Patent Publication Number: US-2016238036-A1

Title: Suppressing device for a vertical pump, vertical pump and method for retrofitting a vertical pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/117,569, filed Feb. 18, 2015, the contents of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The invention relates to a suppressing device for shifting a structural natural frequency of a vertical pump as well as to a vertical pump in accordance with the preamble of the independent device claim. The invention further relates to a method of retrofitting a vertical pump. 
     2. Background of the Invention 
     Vertical pumps, in particular large vertical pumps have been used successfully in a plurality of applications for a very long time. Vertical pumps for specific applications are not infrequently manufactured in accordance with the specifications of the users or are matched in detail to specific requirements. 
     Important fields of application for the use of vertical pumps are, amongst other things, cooling towers, applications in cooling water systems, for example in nuclear or other types of power plants, for the drainage of waste water pools, or overflow basins for the prevention of flooding, or vertical pumps have also been successfully used for the drainage of large areas of land. Vertical pumps are also widely used in the supply of water, in particular in the supply of drinking water, or as main pumps or as auxiliary pumps in open or closed systems. Other applications are firefighting in the offshore area (production of oil and gas), for example on oil platforms or floating production storage and offloading units (FPSO). Moreover, a whole series of further applications for vertical pumps are well known to the person of ordinary skill in the art. 
     Vertical pumps may be designed both as single stage and multistage pumps. They are typically immersed into the liquid reservoir to be pumped, so that at least the intake or suction bell with the adjoining pump rotor is immersed into the fluid to be pumped, so that the pump is directly ready for operation. 
     SUMMARY 
     One typical and well-known setup of a vertical pump (see for example  FIG. 1 ) comprises a pumping unit with an inlet and an impeller for conveying the fluid and being immersed into the reservoir. The pumping unit is connected by a vertically upwards extending column pipe to a discharge unit having an outlet for the fluid. On top of the discharge unit a drive unit is provided for driving the impeller. The drive unit is operatively connected to the impeller by means of a line shaft extending within the column pipe. In some applications the line shaft is surrounded by an additional tube extending coaxially in the interior of the column shaft. Usually the vertical pump is supported by a supporting structure being arranged beneath and in the proximity of the discharge unit, such that the pumping unit and the main part of the column pipe are hanging without further support. 
     One of the problems with vertical pumps is caused by the structural natural frequencies of the pump installation. In former times, vertical pumps were mostly designed by rule of thumb. Due to a lack of reliable analytical methods many of these pumps were designed with structural natural frequencies at or near the running speed of the pump or multiples or half-integral multiples thereof. For example, when the pump is running at 1800 rpm this corresponds to a frequency of 30 Hertz. Thus, if 30 Hertz corresponds or is close to a structural natural frequency of the system the pump is running at a speed corresponding to a structural natural frequency of the pump system. When such a matching occurs a considerable load results especially on the bearings, which causes for example a premature failure of the bearings or the line shaft. In addition, an enhanced wear or other negative degradation effects may occur. 
     In many cases, for example in power plants or nuclear installations the user is not willing to replace an entire existing pump or the user is not interested in a complete redesign of its pumps. Rather, the user favors a retrofit and an after-market solution to solve the problems caused by said resonance effects. 
     To address these issues it is a known measure to attach a point mass at a specific location of the pump, for example at a specific location on the column pipe. The point mass lowers the structural natural frequency of the system. To be effective it is essential to provide as much weight as possible concentrated at a specific location, for example at a node of the oscillation, because the more the weight is spread out and away from the point of interest the less effect it has on lowering the natural frequency. 
     Although this method has proven successful in many applications there are some restrictions or drawbacks. It is possible that the required point mass for effectively lowering the natural frequency is as large that the pump cannot support it structurally. In other applications the spatial circumstances do not allow for the installation of a point mass at the required location. Furthermore, in some applications it turned out that the method with the point mass did not at all yield to the desired result. 
