Boilers

What Are The Benefits Of Chacterizing Deposits In Boiler Water Wall And High-pressure Evaporator Tubes?

By Scott Reeves

ANALYSIS BOILERS COPPER CORROSION DEPOSITION EPRI IRON LABORATORY MATERIALS OF CONSTRUCTION PITTING POWER SCALING SEM STEAM TROUBLESHOOTING TUBES

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Abstract

Weeds are plants that grow where we don’t want them. Similar to weeds, deposits in water wall tubes grow where we don’t want them. Deposition can result from several sources and can cause great harm to the reliability of a steam generator. Low quality make-up water, corrosion products from the feedwater system, and contaminant ingress are all large contributors to the formation of deposits in boiler or heat recovery steam generator (HRSG) water wall or evaporator tubes.

The characterization of these deposits can not only lend insight into deposit removal methods, but also can help to determine the steps necessary to prevent future deposition. The utility chemist is generallyesponsible for monitoring tube deposits in steam generators as well as for interfacing with consultants and representatives of companies that perform chemical cleaning to remove the deposits once they have accumulated to an unacceptable level. There are many sources of information one can use to determine acceptable levels of deposition in water wall tubes. Boiler and HRSG manufacturers, chemical cleaning service providers, consultants, and industry associations such as the Electric Power Research Institute (EPRI), VGB PowerTech (VGB), and the International Association for the Properties of  Water and Steam (IAPWS) all offer advice on acceptable levels of deposition in steam generating equipment.

One must determine how much deposition a unit can tolerate prior to performing a chemical cleaning to remove the deposits. Chemical cleaning is expensive, dependent on deposit characteristics, draws resources away from maintenance activities, and may extend outages.  Long-term steam generator reliability depends on maintaining deposits below the threshold where they begin to influence or exacerbate boiler tube failure mechanisms. The study of boiler tube failure mechanisms is a science in and of itself, and a detailed discussion is outside the scope of this article; however, we will touch on three areas that are influenced by heavy deposition.

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