On cavitation in heat exchangers
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In one of the ethylene plants owned by our company, a dilution steam generator – which uses medium-pressure steam (in the tube side) to heat process water (in the shell side) and cause it to vaporize – had its tube bundle in operation for less than 2 years. In many areas at the fusion-welded joints between the tubes and the tube sheet, severe cavitation occurred; the tubes became thinner, and there were erosion marks on the tube sheet (especially at the areas where the tubes are installed). Since there are gas and liquid phases in the shell side, the area below the mid-plane is filled with liquid. When this liquid from the shell side reaches the flash weld joint (as the heat exchanger tubes are welded to the tube sheet only on the tube side, leaving a gap on the shell side), it is heated by the pressurized steam in the tube side, causing vaporization and expansion that exerts force on the tubes and the tube sheet; as a result, the tubes become thinner and the tube sheet shows signs of erosion. I would like to ask that colleague if they could inform me whether this issue exists with similar heat exchangers in your company, and how it was resolved Thank you so much!
Confirm the following information with the poster: (1) Is this heat exchanger horizontal? (2) The cavitation site mentioned by the poster occurs at the expansion joint near the tube sheet, and it is on the shell side? (3) In the case of a fixed-tube-sheet heat exchanger, it is difficult to see this area; how did the original poster detect corrosion in that spot? The reason you mentioned is possible; I have seen similar cases of corrosion before, only in this case it involved steam heating the fluid in the tube side – the material inside the tubes was vaporized, which caused the tubes in the steam inlet section to thin out significantly and even led to perforations.