Patent Publication Number: US-2011064566-A1

Title: Apparatus for impeller sealing in centrifugal pumps

Description:
The invention relates to a device for impeller sealing in centrifugal pumps for fluid media. The device serves for the purpose of sealing the gap that is present between the impeller and the pump housing—and that is due to the construction of centrifugal pumps—against the transport of material from the pressure side of the impeller to the suction side thereof at the impeller intake. It can be used for the initial equipping of generic pumps when they are manufactured, but also can be used for retrofitting pumps which have not previously been provided with a corresponding gap sealing. 
     Centrifugal pumps of different structural forms are used for the transport of fluid media, particularly liquids in very large amounts. The pumps comprise one or more impellers disposed on a shaft, which, when driven by a motor, move in a corresponding free space of a pump housing (also called a housing bore). Due to its rotation, each impeller produces an underpressure on its intake side, where the inflow for the fluid medium to be transported is found. In this way, the medium is broken up at or in the impeller and transported from it by the rotational motion to the impeller outlet on the pressure side of the impeller. Thus, in order for the impeller to be able to move in the pump housing, a gap between the impeller and the inner wall of the pump housing is indispensable. This gap is formed as an axial or radial gap depending on the form of the housing and of the impeller. 
     A portion of the transported fluid, however, inevitably flows back via the gap to the suction side of the impeller. Hydraulic losses occur in this way. Pump manufacturers are thus attempting to keep the gap dimensions as small as possible. On the other hand, certain minimum gap measurements cannot be further reduced, particularly due to tolerances of the components, since if they were, there is the danger that an impeller that may be running in a somewhat eccentric manner, might strike the inner wall of the housing. This danger also increases over the course of the operating time of a pump because deposits form on the impeller, and these deposits can be added on to a gap dimension that is too small, so that the pump finally“seizes up”. Consequently, there occurs a blocking of the impeller and damage to the machine, which may end up in complete failure of the pump. In the individual case, this causes expensive interruptions in operation. The named problems are additionally increased due to other factors. For instance, material damage to the housing and/or to the impeller occurs over the course of pump operation due to cavitation, i.e., due to implosion predominantly of gas bubbles that arise in the transported medium, and, apart from serious damage, such as the breaking of component parts of the impeller, for example, this can also unfavorably influence gap dimensions. In addition, a particular problem occurs with the use of pumps for the transport of wastewater. Due to the contaminants contained in the wastewater in the form of solids and fibrous components, so-called braid formation occurs on the impellers and as a consequence of this, the gap is stopped up by these added formations, whereupon finally, the impeller is also blocked. Due to the pressure difference between the suction side and the pressure side, the gap between the pump housing and the impeller is a particularly critical region, into which the contaminants are actually pulled. 
     Therefore, in pump manufacture, structural measures are sought, by means of which the gap dimensions are kept small or optimized, but in addition, can be kept constant as much as possible over the long-term operation of the pump. Thus, a device is described in DE 199 60 160 A1, for example, for optimizing the gap width in centrifugal pumps. According to the described solution, a beading is formed on the outer periphery of the free end of a pump impeller. When the impeller is inserted into the pump housing, this beading is introduced behind a sealing collar, which is formed on a gap ring disposed in the housing. A very small gap dimension is achieved in this way. Since the sealing collar involves a rigid element according to this solution, however, there is the danger that, since depositions are formed on it, it may run into the impeller above the beading formed on the impeller. Also, due to the remaining small gap, a transport of material from the pressure side to the suction side is not prevented, so that the danger of braid formation and finally the blocking of the gap is also not eliminated. 
     A solution for reducing the width of a radial sealing gap, by means of which an undesired widening of the gap with increasing operating time of the pump will also be prevented to the greatest extent, is described in EP 1 808 603 A1. For this purpose, a rigid wear ring and an elastic ring are introduced into the radial gap between the pump housing and the outer contour of the impeller. Whereas the wear ring, which comprises, for example, hardened cast steel or a ceramic material, surrounds the impeller with a small residual gap remaining, the elastic ring is disposed around the wear ring and is attached to the inner contour of the pump housing. In this way, a type of elastic suspension is formed for the wear ring acting as the gap ring. Oscillations of the impeller, as they occur primarily when the pump is started up, are especially intercepted by means of this elastic suspension, and thus a damaging of the gap ring or of the outer contour of the impeller is prevented. Further, the gap ring and the inner contour of the pump housing are thermally decoupled from each other in this way. An optimizing of the gap is provided by this solution, which makes it possible to constantly maintain the gap dimension, even over a rather long operating time of the pump with compensation of dimension tolerances of the pump parts and of oscillations that occur in the operation of the pump. In each case, however, in this solution, a residual gap also remains, in which there additionally exists the danger of a braid formation or tressing, in particular, with the use of the pump for transporting wastewater. Therefore, pumps designed corresponding to this solution are suitable for use as wastewater pumps only under certain conditions. 
