Abstract:
A pump apparatus includes at least two main delivery cylinders, a delivery pipeline for conducting viscous material in a flow direction, and a switchable gate mechanism which is moveable between a first position to establish a connection between an outlet port of a first one of the delivery cylinders and the delivery pipeline, and a second position to establish a connection between an outlet port of a second one of the delivery cylinders and the delivery pipeline. Disposed downstream of the gate mechanism is a compensating cylinder having an outlet port for discharge of viscous material into a section of the delivery pipeline. A shut-off valve is disposed in the delivery pipeline in flow direction upstream of the outlet port of the compensating cylinder and constructed in the form of a two-port rotary gate valve.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application claims the priority of German Patent Applications, Serial Nos. 10 2005 008 938.0, filed Feb. 26, 2005, and 10 2005 031 194.6, filed Jul. 1, 2005, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates in general to a pump apparatus, and method for conveying a viscous material.  
         [0003]     Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.  
         [0004]     Pumps of a type involved here are used in the field of concrete conveyance for almost 80 years and operate mechanically by means of crank mechanism, i.e. crankshaft and connecting rod, with a single cylinder. The cubic capacity attainable by this type of crank mechanism is slight because of the very limited piston stroke, when applied for piston diameters of about 200 mm that are still predominantly being used to date. As a consequence, the so-called pump gate, i.e. the valve system, which alternatingly connects the pump cylinder(s) with a concrete reservoir and a delivery pipeline, wear off substantially. Even today, the gate mechanism still represents the most important and most critical component of the concrete pump, save for the typical concrete distributor booms which increasingly become longer in size, thereby increasing manufacturing costs which depending on the boom length can significantly surpass the costs for the actual concrete pump.  
         [0005]     The gate mechanism still represents the single most wearing part of the concrete pump. In view of the ever increasing demand for greater stroke volumes at piston strokes of up to 2500 mm that became feasible through application of respective hydraulic drives as early as about 1950, the switching frequency per conveyed cubic meter of concrete dropped off to a fraction. The gates which necessarily also assumed the function of rock crusher so as to be able to reach their end position, wear off predominantly during the switching operation and less during the subsequent throughflow of concrete.  
         [0006]     U.S. Pat. No. 5,316,453 to Schwing describes a slurry pump having two main delivery cylinders driven by a hydraulic cylinder for alternatingly conducting concrete via a transfer tube to a delivery pipeline. The transfer tube includes a through channel which creates a fluid communication between the outlet of one main delivery cylinder and the delivery pipeline in a first switching mode and a fluid communication between the outlet of the other main delivery cylinder and the delivery pipeline in a second switching mode. Disposed in a movement plane of the inlet port of the transfer tube on both sides thereof are gate valve disks which bar a return flow of concrete into the reservoir during the switch-over phase from pumping phase to intake phase of the respective main delivery cylinder. The gate valve disks also permit a compression stroke for compacting concrete in a main delivery cylinder against the gate valve disks at the start of the pump stroke. A compensating cylinder maintains a flow in the delivery pipeline during the switching phase. Practice has shown that the surfaces of the gate mechanism which slide on one another undergo substantial wear and the forces necessary to overcome the friction resistance during switching operation are high.  
         [0007]     U.S. Pat. No. 6,450,779 to Schwing discloses a pump which prevents a differential pressure on the sliding sealing surfaces during switch-over. The parts of the switching system are subjected during shifting motion either to no load by the concrete pressure (zero pressure at a shut-off valve in the suction line) or are acted upon all-round (outside and inside) by the concrete pressure (balanced pressure in intake swivel pipe in the pressure housing). This type of pump has many shortcomings. The housing must be sized large enough to accommodate the swivel pipe together with the entire concrete content and to allow their rotation, and moreover must be heavy enough because it is under the delivery pressure. In addition, there is only little space available for the flow paths of pumped concrete past the concrete under pressure at the swivel pipe. Thus, on its way from both ports of the delivery cylinders through the housing until exiting into the delivery pipeline, the concrete flow has to undergo several directional changes that are defined by narrow radii of curvature. Depending on the coarseness of concrete, there is a risk of encountering very high flow resistance which could lead to stoppage. This is especially true, as is the case here, when concrete has to flow past standing concrete. When deflected, substantial friction is caused compared to a flow along a pipe wall. Another drawback is the fact that the swivel pipe cannot be sealed against the cylinder ports by a so-called self-adjusting ring which is constructed to automatically compensate for wear.  
