Unanticipated boiler tube failures in steam plants are caused by corrosion and scaling. These failures are attributed wholly or in part to inadequate control of boiler water or steam quality, specifically the maintenance of excessive levels of boiler water impurities, poor application of internal chemical treatment, and inadequate monitoring. As a result, a new set of boiler feed and drum water concentration limits together with a monitoring and control program was developed for boilers operating with demineralized or desalinated water. This program not only improves operating efficiency by minimizing manpower requirements, but it also assures steam plant operating reliability.
Today, corrosion in steam generating systems creates colossal loss for refineries, gas plants and petrochemical plants. This loss can be significantly reduced by: proper design, material selection, chemical treatment program, mechanical deaeration, operation and maintenance. Nevertheless, steam plants will always have to combat corrosion and scaling.
Corrosion in boilers is a function of the oxygen, carbon dioxide, ammonia concentrations, and how well they are controlled. Special circumstances exist where corrosion is caused by specific ions such as free hydroxide, chloride or high concentration of chelant chemicals in the boiler water. However, in a modern steam generating system, control of dissolved gases of oxygen and carbon dioxide results in corrosion minimization on the water side of steam generating systems.
This control transpires because of physical (mechanical) and chemical means. Mechanical methods serve as a primary control (deaerators, air ejectors and decarbonators). Chemical methods, however, serve as back-ups to control the residuals of oxygen and carbon dioxide left by mechanical removal, ammonia generated within the system, and oxygen and carbon dioxide due to leakage.
Typical steam generating systems usually consists of: a pre-boiler section (deaerating heaters, piping, pumps, stage heaters and economizers), the steam generator itself (boiler, superheaters, reheaters), the post boiler section (process equipment, steam piping and condensate traps) and the condensate system (piping, flash tanks, pumps and condensate storage tanks). Although different problems and mechanisms are often encountered, corrosion can occur in any of these sections1.
The paper will review corrosion control in steam generating systems overall in the preboiler and condensate return systems. The paper will then specifically discuss our approach for corrosion and scale control in our steam plants in the refineries and gas plants. Summary of the steam generating system will be reviewed and then boiler water quality and internal chemical treatment will be discussed. Details of boiler water quality control program as well as specific examples of some plants will be related. The paper will be concluded by techniques and methods for oil removal from condensate.