     A solution in which a nearly complete sealing of the gap is achieved by means of several elastic elements that are pressed by screws and springs to contour segments of the impeller is described in U.S. Pat. No. 2,109,679 A. The arrangement, however, is relatively complicated, and since it is not self-sealing without further measures, due to a friction wear occurring on the elastic elements during the operation of the pump, the remaining minimum residual gap would be enlarged very rapidly. Also, the regions in which the springs are disposed are themselves not sealed. In this way, deposits may be formed on the springs and block them, in particular, when the pump is used with wastewater, so that they no longer can exercise the pressure necessary for extensively sealing the gap on the elements pressing against the impeller. Further, there is the danger of a contraction of the elastic elements that are applied to the impeller and in part have somewhat large surface regions into the corresponding bearing surfaces, whereby it is also expected that the sealing will become ineffective after a relatively short operating time of the pump. Finally, radial and axial oscillations of the impeller, as they always occur when the pump is operating, are not equilibrated by the sealing system. As a consequence of this, in general, the gap could in fact almost be sealed, but in each case, particularly with axial oscillations of the impeller, the gap could ocasionally be opened further. In the case of a repeated brief opening of the gap, however, deposits are unavoidably formed in the gap region, particularly when the pump is used for transporting wastewater, which, over the course of time, lead to the braid formation, which has already been mentioned, and finally to the blockage of or even damage to the pump. 
     A more favorable solution, particularly due to its simple construction, is described by DE 196 13 486 C2. According to this solution, when the pump is operating, an elastic sealing ring held in recesses of the pump housing in the gap region thereof slides along a front cover plate of the impeller. However, as is even mentioned in the document, the device does not lead to a sealing of the gap, but rather to an extremely small gap dimension, whereby this dimension would increase in a relatively short time due to abrasion and a wear of the sealing ring that is caused thereby. With respect to the fact that radial and axial oscillations of the impeller also cannot be equilibrated in this solution due to the type of arrangement of the sealing ring in the pump housing, the described arrangement is also not suitable for application in wastewater pumps. There also exists here the danger of a rapid deposition of solids or fibrous contamination in the region of the sealing ring and of its edge resting on the impeller, together with the danger of braid formation in the gap region. 
     The object of the invention is thus to create a device for impeller sealing in centrifugal pumps, by means of which the transport of material between the pressure side of the impeller of a centrifugal pump and its impeller intake is reliably and permanently prevented with a very simple construction of the device, but an unhindered rotational movement of the impeller or impellers in the pump housing is made possible. This type of transport of material and thus braid formation in the gap region, particularly in the case of application in wastewater pumps, will be prevented by the device to be created. In addition, the device will make possible the equipping of pumps in the factory as well as retrofitting pumps already produced. 
     The object is accomplished by a device with the features of the principal claim. Advantageous embodiments or enhancements are given by the subclaims. 
     As has already been mentioned initially, the device according to the invention can be used both in centrifugal pumps with an axial gap as well as in those with a radial gap, depending on the geometry of their elements and of their structural arrangement in the pump. Further, both one-stage as well as multi-stage centrifugal pumps, i.e., centrifugal pumps with more than one impeller, can be equipped with the device according to the invention either in the factory or as the object of a retrofitting. 
