Impact of chloride ions on equipment materials and improvements
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A company’s methanol synthesis shift unit uses shift reactions to adjust the hydrogen-to-carbon ratio of the syngas produced by its gasification unit. The third shift reactor is primarily used for the third-stage shift reaction between the output gas from the second stage shift and 15% crude syngas; the temperature of the shift gas in the outlet pipes is around 310–340°C. The material used for these pipes is SS321, with a diameter of Φ508×16. Cracking and leakage incidents have occurred in multiple sections of these pipes – how can this be resolved? The cracking of the outlet pipeline of the third conversion furnace was likely due to chloride stress corrosion cracking, which is related to the process operating conditions of the facility. During operation, a dry-wet alternating environment is created, leading to the accumulation of chloride ions on the inner wall of the pipe. Microcracks arise at defects such as inclusions and precipitates on the pipe wall, and under the combined action of circumferential, axial, and hoop stresses, these cracks continue to expand and connect with each other, ultimately resulting in the cracking of the pipe wall. After improving the start-up and shutdown procedures, the condensate in the pipelines is completely removed to prevent the formation of a stress-corrosion environment due to chloride ion accumulation, or the materials are replaced with molybdenum steel.
The problem lies in the outlet pipe of the third conversion furnace; it is made of SS321 stainless steel and cracks have appeared in multiple areas of the pipe due to operation at high temperatures of 310–340°C. The main reason is stress corrosion cracking caused by chloride ions. Specifically, the process operations create an environment of alternating dry and wet conditions; chloride ions accumulate on the inner walls of the pipes, and coupled with any impurities or defects in the material itself, cracks begin to form at these weak points. The pipeline is also subjected to various stresses, which cause cracks to continue expanding and ultimately lead to leaks. There are two approaches to solving this issue: 1. Improve the operating procedures, especially during startup and shutdown, to ensure that all condensate in the pipes is completely removed, thereby preventing the accumulation of chloride ions and reducing the corrosive environment. 2. Directly replace the pipe material by using chromium-molybdenum steel that is more resistant to chloride corrosion; materials such as 316L are more suitable for this purpose. Overall, adjustments can be made at the operational level first; if those don’t yield satisfactory results, then considering a change in materials can be an option. This approach allows for cost control while also solving the problem. .
Impact of chloride ions on equipment materials and improvements