Patent Number: 
Section: description

In FIGS. 1 and 2 reference numerals 1, 1xe2x80x2 designate two strainers, which are mounted at the bottom of a reactor containment, whose wall is designated 2 and whose bottom is indicated at 3. The strainers are partly immersed in a pit or depression 4 and are separately connected to conduits 5 for the feeding of water into a cooling system (not shown). In the example, the nuclear power plant is thought to work with a compressed water reactor. This implies that under normal circumstances, the bottom of the reactor containment lacks water. However, at a possible shutdown water may accumulate at the bottom and create a water mass that entirely surrounds the strainers 1, 1xe2x80x2. In case the strainers were mounted to serve a boiling water reactor, the reactor containment would contain a bottom water mass in which the strainers would be fully immersed also under normal running circumstances. Onwards, the invention is described under the presumption that the strainers work in water. To each strainer 1 and 1xe2x80x2, respectively, is connected a measuring device whose main components consist of two pressure indicators or sensors 6, 6xe2x80x2, two tube conduits 7, 7xe2x80x2 and two vessels 8, 8xe2x80x2. As indicated in FIG. 3, the two vessels 8, 8xe2x80x2 are located at one and the same level, i.e., in a common horizontal plane. The first vessel 8 opens substantially directly towards the surrounding main water mass, while the second vessel 8xe2x80x2 is connected with the strainer 1 via a secondary tube conduit 9 that extends horizontally between the vessel and the strainer. Reference is now made to FIGS. 4 and 6, which show the nature of the two vessels 8, 8xe2x80x2 in more detail. In both cases, the vessel comprises a cylinder or a cylindrical wall 10 which is sealed at its opposed ends by means of gable walls 11, 11xe2x80x2. The cylinder is long and narrow and placed horizontally. In practice, the cylinder may have a diameter within the range of 80 to 100 mm and a length that is 3 to 5 times larger than the diameter. Therefore, the volume of the vessel may amount to about 2 to 4 dm3. In the proximity of one of the gable walls 11xe2x80x2 is provided a coupling means 12 connected to the upper part of the vessel, which coupling means may be connected to the previously mentioned tube conduits 7 and 7xe2x80x2, respectively. In the opposed gable wall 11xe2x80x2 is made an aperture 13 serving as an inlet, which aperture is placed near the bottom of the vessel and has a cross-sectional area which is considerably smaller than the cross-sectional area of the cylinder 10. At the vessel 8xe2x80x2 according to FIG. 5, a coupling means 12xe2x80x2 is connected to the aperture 13, which coupling means may be connected to the horizontal secondary tube conduit 9. Contrary thereto, at the vessel 8 according to FIG. 4, a short piece of tube 14 is connected to the aperture 13, which piece of tube is sealed at its free end by means of a gable 15. In the tube wall are recessed a plurality of small apertures 16, through which water can pass into the interior of the piece of tube and via the aperture 13 into the interior of the cylinder or the vessel. By the fact that the piece of tube includes these small apertures, which are substantially evenly distributed over the tube envelope, it is guaranteed that at least some of the apertures enable a liquid communication between the surrounding main water mass and the interior of the vessel, although some other would unintentionally be clogged by impurities. The water designated by 17 forms a surface or a mirror 18, above which there is an air or gas cushion 19. Each one of the two tube conduits 7, 7xe2x80x2 that are connected to the vessels 8, 8xe2x80x2 should have a limited diameter in order to guarantee that the total volume of the tube conduits becomes many times smaller than the volume of the vessels 8, 8xe2x80x2. In this way, it is guaranteed that the vessels receive a sufficient gas volume to keep the conduits filled with gas at all occurring pressure variations and water levels in the surroundings. In practice, the diameters of the tube conduits may be within the range 8 to 20 mm, preferably 10 to 15 mm, or most suitably 12 to 13 mm. As a practical example, it may be mentioned that the longest conduit 7xe2x80x2 may have a length of 5 meters and a diameter of 12,7 mm. Then the conduit obtains a volume of 0,63 dm3. At the same time, the vessel 8xe2x80x2 may have a volume of 3,24 dm3, i.e., a