     Based on that prior art it is an object of the invention to propose a suppressing device for shifting a structural natural frequency of a vertical pump in such an effective manner that the described resonance effects may be avoided. The suppressing device should be easy to construct and easy to be installed. Specifically, the suppressing device shall be suited for retrofitting existing vertical pumps in a simple and cost-efficient manner. In addition, it is an object of the invention to propose a vertical pump that allows for a shifting of its structural natural frequency in a simple and cost-efficient manner. Furthermore, it is an object of the invention to propose a method of retrofitting an existing vertical pump which method allows for shifting the structural natural frequency of the pump. 
     The subject matter of the invention satisfying these objects is characterized by the embodiments described herein. 
     Thus, according to an embodiment of the invention a suppressing device is proposed for shifting a structural natural frequency of a vertical pump, said pump comprising a pumping unit with an inlet and an impeller for conveying a fluid, a discharge unit with an outlet for the fluid, a drive unit for rotating the impeller, a column pipe extending in an axial direction and having at least one column element with an axial length, the column pipe connecting the pumping unit with the discharge unit, as well as a line shaft extending within the column pipe and operatively connecting the drive unit with the pumping unit, the suppressing device being designed and adapted for closely fitting around the entire circumference of the column element, and having an extension in the axial direction which is at least half the axial length of the column element and comprising at least one rib extending essentially in a radial direction being perpendicular to the axial direction. 
     By providing the suppressing device for closely fitting around the entire circumference of the column element a stiffening effect results that shifts the structural natural frequency of the pump to a higher frequency. The close fit around the column element generates a compression that ensures a transfer of stiffness to the column element. This clamping effect is comparable to an increase of the thickness of the wall of the column element. In addition, the at least one rib extending essentially in the radial direction increases the moment of inertia what in turn raises the natural frequency of the pump. Therefore the rib preferably extends with respect to the axial direction along the entire lengths of the stiffening device. By this configuration, the original structural natural frequency can be shifted away from a critical value to a higher value. 
     According to a preferred embodiment the extension of the suppressing device in the axial direction is equal to the axial length of the column element around which the suppressing device is fitting in the assembled state. By this measure a particularly efficient stiffening effect is realized. 
     In order to provide for a very simple mounting of the suppressing device it is preferred that the suppressing device comprises a plurality of stiffening members being curved in a circumferential direction and being connectable to each other and complementing one another in the assembled state to a tubular structure for closely fitting around the entire circumference of the column element. This renders it possible to put the stiffening members around the column member and to fix them with respect to each other, for example by screws. 
     In order to increase the moment of inertia, it is advantageous, when each stiffening member has two lateral ribs extending in the radial direction, for each rib to form an end of the stiffening member with respect to the circumferential direction. 
     According to a particularly preferred embodiment, the suppressing device comprises four stiffening members, each of which has a middle part with a quadrant shaped cross section, the middle part extending between the two lateral ribs. This embodiment provides for an especially simple and fast mounting of the suppressing device. Preferably, the four stiffening members are at least essentially identical. 
     For fixing the suppressing device closely around the column element to achieve the desired clamping effect it is advantageous, when each lateral rib includes a plurality of holes for receiving a fixing device to rigidly connect adjacent stiffening members. The fixing means are preferably screws and screw nuts. 
     In order to realize the desired shift of the structural natural frequency and depending on the respective application it might be advantageous that at least one stiffening member has an intermediate rib extending in the radial direction and being arranged between the two lateral ribs. 
     Furthermore, in accordance with the invention a vertical pump for conveying a fluid is proposed, comprising a pumping unit with an inlet and an impeller for conveying the fluid, a discharge unit with an outlet for the fluid, a drive unit for rotating the impeller, a column pipe extending in an axial direction and having at least one column element with an axial length, said column pipe connecting the pumping unit with the discharge unit, as well as a line shaft extending within the column pipe and operatively connecting the drive unit with the pumping unit, wherein at least one column element of the column pipe includes a suppressing device designed according to an embodiment of the invention, the suppressing device closely fitting around the entire circumference of the column element, and having an extension in the axial direction which is at least half the axial length of the column element. 