         [0008]     In addition, the center distance of the main delivery cylinders is too great for a typical installation between length beams of a truck frame, or a substantial swivel angle of e.g. 250° must be accepted instead of about 70° in the case of discontinuous pumps. A hydraulic drive for such a great angle is bulky, heavy, and expensive and consumes much energy and time for the switching operation so that the already brief time available for the intake operation is even further shortened.  
         [0009]     It would therefore be desirable and advantageous to provide an improved pump apparatus which obviates prior art shortcomings and which is universally applicable for viscous materials to produce a continuous product flow while subjecting moving parts to little wear.  
       SUMMARY OF THE INVENTION  
       [0010]     According to one aspect of the present invention, a pump apparatus for conveying a viscous material includes at least two main delivery cylinders for conveying a viscous material, a delivery pipeline for conducting the viscous material in a flow direction, a switchable gate mechanism moveable between a first position to establish a connection between an outlet port of a first one of the delivery cylinders and the delivery pipeline, and a second position to establish a connection between an outlet port of a second one of the delivery cylinders and the delivery pipeline, a compensating cylinder disposed downstream of the gate mechanism and having an outlet port for discharge of the viscous material into a section of the delivery pipeline, and a shut-off valve disposed in the delivery pipeline in flow direction upstream of the outlet port of the compensating cylinder and constructed in the form of a two-port rotary gate valve.  
         [0011]     The present invention resolves prior art problems by using a compensating cylinder to decrease the drop of the delivery pressure and the delivery flow in the downstream delivery pipeline to the compensating cylinder during the switching operation or to maintain the delivery pressure and delivery flow. The provision of the shut-off valve in the delivery pipeline upstream of the outlet port of the compensating cylinder enables the pump apparatus to prevent a return flow of viscous material pumped by the compensating cylinder into the delivery pipeline so that the viscous material pumped into the delivery pipeline effectively maintains the delivery pressure and delivery flow.  
         [0012]     To ensure clarity, it is necessary to establish the definition of several important terms and expressions that will be used throughout this disclosure.  
         [0013]     The term “viscous material” is used here in a generic sense and the principles described in the following description with respect to concrete are equally applicable to any other type of viscous material, in particular those used in the food industry.  
         [0014]     The term “gate mechanism” is used here in a generic sense and may involve even a system in the absence of an actual slider in a conventional sense so long as the switching modes or connection modes are realized. Thus, a gate mechanism may also include transfer tubes, for example.  
         [0015]     The term “zone of spherical shape” relates to any surface configuration which resembles at least a surface part of a sphere, including conical configuration.  
         [0016]     The term “two-port rotary gate valve” as used in the disclosure relates to a shut-off valve with two switching modes (open/closed) and has a flow cross section which can be opened or closed by turning a swivel body. The rotary motion can suitably be implemented via a sealed stem from outside the valve housing. The swivel body may also have the shape of a disk (rotary flat gate valve).  
         [0017]     The provision of a two-port rotary gate valve as shut-off valve enables realization of a constant volume of the flow rate in the delivery pipeline, even during switch-over of the shut-off valve. Thus, the swivel body does not execute a positive or negative pump function as a result of its switching operation. Another advantage of the application of a two-port rotary gate valve is the complete disposition of the swivel body in the delivery pipeline in the absence of any movement to a lateral space. There is no translatory motion out of the delivery pipeline. Sealing of the rotary motion of a drive shaft can be realized without any problem. In addition, the two-port rotary gate valve is easy to manufacture and has only few components that can easily be controlled.  
         [0018]     A pump apparatus according to the present invention enables a change of the pressure level upstream of the closed shut-off valve in relation to the delivery pressure downstream of the shut-off valve. As a result, the switching operation of the gate mechanism can be implemented in relation to the delivery pressure in the delivery pipeline at a different, preferably lower, pressure level of the viscous material. The pressure level of the viscous materials between a reservoir and the main delivery cylinders and the upstream part of the delivery pipeline between the main delivery cylinders and the shut-off valve can thus be equalized. Normally, the viscous materials surround components of the switching gate mechanism or are situated within the components of the gate mechanism. When realizing a like pressure level of the viscous materials or even lowering the pressure level of the viscous materials, these components of the gate mechanism can be moved easily. Likewise, the shut-off valve can be operated easily. As the concrete volume upstream of the shut-off valve is suitably compressed, to a same pressure level anteriorly of the port of the shut-off valve as in the delivery pipeline downstream of the shut-off valve, equalized pressure is realized during switching operation. Thus, switching takes place under similar favorable condition as the operation of the gate mechanism which is able to switch absent any pressure in the flow medium. This is realized by the shut-off valve and there is no risk of return flow of viscous material from the delivery pipeline when the pressure drops.  
