Abstract:
A pump unit for a nuclear installation, for example a pressurized water reactor, contains a vertical shaft, which is guided through at least one journal bearing. The journal bearing or bearings is or are configured as a spiral groove bearing or bearings, to prevent shaft vibration caused by the bearings and to increase the service life of the pump unit. Preferred uses of the pump units include, for example, the coolant pump of a pressurized water reactor and the circulating pump of a boiling water reactor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuing application, under 35 U.S.C. §120, of copending international application No. PCT/EP2006/006239, filed Jun. 28, 2006, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2005 030 485.0, filed Jun. 28, 2005; the prior applications are herewith incorporated by reference in their entirety. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a pumping device for a nuclear facility, and especially to such a pumping device with a vertically aligned shaft. 
         [0003]    Various pumping devices are necessary in nuclear facilities, as for example coolant pumps of a pressurized water reactor or the recirculation pump of a boiling water reactor. Such pumping devices are in general of large dimensions, possess a vertically aligned shaft, and, not lastly, for safety reasons must meet the toughest quality requirements regarding power capacity and durability. 
         [0004]    In machines with vertically aligned shafts that are guided by sliding bearings and in which a small, radially-directed hydraulic force is present, as is generally the case in axial pumps and semiaxial pumps, oscillations appear as a rule that are known as half-frequency whirl, also commonly under the designation ω/2 whirl. Such periodic excitations cause operational instability that can also have a negative effect on adjoining welded seams of housing components or support structures. 
         [0005]    In such pumping devices a danger also exists as well of punching through the lubricating film, also known as whip. This instance occurs when the residual imbalance is equal to the radially-directed force, and cannot in principle be precluded. Owing to punching through of the lubricating film, the shaft and bushing can come into material contact, which ultimately can lead to destruction of the sliding bearing. Only in individual instances can these material contacts be ameliorated by damping via the lubricant; the oscillatory behavior itself remains unaffected by this, however. 
         [0006]    The problem named above can in principle be avoided by special configurations of sliding bearings as so-called multi-surface bearings, as multi-wedge bearings, as multi-wedge pocket bearings or as sliding bearings with multiple individually suspended bearing segments. However, these known types of bearings are very expensive to manufacture and thus cost-intensive. Therefore, generally it is the practice to dispense with such types of sliding bearings and efforts are made to check the condition of the sliding bearing using so-called shaft-path monitoring. This measure, however, does not eliminate the problem described. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    It is accordingly an object of the invention to provide a pumping device for a nuclear facility that overcomes the above-mentioned disadvantages of the prior art devices of this general type. 
         [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a pumping device for a nuclear facility. The pumping device contains at least one sliding bearing being a radial bearing and embodied as a spiral flute bearing; and a vertically directed shaft being guided by said at least one sliding bearing. 
         [0009]    Therefore the task that is the basis of the present invention is to make available a pumping device for a nuclear facility with a vertically aligned shaft, that is guided by at least one sliding bearing, by which the problem named above that half-frequency whirl in the bearing appears, is avoided in simple fashion, or at least minimized. 
         [0010]    In the pumping device for a nuclear facility with a vertically directed shaft that is guided by at least one sliding bearing, the at least one sliding bearing is embodied according to the invention as a spiral flute bearing. 
         [0011]    The spiral flute bearing, which has previously found application exclusively in special cases of smaller bearings, is distinguished by various advantageous properties that according to the invention are to be also used in a large pumping device of a nuclear facility with a vertically directed shaft. In particular, the properties of the spiral flute bearing result in the pump shaft being well centered and the half-frequency whirl being avoided or at least minimized, so that ultimately also the above-named material contacts in previous pumping devices can be avoided. 
         [0012]    The sliding bearing embodied as a spiral flute bearing can also be used both as a radial bearing and as an axial bearing. 
         [0013]    The invention-specifically embodied pumping device is used in advantageous fashion for example as a coolant pump of a pressurized water reactor or as a recirculation pump of a boiling water reactor. 
         [0014]    The above ones, as well as further tasks, features and advantages of the invention are made more understandable from the following specification of a preferred, non-limiting embodiment example, with reference to the appended drawing. 
         [0015]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0016]    Although the invention is illustrated and described herein as embodied in a pumping device for a nuclear facility, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0017]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0018]    The single FIGURE of the drawing is a schematic depiction of a configuration of a pressurized water reactor in which the present invention is used. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring now to the single FIGURE of the drawing in detail, there is shown a pressurized water reactor  10  that uses normal “light” water simultaneously as the moderator and as a coolant. Therefore, it belongs to the class of light water reactors. 
         [0020]    Pressurized water reactors  10  have two main coolant loops, the primary coolant loop and the secondary coolant loop. The primary loop  14  is formed of a pressurized reactor vessel  12  with reactor core, a main coolant pump  16  and a steam generator  18  as well as the connecting pipes. Additionally, in a known manner, a pressurizer  20  is provided. 