     As already explained in connection with the suppressing device, a vertical pump including a suppressing device according to an embodiment of the invention renders it possible to easily suppress critical structural natural frequencies of the pump by shifting these critical frequencies to higher values. 
     It is preferred that the suppressing device be releasably connected to the column element, because by this measure it is also possible to modify already existing pumps in such a manner that they become vertical pumps according to the invention. Furthermore, this measure is advantageous from the constructural point of view. 
     In order to realize a particularly efficient shift of the structural natural frequency it is preferred that the extension of the suppressing device in the axial direction is equal to the axial length of the column element, i.e. the suppressing device covers the column element along its entire axial extension. 
     In case the column pipe comprises a plurality of column elements it is an advantageous measure when at least two column elements include a respective suppressing device. In many applications it might be preferred when each of the individual column elements includes a respective suppressing device. 
     Furthermore, in accordance with the invention, a method of retrofitting a vertical pump is proposed, the pump comprising a pumping unit with an inlet and an impeller for conveying a fluid, a discharge unit with an outlet for the fluid, a drive unit for rotating the impeller, a column pipe extending in an axial direction and having at least one column element with an axial length, said column pipe connecting the pumping unit with the discharge unit, as well as a line shaft extending within the column pipe and operatively connecting the drive unit with the pumping unit, said method comprising the steps of:
         providing a suppressing device for shifting a structural natural frequency of the vertical pump, said suppressing device being designed in accordance with the invention   selecting at least one of the column elements   mounting the suppressing device to the column element in a closely fitting manner around the entire circumference of the column element.       

     The suppressing device according to an embodiment of the invention is particularly suited for retrofitting already existing vertical pumps. If there is a resonance issue at a specific vertical pump, e.g. because a structural natural frequency of the pump installation is equal or very close to the running speed of the pump the method according to the invention provides for an efficient, very simple and cost-efficient solution for suppressing said natural frequency by shifting the structural natural frequency to higher values. Thus, there is no need for a complete redesign of the pump. The resonance issue may be solved by providing a suited suppressing device to the column pipe of the pump. 
     By the same reasons as already herein before mentioned, it is preferred that the suppressing device includes an extension in the axial direction that is equal to the axial lengths of the column element. 
     In order to facilitate the mounting of the suppressing device, it is preferred when the suppressing device comprises a plurality of stiffening members, preferably four stiffening members, and when the stiffening members are laid around the column element and fixed to each other to closely fit around the entire circumference of the column element. 
     For applications where the column pipe comprises more than one column element, it is preferred that a plurality of column elements is selected and each of the selected column elements is provided with a separate suppressing device. 
     Further advantageous measures and embodiments of the invention will become apparent from the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail hereinafter with reference to the drawings. 
         FIG. 1  is a cross-sectional view of a vertical pump known from the prior art, 
         FIG. 2  is an exploded perspective view of a first embodiment of a suppressing device according to the invention together with an column element of a vertical pump, 
         FIG. 3  is a perspective view of a stiffening member of the first embodiment shown in  FIG. 2 , 
         FIG. 4  is a cross-sectional view of the stiffening member shown in  FIG. 3  in a section perpendicular to the axial direction, 
         FIG. 5  is a perspective view of a column element with the first embodiment of the suppressing device mounted on it, 
         FIG. 6  is schematic, cross-sectional view of an embodiment of a vertical pump according to the invention, 
         FIG. 7  is similar to  FIG. 4 , but for a variant of the stiffening member, and 
         FIG. 8  is a cross-sectional view of a second embodiment of a suppressing device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the drawings of the different embodiments identical parts or parts having the same function or an analogously same function are designated with the same reference numerals.  FIG. 1  shows a cross-sectional view of a vertical pump which is designated in its entity with reference numeral  1 . The vertical pump  1  as such is known from the prior art. However the general description given with reference to  FIG. 1  is also valid for an embodiment of a vertical pump  1  according to the invention as illustrated in  FIG. 6 . 
