Abstract: Although protective agents have been applied in many power plants to inhibit corrosion during unit shut-down process, related research and operation experience seem to be inadequate regarding the effect of the protective agents containing organic compound on the TOCi (total organic carbon ion) values of steam and water in boiler. Thus experimental studies have been performed for several protective agents used in subcritical, supercritical and ultra-supercritical units in the aspects of the TOCi and cation conductivity values changes. This study result revealed that TOCi and cation conductivity of feedwater and main steam would exceed standard values severely. Specifically, the TOCi concentration would reach more than ten times of the standard limit while the cation conductivity would be more than three times of the maximum limit value respectively, which definitely increased the risk of corrosive hazard of steam and water system. Therefore, the protective agent free of chained carbon component should be selected whenever possible. If organic agents had to be a choice, the TOCi and cation conductivity must be placed under supervisory control during shutdown or initial start-up processes so as to provide directions for the addition of agents as well as the system flushing operation. It was also suggested that condensate polishing system should be put into operation during unit lay-up and start-up process such that the TOCi values of steam and water could be constrained within the standard range.

Key words: thermal power plant, lay-up protection, protective agent, TOCi, cation conductivity