Abstract:
A centrifugal pump (10) conveys a fluid from a rotor wheel inlet opening (8) via a rotor wheel outlet opening (9) into a pressure nozzle (4). The fluid has a velocity profile between the outer flow line (c) and the inner flow line (d). The method allows a fluid to be conveyed with a centrifugal pump (10) in such a manner that the velocity profile is continuously changed in dependence on the delivery flow (Q) which means that an abrupt change of the velocity profile when transferring from one load condition, e.g. at maximum efficiency, to another load condition, e.g. a partially loaded region, is avoided which results in a stable H/Q)-characteristic.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for conveying a fluid and an apparatus, in particular a centrifugal pump, for conveying a fluid in accordance with the method of the invention. 
     A centrifugal pump is a conveyor machine in which mechanical energy is transmitted to a liquid with a rotating rotor wheel. The energy of motion is converted into pressure energy partly in the rotor wheel and partly in the subsequent guide apparatus. Depending on the angle of outlet of the fluid, a distinction is drawn between radial, half-axial and axial rotor wheels. Centrifugal pumps are built as single stage pumps or multistage pumps (with stacked rotor wheels arranged in series) and also as single flow or multiflow pumps (with parallel arrangements of the rotor wheels). The throughflow or delivery can be regulated by restricting the flow in the pressure line, by changing the rotational speed or, in the case of axial and half-axial rotor wheels, via adjustment of the :rotor blades. In single stage centrifugal pumps for larger conveying flows, pre-twist regulation is used by generating a twist with an upstream adjustable guide wheel. 
     The operational behavior of centrifugal pumps can be described by characteristic lines or curves as a function of the delivery flow Q. For example, the pressure head H can be described in terms of a H(Q)-curve. When the H(Q)-curve falls steadily as a function of the delivery flow Q, it is termed a stable characteristic. If the H(Q)-line has a positive gradient in the curve, this is termed an unstable characteristic. When operating the pump with an unstable characteristic, it is for example possible that a full-load region is not reached because, under some circumstances, the pump cannot be operated in the unstable region when being run up to speed. Moreover, it may be impossible to operate pumps with unstable characteristics in parallel. Furthermore,.flow dependent pulses can occur in an unstable region of the characteristic. The causes of an unstable characteristic are complex flow phenomena in the centrifugal pump. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide method for conveying a fluid so that the operational behavior of a centrifugal pump is improved, in particular in partial part load operation, by constructing rotational pumps in a manner which prevents unstable operating characteristics.. 
     The advantages of the present invention are that a centrifugal pump with the features of the invention has a stable H(Q)-curve. Consequently, a stable operation of the centrifugal pump is achieved, in particular in the partial load region. 
     A centrifugal pump can comprise a closed, half-open or open rotor wheel. A rotor wheel comprises at least one blade and one carrier disc. A closed rotor wheel additionally comprises a cover disc. A fluid conveyed through the rotor wheel of the centrifugal pump flows through the centrifugal pump while forming a flow or velocity profile. The flow profile at the rotor wheel outlet strongly influences the pressure recovery in the guide apparatus (diffusor) and thus also the overall delivery head, particularly in part load operation. An unstable H(Q)-characteristic arises in a centrifugal pump due to the fact that the velocity profile does not smoothly change as the delivery flow Q changes, but rather changes abruptly or suddenly. A change of the velocity profile in dependence on the delivery flow Q arises since, in the region of maximum efficiency of the centrifugal pump, the flow for the most part hugs to or follows the rotor wheel and guide apparatus, whereas, in the intermediate part load region, flow separations occur in the rotor wheel and guide apparatus, and in that fully developed recirculation zones are present when the slider is shut both at the rotor wheel inlet and outlet. 
     An advantage of the present invention is that the velocity profile is continuously changed in dependence on the delivery flow so that a sudden change in the velocity profile on a transition from one load state (e.g. the maximum efficiency position) to another load state (e.g. a part load region) is avoided. A stable characteristic can for instance be achieved by ensuring that a recirculation zone at the rotor wheel outlet cannot jump from the carrier disc to the cover disc. In accordance with the method of the invention, a centrifugal pump can be designed in such a way that the velocity profile changes continuously in dependence on the conveying flow from the design delivery flow to zero delivery flow. 
     An advantage of the present invention is that the velocity profile at the rotor wheel outlet aperture can be influenced in such a manner that a low energy flow zone does not simultaneously arise at the carrier disc and at the cover disc which would result in an unstable velocity profile which could change suddenly in a discontinuous, uncontrolled manner. The method of the invention results in the rotor wheel and/or the guide wheel and/or the centrifugal pump being formed in such a manner that an asymmetric velocity profile is produced outside the region of maximum efficiency and in particular shortly before flow separation in the guide apparatus at the rotor wheel outlet aperture which avoids the destabilizing influence of low energy zones occurring simultaneously at the carrier disc and the cover disc. 
