Chapter 6 - Chemical Cleaning of a Steam Generator

Abstract

Deposits are responsible for overheating and corrosion of tube metal surfaces and impair boiler water circulation. Scale formed by precipitation of salts and sludge consists of suspended insoluble particles. Films of oil, grease, and temporary protective coatings in tubes render an insulating effect to heating surfaces, causing failure of tubes due to overheating. Absorption of hydrogen into tube metal results in loss of ductility with consequent failure of tubes. To get high purity steam, a boiler is required to be chemically cleaned by employing either a mineral acid or an organic acid/chelant. Inhibitor is used to prevent the acid from aggressively attacking the metal surface. The soaking or circulation method is followed for chemical cleaning. The chemical cleaning process is sequentially divided into “water flushing,” “alkali boil-out,” “soaking with acid solution or circulation of acid solution,” “rinsing,” “first-stage passivation,” “second-stage passivation,” and “inspection.” Waste disposal following chemical cleaning is a matter of concern.