     In order to accomplish the object according to the invention, a sealing ring is disposed between the pressure side and the suction side of the at least one impeller in the pump housing of a centrifugal pump, which is either equipped in the factory or retrofitted with this device, this sealing ring being composed of a rigid pressure ring and an annular elastic sealing lip joined with the pressure ring. The sealing lip is joined with the pressure ring in the region of its inner diameter. In a region projected on by the pressure ring or/and projecting over the pressure ring, with respect to its diameter, the sealing lip is inclined against a sliding surface formed on the impeller at its peripheral contour in the region of the impeller intake. According to the invention, an adjusting member on the pump housing acts on the sealing ring formed as described above. The sealing ring is positioned relative to the sliding surface by this adjusting member, and at the same time, its sealing lip is pre-stressed against the sliding surface of the impeller in the region projected on by the pressure ring or/and projecting over the diameter of the pressure ring. The sealing ring is thus positioned in such a way and its sealing lip is pre-stressed such that it is applied to the sliding surface by a radially outer end of the above-named region when the pump is in operation. The adjustment to be carried out at the adjusting member can be such that the sealing lip is applied only slightly (very small pre-stressing) to the corresponding sliding surface of the impeller when the pump is in its resting state, while the application of the radially outer end of the sealing lip to the sliding surface is reliably assured by the pressure difference existing additionally between its pressure side and the suction side when the pump is in operation. The sealing device is thus configured such that, due to the deformability in a length sufficient for this purpose of the region of the sealing lip projected on by the pressure ring or/and projecting over it to a sufficient extent relative to its diameter, and based on the pre-stressing produced in the sealing lip by means of the adjusting member, a permanent as well as a self-adjusting complete sealing of the gap is provided. In an advantageous manner, due to the possible large-scale deformation of the elastic region projecting over the pressure ring or/and projected on by it, the radial and axial oscillations occurring during pump operation are also equilibrated so that they do not lead even to only a brief opening of the gap. In new pumps equipped with a corresponding gap seal or in pumps freshly retrofitted with the corresponding gap seal, the sealing ring is thus adjusted by means of the adjusting member with respect to its position and the pre-stressing of its sealing lip in such a way that the radial outer end of the sealing lip very reliably contacts the wear ring with pre-stressing that is as small as possible. 
     According to one possible embodiment of the invention, the sliding surface is part of a rigid wear ring attached in the region of the impeller intake on the peripheral contour of the impeller. Said wear ring can thus be disposed on the impeller in a pump equipped with the device according to the invention at the factory or it can be attached later by a corresponding reworking of the impeller as the object of a retrofitting. The wear ring comprises a particularly wear-resistant material, such as stainless steel/cast iron, ceramic or a special composite material, or it is provided at least with a special wear-resistant coating. It is also conceivable, of course, to configure at least the sliding surface of the impeller in a particularly wear-resistant manner by a suitable surface treatment or coating of the material or to form the impeller overall from a corresponding wear-resistant material, but this optionally leads to increased costs. Particularly in connection with retrofitting, however, in practice, the wear-resistant sliding surface is to be realized preferably by introducing a wear ring onto the impeller in the region of the impeller intake. 
     As already mentioned, the sealing ring is realized by a rigid pressure ring and an elastic sealing lip joined to it, whereby the pressure ring also is composed of a wear-resistant material, preferably metal. In the case of a metal pressure ring, according to a preferred embodiment of the invention, the sealing lip is composed of an elastomer and is joined with the pressure ring by vulcanization. In this way, the invention can be enhanced advantageously still further due to the fact that a reinforcing material, preferably a metal wire mesh, is embedded in the elastomer for mechanical stabilizing. It is also possible to vulcanize a metal washer in the elastomer for stabilization. 
     The geometry of the elastic sealing lip, preferably composed of an elastomer or of rubber is advantageously configured such that its thickness decreases radially toward the outside. This configuration of the geometry of the sealing lip and the measures for its mechanical stabilization, which have been explained above, serve for the purpose that the sealing lip is applied in a sliding manner to the wear ring or to the sliding surface, respectively, only by its radial outermost region according to the basic concept of the invention, even when there are large pressure differences between the pressure side and the impeller intake side. It has been shown that without appropriate measures, due to the high pressure acting on the sealing lip, the danger exists that the sealing lip will be bent or snapped off and in this way would come to rest with a large surface region on the wear ring, whereupon the danger is increased that the sealing lip would enter further into the sliding surface over the course of time or would work into the wear ring. Due to the pressure difference that exists between the pressure side and the impeller intake on the suction side, however, it is nevertheless assured that the sealing lip will be applied by sliding on the sliding surface of the impeller associated with it as well as completely sealing the gap, at least when the centrifugal pump is in operation. 
