Severe cracking problem in the methanol synthesis tower
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The isothermal tubular synthesis tower at our plant has been in use for less than a year. First, through-cracks appeared in the circumferential welds of the lifting lug gussets, and then cracks up to 700 millimeters in length appeared in the circumferential welds of the lower cylinder. During repairs, when the insulation layer was removed, it was found that steam was leaking from the steam rising pipe, with water droplets falling onto the body of the synthesis tower. Additionally, it was discovered that the tower continued to operate despite the leakage, running under faulty conditions for half a month. Were the cracks caused by manufacturing defects or by the steam pipe leakage? Is the occurrence of circumferential weld cracks related to operating the tower under faulty conditions?
Based on the observed phenomenon, a manufacturing defect is considered more likely; it is estimated that there is stress concentration at the lifting lugs, coupled with the continuous expansion caused by operating temperature and pressure. Furthermore, operating the equipment while it is faulty and focusing solely on production can also lead to an increase in crack expansion; a thorough inspection is necessary and cannot be ignored!
It’s been less than a year and already this happened:L. Could LZ explain first: Does the suspension ring still experience stress after the synthesis tower is put into operation? Leak in the synthesis tower – where is the leak? Are the normal operating temperature, pressure, etc. within normal ranges? Whether the cracks found in the tower are on the outside or inside, in my opinion, those in LZ should be on the outside. In any case, the discovery of cracks in these three types of containers is definitely a serious issue; please first investigate the cause on your own and simultaneously contact the manufacturer to address it as soon as possible.
I think it might be 1. Excessive stress on the components, 2. Large amounts of thermal expansion and contraction, 3. Corrosion of the material by the substance in question. For reference only
Return to the 3rd floor; there is air leakage outside the synthesis tower. The lifting lugs are only used during installation, and the steam pressure remains stable during normal operation. We perform frequent start-up and shutdown operations, which results in significant fluctuations in process parameters during these operations. Steam pipes leaked water onto the tower for a long time.
Based on the above description, this is a typical stress corrosion crack (SCC), which is related to both manufacturing and use. First, the sources of stress: rolling and welding stresses from the manufacturing process have not been completely eliminated; residual welding stresses remain in the welds, as well as thermal stresses and internal pressure stresses during use ； It is possible that there is an issue with the quality of the water supplied to the boiler; steam condensation drops onto the shell of the synthesis tower. Alkali ions such as potassium in the deionized water concentrate in the weld areas, leading to electrochemical corrosion that results in corrosion pits or cracks. At the points of stress, these cracks expand, eventually forming full-scale fractures. Stress corrosion cracks are difficult to predict, and accidents often occur suddenly; therefore, sufficient attention should be paid to them. In addition to strengthening the control of process parameters, the frequency of inspections should also be increased to nip accidents in the bud. Different metals are subject to different stress corrosion environments; for example, brass suffers from stress corrosion in ammonia solutions, carbon steel in caustic alkalis or halogen-nitrogen compounds, stainless steel and aluminum alloys in chlorine-containing solutions, and titanium alloys in methanol and nitric acid solutions. Therefore, careful analysis is necessary during use to ensure that the materials operate in a safe environment.
We have a total of 3 shell-and-tube methanol synthesis towers, all of which have developed cracks and leaks. If it were a stress-related issue, it’s impossible for all three to leak; moreover, most of the leaks occur at the welds of the lower tube sheet. If it’s due to the material, the test analyses show no issues; the main material is 13MnNiMoNbR. All possible reasons have been considered, but it’s not possible to determine the cause at this time. For now, the situation is being temporarily managed by using a method that allows pressure to be maintained, but this isn’t a sustainable solution. It’s really worrying, and I hope the more experienced colleagues can offer some advice. QQ: 467853036