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The integrity of water/steam circuits of thermo-electric power plants is critically dependent on chemistry issues to ensure the avoidance of excessive corrosion damage and deposition. The consequences of inappropriate chemistries can be severe irreversible plant damage requiring extensive repairs and remedial actions. Accordingly there has been widespread development, generally on a country by country and in some cases on a company by company basis, of guidelines for the chemistry of fossil fired plants. Discussion of this subject on a seminar (at the Israel meeting of THERCHIM, Group of Experts meeting in 1995) showed a convergence of national guidelines comparing to the earlier inquiry [1] in 1970. Because of differences in national standards at that time, joint UNIPEDE guidelines could not be prepared. The current agreement between countries motivated formation of a Working Group for the preparation of the UNIPEDE guidelines for the chemical control of steam water circuits. This includes the establishment of guidance for the optimisation of plant integrity and availability when operating for periods outside the normal specification range.The approach adopted has been to encompass all of the national and company reference guidelines for the chemical control of large generating plant available from the utilities represented in UNIPEDE. These guidelines have been developed and defined taking account of both operational experience and the recommendations of the plant suppliers.The concept of action levels for important chemical control parameters has been adopted, and a procedure for further development of the use of these in terms of plant lifetime assessment is suggested. In some of the national and company guidelines action levels are already identified for some parameters for the out of specification operation of the water, steam circuit. These set out, in broad terms, the time limits for operation out of specification. In general the action level approach is not supported by the plant suppliers who tend to specify normal operating parameters only. In Europe the VGB recommendations[2], which apply to industrial plants in addition to power systems carry international authority.The concept of action levels has also been adopted by the Electrical Power Research Institute (EPRI) in the USA[3]; these are probably the most widely adopted guidelines world wide.The UNIPEDE guidelines will only address fossil fuelled power plants. The action level approach adopted covers chemical control parameters from normal acceptable operation, through minor perturbations, to more serious deviations and ultimately plant shut down. Control parameters are chosen as those which are directly influenceable by operator intervention.1.2 Review of Current Practice of UNIPEDE MembersGuidelines from several countries/utilities have been collected together and compared. Only the most significant chemical parameters for feed-water, boiler water, and steam have been considered, and although a few UNIPEDE members already use the action level approach, the normal operating values in individual guidelines have been found to be quite similar (see Appendix 1). It has therefore proved feasible to propose action levels which are broadly applicable across UNIPEDE members.Data from the inquiry relate to high pressure plants, including both once through and drum type boilers. The following operational regimes are covered:- All volatile treatment, AVT, - reducing regimes in which pH is controlled with ammonia and an oxygen scavenger (usually hydrazine) may be added.- Combined water treatment, CWT, or oxygenated treatment, OT, - ammonia under slightly oxidising conditions usually achieved by oxygen dosing with very restrictive limits on acid conductivity.(In these guidelines the acronym OT will be used.)
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