Issues with the lining of the urea synthesis tower

The lining should be 316L; our factory has been in operation for nearly 12 years without such issues occurring. It should be a quality issue with the equipment.

The lining is 316L MOD or Ti material; it might be a problem with the material. Check the original equipment handover documents to see if they are complete, especially the ammonia leakage test data!

What factors cause cracks to appear on the lining of the urea synthesis tower? I hope fellow experts can provide some guidance. Describe the full circulation process of aqueous solutions and steam leak detection.

If the chlorine ion content in the leak detection steam is high, it can cause stress corrosion cracking; many manufacturers have encountered this problem, and it is recommended to change the leak detection medium.

"The leak detection steam contained ammonia, and inspection revealed cracks in the lining wall plates. The crack is about 40 mm long, while the crack on the back side has been measured to be 200–300 mm long. This is a very serious issue; fortunately, it was detected in time and the equipment was stopped, otherwise the consequences could have been severe. There have been several cases of explosions in urea synthesis towers in various companies. When dealing with such situations, it is absolutely not advisable to take shortcuts to save time or effort. Based on our experience, patch welding on site cannot resolve the problem fundamentally and will only lead to further issues. The best approach is to return to the manufacturer to replace the faulty section of the tower, and after welding, the entire tower must undergo heat treatment.

If it is merely a lining issue (probably weld cracking), the repair will not affect the strength of the equipment. When the lining is only 8–10 mm thick, it serves only a corrosion-resistant purpose and is not designed to bear pressure. Many old factories in the country have had their linings replaced; it’s not technically difficult. But you need to pay special attention to the reminders from some of the comrades above. For small urea production facilities (based on your 33-cubic-meter urea tower, this would correspond to an urea production plant with an annual output of 80,000 tons), steam is used for leak detection; the quality of the steam, as well as the design of the leak detection holes and the welding techniques employed, can have an impact on the base material. The concentration of salts in the steam can cause stress cracking in the pressure-bearing plates ； An unreasonable structure of the leak detection holes will exacerbate the impact. In the analysis report following the explosion of the urea synthesis tower at Luxi Fertilizer Factory in Pingyin, Shandong Province in 2005, improper design of steam leakage detection systems and leakage detection holes were identified as two main causes (Nanhua Machinery’s urea towers). Therefore, after cutting the lining plate, the following tasks must also be carried out: 1) Check the quality of the material that has leaked out of the cylinder; perform PT testing on the surface and UT testing on the inner layer ； Superficial cracks can be smoothed out by grinding; heat treatment and welding may be necessary if required ； 2) Check the drawings to see the welding structure of the leak detection holes. Compare the leak detection holes of the Pingyin urine tower where the incident occurred, identify the shortcomings, and take corrective actions (this will be difficult to handle) ； 3) Blow through the leak detection channel ； 4) Try to change the leak detection conditions, by switching from steam leak detection to nitrogen leak detection. If a leak is detected, introduce steam again—to prevent urine from accumulating in the channels. From your description, it seems that the issue was addressed promptly once it was detected; therefore, the pressure housing should be fine. This post was last edited by lxq700918 on 2009-2-24 13:16.]

Look at the statement on the first floor: \"The cracks on the backside are currently measured at 200-300mm.\" It’s clear that it’s not just a lining issue.

Based on experience, for backside cracks in the urea synthesis tower lining, leak detection steam is a significant cause. Some manufacturers that have used urine towers from different producers have experienced dorsal cracks. At present, we are able to effectively detect dorsal cracks in the urine towers manufactured by certain companies using ultrasound. Practice has shown that, provided the appropriate parameters are used, ultrasound can accurately identify cracks on the back side of austenitic stainless steel linings, thereby providing good validation for the issues discussed earlier in this section. Regarding the treatment of urine towers with dorsal cracks, it is still recommended to replace the lining. First, a non-destructive inspection should be conducted; if there are few sections with problems, the linings of those sections can be replaced. If many sections have dorsal cracks, it is advisable to replace the entire lining. It’s best to return to the factory; the reason for this is actually quite clear to everyone.

Based on the answers provided by experts earlier, there are roughly the following categories; I hope those with more knowledge can add to this: 1. Issues related to equipment manufacturing; 2. Gaps between the lining and the cylinder, which cause stress on the lining； 3. Operational issue: Severe temperature differences occur between the inside and outside of the lining ； There are severe pressure changes on the inside and outside of the lining. 4. The leak detection steam is impure, containing Cl-, and combined with tensile stress, this leads to stress corrosion. Tests have shown that stress corrosion cracks are not only present around the leak detection area; since the leak detection steam can accumulate in other places and even condense, accumulating in large quantities above the welds, cracks can form in any part of the austenitic stainless steel lining. Most of these cracks are longitudinal, but there are also some transverse cracks.

The poster found the leak point and discovered the crack on the back side after opening it, right? In my opinion, if it is confirmed to be stress corrosion cracking, then it is likely due to issues with the steam used for leak detection; in that case, the problem with that tower could be quite serious and requires proper attention. There are probably multiple cracks, but so far they have not penetrated completely, so the problem has not become apparent yet. Moreover, once cracks appear, their propagation rate is unknown, posing a significant risk to the tower. It is recommended to conduct a thorough inspection of the lining in order to detect cracks promptly and eliminate them. Our unit used linings to create backside crack test pieces in order to simulate backside crack defects. Based on extensive experimental work, and by adjusting the parameters of the UT testing equipment and probes, specialized probes for detecting cracks in austenitic stainless steel plates were developed. Through numerous laboratory tests, it became possible to detect backside cracks in the linings, with the ability to identify cracks with a depth of over 0.3 mm. Tests on several urea leakage towers have shown that ultrasound can effectively detect cracks on the back side of urea synthesis towers, which has earned praise from the manufacturers. It provides technical support for promptly eliminating potential hazards in the equipment, ensuring the safety of on-site workers, and safeguarding the safe operation of enterprises. Colleagues who are interested can discuss this together and work toward ensuring safety in the urea industry.