         [0019]     The use of two main delivery cylinders is currently preferred and is applicable in concrete pumps installed on trucks because the two main delivery cylinders can be arranged in inclined relationship to the horizontal and guided through the carrier frame of the truck. Thus, existing trucks can be retrofitted, allowing even the use of existing components such as main delivery cylinders, existing gate mechanism, and the essential parts of the delivery pipeline and the distributor boom. Of course, a pump apparatus according to the present invention may also be constructed with more than two main delivery cylinders.  
         [0020]     According to another feature of the present invention, the gate mechanism may be constructed to connect the second delivery cylinder with a reservoir, when assuming the first switching position, and to connect the first delivery cylinder with the reservoir, when assuming the second switching position. Thus, as a result of the alternating operation, in the first switching mode the first main delivery cylinder feeds viscous material into the delivery pipeline, while the second main delivery cylinder draws viscous material from the reservoir, whereas in the second switching mode the second main delivery cylinder feeds viscous material into the delivery pipeline, while the first main delivery cylinder draws viscous material from the reservoir. The gate mechanism may hereby be constructed in the form of a transfer tube. As a consequence of this construction, the one main delivery cylinder that is not in pumping mode is able to draw viscous material from the reservoir while the other main delivery cylinder is in pumping mode to feed viscous material into the delivery pipeline.  
         [0021]     The gate mechanism may be constructed of several parts. For example, the gate system may include separate slides which respectively open the connection of the first main delivery cylinder to the delivery pipeline, while closing the respective port of the second main delivery cylinder. A further slide closes the connection of the first main delivery cylinder to the reservoir at the same time and opens its connection to the delivery pipeline. Currently preferred is however a gate mechanism using a transfer tube. An example of a transfer tube is described in U.S. Pat. No. 4,373,875 to which reference is made herewith and the entire specification and drawings of which are expressly incorporated herein by reference. Other possible constructions of a gate mechanism for incorporation in a pump apparatus according to the present invention are described in German Offenlegungsschriften DE 26 32 816, DE 21 62 406 and DE 1 278 247, to which reference is also made herewith. Of course, any of the gate systems used in discontinuously operating pump devices for viscous materials, such as concrete, may be used as well.  
         [0022]     The compensating cylinder through which the delivery flow circulates may be constructed to have an inlet port separate to the outlet port. Other examples of a compensating cylinder for incorporation in a pump apparatus according to the present invention are described in U.S. Pat. No. 3,663,129 or U.S. Pat. No. 3,963,385 or U.S. Pat. No. 5,316,453, to which reference is made herewith.  
         [0023]     According to another feature of the present invention, the shut-off valve may be disposed in flow direction downstream of the gate mechanism. Although an arrangement of the shut-off valve within a switchable gate mechanism, e.g., a transfer tube, is conceivable, a construction of the shut-off valve separate from the gate mechanism and downstream of the gate mechanism is simpler to implement.  
         [0024]     The shut-off valve is intended to completely prevent a return flow or at least limit the amount of viscous material being conveyed back in opposition to the desired flow direction as a result of the pump stroke of the compensating cylinder. Depending on the configuration of the pump apparatus and the gate mechanism which may have Y-shaped pipes with branches routed to the main delivery cylinders, several shut-off valves may be provided in the various branches for example. Regardless whether one or more shut-off valves are provided, care should be taken to separate the conveyance of viscous material as maintained by the compensating cylinder from the area of the pump apparatus where no conveyance takes place as a result of the switching operation.  
         [0025]     Suitably, the shut-off valve may be controlled by a control circuit (hydraulic, pneumatic, electric or other manner) used separately for control of the main delivery cylinders and/or switching of the gate mechanism.  
         [0026]     According to another feature of the present invention, the gate mechanism may be provided with a self-adjusting ring in an area of its inlet port for connection to the outlet ports of the first and second delivery cylinders. A self-adjusting ring of a type involved here is described for example in U.S. Pat. No. 4,465,441, to which reference is made herewith, and involves a cutting ring which is supported by a flexible rubber ring.  
