1. Field of the Invention
An object of the present invention is a water cleansing tank for steam generators.
In steam generators, the water in the secondary circuit contains particles which are mainly products of oxidation such as magnetite. These particles tend to get deposited in zones where there is a low flowing speed, especially near the tube plate, thus promoting the corrosion of the heat exchange tubes.
2. Description of the Prior Art
This is why attempts have already been made to build devices comprising cleansing tanks in which these particles accumulate pending their removal. The prior art in this field includes three patents filed on behalf of the firm Westinghouse and respectively extended to France under Nos. 2 462 655 and 2 564 949 and to Europe under No. 0 185 174.
The inventions of these three patents concern cleansing tanks placed on the horizontal roof of a casing containing the tubes in which the primary fluid flows. The water of the secondary circuit of the steam generator, which undergoes cleansing, flows downwards between the shell of the steam generator and the casing of the bank of tubes, then flows upwards inside the casing. It then gets evaporated in contact with the tubes and rises in the vertical conduits that go through the roof of the casing. Various devices placed at the top of the steam generator, in the two-phase zone, are used to dry the steam produced by separating it from the water that it still contains. Devices of this type are cyclone devices which work by centrifugation and herringbone-structure driers.
This water, separated from the vapor phase, is more charged with particles than the water in the water-return conduits or the water at the bottom of the bank because the boiling process concentrates the solid particles in the non-evaporated water. As a rule, it flows up to a cleansing tank located on the roof of the casing of the bank where it is decanted before being recycled. The particles are deposited at the bottom of the tank and are allowed to stay there in this state until the tank is drained. These draining operations can be done between two operating cycles or during one cycle, by means of a special device comprising a water-sprinkling tube and a drainage or extraction tube.
In the first of these patents, the cleansing tank is open at the top. Its bottom consists of the roof of the casing and its side wall is cylindrical and vertical. The water accumulates until it overflows, and flows in a cascade above the upper shoulder of the side wall. It is then recycled. The drainage is provided by an extraction tube that opens out into the center of the bottom of the tank.
This device has the following disadvantage: since the cyclone devices and driers are distributed throughout the surface of the steam generator, the side wall of the tank should have a large diameter. In practice, this diameter should be almost as large as that of the shell of the steam generator itself, so as to encompass all the paths through which the water, which is separated from the steam, runs. The result of this is that, during construction, it is necessary to move and set up a very large-sized plate in the upper parts of the steam generator which are difficult to reach.
The other disadvantage is due to the extraction tube which makes it necessary to pierce the casing at two places: through the roof and on the side wall. This extraction tube also makes it necessary to pierce the shell of the steam generator itself, thus creating brittle zones.
The second patent proposes a less cumbersome method since the single tank is replaced by small closed tanks with perforated tops. These tanks are placed entirely in liquid phase. The particle-charged water flows above them, and a part of this water enters them. Since the flowing speed is very low inside, the particles are deposited at the bottom of the tanks. The extraction device is complemented by a spraying device consisting of a tube that enters the tank through the top and ends in radial nozzles which spray its interior like a shower sprinkler.
This device removes the drawback of the big tank. However, since the extraction device is the same as that of the previous approach, it has the same drawbacks and it must be acknowledged that another drawback appears with this approach: the particle-charged water flows erratically over the perforated bottoms of the various tanks, and the quantity that actually goes therein is not known. The efficiency of the device is therefore uncertain.
In the third patent, the casing that contains the bank of tubes has a triple top in the form of three horizontal plates. The top plate is perforated, the median plate has a central hole and does not extend radially up to the edge of the casing, and the lower plate, which is impervious to fluids, corresponds to the roof of the casing.
The water to be cleansed flows, as in the above approach, over the upper perforated plate and penetrates this plate. It goes towards the central hole of the median plate which it therefore crosses, also downwards. It then enters a device of deflectors which give it a laminar flow suitable for the depositing of the particles. The water then flows towards the edge of the envelope and is recycled after having gone through a vertical movement which makes it flow out of the median plate and through the edge holes of the perforated plate. In this device, no special drainage or extraction means are provided.
The drawbacks of this approach can be summarized as follows: as in the preceding approach there is no basis whatsoever for asserting that the particle-charged water actually penetrates the holes of the perforated plate and that it is not recycled directly. And, above all, the deflector devices have to be placed in relatively narrow spaces between two horizontal plates and the big tubes, conveying the water/steam mixture, which go through these plates. It can therefore be concluded that the assembly of these deflectors is very complicated.
Finally, the absence of the drainage device may entail relatively short operating cycles.
An object of the invention is to overcome these various drawbacks and, more especially, the drawback related to perforated lids which do not ensure high efficiency in the cleansing tank.
A special object of the invention is, therefore, to propose a cleansing tank which is practically an enclosed casing, with the exception of holes which may be small-sized if necessary and are used for discharging, and an upper conduit enabling a tube, through which the particle-charged water flows, to open out into the tank.