Seeking help regarding the cause of leakage in the short-radius elbows of the ammonia water pipeline in coke ovens
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The new circulating ammonia water pipeline has been in operation for about 4 months now, and leaks have occurred at the elbows recently. Please help analyze this issue. The elbows that have appeared are two 1D elbows of 426*8, and two 1D elbows of 273*7; all of them are made of 20# steel. The medium used is ammonia, with a concentration of around 20%, and the temperature is approximately 70 degrees. These elbows are part of the piping system used for cooling the coke ovens. So far, these four elbows have appeared among the pipe fittings in this batch. Please help analyze the reasons behind this and suggest any remedial measures. Thank you! Another one is at roughly the same position on the inside of the other end, and another one is at the center of the outer arc of the elbow. All elbows are manufactured from seamless steel pipes in accordance with GB/T8163, using 20# material.
Unconventional methods: I have seen people performing repairs under pressure! The reason is unclear. Based on what you’ve said, the wall thickness of the pipes should be sufficient; it’s normal to use 20# steel for the ammonia spray pipes in coke ovens. We look forward to analysis from professionals
Ammonia water is corrosive to carbon steel. And it seems like this situation is quite common in your case as well. I found a similar post. https://bbs.hcbbs.com/thread-1079909-1-1.html It seems that leaks occur particularly easily at the welds. I’m not sure if it’s a weld quality issue or some other problem. I’m just throwing out some ideas to encourage experts to provide a summary.
My situation is similar but different; in my case, there are some areas near the weld seam as well as those away from it
20 steel is also used for ammonia pipelines. But the pipe wall might need to be thicker, with more margin left for corrosion resistance; your pipe wall is probably too thin, and the 1D elbow causes excessive thinning, which is why the elbow area gets corroded through first. I guess that a temporary solution is to replace the elbow, while a permanent solution would involve changing the material or increasing the wall thickness.
That’s true, but the reasons still haven’t been properly analyzed I guess
There are two reasons why ammonia corrodes metals: one is that ammonia can form complex ions with metal ions. Second, the atmosphere contains a large amount of oxygen. Since ammonia can form relatively stable complex ions with many metal ions, it reduces the standard electrode potential between metal ions and metallic elements. In terms of high school chemistry, this means that metals are more easily oxidized. Additionally, Earth’s atmosphere is an oxidizing atmosphere, which further facilitates the oxidation of metals, leading to corrosion. In simple terms, ammonia has a certain coordinating ability and can form complexes with metal ions. For example: as mentioned above, 2+, the fact that complexes are generally more stable than metal ions means that metallic elements are more prone to oxidation. Ultimately, it is the oxygen in the air that actually corrodes metals; ammonia’s role is to make the metals more susceptible to oxidation. (The above content is from Baidu Knows; by the way, I’m also learning* it…)