AU2012267258A1 - Multistage comminuting pump - Google Patents

Abstract

A multi-stage comminuting pump used for example with agricultural effluents arising from a cowshed or feedlot pad is capable of developing an output pressure up to 100 to 140 metres water while breaking up any suspended fibrous solids such as straw. In each stage, the vanes of a non-rotating diffuser are located in close proximity to a turning disk comprising the rear of a previous impeller stage and having slots for comminution of suspended solids. The impeller of each stage turns close to a fixed plate also carrying comminuting slots. A venturi-assisted suction stage option is described.

Description

WO 2012/169904 PCT/NZ2012/000085 TITLE: MULTISTAGE COMMINUTING PUMP FIELD: The invention relates to rotary pumps for non-homogenous liquids, to centrifugal pumps, especially multi-stage centrifugal pumps, and to the class of pumps that include comminuting means for breaking down solid material (such as fibre) that may be included in the material, such as fibrous material, manure or effluent to be pumped. DEFINITIONS "Comminuting" means that this type of pump is capable of macerating or chopping fibrous material 10 suspended in a liquid being pumped into a finer suspension. "Effluent" is used as a collective term to refer to all liquids, including water, and suspensions, including suspensions of fibrous material that are pumped by pumps according to this invention. "Obverse side" as applied in this specification to a disk-shaped impeller or diffuser refers to the side carrying vanes, as shown for example in Figs 3 or 7, that is first encountered by incoming effluent, and the term "reverse side" refers to the other side. BACKGROUND For a considerable time, dairy shed and feed pad (or feed lot) effluents have been disposed of by shifting them away, such as on to a paddock which is not currently being grazed. Sometimes the effluent is carried by tanker. Alternatives include pumping the material in a relatively specialised 20 form of centrifugal pump; one that is capable of shifting watery materials including suspended fibrous material (such as dung, or spilt grass, hay, straw or silage) without fear of blockage. Such pumps generally overcome the possibility of becoming blocked by fibre by including means to break down, or comminute, the fibrous material. More recently the problem that has arisen is to efficiently pump the material against a pressure head of up to about 130-140 metres, whereas the previous requirement was for a much lower head of pressure. One way to get an increased flow against a pressure is to speed up the pump, but the greatly preferred motive power for fixed pumps is an AC induction motor which has a relatively constant speed dependent on the local mains power frequency: 50 Hz = 3000 rpm, 60 Hz = 3600 rpm. Another way to get an increased flow against a 1 WO 2012/169904 PCT/NZ2012/000085 pressure is to use larger diameter rotors and stators, so that the tip speed is greater. There are 30 disadvantages associated with single-stage pumps, such as the rising motor load - see below under "Results" in relation to Fig 9, and increased wear caused by erosion and the like. GB 1147288 Baum describes a comminuting pump in which a left-handed and a right-handed screw stage, on the same shaft, provide converging flows to a common centrifugal stage. That design would remove an axial load on the shaft. Each screw is faced with a hard material. Note that the waste water treatment industry and the pulp and paper industry use single stage centrifugal comminuting pumps of a larger scale than the present multi-stage example. OBJECT An object of the present application is to provide a pump for reliably pumping suspensions in water such as dairy shed or feed pad effluent against a pressure head at relatively low to medium 40 flows, or at least to provide the public with a useful choice. SUMMARY OF INVENTION In a first broad aspect the invention provides a multi-stage comminuting centrifugal type pump having a housing and an inlet for receiving a liquid to be pumped and an outlet for pressurised liquid and within pump which a rotatable shaft is made to rotate by a turning force provided by a motor, wherein the pump includes at least two stages; each stage including an impeller having a preferred direction of rotation mounted on to the rotatable shaft and confined within a housing, and a diffuser mounted on to the housing; where both the impeller and the diffuser are made from a hard material capable of retaining an edge, and wherein each stage includes comminution means each comprising a first radial surface, perpendicular to an axis of the shaft, including spiral vanes; 50 said surface being placed close to a