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We have a DIesel Hydrotreating Unit. In feed furnace we have Austinitic steel coils. During short shut down, UOP manual says to maintain the bridge wall temperature as 205 C, to prevent the polythionic acid corrosion.
Can anyone tell the basis of temps as 205 C?
 
Answers
13/08/2018 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I would assume that is to prevent an acid dew point forming. That dew point can be well above the water dew point, and their will be a lot of conservatism built into the numbers because you can have cold spots in the furnace that are significantly lower than measured temperatures at thermocouple locations.
07/08/2018 A: Emmanuel Aunde, DORC, lemma012@yahoo.com
Polythionic acid is formed in the presence of water, oxygen and iron sulphides. To prevent the formation of polythionic acid during shut down, the temperature in the heater should be high enough to prevent condensation of water vapour i.e it should be above the water dew point. While I don't know why UOP recommended 205 C, it's well above water dew point at atmospheric pressure (am assuming the pressure during shut down is atmospheric). In addition, a bridgewall temperature of 205 C will ensure that the convection tubes will be above 100 C too which will in turn break the cycle for polythionic acid formation in the convection section.