         [0027]     According to another feature of the present invention, the shut-off valve may have a casing and a valve body, wherein the casing is provided with an area disposed in surrounding relationship to a port of the shut-off valve and constructed to have a spherical shape, with the valve body being swingable about a pivot axis and constructed to have a zone of spherical shape complementing the spherical shape of the casing area. The casing area in surrounding relationship to the port of the shut-off valve includes also regions which are formed by components installed in the casing, such as, e.g., the region formed by at least a portion of the valve body confronting surfaces of a self-adjusting ring which is placed in the casing. In this way, the casing itself may for example have a cylindrical configuration, and the zone of spherical shape may be formed by the self-adjusting ring placed in the cylindrical wall surface of the casing.  
         [0028]     For proper operation of the shut-off valve, a pipeline guided through the valve body should be brought into registration with the port of the shut-off valve in a switching situation. The shut-off valve is then open. Closing of the shut-off valve requires a sealing of the port by surface zones of the valve body. The complementing zones of spherical shape between the shut-off valve casing and the valve body permit easy swiveling of the valve body and use of a rotation symmetrical self-adjusting ring. As only part of the valve body and the shut-off valve casing have zones of spherical shape, those portions of the valve body and shut-off valve that do not contribute to a sealing of the port may be configured in any form or shape. Surface finishing for effecting a proper seal can thus be limited to the zone of spherical shape. In particular, when the zone of spherical shape is small, such as for example the valve body confronting surface of a self-adjusting ring, the advantages achieved by a surface having a zone of spherical shape can also be attained, even when the surface only resembles a spherical shape, like, e.g., a straight conical shape, because any adverse effect is negligible. Currently preferred is however a spherical shape.  
         [0029]     According to another feature of the present invention, a self-adjusting ring may be provided in surrounding relationship to the port of the shut-off valve. This self-adjusting ring may be configured as described above and referred to in U.S. Pat. No. 4,465,441, with the difference residing in the disposition of the self-adjusting ring on the casing instead of a disposition on the sides of the moving element (valve body).  
         [0030]     Suitably, the self-adjusting ring has a surface in confronting relationship to the valve body, with the surface of the self-adjusting ring formed with a zone of spherical shape to complement the zone of spherical shape of the valve body.  
         [0031]     According to another feature of the present invention, the casing and the valve body may be flattened in a plane perpendicular to the pivot axis. In this way, the shut-off valve may exhibit a slender configuration.  
         [0032]     According to another feature of the present invention, the compensating cylinder may include a displaceable pipe bend which forms part of the delivery pipeline and is constructed to increase or decrease a volume of the delivery pipeline in dependence on an adjustment of the pipe bend. Resembling the U-shaped outer tube of a trombone, the compensating cylinder is able to store viscous material in the entire delivery pipeline through increase of the volume of the delivery pipeline by moving the pipe bend out. Persons skilled in the art will understand that the term “pipe bend” should not be limited to a U-shaped configuration but may involve also other constructions which generally follow the concepts outlined here.  
         [0033]     The compensating cylinder executes a pump stroke after the pump stroke of a main delivery cylinder is over. As a result of the pump stroke of the compensating cylinder, the pipe bend is moved back to thereby decrease the volume of the delivery pipeline and maintain the further conveyance in flow direction. By adjusting the sliding motion of the compensating cylinder, the desired delivery flow and the necessary delivery pressure can be maintained.  
         [0034]     As viscous material conveyed by the main deliver cylinders flows through the compensating cylinder and material can be received in the compensating cylinder through increase of the volume or material may be delivered through decrease of the volume, the flow direction of viscous material remains substantially the same. There is no partial flow in a compensating cylinder adjoining the delivery pipeline, as encountered in the prior art, and thus there are no flows about acute angles.  
         [0035]     According to another feature of the present invention, the pipe bend may have an upstream inlet port which embraces an upstream part of the delivery pipeline, and a downstream outlet port which is embraced by a downstream part of the delivery pipeline. In this way, the pipe portion of smaller cross section is always received−as viewed in flow direction−in the pipe portion of greater cross section. As a consequence, the flow is prevented from flowing frontally onto a seal, as would be the case if an upstream pipe section of greater cross section were to be placed over a downstream pipe portion of smaller cross section.  
         [0036]     According to another aspect of the present invention, a method of conveying a viscous material includes the steps of pumping viscous material during a pump stroke in a first switching mode from a main delivery cylinder to a delivery pipeline, while a compensating cylinder executes an intake stroke to receive viscous material, closing a shut-off valve disposed in the delivery pipeline in flow direction upstream of an outlet port of the compensating cylinder after conclusion of the pump stroke by the main delivery cylinder, and initiating an intake stroke by the main delivery cylinder in a second switching mode only after the shut-off valve has been closed.  
