Iron, copper and hardness scale deposition occurring on internal boiler generating surfaces seriously impact steam generation in industrial steam boilers and generators. Deposits lead to a loss in efficiency, serious corrosion, and short and long term overheating problems leading ultimately to tube failures. Corrosion problems associated with deposits are generally ductile gouging or caustic gouging (underdeposit corrosion or UDC). This may also lead to hydrogen damage. Pertinent mechanisms of scale and deposition control processes as well as the sources of contaminants are discussed. Well established methods of prevention and control parameters are demonstrated with laboratory and field results.
The major problems associated with deposits and scale are corrosion and overheating. Plant profitability is seriously affected by unscheduled boiler shutdowns due to the failures and subsequent acid cleaning. Also, corrosion in the condensate and economizer/boiier system may be a major contributor to internal deposition on generating tubes.1,2
Corrosion damage is generally related to ductile gouging or caustic gouging associated with underdeposit corrosion (UDC). Caustic and acid gouging has also been associated with hydrogen damage. A significant factor is the amount of corrosion product or other deposits on the tube wall. Corrosion is caused or enhanced by the breakdown and/or the inability to form a protective magnetite film. Figures 1 and 2 show a tube failure with both thinning and gouging.
The major cause of deposition can be traced to suspended and soluble contaminants in boiler feedwater. The majority of industrial boiler systems operate on good quality feedwater consisting of at least zeolite softened makeup with a 30-70 percent condensate return. Demineralized water systems coupled with RO are recommended for higher pressure industrial systems (>900 psig). Zeolite soft makeup coupled with >30% condensate return provides good quality feedwater. Industriai boilers require chemical treatment due to the contaminants present in the feedwater that may cause major deposition.