Process for purifying and deaerating the condensate/feed water in the circulation system of a power-generating plant

#The invention relates to a process for purifying and deaerating the condensate/feed water in the circulation system of a power-generating plant. A main circuit consists essentially of a steam generator, a turbine, a condenser, a condensate purification system, low-pressure and high-pressure preheaters, and the necessary circulation pumps. A secondary circuit includes preparation, deaerating, and storage capacities for the cold condensate and the make-up water.

#Oxygen dissolved in the feed water, dissolved copper, and further corrosion products carried along by the feed water have recently been identified as the promoters of corrosion in steam generators of pressurized-water reactor installations. Consequently, there have recently been great efforts to keep the oxygen content in the feed water as low as possible, to eliminate dissolved copper by removing all pipes made of copper alloys from the feed water circulation system, and to separate out corrosion products before they reach the steam generator. These efforts are all the greater because the replacement of steam generators in a power station in the 1,000-MW class results in costs of approximately 100 million dollars. In addition, a reduction in the oxygen content of the feed water lessens the corrosion in the feed water line and the preheaters.

#Oxygen is dissolved in the feed water or in the condensate when air comes in contact with the water. For example, this occurs:

#An exemplary embodiment of the invention is shown diagrammatically in the drawing. None of the plant parts unimportant for an understanding of the invention, such as, for example, the actual steam circuit and the many self-evident shut-off fittings in the lines, are shown. The direction of flow of the various media is indicated in each case by arrows.

#1 denotes a steam generator heated by nuclear power. In normal operation, the saturated steam produced in it is expanded to the condenser pressure in the turbine installation 2, which is not shown in detail. In a condenser 3, the steam in a condensation space 4 is precipitated on the water-cooled tube bundles 5. The condensate is collected in the condensate-collecting vessel (hot well) 6. The level triangles indicate the water level 7 in the hot well 6. A condensate pump 8 conveys the condensate through a condensate purification apparatus 9, from where it passes via a hot-well regulating valve 10 into a low-pressure preheater 11, only one of which is represented symbolically. It is also assumed for the purpose of the present example that the relevant plant has no combination preheater and therefore also no conventional combination-preheater/deaerator stage. Accordingly, it is a plant such as is encountered frequently in the USA. From the low-pressure preheater 11, a feed pump 12 conveys the feed water to the steam generator 1 via a high-pressure preheater 13, of which again only one is shown.

#The water losses occurring in operation at normal rating as a result of leakage and elutriation are on the order of magnitude of 1% of the circulation quantity. These losses are made up from a cold-condensate vessel 14, from which they are introduced into the condensation space 4 of the condenser 3 through a cold-condensate line 16 by means of a feed pump 15. It goes without saying that there is no need for the feed pump 15 if the vessel 14 is located at an appropriate height. The vessel 14 obtains its content from a make-up water vessel 17 via a make-up water line 18 with a feed pump 23 located in it. During the preparation of the make-up water (not shown), the demineralised water is flushed with air to expel carbon dioxide. Consequently, the make-up water is as a rule saturated 100% with air and therefore correspondingly with oxygen. However, this does not have any effect in the case of normal load operation of the plant (which is still being considered), since the condensation space 4 is scavenged so thoroughly that the deaerating of this small quantity of make-up water can be carried out in the condenser 3 itself.

#Of course, the cold-water system contains a conventional vacuum deaerator 19 which is connected on the in-flow side to the make-up water line 18 and on the outflow side via a feed pump 20 in a deaerator outflow line 26 to the cold-condensate line 16. Via the vacuum deaerator 19, it is possible, on the one hand, initially to deaerate the make-up water from the vessel 17 to be introduced fresh into the cold-condensate vessle 14 and, on the other hand, if necessary, to circulate and thus deaerate the content of the cold-condensate vessel 14. To prevent the water, once deaerated, in the cold-condensate vessel 14 from absorbing oxygen, an inert-gas atmosphere is generated above the water level. A nitrogen connection suitable for this purpose is denoted by 21. A floating cover 22 can also protect the condensate surface against the effects of oxygen in the event of an accidental inrush of air. The latter measure can be taken in isolation or simulataneously with the first-mentioned measure.