second radial surface including at least one comminution slot comprising a shallow radially or tangentially directed groove cut into the radial surface having at least one sharp or cutting edge on a trailing edge between the groove and the radial surface; comminution thereby occurring within all stages during relative movement of the first and second radial surfaces, and wherein each stage receives the liquid to be pumped in a sequence so that the pressure differential across all stages is added. In a related aspect, each impeller comprises a disk having an obverse face directed forwardly toward the inlet and a reverse face directed away from the inlet; each obverse face includes a 2 WO 2012/169904 PCT/NZ2012/000085 plurality of curved vanes extended forwardly from the obverse face; each curved vane commencing near a central shaft mount of the impeller and terminating at an outer periphery of the impeller; a 60 forward edge of every vane is finished with a flat radial surface in a common radial plane; each impeller has a smooth radial surface in another radial plane on the reverse face of the impeller save that the smooth surface is interrupted by a plurality of radially extended grooves, the trailing edge of each of which is provided with a cutting edge; each diffuser has an obverse face includes a plurality of curved vanes extended forwardly from the obverse face; each curved vane commencing an outer periphery of the diffuser and terminating near a central aperture of the diffuser, at least some of the curved vanes are finished with a flat radial surface in a common radial plane, and each diffuser has a smooth surface in a radial plane on a reverse face directed away from the inlet that is placed close to an obverse face of an adjacent impeller; the smooth surface including at least one comminution slot; thereby allowing a pump having more than one stage to be constructed and 70 wherein comminution means are provided at the obverse face and at the reverse face of each impeller so that fibrous material does not accumulate within any stage of the pump. In another related aspect the invention provides a multi-stage centrifugal type pump having a housing and within which a shaft is made to rotate, wherein the rotating shaft 101 has a base and a free end extended into the housing and engaging with each impeller; the rotating shaft has a relatively large diameter and is stiff; the rotating shaft is rotatably supported from near the base only, by bearing means including more than one rolling bearing (102) spaced apart and protected by sealing means between the bearings and the space containing pressurised liquid. In yet another related aspect the invention provides that the last impeller comprises a disk of greater diameter than that of the other impellers, and pressurised liquid is permitted to access a 80 space behind the reverse face of the last impeller; however back vanes and wear rings on the reverse face of the impeller reduce the pressure exerted by the liquid so that an axial force applied along the shaft is at least partially compensated. In a still further aspect the pump is provided with a suction box comprising a closed cavity sealed around the first impeller, and an inlet pipe is connected on to the suction box at a position above the axis of the shaft, so that when in use a pool of liquid will collect within the suction box and allow the pump, when operation commences, to prime itself and commence suction. In an even further aspect the pump is provided with an assembly comprising a venturi eductor and 3 WO 2012/169904 PCT/NZ2012/000085 coaxial venturi jet mounted coaxially adjacent the free end of the shaft and directed toward the first impeller; the eductor substantially enclosing the input to the impeller, the assembly being 90 connected by a pipe to the outlet of the pump, so that when in use the venturi jet is supplied with a proportion of the pumped, pressurised and comminuted liquid at the outlet of the pump and ejects the liquid through the eductor, so that a greater amount of suction is developed within the suction box. Preferably the number of slots and adjacent vanes between which comminution may take place are not the same number, so that vibration and noise are reduced. For instance, three vanes may pass closely over four slots thereby comminuting any suspended solids caught between the two. Preferably the sealing means is placed over the rotating shaft at one end; the sealing means comprising a dust protection seal for the bearings, a finger for throwing off any drips, the finger 100 being exposed to the external environment, a wiping seal, a space filled with grease, and most importantly, a mechanical seal leading to the area filled (when in use) with a liquid at high pressure; the grease serving to prevent intake of air, and no other sealing means are applied along the interior side. PREFERRED EMBODIMENT The description of the invention to be provided herein is given purely by way of example and is not to be taken in any way as limiting the scope or extent of the invention. Throughout this specification unless the text requires otherwise, the word "comprise" and variations such as "comprising" or "comprises" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step 110 or group of integers or steps. Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference. Reference to cited material or information cited in the text should not be understood as a concession that the material or information was part of the common general knowledge or was known in New Zealand or any other country. DRAWINGS Fig 1: A cross-section through an example pump 4 WO 2012/169904 PCT/NZ2012/000085 Fig IA: Detail of Fig 1; a cross-section through the sealing means. Fig 2: An external view of a pump with a motor attached. Fig 3: Obverse side of an impeller disk from one stage. 120 Fig 4: Reverse side of an impeller disk from one stage. Fig 5: Edge view of an impeller disk from one stage. Fig 6: Reverse side of a diffuser ring from one stage. Fig 7: Obverse side of the diffuser ring showing directing vanes. Fig 8: A vertical sump pump. Fig 9: Graph of performance, compared with existing effluent pumps. Fig 10: A self-priming version. Fig 11: A self-priming version including a venturi jet. INTRODUCTION The present invention is a comminuting pump, which can macerate suspended fibrous solids such 130 as straw in effluents such as manure, or washdown from a cowshed or feedlot pad. It is a multi stage centrifugal pump which can produce an output pressure of the order of 100 to 140 metres of water. It would be useful to reduce any axial force on the impeller shaft, which would become significant if several stages of impeller upon a common shaft are exposed, during use, to a pressure head on one side only. These examples use back vanes on the last of the impellers so that the axial force on the impeller shaft, is reduced, and use a wear ring. In each stage, the vanes of a non rotating diffuser are located in close proximity to the rear of a turning impeller disk. By "close proximity" is meant a clearance of perhaps 0.25 to 1 mm. Each non-rotating diffuser has slots for causing comminution of suspended solids. Since comminution is provided for at two planes (both obverse and reverse) around each impeller of each stage, there is little opportunity for suspended 140 solids such as plant fibre to accumulate. EXAMPLE 1 This example is a three-stage centrifugal pump suitable for use with liquids and liquids carrying 5 WO 2012/169904 PCT/NZ2012/000085 suspended solids. Fig I shows a longitudinal cross-section of the pump 100. A sturdy rotatable shaft 101 extends to the left side of the drawing and includes a spline for attachment of a driving means such as a 11 kW directly coupled three-phase AC induction motor (as in Fig 2) or a belt, chain or gear drive. Such a motor has an almost constant speed. Two sets of roller or ball bearing modules 102 inside a casing 102A support one end of the shaft despite overall sideways or axial forces applied to the shaft during use and serve as a thrust bearing. The shaft is cantilevered from the base, supported by the bearings 102. Unlike some prior art devices, the far end 10IA of the 150 shaft 104 is not further supported by bearings. A bell-shaped housing member 105, the interior of which may be open to the air, is used to mount the base of the pump housing on to the casing 102A. The housing 106 is comprised of a series of cylindrical castings 108 109, 110 each forming part of a corresponding rotary diffuser. are secured to the base 104 and are tightened together by bolts such as 111 so as to seal the interior of the pump. The last cylindrical casting 110 is preferably also formed into an inlet port 112 for connection to an inlet pipe for receiving liquids or suspensions (hereinafter referred to as "effluent") to be pumped. The outlet port is at 124. Sealing means for this pump is shown separately in Fig 1 a, which is an enlarged portion of the middle of Fig 1. Seals 125-132 are provided around the rotatable shaft 101. They separate the interior 103 of housing member 105, which may be air at atmospheric pressure, from the high 160 pressure or outflow cavity of the pump 123 holding effluent at a pressure, which may, when in a steady operational state, be at ten times atmospheric pressure or more, depending on the actual head and resistance to flow within the pipe. Transients may further raise this pressure. With reference to the cross-sectional view in Fig 1, the seal components from the right are: 125 is a rotating metal disc, which has close running clearances against a sealing ring 126 to provide pressure reduction in the seal chamber. 