Here Are Key Aspects of Scale and Corrosion Mechanisms and Control Programs in Industrial Boiler Systems
By Jack Kelly
Deposits on boiler heat transfer surfaces can lead to a loss in efficiency, serious corrosion, short- and long-term overheating problems, and ultimately tube failures. Plant profitability and production losses generally cannot be recovered when tube failures occur. Production losses resulting from reduced steam capacity usually are far greater than the actual repair and maintenance costs incurred during the shutdown. The major problems associated with deposits and scales are corrosion and overheating.
Corrosion damage to boilers caused by deposits is generally related to ductile gouging or caustic gouging. Hydrogen damage is less likely at pressures around 1,000 pounds per square inch gauge (psig) and greater in industrial systems. 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. This is because of the concentration of boiler dissolved solids within the deposit layer during steam bubble generation. Proper chemical control alleviates this situation.
This is due to the fact that clean tubes are reportedly not susceptible to concentrated chemical attack unless very high concentrations of corrodents are present. Figure 1 shows recommended control range for sodium hydroxide alkalinity control. Long term overheating failures are primarily caused by deposition on boiler tubes, circulation problems, or flame impingement. In deposit-caused overheating, local tube wall temperatures rise gradually to the extent that specific points begin to creep before others resulting in blisters. Accelerated deposition and scale occurs due to the changing circulation pattern caused by tube distortion. Blistering can be caused by deposition but at least two other basic conditions may result in overheating failures as well as enhancing scaling rate. Any one or combination of the following effects can lead to overheating failures:
- Direct flame impingement results from a change in flame shape or flame pattern in the boiler unit.
- Distortion of water circulation caused by a change in flame pattern or shape leads to similar, but generally less severe effects as flame impingement.
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