       FIG. 1  shows the vertical pump  1  in its usual operating position, i.e. in a vertical orientation. Hereinafter relative terms regarding the location like “above” or “below” refer to this operating position shown in  FIG. 1 . 
     The vertical pump  1  ( FIG. 1 ) comprises a pumping unit  2  located at the lower end of the pump  1 . The pumping unit  2  is provided with an inlet  21  and an impeller  22  for conveying a fluid, for example water. From the upper end of the pumping unit  2  a tubular column pipe  3  is vertically extending upwards to connect the pumping unit  2  to a discharge unit  4  having an outlet  41  for discharging the pumped fluid. The column pipe  3  extends in an axial direction A which is defined by the axis of the column pipe  3  and coincides with the vertical direction when the pump  1  is in its usual operating position. The column pipe  3  consists of a plurality—in this embodiment five—column elements  31  being arranged one after the other with respect to the axial direction A. Each of the tubular column elements  31  is provided at its axial ends with a flange  32  for connecting the column element  31  to the adjacent column element  31  or the pumping unit  2  or the discharge unit  4 , respectively. 
     On top of the discharge unit  4  a drive unit  5  is arranged for driving the impeller  22  of the pump  1 . The drive unit  5  may be for example an electric motor or any other motor. The drive unit  5  is operatively connected to the impeller  22  by means of a line shaft  6  extending in the center of the column pipe  3  and coaxially therewith. The line shaft  6  is supported by a plurality of shaft bearings  61 . Usually adjacent to each connection of two neighboring column elements  31  a shaft bearing  61  is arranged for supporting and guiding the line shaft  6  along its entire axial extension. Depending on the specific application it is possible that an additional tube is arranged around the line shaft  6  extending coaxially with the column pipe  3  and delimiting the line shaft  6  from the remainder of the interior of the column pipe  3 . This measure can be used for example if a contact of the fluid to be conveyed with the line shaft  6  should be avoided. 
     During operation the vertical pump  1  is supported by a supporting structure (not shown in  FIG. 1 ) which is usually supporting the pump  1  in the vicinity and below the discharge unit  4 , for example at a level indicated by arrow B in  FIG. 1 . The lower part of the pump beneath the supporting structure is freely hanging, i.e. without additional support, into the reservoir of the fluid to be pumped. If the fluid is water the reservoir may be a river, a lake, a sea, a sump or a cooling water pool. The sump level which is the level of the fluid surrounding the vertical pump is usually at a given level located between the upper end of the pumping unit  2  and the lower end of the discharge unit  4 . Thus, during operation of the pump at least the pumping unit  2  is completely immersed into the fluid to be conveyed. The impeller  22  driven by the drive unit  5  and the line shaft  6  sucks the fluid through the inlet  21  and conveys it through the column pipe  3  to the discharge unit  4  and to the outlet  41 . 
     In known vertical pumps  1  resonance effects can occur which are detrimental for the pump  1 . In particular, a premature failure of the bearings like the shaft bearings  61  and of the line shaft  6  may be caused by resonance effects. These effects arise for example when the running speed of the pump  1  or the rotational speed of the impeller  22  corresponds to a frequency which is equal or very close to a structural natural frequency. These resonance effects may occur both in already existing and operating pumps  1  and in newly constructed pumps  1 . 
     In order to resolve these resonance problems the present invention proposes a suppressing device for shifting a structural natural frequency of a vertical pump  1  and thus suppressing the original natural frequency. 
       FIG. 2  shows in an exploded perspective view a first preferred embodiment of a suppressing device according to the invention which is designated in its entity with the reference numeral  10 . For a better understanding  FIG. 2  additionally shows a perspective view of a column element  31  of the column pipe  3 . The suppressing device  10  is designed and adapted for closely fitting around the entire circumference of the column element  31 . The suppressing device has an extension L in the axial direction A which is at least half the axial length C of the column element  31  the suppressing device  10  is surrounding. It is preferred as shown by the first embodiment of the suppressing device  10  if the extension L in the axial direction A is equal to the axial length C of the column element  31 , i.e. in the mounted state the suppressing device  10  covers the entire lateral surface of the column element  31  (see  FIG. 5 ). 