     As has already been mentioned, due to the pre-stressing of the elastic sealing lip against the sliding surface, the gap seal, insofar as it is self-adjusting, is continually applied more closely to the sliding surface as the sealing lip, even if it is gradually abraded over the course of operation, due to the pressure difference between the pressure side and the suction side provided during operation, up to a certain extent of wear, when the pump is in operation. A complete sealing of the gap during the operation of the pump, by means of which a deposition of solids and/or of fibrous components contained in the transported medium in the gap region and thus a blockage due to braid formation when it is applied in wastewater is effectively prevented, can then also be provided in this case, if, when the pump is turned off, the sealing lip is no longer applied to the sliding surface, due to the wear of material that has occurred. 
     According to the basic concept of the solution, the pre-stressing in the elastic sealing lip of the sealing ring is achieved by suitable positioning of the sealing ring in the pump housing or positioning relative to the sliding surface, whereby the sealing ring is supported on an abutment present inside the pump housing so that a certain pressure is exercised on its sealing lip. Insofar as the solution according to the invention can also be realized optionally by a gap ring that is already present and fixed in the pump housing acting as an abutment, as is usual for the adjustment of a pre-determined gap width in pumps of the prior art, after an appropriate retrofitting according to the invention, this gap ring is joined by mechanical fastening or vulcanization with a sealing lip inclined against a wear ring of the impeller or the sliding surface thereof, which has also been retrofitted. 
     According to a practical embodiment of the invention, preferably and particularly provided for use in one-stage centrifugal pumps with an axial gap, the adjusting member acting on the sealing ring is formed as a collar that can be adjusted in the axial direction with respect to its position, and it acts directly on the sealing ring. In this embodiment, at least when the centrifugal pump is operating, the sealing lip is applied in a sliding manner to an outer surface of the wear ring, which is disposed preferably orthogonal to the axial direction or pump axis, whereby the axial direction is given by the direction of the lengthwise extension of the motor shaft driving one impeller or optionally several impellers. Corresponding to one configuration of this embodiment that is provided, the position of the collar can be adjusted by means of two or more adjusting screws disposed approximately equally distributed relative to the periphery of the collar, and each of these screws is engaged with a thread extending in the pump housing in the axial direction. The threads for the adjusting screws are thus preferably formed in threaded bores, which are guided through the pump housing in the axial direction. 
     In particular, when pumps are retrofitted with the gap seal according to the invention, however, it may be necessary to insert into the pump housing an additional housing ring applied to the collar on the side opposite to the sealing ring. This is the case particularly in pumps which have a relatively large housing bore or housing chamber, respectively, on the intake side of the impeller. In these pumps, it is not possible, without the placement of the additional housing ring, to position the adjusting screws in such a way that the collar acts on the sealing ring at an appropriate place, namely, in particular, in the region of the sealing lip projecting over the rigid pressure ring of the sealing ring or/and the region projected on by it. In order to make this possible, said housing ring, which has several circular segment-shaped recesses on its periphery, is inserted. Each of the threads for the adjusting screws of the adjusting member here are in part formed in the circular segment-shaped recesses disposed on the outer periphery of the housing ring and in part in annular recesses of the pump housing disposed corresponding to these first recesses of the housing ring and preferably enclosing together with these a full circle in each case. The housing ring is simultaneously fixed in the pump housing by the adjusting screws engaged with the threads. 
     In a particularly advantageous embodiment of the invention, the collar has an outer thread and is engaged with an inner thread of the pump housing that is correspondingly disposed for this purpose. The axial position of the collar can thus be adjusted inside the centrifugal pump by turning the collar in the inner thread of the pump housing. Independently of which of the above-described embodiments are used for the collar and/or the means serving for the positioning thereof or for establishing the contact force acting on the sealing ring, according to an advantageous enhancement, the device according to the invention has set screws, by means of which the collar can be fixed in the place and position adjusted for it each time. 
     As has already been mentioned several times, the device according to the invention is suitable for both equipping centrifugal pumps in the factory as well as for retrofitting them. Therefore, the device according to one embodiment provided for retrofitting is designed in such a way that its respective elements are components of a kit, in which is also included a drilling template or a hole gauge for correct positional arrangement of the threaded bores to be formed for the adjusting screws and optionally for the set screws. 