         [0037]     As the main deliver cylinder begins the intake stroke, when the shut-off valve is closed, pressure relief is realized in the part of the pump apparatus upstream of the shut-off valve. Thus, the gate mechanism can be switched in an environment of low pressure and not at delivery pressure. Energy is thereby reduced for switching the gate mechanism and the presence of substantial friction forces to expose the gate mechanism is eliminated.  
         [0038]     According to another feature of the present invention, the switching operation from one switching mode of the gate mechanism to the other switching mode thus starts only after commencement of the intake stroke of the main delivery cylinder which has previously pumped viscous material in the one switching mode into the delivery pipeline.  
         [0039]     According to another feature of the present invention, the main delivery cylinders are operated in opposite mode. This greatly simplifies the control for the main delivery cylinders.  
         [0040]     According to another feature of the present invention, the pressure of a working fluid of a delivery cylinder may be reduced during switching operation of the gate mechanism. This saves time as drawn concrete volume is already slightly compressed during switching operation. As a consequence of a reduction in pressure, there is no interference in the switching operation of the gate mechanism.  
         [0041]     According to another feature of the present invention, the pressure of the viscous material conducted upstream of the shut-off valve may be increased before opening the shut-off valve. Suitably, the pressure is increased to a pressure level corresponding to a pressure of the viscous material conducted downstream of the shut-off valve. As a result, the shut-off valve can be operated in an environment almost free of any differential pressure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0042]     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:  
         [0043]      FIG. 1  is a partly sectional schematic illustration of a pump apparatus according to the present invention;  
         [0044]      FIG. 2  is a schematic sectional view of a shut-off valve of the pump apparatus;  
         [0045]      FIG. 3  is a schematic sectional view of the shut-off valve, taken along the line III-III in  FIG. 2 ; and  
         [0046]      FIG. 4  is a schematic sectional view of the shut-off valve in closed position. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0047]     Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.  
         [0048]     Turning now to the drawing, and in particular to  FIG. 1 , there is shown a partly sectional schematic illustration of a pump apparatus according to the present invention, including two main delivery cylinders  1 ,  2 . Each main delivery cylinder  1 ,  2  accommodates a piston  14  which can move into the main delivery cylinder  1 ,  2 , as indicated by arrow A, to realize a pump stroke, or in a direction out of the main delivery cylinder  1 ,  2 , as indicated by arrow C, to realize an intake stroke. Connected to the main delivery cylinders  1 ,  2  is a gate mechanism, generally designated by reference numeral  3  and having a swingable transfer tube  12  with an inlet port  15  for selective fluid communication with either one of the main delivery cylinders  1 ,  2 . In an area of the inlet port  15 , the transfer tube  12  has a self-adjusting ring  17  for connection to the outlet ports of the delivery cylinders  1 ,  2 . The transfer tube  12  is fluidly connected to a first section  4  of a delivery pipeline, generally designated by reference numeral  5 , with a shut-off valve, generally designated by reference numeral  6  being disposed in the pipeline section  4 . A compensating cylinder, generally designated by reference numeral  7 , connects the pipeline section  4  with a second pipeline section  8  of the delivery pipeline  5 .  
         [0049]     The gate mechanism  3  includes a housing  10  having an interior space  11  for accommodating the transfer tube  12  which is operated by a hydraulic cylinder  13  via a connecting rod  16 . The transfer tube  12  can be shifted between a first switching mode for establishing a connection between the outlet of the main delivery cylinder  1  and the pipeline section  4 , and a second switching mode for establishing a connection between the outlet of the main delivery cylinder  2  and the pipeline section  4 . Although not shown in detail, a reservoir or priming tank is fluidly connected to the interior space  11  of the gate mechanism  3 .  
         [0050]     The shut-off valve  6  is configured in the form of a two-port rotary gate valve and includes a casing  20  and a valve body  21  received in the casing  20  and formed with a through channel  23 . The valve body  21  can be moved by a hydraulic cylinder  22  between an open position in which the through channel  23  is aligned with two opposing ports  24 ,  25  of the shut-off valve  6 , as shown in  FIG. 2 , and a closed position in which the connection between the through channel  23  and the ports  24 ,  25  is cut, as shown in  FIG. 4 .  
