Scale and Corrosion Debris Transport

The pickup, transport, and redeposition of corrosion debris and deposits can happen anywhere in steam distribution and condensate return systems and are not confined to any particular boiler plant size or pressure rating. For example, deposit pickup may occur in a superheater with redeposition taking place perhaps in a pressure reducing station, steam trap, or condensate line. The starting point for transport mechanisms is often a combination of BW carryover and condensate line corrosion. 

Where carryover occurs, much of the solids content is deposited in the first parts of the steam and condensate system, such as superheaters, but the balance can be transported all the way back to the pre-boiler system and from there to the boiler itself. Thus, a chain of cause and effect may once again develop in a manner similar to the progression of problems in other areas of the boiler system. 

Corrosion of condensate lines is a serious problem. It is compounded where both oxygen and carbon dioxide are present because it causes considerable quantities of hematite (Fe203) to develop. Corrosion of other boiler plant components, such as FW heaters, adds more metals to the mix, and corrosion debris typically includes iron, copper, nickel, zinc, and chromium oxides. 

Corrosion and mineral debris can form in condensate lines from a variety of means, and it is not uncommon for the resultant debris accumulated over many years to slough off and return to the boiler when operating conditions change. The result is often a thick boiler-bottom sludge that settles out in the water space or baked-on sludge, which mars efficient combustion and water tube heat transfer. 

Filming amines generally produce a much worse fouling condition than neutralizing amines so where fdmers are required, the typical prac- 

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Scale and corrosion debris transport results in FW line/feed pump blockages Impingement and erosion Poor deaerator venting Sticking check valves Fe, Cu, Ni, Zn, Cr oxides Crystalline magnetite... 

Localized pre-boiler scale and corrosion debris deposits. Combination of New phosphate, iron, copper, and silica deposition Old re-deposited debris Transport of Fe, Cu, Ni, Zn, Cr oxides to HP boiler section, leading to deposition, fouling, and possible tube failures Transport of minerals and debris including malachite, ammonium carbamate, basic ferric ammonium carbonate Precipitation in FW line of phosphates, iron, and silicates... 

Where deposits occur in the final FW system, analyses often show that these deposits are composed of both new localized scale and corrosion products, plus old redeposited debris transported from other parts of the boiler plant system. 

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