     The expression “axial length” with regard to the column element  31  means the distance between the two flanges  32  delimiting the column element  31 , that is the distance between the lower edge of the upper flange  32  and the upper edge of the lower flange  32  of the respective column element  31 . 
     The suppressing device  10  comprises at least one rib  11  extending essentially in a radial direction being perpendicular to the axial direction A. 
     The first preferred embodiment of the suppressing device  10  comprises a plurality and more specifically four stiffening members  12 . The four stiffening members  12  are essentially identical. For a better understanding  FIG. 3  shows one of the stiffening members  12  in a perspective view and  FIG. 4  a cross-sectional view of one of the stiffening members  12  in a section perpendicular to the axial direction A. In addition,  FIG. 5  shows a perspective view of the column element  31  with the suppressing device  10  being mounted on it. 
     Each of the stiffening members  12  is curved in a circumferential direction and connectable to its adjacent stiffening members  12 . The four stiffening members  12  are complementing one another in the assembled state to form a tubular structure having an inner diameter which corresponds or is slightly smaller than the outer diameter D of the column element  31 . Thus, in the assembled and mounted state the four stiffening members  12  are closely fitting around the entire circumference of the column element  31  and generating a clamping effect. By this close fit around the entire circumference of the column element  31  the suppressing device  10  provides for an increase of the thickness of the wall of the column element  31  which in turn stiffens the column element  31  and thus shifts the structural natural frequency to a higher value. 
     As can be best seen in  FIG. 3  and  FIG. 4  each stiffening member  12  has a middle part  121  having a quadrant shaped cross-section as well as two lateral ribs  11  extending in the radial direction. Each of the lateral ribs  11  forms an end of the stiffening member  12  with respect to the circumferential direction. The middle part  121  of each stiffening member  12  extends between the two lateral ribs  11 . 
     The stiffening member  12  may be manufactured by selecting a material, preferably a metallic material, which is compatible with the material of the column element  31 . Starting with a rectangular sheet of the material two parallel end edges of the rectangular sheet are bent until each of them extends perpendicular to the remainder of the sheet. After that the rectangular sheet is bent until the middle part  121  is curved such that it has a quadrant shaped cross section. 
     As best seen in  FIG. 4  the middle part  121  has a curvature that corresponds to a radius R which is predefined by the outer diameter D of the column element  31 . As already said the radius R is most preferred equal to half the outer diameter D of the column element  31 . The stiffening member  12  has a thickness T which is preferably—but not necessarily—uniform over the entire stiffening member  12 . The extension RL of the lateral ribs  11  in the radial direction is preferably the same for both lateral ribs  11 . However it is also possible that the two lateral ribs  11  have different extensions RL in the radial direction. 
     With respect to the axial direction A, it is preferred that both lateral ribs  11  have the same axial length AL ( FIG. 3 ) and that the axial length AL is equal to the extension L of the suppressing device  10  in the axial direction L. Thus, it is preferred that each lateral rib  11  extends in the axial direction A over the entire length of the stiffening member  12 . However, depending on the specific application it is also possible that the axial length AL of the lateral ribs  11  is smaller than the extension L of the suppressing device  10  in the axial direction A. Thus, the middle part  121  of the stiffening member  12  may be longer in the axial direction A than the lateral ribs  11 . This might be advantageous for example in such applications where the spatial circumstances are not sufficient for lateral ribs  11  extending over the entire extension L of the suppressing device  10 . 