     A large number of flow profiles occur in a centrifugal pump. The velocity profile preferred in the present invention lies between an inner flow line and an outer flow line of the centrifugal pump. A centrifugal pump has a rotor wheel with a center of rotation. The effective velocity of a fluid in a centrifugal pump is represented by a velocity vector which can be split up into a radial component of the velocity vector and a tangential component of the velocity vector. The radial component of the velocity vector lies in a common plane with the center of rotation of the rotor wheel and is particularly well suited for detecting low energy flow zones. A velocity profile in another direction could naturally also be suitable, for example a velocity profile which includes the entire velocity vector, i.e. both the radial and the tangential components of the velocity vector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram which illustrates a stable and an unstable delivery characteristic of a centrifugal pump; 
     FIGS. 2a, 2b, 2c, 2d are diagrams which illustrates velocity profiles for various delivery flows; 
     FIG. 3 is a sectional view of a radial rotor wheel with guide wheel, 
     FIG. 4 is a plan view of a carrier disc of a rotor wheel, 
     FIG. 5 is a plan view of a guide device; 
     FIG. 6 illustrates the arrangement of an intermediate blade in a rotor wheel; and 
     FIG. 7 illustrates various designs of a gap. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a characteristic of a centrifugal pump, namely the delivery head H as a function of the delivery flow Q, or more precisely as a function of Q/Q BEP  which is the ratio of the delivery flow Q to the delivery flow Q BEP  at the best point. The shape of this H(Q)-curve depends on the flow profile of the fluid in the centrifugal pump, in particular on a large number of geometrical parameters which relate to the design of the centrifugal pump. Centrifugal pumps often have an unstable H(Q)-characteristic as is shown by a second characteristic A i  over the range A1. 
     This unstable region A1 proves to be disadvantageous when running a centrifugal pump up to speed and also when operating under part load within the unstable region, since a centrifugal pump can only be reliably operated within a stable region. The first characteristic A s  shows a H(Q)-curve with a steadily falling profile which is termed a stable H(Q)-characteristic within which the centrifugal pump can be operated reliably. 
     FIG. 3 shows a cross-section through a centrifugal pump 10 with a radial, closed rotor wheel 1, a shaft 2, a pressure housing 16 and a guide wheel 5. The fluid flows from an intake duct 3 into a rotor wheel inlet aperture 8, through the rotor wheel 1, and enters at the rotor wheel outlet aperture 9 into the pressure duct 4 via a guide wheel 5. The closed rotor wheel 1 comprises a carrier disc 6, a cover disc 7 and also at least one blade 7a arranged between them. The present embodiment shows a guide wheel 5 with guide blades. The transition between the rotor wheel outlet aperture 9 and the spiral housing 19 or pressure duct 4 can however also be formed as a bladeless ring chamber. 
     A flow with a velocity profile 11 forms between the carrier disc 6 and the cover disc 7 when a fluid is flowing and is bounded by an inner flow line d and an outer flow line c. FIGS. 2a-2d show various velocity profiles 11 arising between the inner and outer flow lines (c, d) as a function of the delivery flow Q, or more precisely as a function of the ratio Q/Q BEP . In the above embodiment it is assumed that the components of a rotor wheel 1 or of a centrifugal pump 10 which determine the velocity profile 11 are designed in such a way that an approximately symmetrical velocity profile exists at the design point of the centrifugal pump, i.e. at Q/Q BEP  =1, as shown in FIG. 2d with the profile A s . 
     FIGS. 2c, 2b and 2a each show one velocity profile 11 with a stable profile A and one with an unstable profile A i  with a reduced flow rate Q, with FIG. 2a showing a flow rate Q/Q BEP  =0. The stable profile A s  changes continuously as a function of the flow rate Q, the back flow contribution increasing towards the outer flow line c. In the present patent document, a continuously changing velocity profile is to be understood as a profile with the properties that the flow direction does not change abruptly in certain regions of the velocity profile as a function of the flow rate Q, as can for instance be seen in the unstable profile A i  in FIGS. 2b and 2c towards the outer flow line c, but rather that the velocity profile as a function of flow rate Q varies without an abrupt change in the flow which, in the present example, manifests itself in that the back flow contribution of the fluid for the stable profile A s  steadily increases with a decreasing flow rate Q towards the outer flow line c. In contrast, the unstable profile Ai, which is representative of the flow behavior of known centrifugal pumps, shows an unstable discontinuous behavior from FIG. 2d to FIG. 2a. The unstable profile Ai of FIG. 2c has a back flow current at both flow lines c, d. The unstable profile Ai changes in such a way that with a further reduction of the flow rate Q/Q BEP  =0.25 of FIG. 2b, no more back flow current arises. On further reduction of the flow rate Q/Q BEP  =0 the flow strongly rises at the outer flow line c whereas a marked back flow arises at the inner flow line d. This abrupt change in the flow of the unstable profile A i  is the reason for an unstable H(Q)-curve as shown with A i  in FIG. 1. 
