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In our hydrotreater units SWS reboiler we are having SS 316L tubes while shell is of carbon steel. Reboiler is kettle type, tube side is MP steam(Pr-10 bar and Temp-220 degC) and condensate(Temp-170 degC). During our shutdown we have opened reboiler and found 60-70 percent tube thickness loss from OD side(By ECT method) that is part of tubes which is in contact with shell side fluid and thinning is only on upper portion of tubes i.e. portion where steam is present while bottom tube portion where condensate is in tubes is not having thinning. We want to know following-
1) Reasons for such heavy metal loss/tube thinning from OD side and only on upper portion of tubes
2) Path forward to avoid such heavy loss of metal /tube thinning
3) Any up-gradation required in tube bundle metallurgy of Reboiler.
 
Answers
25/08/2014 A: Egbert van Hoorn, Hocon B V, Egbertvh@hotmail.com
The corrosion of the reboiler is likely due to the composition of the sour water. It is suspected that acids are present. Are there pH measurements available? The pH should be above 7, but it is likely to be lower. the steam temperature seems to be quite high. Normally LP steam is applied in this application.
Path forward: understanding and measurements of the water composition.
Up-grading of material: not directly required. This corrosion can be handled by the correct choice of material combined with chemical treatment (which does not need to be expensive). As consultants we do not provide chemicals, but we only provide understanding.