     In order to mount the suppressing device  10  to the column element  31  each lateral rib  11  is provided with a plurality of holes  111  for receiving fixing means to rigidly connect adjacent stiffening members  12 . Most preferred the suppressing device  10  is bolt on the column element  31  (see  FIG. 5 ). The holes  111  in adjacent ribs  11  of the stiffening members  12  are aligned such that they can receive screws  13  as fixing members which are secured by means of screw nuts  14 . This measure renders possible a very simple and reliable mounting of the suppressing device  10 . In addition, by bolting the suppressing device  10  on the column element  31  there is no need to weld additional parts to the column pipe  3 . Welding something to the column pipe  3  always includes the danger to distort the column pipe  3  or parts thereof and render these parts unusable. Furthermore, using screws  13  and screw nuts  14  renders possible to tightly fit the suppressing device  10  around the circumference of the column element  31  thus providing the desired clamping effect that stiffens the column element  31 . 
     Whereas the closely fitting stiffening members  12  correspond to an increase in the wall thickness of the column element  31  the lateral ribs  11  extending in the radial direction have the additional effect to change and especially to increase the moment of inertia of the column element  31 . The increase of the moment of inertia shifts the natural frequency of the structure to higher values. 
     The respective dimensions of the suppressing device  10  or the stiffening members  12  for generating the stiffening effect and the increase in the moment of inertia depend on the specific application and have to be determined for the respective application. A preferred method for the determination of appropriate dimensions and locations of the suppressing device  10  is a finite element analysis (FEA). This well-known method is especially suited for the determination of structural natural frequencies of given structures. 
     Thus, using a FEA, the original structural natural frequencies of a vertical pump can be determined. After that the FEA renders it possible to find appropriate parameters for the dimensions of the suppressing device  10 , which results in an efficient increase of the structural natural frequency to move it away from critical values—like the running speed of the pump  1 —thus, suppressing detrimental resonance effects. 
     Of course there are also other suited methods for the investigation and the determination of structural natural frequencies, for example methods based upon simulations or other analytical methods which are as such known to a person skilled in the art. Furthermore, it is possible to use empiric data, historical data of vertical pumps or general knowledge or know-how both alone and in combination with analytical methods or simulations in order to determine appropriate dimensions for the design of the suppressing device  10 . The parameters that can be varied in order to find appropriate dimensions and/or locations for the suppressing device  10  include: the extension L of the suppressing device in the axial direction A, the axial length AL of the stiffening members  12 , the thickness T of the stiffening members  12 , the extension RL of the ribs  11  in the radial direction, the number of the ribs  11 , the number of the suppressing devices  10 , the location of the suppressing device  10 , i.e. to which column element  31  or column elements  31  a suppressing device  10  is mounted. 
     According to an embodiment of the invention, also a vertical pump  1  having at least one suppressing device  10  is proposed. The suppressing device  10  is suited both for already existing pumps  1  to shift structural natural frequencies away from critical values and for newly constructed vertical pumps  1 .  FIG. 6  shows in a more schematic, cross-sectional view a preferred embodiment of a vertical pump  1  according to the invention. In  FIG. 6  the same reference numerals are used as in  FIG. 1  to  FIG. 5  and they have the same meaning as already explained herein before. 
     The vertical pump  1  for conveying a fluid, for example water, comprises the pumping unit  2  with the impeller  22  (not shown in  FIG. 6 ) and the inlet  21  (not shown in  FIG. 6 ) and the discharge unit  4  with the outlet  41 . On top of the discharge unit  4  the drive unit  5  is arranged. The column pipe  3  is vertically extending and connects the pumping unit  2  with the discharge unit  4 . The line shaft  6  extends within the column pipe  3  and operatively connects the drive unit  5  with the impeller  22  in the pumping unit  2 . 
     The column pipe  3  comprises a plurality, here four, column elements  31  being arranged one after another in the axial direction A. 
     According to an embodiment of the invention at least one column element  31  of the column pipe  3  includes a suppressing device  10  which is designed in accordance with the invention. The suppressing device  10  is closely fitted around the entire circumference of the column element  31  and has the extension L in the axial direction A which is at least half the axial length C of the column element  31 . 