     The components of the centrifugal pump 10 which determine the velocity profile 11 of the delivery flow Q are formed in such a way in the invention that the velocity profile 11 steadily changes as the flow rate Q changes, as is shown by the stable profile As. Along with the design of the rotor wheel 1 with at least one blade 7a, carrier disc 6 and cover disc 7, further components can also be correspondingly provided such as the rotor wheel inlet 8 and the rotor wheel outlet 9, and also components fitted in front of or behind the rotor wheel 1 such as a guide wheel 5. The velocity profile 11 can be influenced by further measures such as by apertures 15 provided in the rotor wheel 1 through which liquid flows, or through a design of the inlet flow gap 14a of such a kind that liquid flows out of the gap 14 so that the velocity profile 11 is influenced, particularly in the region of the rotor wheel inlet 8. 
     The design of the components of the rotor wheel 1 or of the centrifugal pump 10 determining the velocity profile 11 of the conveying flow Q between the inner and outer flow lines c, d allows many design options for the implementation. However, a person skilled in the art knows how to design the components so that a steadily changing velocity profile 11 is achieved. For instance, a person skilled in the art uses numerical calculation methods in order to select components appropriate for a desired specification. The rotor wheel 1 has a center of rotation la. The velocity profile shown in FIGS. 2a to 2d lies between the inner and outer flow lines c and d and in a common plane with the center of rotation la and therefore includes the radial components of the velocity vector. 
     The rotor wheel and, if provided, also the guide apparatus are designed in such a way that a steady development of the velocity profile 11 as a function of the delivery flow Q results, in particular at the rotor wheel inlet 8 and at the rotor wheel outlet 9. In the embodiment shown in FIGS. 2a to 2d, a low energy flow zone at the outer flow line c is avoided in order to avoid destabilizing flow discontinuities, in particular for half-axial or axial centrifugal pumps. Moreover, the rotor wheel 1 and the guide apparatus 5 can be designed in such a manner that flow separation in the guide apparatus does not take place in the region where the characteristic of the static pressure at the rotor wheel outlet is flat or unstable. 
     The shape of the rotor wheel and the guide apparatus is made such that an asymmetric velocity profile is produced at the rotor wheel outlet 9 shortly before a flow separation in the guide apparatus to avoid the destabilizing influence of low energy flow zones appearing simultaneously at the inner and outer flow lines c, d. This is also important for the stability of the characteristics for pumps with twin-flow rotor wheels as these usually have a symmetrical design and thus, by reason of that geometry, produce a symmetrical velocity profile. A closed rotor wheel is shown in FIG. 3. For half-open or open rotor wheels, the outer and inner flow lines c, d would partly follow or be attached to the housing 16. 
     FIG. 4 shows a plan view onto a carrier disc 6 of a rotor wheel 1 with blades 7a. The velocity profile 11 can be determined by, amongst other things, the blade arrangement, for example by the blade separation A2, and/or the blade outlet angle beta2 and/or the angular profile of the blade 7a and/or the rotor wheel outlet width B2. Apertures 15 can be seen which are arranged in such a manner that the velocity profile 11 is changed in a specific manner, for example by arranging the apertures or perforations 15 in a low energy flow zone. 
     FIG. 5 shows a plan view onto a guide device with a guide wheel 5, wherein the choice of the inlet width A3 of the guide wheel 5 also affects the velocity profile 11. 
     As shown in FIG. 6, intermediate blades 7b can also be arranged in the rotor wheel 1 which extend over a part of the width B3 of the rotor wheel outlet width B2 in order to influence the flow of the fluid. 
     FIG. 7 shows a gap 14 of a centrifugal pump 10 with a fluid flow 23a, 23b, 23c emerging from between the rotor wheel 1 and the housing 16 or a sealing ring 20. The flow direction of the emergent fluid flow can be influenced with a guide element 21 so that, depending on what is required, a fluid flow 23a, 23b, 23c in different flow directions is produced in order to thereby influence the velocity profile 11. A trip edge 22 clan be used to influence the velocity profile 11 and, in the present embodiment, is arranged at the outer flow line c of the rotor wheel 1. 
     Further examples of possibilities for influencing the velocity profile 11 can for example be achieved by the following measures: 
     The components of the centrifugal pump which determine the flow behavior are specifically designed so that, for an axial or half-axial centrifugal pump, a high energy zone is produced at the outer flow line c. 
     The twist of the blade 7a in half-axial or axial rotor wheels is designed in such a way that flow separation at the hub takes place before a low energy zone arises at the outer flow line c. 
     The relationship between the blade length and blade pitch is selected for half-axial or axial rotor wheels such that a low energy zone at the outer flow line c is avoided. 
     The inlet width A3 of the guide wheel 5 is varied in its width between the outer flow line c and the inner flow line d in order to effect an asymmetric velocity profile 11. 
     The inlet opening 18 of the guide wheel 5 and the rotor wheel outlet opening 9 are offset relative to one another in a direction axial to the shaft 2. 
     The components of the centrifugal pump which determine the flow behavior are specifically designed so that flow separation occurs in the guide wheel 5 before the gap characteristic becomes flat or unstable. 
     The blade separation A2 at the rotor wheel outlet is varied[in its width between the inner flow line d and the outer flow line c in order to produce an asymmetric velocity profile.