         [0021]    The main coolant pump  16  conveys the coolant, water prepared at about 290° C., into the thick-walled reactor pressurized reactor vessel  12  made of steel, and there initially downward in an annular channel. At the vessel bottom, the coolant is redirected and then flows back from below upwards around the fuel rods in pressurized reactor vessel  12 . The coolant cools the fuel rods, by which it itself is heated to about 325° C. The heat thus admitted is released in steam generator  18  to the secondary coolant loop  22 . The water is again cooled down to 290° C. and is brought back through main coolant pump  16  to the pressurized reactor vessel  12 . While this is occurring, the high operating pressure of about 160 bar prevents the formation of film boiling. 
         [0022]    In steam generator  18 , the thermal energy is radiated off via the heating pipes to the secondary cooling loop  22  and at about 70 bar leads to formation of 280° C. steam. The steam transfers the thermal energy to the turbine  24 , where, by a connected generator  26 , electrical energy is generated. In a condenser  28  connected downstream, the cooled steam condenses at about 30 to 35° C. into water, which is then pumped back again as feed water into steam generator  18 . The condensation heat is normally released via an additional condenser coolant loop  30  into the environment via a cooling tower. 
         [0023]    An additional known type of nuclear reactor is the so-called boiling water reactor. In a boiling water reactor, which is not depicted, the water used as a coolant is already boiling at the fuel rods of fuel elements in the pressurized reactor vessel, so that the steam for the turbine-generator set is already produced in the reactor, and no separate steam generator is necessary. The water fed into the pressurized reactor vessel is brought by suction through appropriate recirculation pumps into the annular channel, and fed into the lower part of the pressurized reactor vessel. From there, the water flows upwards through the reactor core, absorbs heat while doing so, and leaves the pressurized reactor vessel as a steam-and-water mixture. 
         [0024]    Since a nuclear facility itself is not the subject of the present invention, we shall dispense with a detailed description of its design and method of functioning, and make reference to appropriate literature. 
         [0025]    Both the above main coolant pumps of the pressurized water reactor and the recirculation pumps of the boiling water reactors—as well as other pumps, possibly—have vertically aligned shafts that are guided in sliding bearings—radial bearings and/or axial bearings. These sliding bearings are embodied as so-called spiral flute bearings according to the invention. 
         [0026]    The spiral flute bearings here mentioned are completely surrounded by a fluid like oil, grease or water, and therefore have no so-called “free surfaces.” In a limited lubrication wedge, a pressure buildup occurs over only a part of the shaft circumference, but due to the flutes distributed over its entire circumference, in a spiral flute bearing, pressure likewise builds up over the entire circumference. Due to this, restoring forces form that are distributed largely uniformly over the entire circumference. These restoring forces distributed uniformly over the entire circumference cause the shaft to be well centered and the half-frequency whirl highly likely to be suppressed. By this measure, the equilibrium running of the shaft is improved, and the oscillatory behavior in previous sliding bearings described at the outset is totally avoided, which leads to improved power capacity of the particular pumping device and to a reduction in material loading and thus ultimately to this pumping device having a longer service life. 
         [0027]    Additionally, through the pumping action of spiral flute bearings, the starting behavior of the pumping devices is improved, since the mixing range is gotten through very quickly. 
         [0028]    Preferably, each spiral flute bearing is lubricated with water, so that free surfaces especially can be largely avoided. 
         [0029]    An additional advantage is that the spiral flute bearing possesses the properties of a normal sliding bearing in unlimited fashion, so that even in the event of total loss of lubricant, the emergency operating properties of the normal sliding bearing can be utilized with appropriate precautions. 
         [0030]    Additionally, the bearing shells of the spiral flute bearing are preferably configured to be cylindrical and thus easy to produce. Thus for manufacture of the spiral flute bearing, no special tools or machinery is required. 
         [0031]    The spiral flute bearing can be used both as a radial bearing and as an axial bearing. In axial bearings, the advantage is in a considerably higher carrying capacity as opposed to the customarily used Mitchel bearings. 
         [0032]    It is true that spiral flute bearings are already known from the state of the art, but previously they have been used exclusively for relatively small-dimension precision bearings. To be named here as an example is the rotary-motion support of polygonal mirrors of scanners in laser printers, for which see for example German patent DE 197 12 432 C2. 
         [0033]    It is precisely in applying them in a nuclear facility that pumps equipped with spiral flute bearings have the following especially advantageous features:
       a) the bearing is completely surrounding by cooling and lubricating fluid;   b) provision is made for axial flow-through of the bearing (radial or axial bearing); and   c) water is provided as the lubricant.       
 
         [0037]    This can be applied especially for large-dimension, rapidly rotating shafts.