     In the preferred embodiment shown in  FIG. 6  each of the four column elements  31  is provided with a respective suppressing device  10 . Each suppressing device  10  is designed in the same manner as explained herein before with reference to  FIG. 2  to  FIG. 5 . Each suppressing device  10  is adapted to the axial length C of the column element  31  it is surrounding, i.e. the extension L of the suppressing device  10  in the axial direction A and the axial length AL of the stiffening members  12  is equal to the axial length C of the column element  31 . The suppressing devices  10  are bolt on the respective column element  31  as already explained and thus the suppressing devices  10  are releasably connected to the column element  31 . The connection between the suppressing device  10  and the respective column element  31  is a tight fit that provides for a clamping effect which stiffens the respective column element  31 . 
     In vertical pumps  1  having a column pipe  3  comprising a plurality of column elements  31  it is not necessary that each column element  31  includes a suppressing device  10 . It is also possible that only one or two or any other number of column elements  31  includes a suppressing device  10 . The number of column elements  31  and the choice of the specific column element  31  or elements  31  that are provided with a suppressing device  10  are depending on the specific vertical pump  1  and/or the specific application. The appropriate number and location of the suppressing device  10  or the suppressing devices  10  may be determined by way of a FEA method or any other method herein before mentioned. It may also be the case that the circumstances, especially the spatial circumstances at the location where the vertical pump is operating, require some restrictions regarding the number and the location of the suppressing devices  10  mounted to the column pipe  3  of the pump  1 . 
       FIG. 7  shows a variant of the stiffening member  12  in a like representation as  FIG. 4 . The variant is illustrated in a cross-sectional view wherein the section is taken perpendicular to the axial direction A. The variant of the stiffening member  12  differs from the stiffening member  12  shown in  FIG. 4  by an additional intermediate rib  15  extending in the radial direction and being arranged on the middle part  121  of the stiffening member  12  between the lateral ribs  11 . The intermediate rib has an extension RL in the radial direction which may be the same as or different from the extension of the lateral ribs  11  in the radial direction. 
     Regarding the extension of the intermediate rib  15  in axial direction A, it is preferred that this extension is the same as the corresponding extension of the lateral ribs  11 . It is preferred when the extension of the intermediate rib  15  in the axial direction A is equal to the axial length AL of the stiffening member  12 . The intermediate rib  15  is preferably fixed to the stiffening member  12  by means of welding. 
     Of course it is also possible to provide more than one intermediate rib  15  on a stiffening member  12  and to combine stiffening members  12  having at least one intermediate rib  15  with stiffening members  12  having no intermediate rib  15 . 
       FIG. 8  is a cross-sectional view of a second embodiment of a suppressing device  12  according to the invention. In the following description only the differences to the first embodiment are explained. The explanations with respect to the first embodiment are also valid in analogously the same way for the second embodiment shown in  FIG. 8 . 
     The second embodiment of the suppressing device  10  is designed as a single part for closely fitting around the entire circumference of the column element  31 . The suppressing device  10  comprises two essentially identical stiffening members  12 , each being designed in a similar matter as the stiffening member  12  discussed with reference to  FIG. 4 . However, in the second embodiment each stiffening member  12  has a middle part  121  having a semicircular cross-section in a section perpendicular to the axial direction A. Analogously to the first embodiment the middle part  121  of each stiffening member  12  extends between the two respective lateral ribs  11 . The two stiffening members  12  are connected to each other in an articulated manner. This is realized by providing a hinge  16  that connects two adjacent lateral ribs  11  belonging to different stiffening members  12 . Thus, the suppressing device can be moved from an open position as shown in  FIG. 8  to a closed position, in which the two lateral ribs  11  that are not connected by the hinge  16  are contacting each other. The lateral ribs  11  are provided with a plurality of holes  111  for receiving fixing means. Preferably the fixing means are screws and screw bolts. 
     In order to mount the suppressing device  10  to the column element  31  (not shown in  FIG. 8 ) the suppressing device is brought into the open position, laid around the column element  31  and afterwards moved to the closed position. Then, the screws  13  (not shown in  FIG. 8 ) are put through the holes  111  and secured by the screw nuts  14  (not shown in  FIG. 8 ) such that the suppressing device  10  is closely fitted around the entire circumference of the column element  31 . 
