An unfortunate incident happened at our triple effect caustic evaporator (32w.t% to 51w.t%). The third effect (highlighted red) where major steam (~12-13bar depending on flowrate) assisted evaporation take place, here for some reason Ni tubes get severely collapsed, while the plant was offline Construction ..generally for falling film internals P&ID (illustrated) www.mediafire.com/view/6dumme618wr63o6/Evp2_P-1442_P_I_Diagram_process.jpg Damage done http://www.mediafire.com/view/38zlrmb8hm360hk/P_20151210_152624.jpg http://www.mediafire.com/view/2r1izd5r3doylts/P_20151210_152653.jpg http://www.mediafire.com/view/xxwqft25j17f5b1/P_20151210_152754.jpg http://www.mediafire.com/view/93hlv4byr6x6t6h/P_20151210_152826.jpg So it went like this @day1 ..we drained the evaporation while cooling it slowly (~2hrs) to ~60celsius for mechanical maintenance of effect_3. During this time level transmitter was dismantled & taken out from span of effect_3 (a dp transmitter) this therefore produced two potholes in our Ni made span which was now open to atmosphere ..& so were Ni tubes (see internal construction fig.) @day2,3 ..crane to bring down this mammoth (6.3m length ~6000mm Ni 81 tubes) effect_3 didn't arrive, it was idly sitting there ..ambient temperature here in winter did approach ~3celsius. Do recall span of effect_3 is open to atmosphere so vacuum (via condensation) isn't possible @day4 ..crane arrived ..supports unbolted & effect_3 was on the floor but the mild steel shell appeared to be ~80celsius indicating minor steam passing into shell (diaphragm steam valve is passing). We (process engineers) didn't notice the tubes though. Had this being done we would have known whether the tubes are inwards bulged merely by viewing across them @day5 ..in mechanical workshop our jaws dropped when we were asked to view Ni tubes of effect_3. HERE STARTS OUR DISCUSSION Mechanical department was to replace any leak Ni tubes so they have to wash every tube because it was laced with NaOH 50w.t% ..Is it possible that because of temperature gradient of washing water & skin temperature of Ni tubes (recall ~80celsius) Ni abnormally expanded & while its two extreme ends are welded indirectly to MS shell it may have twisted & collapsed. What other reasons you think of?