volume that is about 5,2 times larger than the volume of the tube conduit. In practice, the volume of the vessel should be 4 to 7, prefereably 5 to 6 times larger than the volume of the tube conduit. It is essential that the two vessels 8, 8xe2x80x2 be located in a common horizontal plane, so that differences in the statical liquid columns between the vessels do not influence measurements of pressure differences. More specifically, the two inlet apertures 13 shall be located at one and the same level. Assume that a shutdown occurs and that the strainers 1, 1xe2x80x2 start working in connection with an accumulation of water at the bottom of the reactor containment. The two vessels 8, 8xe2x80x2, which are located approximately on a level with the appurtenant strainer, will then be filled with water substantially simultaneously. Depending upon how high the surrounding main water mass rises in relation to the level of the vessels, a more or less high gas pressure will be created in the air cushion 19 and the interior of the tube conduits 7, 7xe2x80x2. These gas pressures may be detected by the sensors 6, 6xe2x80x2 (or by a gauge indicator for pressure differences common for both conduits). As long as the individual strainer, e.g. the strainer 1, is clean, equally large air or gas pressures prevail in the vessels 8, 8xe2x80x2. However, if impurities start clogging the apertures in the strainer, the water pressure in it will decrease in relation to the pressure in the surrounding main water mass. This will have the consequence that the water level 18 in the vessel 8xe2x80x2 sinks somewhat and that the gas pressure in this vessel is reduced. In other words, a difference arises between the gas pressures in the vessels 8, 8xe2x80x2 and the appurtenant tube conduits, which difference may be read by the sensors 6, 6xe2x80x2. Here, the magnitude of the gas pressure difference constitutes a measure of the degree of clogging of the strainer. When the pressure difference has reached a predetermined magnitude, reverse flushing or any other suitable cleaning of the strainer is initiated in a suitable way. In practice, the cleaning of the strainer may be accomplished either in an automatic way or by the sensors starting an alarm which in turn is utilized by the staff to manually start the cleaning operation. An essential advantage of the device according to the invention is that the vertical parts of the measuring legs, i.e., the tube conduits 7, 7xe2x80x2, always are kept dry, whereby the pressure measuring or indicating is not influenced by varying liquid columns. By the fact that the volume of the vessels 8, 8xe2x80x2 is many times larger than the volume of the tube conduits, this condition is guaranteed also if the level of the surrounding main water mass would rise to a considerable level above the vessels. From the individual strainer extends the tube conduit 9, which serves as a measuring leg, horizontally to the appurtenant reference level vessel; something that involves that the measurement is not influenced by the fact whether this tube conduit is wholly filled with water or contains air. The pressure difference at the difference pressure gauge indicator or the sensors is the same as at the measurement sites, independently of the surrounding pressure or liquid level. Sometimes wave formations or other sudden water motions may arise in the containment, which in extreme cases may lead to that water in an uncontrolled way is pressed up into the conduits 7, 7xe2x80x2. However, by the fact that these conduits have a pronounced, albeit limited diameter (e.g. 12 to 13 mm), it is guaranteed that the water is not withheld by the surface tension, but may flow back down into the vessel when the conditions return to normal. The invention is not restricted solely to the embodiment as described and shown in the drawings. Thus, in a wider aspect the invention is also applicable to the measuring of absolute gas pressures, i.e., without difference pressures being measured or indicated. In such cases, one single vessel may be included in a tube conduit and form a reference point or a reference vessel that forms a starting point from which a measurement may take place, independently of the level of the surrounding water or liquid mass. In this context it is underlined that the vessel, by being elongated and horizontal, guarantees that the level of the liquid enclosed in the vessel will vary within narrow limits even if the level of the surrounding liquid mass varies quite considerably.