Cavitation problem 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 erosion marks were visible on the tube sheet (especially at the holes 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 fusion-welded joints (as the heat exchanger tubes are welded to the tube sheet only on the tube side, with 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 problem exists with similar heat exchangers in your company, and how it was resolved Thank you so much!
Based on what the poster has described, cavitation mainly occurs in the gap between the tube and the tube sheet. I think by applying gasketing to the pipe-to-tube sheet joint, it is possible to eliminate the gap, and thus the cavitation issue mentioned by the original poster will not occur.
I was wondering whether your steam generator is vertical or horizontal – does cavitation occur on both side plates? However, based on your description, it seems that leakage at the joint where the tube sheet meets the tube bundle is the cause of the water hammer phenomenon. You can first conduct a hydrostatic test to check for leaks, and then weld the joint between the tube sheet and the tube bundle to eliminate the leakage. I believe that the cavitation phenomenon you described will no longer occur. We also have waste heat boilers here, and such phenomena have not appeared there.
Personally, I think the analyses of those two netizens above deserve further consideration. The tube walls thin out and the tube sheets show signs of erosion because the steam in the tube side is condensed into condensate water; furthermore, once this water enters the heat exchange tubes, its flow pattern changes drastically, causing the water droplets to strike the tube walls with force (since the convective heat transfer coefficient of water is much higher than that of steam, the wall temperature approaches that of the water side). The best solution is to change the direction of the steam flow in the pipeline, introducing steam from the top of the equipment; it’s preferable if the steam is slightly superheated.