Patent Number: 042499941
Section: description

In FIG. 1, the electromagnetic filter is designated by the reference 2. As shown diagrammatically, said filter comprises a casing 4 filled with a packing 6 and placed within a winding 8 which is supplied from a voltage source 10. The constructional details of this electromagnetic filter are already known and do not form part of the invention. The winding 8 can be constituted by a single winding or by a stack of discs which are supplied in parallel, for example. The filter 2 is placed in a duct 12 through which the liquid to be filtered is circulated. By way of example, said duct can form part of the primary circuit of a nuclear reactor. Valves 14 and 16 serve to isolate the filter from the duct 12. Valves 18 and 20 serve to connect the filter to a loop circuit 22. This circuit comprises an accelerating pump 24 and is connected by means of a pipe 26 to a source 28 of hot water under pressure through a valve 30. By way of example, the source 28 can be the primary circuit of a nuclear reactor or alternatively the pressurizer of said circuit as will become more readily apparent in connection with FIG. 4. A drain-off valve 32 is placed at the lower end of the accelerating pump 24 in a discharge duct 34 which is connected to cooling devices and to effluent tanks (not shown in the figure). The operation of this installation is as follows: in order to carry out unclogging of the filter 2, the main valves 14 and 16 are first closed. The magnetizable packing 6 is then demagnetized by applying a low-frequency current to the winding 8 in known manner, the amplitude of said current being progressively reduced to zero. The valves 18 and 20 of the unclogging circuit are then opened as well as the valve 30 in order to permit the addition of hot water under pressure which is delivered by the means 28. The pump 24 is then started up whilst the valve 30 can remain open if this is permitted by the conditions of pressure within the circuit or can be closed. The output of the pump is so adjusted as to ensure that the liquid which flows into the filter attains a velocity which is just sufficient to cause lifting of the bed of beads when no magnetic field is present. The drain-off valve 32 is opened several times in succession. Each time this valve is opened, a fraction of the unclogging sludges is discharged by expansion through the duct 34. Each opening of the valve 32 causes at the same time a reduction in flow velocity of the liquid, with the result that the bed of beads is deposited and comes to rest on the bottom of the filter. Conversely, each time the valve 32 is closed, the bed of beads moves upwards under the action of the increase in flow velocity of the liquid circulated within the filter. The succession of opening and closing movements of the valve 32 thus causes the bed of beads to undergo a succession of movements of small amplitude and in turn gives rise to unclogging and washing-out of the sludges. After a few cycles of washing and draining-off operations, the filter is finally cleared to a sufficient extent to be put back into service. This recommissioning of the filter is carried out by closing the valves 18 and 20, by restoring the magnetizing field and finally by opening the main valves 14 and 16. While the alternative embodiment of the method of unclogging in accordance with the invention which has just been described proves satisfactory in the majority of cases, a disadvantage does nevertheless arise from the need for an accelerating pump (namely the pump 24 shown in FIG. 1), the operating regime of which may be affected by contamination of the wash water, especially if this water contains a high concentration of metal oxide sludges. In the second alternative embodiment which is illustrated in FIG. 2, said accelerating pump is dispensed with. In FIG. 2, the filter (the illustrattion of which is simplified) is again designated by the reference 2 and mounted in a duct 12 from which it can be isolated by means of main valves 14 and 16. A drain-off valve 40 is placed at the lower end of a Y-shaped pipe 42, the upper end of which contains an introduction valve 44. The operation of the installation described above is as follows: unclogging of the filter 2 first consists in carrying out the operations already described in connection with the alternative embodiment of FIG. 1, namely demagnetization of the packing followed by isolation of the filter by closure of the main valves 14 and 16. Since the valve 44 remains closed, the drain-off valve 40 is then opened so as to empty the filter by expansion through cooling devices which are not shown in the figure. A part of the metal oxides retained in the filter packing is then carried away. Stopping of the draining-off process is carried out by closing the valve 40 when the filter finally contains steam alone. The valve 44 is then opened so as to allow the wash water to penetrate into the filter. This water is withdrawn for example from the main circuit of the reactor in which the filter is located, or alternatively from the pressurizer as will be more readily apparent from a study of FIG. 4. This introduction of water into the filter takes place abruptly since the water undergoes sudden expansion from the pressure of the primary circuit (which can be of the order of 150 bar, for example) to the pressure of the steam remaining within the filter (which is usually lower than 100 bar). There accordingly results a turbulent flow motion of the injected water which is imparted to the beads and causes mechanical separation of the sludges. The turbulent flooding thus produced is immediately followed by further draining-off, this being produced by closing the valve 44 and opening the valve 