    
    
     
       The invention will be explained in more detail below on the basis of embodiment examples. In this case, the explained examples concern variants that are particularly suitable for retrofitting pumps that have not been provided previously with a corresponding device. The following are shown in the appended drawings: 
         FIG. 1 : a centrifugal pump retrofitted with a device according to the invention corresponding to one possible embodiment of this device, 
         FIG. 2 : an enlarged excerpt of the pump according to  FIG. 1  with the device for impeller sealing, 
         FIGS. 3   a ,  3   b : the housing ring additionally inserted into the pump housing according to the embodiment according to  FIG. 1  and  FIG. 2 , 
         FIG. 4 : another basic embodiment of the invention, with a specially configured collar. 
     
    
    
       FIG. 1  shows by way of example a centrifugal pump retrofitted with the device according to the invention, wherein the pump is shown with a cut-out in a pump housing  1  for better illustration. The example shown relates to the retrofitting of a centrifugal pump, in which an impeller  2  is designed in the form of a channel impeller. This embodiment as well as the other embodiment explained in connection with the drawings refers to a design, in which the annular elastic sealing lip  8  joined to pressure ring  7  and forming the sealing ring together with this is pre-stressed by means of a collar  9  acting on the sealing ring, and sliding surface  6 ′ is formed on a wear ring  6  attached to impeller  2 . 
     The pump is composed of pump housing  1  in which impeller  2  or the channel impeller, which is disposed on a motor shaft that is not shown here, moves in the peripheral direction u when the pump is in operation. The motor, which is also not shown here, is flange-mounted onto pump housing  1  on the right side of the pump in the drawing, so that the motor shaft extends in the axial direction a. The gap which is to be sealed is formed between pump housing  1  and impeller  2 , and this gap involves an axial gap based on the structural form of the pump shown in the example. This gap is completely sealed by means of the device according to the invention, which is still better illustrated in the enlarged excerpt according to  FIG. 2 , against transport of material between the pressure side  3  of the pump and the impeller intake  5  on the suction side  4  of the pump. 
       FIG. 2  shows an enlarged excerpt of the centrifugal pump according to  FIG. 1 , which has been retrofitted with the device for impeller sealing according to the invention, in a sectional view. A shoulder has been formed for the retrofitting by working the outer contour of impeller  2 . A highly wear-resistant wear ring  6  provided with a complementary step has been applied to this shoulder and attached by means of several screws to impeller  2 . For this purpose, threaded bores corresponding to the counter-bores of wear ring  6  are introduced into impeller  2 , distributed on the periphery. In order to reduce the relatively large diameter of the housing bore for the pump shown in the example on the side of impeller intake  5 , a preferably metal housing ring  11  has been inserted additionally into pump housing  1  of this pump, and the design of this housing ring is illustrated in  FIGS. 3   a  and  3   b . Housing ring  11  has several annular recesses  12 , each of which is provided with a first part of a thread and, together with the corresponding recesses worked into pump housing  1  and provided with the respective second part of the thread, each enclose a full circle or circular cylinder. Each of adjusting screws  10  is engaged with the inner threads formed in part in recesses  12  of housing ring  11  and in part in the recesses of pump housing  1 . Collar  9  can be moved in the axial direction a by means of adjusting screws  10  that can be adjusted with an Allen wrench. Adjusting screws  10  in this case serve simultaneously for fastening housing ring  11  in pump housing  1 . Collar  9 , which is provided with shoulders, presses against the flexible region  8 ′ of sealing lip  8  of the sealing ring disposed between collar  9  and wear ring  6 , whereby the compressive force by which sealing lip  8  is pressed against wear ring  6 , is dependent on the position of collar  9 , which can be adjusted by means of adjusting screws  10 . The position of collar  9  that is adjusted each time, and that can be re-adjusted if necessary, is fixed by means of set screws  15 , which are formed in the present case as tension screws, and which are guided through an inner bore of adjusting screws  10  and engaged each time with a threaded bore in collar  9 . In this way, the heads of set screws  15  that are disposed in captive manner come to rest on a shoulder of the inner bore of the respective adjusting screw  10 . 