     As an optional measure the two lateral ribs  11  that are connected by the hinge  16  may also be provided with holes  111  to receive screws  13  that are secured by screw bolts  14 . This measure may improve the clamping effect of the suppressing device  10  and release the hinge  16  from its load. 
     According to a variant of the second embodiment, the two stiffening members  12  are not connected in a hinged manner but separate parts. Thus, the hinge  16  is dispensed with. The two separate stiffening members  12  are then mounted on the column element  31  in an analogous manner as described with respect to the first embodiment. 
     Of course, there are other embodiments possible with a different number of stiffening members  12  complementing one another to a tubular structure in the assembled state, for example the number of stiffening members  12  may be three or more than four. 
     The suppressing device  10  according to an embodiment of the invention is especially suited for retrofitting vertical pumps  1 . 
     According to an embodiment of the invention, the method of retrofitting a vertical pump  1  comprises the steps of providing a suppressing device  10  for shifting a structural natural frequency of the vertical pump  1 , said suppressing device being designed in accordance with the invention, selecting at least one of the column elements  31  of the column pipe  3  and mounting the suppressing device  10  to the column element  31  in a closely fitting manner around the entire circumference of the column element  31 . 
     The tight fitting of the suppressing device  10  provides for a clamping effect that results in a stiffening of the column member  31  or the column pipe  3 , respectively. Also when retrofitting existing axial pumps it is preferred that the suppressing device  10  comprises a plurality of stiffening members  12  and most preferred four stiffening members  12 . Furthermore, it is preferred that the axial length AL of the stiffening members  12  or the extension L of the suppressing device  10  in axial direction A, respectively, is equal to the axial length C of the column element  31 . The stiffening members  12  are laid around the column element  31  and fixed to each other. The preferred means for fixing the stiffening members  12  to each other and to the column element  31  are screws  13  and screw nuts  14 . Thus, the method provides for a very simple, safe and efficient bolt-on solution to resolve resonance problems in vertical pumps. 
     When the column pipe  3  of the axial pump  1  comprises more than one column element  31 , for example four or seven column elements, it is preferred to select a plurality of column elements  31 , for example all column elements  31 , each of which is provided with a separate suppressing device  10 . 
     In order to select the appropriate column element  31  or column elements  31  that shall include a suppressing device  10  as well as to determine appropriate dimensions and designs for the individual suppressing devices  10  it is preferred to use an analytical method and preferably the method of a FEA. As already herein before mentioned, the determination of the specific dimensions and locations of the suppressing device  10  may be based upon other methods, e.g. simulations or other analytical methods which are as such known to a person skilled in the art. Furthermore, it is possible to use empiric data, historical data of vertical pumps or know-how for the determination. 
     The method according to an embodiment of the invention is suited both for resolving resonance problems in already existing and operating pumps and for avoiding resonance problems in newly manufactured pumps. Especially in view of retrofitting existing axial pumps it is advantageous that there is no need for a complete redesign of the vertical pump to overcome resonance issues. The invention provides a solution with a very simple and effective design. This method is very flexible and can be applied to all vertical pumps  1  having a column pipe  3 . The installation of the suppressing devices  10  is very fast and easy. In addition, there is no need for welding parts to the vertical pump  1 . Thus, welding related distortions or other detrimental effects are avoided. 
     Furthermore, the installation of the suppressing device  10  may be performed on site at the location where the vertical pump is running. Since the proposed method comprises a very simple bolt on solution, it is not necessary to disassemble the entire vertical pump or to disassemble the column pipe  3  of the vertical pump  1 . This drastically reduces the costs and the required time for resolving existing resonance problems in vertical pumps. 
     In a specific example where a resonance problem occurred in a vertical pump already in operation because the running speed of the pump was very close to a structural natural frequency of the pump the method according to the invention successfully suppressed the original natural frequency by increasing the natural frequency to a value that is now 15% away from the running speed of the vertical pump.