         [0051]     The compensating cylinder  7  includes a pipe bend  30  having an upstream end  31  of greater cross section than a downstream end  32  of the pipeline section  4  so that the upstream end  31  of the pipe bend  30  can be pushed over the downstream end  32  of the pipeline section  4 . The downstream end  33  of the pipe bend  30  has a cross section which is smaller than an upstream end  34  of the pipeline section  8  so that the downstream end  33  of the pipe bend  30  can be inserted into the upstream end  34  of the pipeline section  8 . Seals  35 ,  36  are provided between the pipe bend  30  and the pipeline sections  4 ,  8 . A movement of the pipe bend  30  in and out in relation to the delivery pipeline  5  is realized by a hydraulic cylinder  37  via a connecting rod  38  secured to the pipe bend  30 .  
         [0052]      FIGS. 2 and 3  show the shut-off valve  6  in open position. The shut-off valve  6  has a self-adjusting ring in the form of a cutting ring  26  which is supported by a flexible rubber ring  27  upon a pipe flange  28 . The casing  20  of the shut-off valve  6  has a zone of spherical shape in an area surrounding the port  24 , whereas the pivotable valve body  21  has a complementary zone  29  of spherical shape. The valve body confronting surface of the cutting ring  26  of the self-adjusting ring is also configured to have a zone of spherical shape. As shown in  FIG. 3 , the casing  20  may otherwise have flattened areas to save installation space.  
         [0053]      FIG. 4  shows the shut-off valve  6  in closed position. The spherical-shaped zone  29  of the valve body  21  is moved to a position in front of the port  24 . Thus, the valve body  21  seals off the port  24  in cooperation with the cutting ring  26 , urged by the flexible rubber ring  27  against the valve body  21 .  
         [0054]     The mode of operation of the pump apparatus according to the present invention is as follows:  FIG. 1  shows the main delivery cylinder  1  of the pump apparatus in a starting phase of the pump stroke. The transfer tube  12  is moved by the hydraulic cylinder  13  in the direction of the outlet port of the main delivery cylinder  1 . As indicated by arrow A, the main delivery cylinder  1  has already started its pump stroke. As a consequence of the partial overlap between the inlet port  15  of the transfer tube  12  and the outlet port of the main delivery cylinder  1 , concrete from the main delivery cylinder  1  is slightly pre-compressed against a cutting ring of the transfer tube  12 . The shut-off valve  6  is closed so that concrete being pumped into the pipeline section  4  build up pressure in the pipeline section  4 . Compression to the actual pressure level is realized after the transfer tube  12  reaches the end position. When the pressure on both sides of the shut-off valve  6  is the same, the hydraulic cylinder  22  is activated to pivot the valve body  21  so as to open the shut-off valve  6 . The operation is executed without a need to overcome great friction forces as the concrete on both sides of the shut-off valve  6  is under the same pressure.  
         [0055]     As the transfer tube  12  has reached the switching position in which a connection is established between the outlet of the main delivery cylinder  1  and the pipeline section  4  across the entire cross section of the outlet, the main delivery cylinder  1  is now able to pump concrete through the pipeline section  4  and the compensating cylinder  7  into the pipeline section  8 . During the pumping operation, the hydraulic cylinder  37  moves the pipe bend  30  to the outside by the concrete pressure acting thereon (to the left in  FIG. 1 ) to thereby increase the volume of the delivery pipeline  5 . The thus extended pipe bend  30  is now able to store concrete.  
         [0056]     Once the main delivery cylinder  1  has reached the end of its pump stroke, the shut-off valve  6  is closed. As the pipe bend  30  retracts in a direction indicated by arrow B, the concrete stored in the pipe bend  30  is pumped into the pipeline section  8  so that pressure in the delivery pipeline  5  is maintained. Return flow of concrete into the pipeline section  4  is prevented by the closed shut-off valve  6 .  
         [0057]     The main delivery cylinder  1  commences the intake stroke immediately following the closing of the shut-off valve  6 . As a consequence, concrete in the transfer tube  12  and the pipeline section  4  up to the shut-off valve  6  relaxes. The transfer tube  12  is then moved in the direction of the other switching position by the hydraulic cylinder  13 . In view of the pressure relief, the pressure difference between concrete in the transfer tube  12  and in the interior space  11  of the gate mechanism  3  is substantially negated so that the transfer tube  12  can easily be moved.  
         [0058]     As further shown in  FIG. 1 , the main delivery cylinder  2  executes the intake stroke, as indicated by arrow C, to draw concrete from the interior space  11 , whereby additional concrete is able to flow from the unillustrated reservoir to the interior space  11 .  
         [0059]     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suite[d to the particular use contemplated.