     The sealing ring is composed of a rigid metal pressure ring  7  to which is screwed the annular sealing lip  8  comprised of rubber in the example shown. However, a joining of rigid pressure ring  7  with sealing lip  8  by vulcanization is more advantageous. Sealing lip  8  is subjected to a pre-stressing by collar  9  in such a way that it is applied to wear ring  6  only by the outer end  8 ′ (relative to its periphery) of its flexible region  8 ′ projecting over pressure ring  7 , as can be seen from the figure, when the pump is turned off. When the pump is operating, sealing lip  8  is pressed more intensely on wear ring  6  with complete sealing of the axial gap based on the pressure difference existing between pressure side  3  of the pump and the impeller intake  5  of suction side  4 . Based on its elasticity, sealing lip  8 , however, is applied only in sliding manner on wear ring  6  by outer end  8 ′ of region  8 ′, so that the rotational movement of impeller  2  of the pump is additionally made possible. Based on the pre-stressing introduced on sealing lip  8 , its sliding contact with sliding surface  6  continually remains, even with axial and/or radial oscillations of impeller  2  that occur and are caused by pump operation. Therefore, a reliable and above all permanent, self-adjusting, complete seal of the gap between pump housing  1  and suction side  4  of impeller  2  or impeller intake  5  is provided despite possible oscillations. With the application of a centrifugal pump furnished with the device according to the invention as a wastewater pump, a braid formation in the gap region and thus a later blockage of the pump are reliably prevented in this way. 
     As long as the inner diameter of pump housing  1  is not too large, differing from the previously explained example of embodiment, collar  9  can also be inserted directly into pump housing  1  without an additional housing ring  11 . Adjusting screws  10  are guided through axially running threaded bores in pump housing  1 . In this way, in the case of equipping centrifugal pumps with the device according to the invention carried out in the factory, the embodiment without additional housing ring  11  is preferred, of course. The representation of the embodiment shown in  FIG. 1 to 3   a  or  3   b , respectively, however, will serve for the purpose of showing that even pumps already in use can be well retrofitted with the device according to the invention. 
     Another embodiment of the device according to the invention is shown in  FIG. 4  in a representation comparable to  FIG. 2 . In this embodiment, collar  9  and thus the sealing ring are positioned, and the contact force acting on the sealing ring made of pressure ring  7  and sealing lip  8  is established by means of an outer thread  13  disposed on the front side of collar  9 . This thread engages in a corresponding inner thread  14  formed in pump housing  1  (optionally. i.e., in the case of retrofitting, by a corresponding re-working of pump housing  1 ). By turning in inner thread  14 , the axial position of collar  9  can be adjusted corresponding to the requirements, i.e., application of the radially outer end  8 ′ of the region  8 ′ of sealing lip  8  projected on by pressure ring  7 , with pre-stressing on wear ring  6  as small as possible. In the corresponding set position, collar  9  can be attached by means of set screws  15  (one or more set screws  15  is or are disposed in the outer region of the collar), and these screws act as clamping screws and brace the sides of threads  13 ,  14  of collar  9  and of pump housing  1  against one another. For sealing, an O-ring  16  made of rubber is disposed between pump housing  1  and collar  9 . 
     By means of the device according to the invention, a clear increase in efficiency is achieved for appropriately equipped centrifugal pumps, due to the complete prevention of backflow, i.e., backflow from pressure side  3  to suction side  4  in impeller  2 , and by the elimination of cross-current losses. Suction-side blockages are effectively prevented, particularly in wastewater pumps with one or more channel impellers. The operating, energy and repair costs with the use of centrifugal pumps are reduced thereby. The device is also characterized by a simple construction and can be well retrofitted in pumps already in use. Due to the automatic equilibration of the structural tolerances of the components and/or those occurring as a consequence of wear, the proposed solution is also advantageously self-adjusting and thus permanently self-sealing. 
     LIST OF REFERENCE NUMBERS 
     
         
           1  Pump housing 
           2  Impeller 
           3  Pressure side 
           4  Suction side 
           5  Impeller intake 
           6  Wear ring 
           7  Pressure ring 
           8  Sealing lip 
           8 ′ Region of the sealing lip 
           8 ″ End (radial outer end of the sealing lip) 
           9  Adjusting member, for example, collar 
           10  Adjusting screw 
           11  Housing ring 
           12  Recess 
           13  Outer thread 
           14  Inner thread 
           15  Set screw 
           16  O-ring 
         a Axial direction 
         u Peripheral direction