40. As in the preceding alternative embodiment, this cycle of operations is repeated a number of times. After a few cycles (4 or 5), the filter is regenerated to a sufficient extent to be put back in circuit for further operation. The valves 40 and 44 are then closed, the magnetizing field is restored and the main valves 14 and 16 are opened. In this alternative embodiment, it is not necessary to ensure that the packing is set in motion in order to produce effective detachment of corrosion products; this form of the method is therefore equally applicable to packings of different types, especially the fixed packings which were mentioned earlier. When this variant of the method of unclogging is applied to a filter which is mounted in the secondary circuit of a nuclear reactor, it is preferable in order to obtain a high degree of turbulence of the water to ensure that the residual vapor pressure within the filter is lower than the water pressure of the secondary circuit. The two variants of the method which have just been described apply to any electromagnetic filter which is placed in one of the circuits of a nuclear power plant. By way of explanation, an installation fitted with filters of this type is illustrated in FIGS. 3 and 4, said filters being placed respectively in the secondary and primary circuits. In the installation which is illustrated in FIG. 3, two filters are placed in the secondary circuit of a pressurized-water reactor. These filters are employed for filtration of condensates downstream of the high-pressure heaters and for filtration of the purge water flow in a steam generator, the temperature of which is of the order of or higher than 260.degree. C. The steam generator 50 is purged with a flow at a rate within the range of 1% to 1.5% of the supply flow rate. The purge flow is filtered directly within a first electromagnetic filter 52 and is reinjected into the steam generator with the water of the high-pressure heaters 54 by means of a pump 56. However, a fraction is withdrawn and cooled within a heat-exchanger 58, then directed into the demineralizer 60 which consists of a mixed bed of ion-exchange resins, at a temperature of 50.degree. C. This fraction is then directed into the condensate circuit towards the heaters. The condensates from the condenser 62 are demineralized on the ion-exchange resins 64 and directed to the low-pressure heaters 66 by a pump 68. The condensates are then directed with the make-up water derived from the feedwater tank 70 to the high-pressure heaters 54. The water which is heated to the injection temperature within the steam generator 50 is filtered by means of a second electromagnetic filter 72, with the result that the undissolved corrosion products (the heaters are one of the main sources of said products in the condensate circuit) are retained by the filter 72 before finally reaching the steam generator 50. The circuit is completed by a certain number of valves which need not be described in detail since an installation of this type is known to anyone versed in the art. In the installation which is illustrated in FIG. 4, the electromagnetic filter is placed in the primary circuit of a pressurized-water reactor. The reactor core which is shown diagrammatically is designated by the reference 80; the primary circuit comprises a pressurizer 82, a steam generator 84, a circulating pump 86. The filter 88 receives through the valve 87 the water which is taken from the discharge side of the pump 86, the temperature of the water being about 300.degree. C. The rate of withdrawal can vary between 1% and 10% of the nominal flow rate within the loop constituted by the reactor core, the pump and the steam generator. The water discharged from the filter 88 is reinjected into the suction side of the pump through the valve 90 but a fraction can be withdrawn and directed through the valve 92, first towards a heat-exchanger 94 which cools the water to 50.degree. C. and then to a mixed bed of ion-exchange resins 96 for separating ionic impurities and any fission products which may be present. The extent of the above-mentioned withdrawal can be adjusted as a function of the ionic purity of the water of the primary circuit; said withdrawal is usually of the order of 0.1% of the nominal flow rate within a primary loop. After passing through the mixed bed 96 of resins, the water is reinjected into the primary circuit by means of a circulating pump 98. The circuit for unclogging the filter 88 comprises a pipe 100 which connects the pressurizer 82 to said filter by means of an introduction valve 102. A drain-off valve 104 connects the lower end of the filter to an effluent tank 106. In order to carry out unclogging of the filter, the packing is demagnetized and the filter is then isolated by closing the valves 87 and 90. The drain-off valve 104 is then caused to open so that the contents of the filter are discharged from the lower end of this latter to the effluent tank. The draining-off operation is stopped by closing the valve 104. The valve 102 is then caused to open in order to enable the water withdrawn from the pressurizer 82 to carry out a washing operation at the pressure and temperature of withdrawal (subject to pressure and temperature drops). Washing is followed by a further bottom-draining as a result of closure of the valve 102 and opening of the valve 104. It will be observed that the general arrangement of an installation of this type dispenses with the need for the wash water duct and auxiliary source while providing a direct connection between the wash pipe and the primary circuit as well as draining of the filter from the lower end, which are essential features of the unclogging installation in accordance with the invention.