Post
by fredrosse » Sat Mar 28, 2015 2:45 pm
Steel will rust quickly if exposed to the oxygen in air, and water/wetness speeds this process very much. Steel will corrode much more slowly if submerged in water which contains dissolved oxygen. If a boiler is bottled up with water having dissolved oxygen, then there is a little rusting, this consumes the dissolved oxygen, then corrosion stops.
Steel will not corrode if submerged in water that has no disolved oxygen, and it is standard practice to fill industrial boilers completely full of deaerated water (no dissolved oxygen) for layups that will be for several days or several weeks.
I have attempted to get my VFT completely dry, with hot blowdown, opening several bottom blow valves, and opening the top mounted steam outlets. This method never seems to get all the wetness out, and that makes for ideal corrosion conditions, wetness and air circulation. Running a mild fire to evaporate all of the wetness is risky business in a dry boiler, especially a VFT. Differential expansion may cause damage, the mild fire may overheat a thin wall tube well before the thick boiler shell metal is heated.
As a result, I always fill my boiler completely full with deaerated water on every shutdown during the operating season. In winter, with the possibility of freezing, I drain and dry the boiler, applying a high vacuum to boil away all wetness within the system before bottling up the boiler. This takes a long time with a refrigeration high vacuum pump.
I think the best approach is to put an inert gas (Nitrogen) blanket on a drained boiler, regulated to 1/2 PSIG. This will keep air out, but the purge process to get all the air out before blanketing needs proper attention. Previous forum posts here describe methods, either apply the Nitrogen while there is still nothing but steam in the boiler (immediately after a hot blowdown), or purge the boiler interior with about 15 volumes of Nitrogen, to sweep out all the air before bottling up the boiler.