Analysis of the internal leakage in the heat exchange tubes of the low-temperature variable-pressure waste heat boiler in an ammonia synthesis plant
Thread Content
In the decarbonization section of ammonia synthesis plants, the low-temperature waste heat exchanger uses a condensate containing a certain amount of potassium carbonate as the fluid flowing through its tubes, and this fluid is used to generate steam. However, after about 5 years of operation, leaks begin to occur in the heat exchange tubes. Upon inspection, it was found that the portion of the tubes where they meet the tube sheet was perforated; all other tubes that come into contact with the solution as well as the shell itself were intact. The perforated areas corresponded to those parts of the tubes where expansion fitting was not carried out after welding to the tube sheet. What could be the reason for this? For reference: the material used is 304.
It is likely caused by thermal stress corrosion, and the heat shock is greatest at the inlet of the heat exchanger.
Is it due to the direct impact of steam? It’s difficult to determine since the specific operations are not clear
In stainless steel materials, this phenomenon is mostly caused by stress corrosion. The key to resolving this issue is to reduce or even eliminate stress, for example by using bellows and undergoing heat treatment
It may be the unexpanded portion; there is a gap between the heat exchange tube and the tube sheet, which leads to the concentration of chloride ions and thus stress corrosion