? (5.30--6.5)
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This post was last edited by dongjiang1001 on 2011-6-22 09:32. The boiler is of the JG-130/3.82-Q type; it burns blast furnace gas entirely, with coke oven gas being used only for stable combustion during ignition and at low loads. Since operation began, the elbow at the outlet pipe of the secondary economizer leading to the steam drum has required frequent thinning and replacement. This outlet pipe consists of two pipes with dimensions of 133*6; after about one year of operation, their thickness would reduce to just two or three times the original value, forcing the need to replace the elbows. Two years ago, the two outlet pipes were modified to 159*10. It has been two years since they were put into use, and after thickness measurements taken while the equipment was shut down, the wall thickness has decreased to one-third of its original value. Although replacing the pipes helps to extend the interval between replacements, it does not address the root cause of the problem. How to solve the frequent thinning problem at the elbow where the outlet pipe of the secondary economizer connects to the steam drum?
First, it is necessary to determine whether the thinning is caused by corrosion of the outer wall or the inner wall. If the outer wall is the one that is thinning, the fundamental solution is to desulfurize the fuel gas so that its sulfur content is such that the boiler’s exhaust temperature is at least 20 degrees above the dew point. If desulfurization is not possible, the only option is to replace the material with one that is resistant to sulfur corrosion； Second is the corrosion of the inner wall of the pipe. It is necessary to properly monitor the oxygen content in the deoxygenated water, keeping it within acceptable limits to prevent oxygen-induced corrosion. The pH level of the incoming water also needs to be controlled to avoid anionic corrosion
In my opinion, it should be considered from two main aspects: First, in terms of wear on the flue gas side: 1. The erosion caused by flue gas and dust on the outer wall of the pipes; it is recommended to find ways to install anti-wear inserts； 2. Acid gases such as SO2 in the flue gas cause corrosion; it is recommended to increase the flue gas temperature appropriately or to carry out desulfurization inside the furnace. II. Regarding erosion on the inner side of the pipe: 1. The quality of water inside the pipe causes erosion of its inner wall; it is necessary to improve the deoxygenation of the boiler feed water ； 2. The pressure of the water inside the pipe is too low, resulting in the formation of numerous bubbles. These bubbles burst against the wall of the pipe at the bends, creating instantaneous shocks within the pipe. Over time, this leads to thinning of the pipe walls; it is recommended to increase the pressure of the boiler feed water accordingly.
Reply to 3# dongjiang1001 regarding the corrosion issue: it is recommended to use 316L material or titanium composite plates
1. Internal corrosion: control the boiler water parameters properly. 2. External corrosion prevention: white steel anti-wear plates are installed on the side facing the flue gas.
First, it is necessary to identify the cause (flue gas erosion, external corrosion, internal steam-liquid boiling, pipe material issues). . ) before proceeding to analyze countermeasures.
If the factor of external wall corrosion can be ruled out, I believe the reason for the thinning of the wall thickness at the elbow where the outlet pipe of the economizer leads into the steam drum is as follows: The economizer operates in a boiling mode with a high degree of boiling; there is two-phase flow of steam and water inside the tubes. The high-speed steam carrying saturated water possesses significant kinetic energy, and due to centrifugal force, the saturated water is thrown toward the back arc of the elbow, thereby impacting that area. Over time, this results in continuous thinning of the back arc of the elbow, which is similar to the mechanism by which the wall thickness of elbows in drain pipes thins out, leading to leaks. There are two ways to solve this problem: 1) First, controlling the boiling temperature of the water at the outlet of the secondary economizer can reduce the heating surface area of the economizer; however, this seems difficult to achieve and may not be worth the effort ； 2) Increase the bending radius of the outlet elbow of the secondary economizer, and use medium-frequency bending to create the elbow ； 3) Change the elbow at the outlet of the secondary economizer to a tee-type plug, and increase the wall thickness of the plug. Since there is a \"hydraulic cushion\" in front of the plug to absorb impacts, and thanks to the thick wall thickness of the plug, the aforementioned leakage problems will not occur. In fact, power plants use this method to address the leakage issues caused by the gradual thinning of the elbows in the heater drain pipelines. Although this method results in some loss of pressure due to resistance when applied to the economizer outlet pipelines, it is still more cost-effective compared to the downtime losses resulting from leakage due to thinning elbows. The original poster might want to give it a try.
Note that the thinned areas are concentrated at the bent sections; if it is corrosion, then other areas will also become thinner as a result
What is the outlet temperature of the economizer? It should be tube wall thinning; check the water quality as well as the efficiency of the deaerator
It seems to be caused by cavitation. However, gin1970’s approach will definitely solve some of the problems; I’ve learned that
If corrosion occurs on the gas side, it is due to the presence of sulfur dioxide in blast furnace gas; coke oven gas, on the other hand, has been desulfurized, so the issue may stem from the fact that blast furnace gas has not been desulfurized or has not been desulfurized thoroughly. Although blast furnace gas is inexpensive, it corrodes the contacts. If that is the case, blast furnace gas should be desulfurized to prevent equipment corrosion as well as environmental pollution. If only the bend at the pipe connecting the economizer outlet to the steam drum is corroded and thinned, with no severe corrosion damage observed in other areas, it is possible to replace it with better-quality material, which results in lower costs.