127 is mechanical seal rotating assembly with a silicon carbide rotary face, which seals against the shaft with a rubber boot, and contains a spring to maintain seal face contact pressure. 128 is a mechanical seal stationary seat made of silicon carbide which does not contact the shaft 101. One preferred kind of seal is the "Type 21" elastomer bellows seal, supplied by John Crane of Slough, United Kingdom, in a selected size. 129 indicates a circumferential space which is 170 maintained full of grease into the space between seal 128 and a further seal 130, using a grease nipple opening into cavity 103 from where it may be accessed. Any effluent that passes this point flows on to the finger 131 and either drops down if the shaft is not rotating, or is flung off in any direction into the interior of housing 105 if the shaft is rotating. A final seal 132 has the purpose of protecting the main bearings 102 from contamination arising in the air space 103 and of holding 6 WO 2012/169904 PCT/NZ2012/000085 lubricating grease in place. During use the effluent received through inlet 112 is thrown to the periphery of the interior of the first housing 110 by rotation of vanes 117 of the first impeller 114 of three (in this Example) impellers, For details of impellers see below, in relation to Figs 3, 4 and 5. The rotating and partially pressurised effluent is guided back towards the shaft by curved vanes 115 forming part of 180 the first diffuser ring 116; the exterior of which is housing element 109. The rotation is largely removed by the angles of the curves as shown at 704 in Fig 7. Now the more pressurised effluent is thrown to the periphery of the interior of the second housing 109 by vanes 117 attached to the second impeller 118. Again, the rotating and twice pressurised effluent is guided towards the shaft by curved vanes 119 forming part of the second diffuser ring 120; the exterior of which is housing element 108. A final impeller 122 using vanes 121 applies a third increment of pressure to the effluent, which is carried in space 123 that is connected to exit port 124 and in turn to a distribution pipe. As will be explained below, each stage of the multi-stage pump includes comminution means. Note that this design places little radial load on the shaft 101 as a result of pumping action, but there is an axial force directed toward the right of the drawing; toward the inlet port 112 as a result 190 of differential pressures across each of the impeller plates, most particularly resulting from the pressure within the space 123 following the last impeller and leading to the output port. Bearing modules 102 transmit this force from the rotating shaft to the fixed casing 102A and in turn to the base. Each impeller is forced towards the right hand side of this drawing. As a result, the flat outer edges of the vanes tend to make sliding contact with the flat left hand side of each diffuser plate. See below for further details of the impellers and diffusers. Fibrous material will become comminuted or macerated as a result of shearing between the centripetal surfaces of the vanes on the side of each impeller closest to the input 112, which side is manufactured to comprise a flat plane, and the fixed facing surface attached to or comprising part of the housing ring such as 108 or 109. Material which becomes lodged around the rotating 200 impeller vanes that are slanted (see vane 117 in Fig 3) will tend to move towards the space between the vanes and the fixed facing surface 114A. This surface is another flat plane into which a series of slots have been cut or ground as shown in Fig 6, such that the trailing edge is relatively sharp. The axial clearance between all rotating and fixed parts in this pump is deliberately made small so that shearing will occur. The existence of cutting slots drawn as 402 and 605 may reduce efficiency by perhaps 10%, but allows the pump to work without interruption even if used to transfer feedlot 7 WO 2012/169904 PCT/NZ2012/000085 effluent which often has a high fibrous content arising from straw and cattle dung. Clearly, reasonably close tolerances of all impeller and diffuser heights must be maintained since errors along the axis of the pump may accumulate. A typical axial clearance across any pair of plane surfaces is of the order of 0.1 to 1 mm, with clearances on the obverse side being smaller than 210 on the reverse side. A working clearance of about 0.25 to 1 mm is set by mounting the impellers with shims. Thermal expansion along the length of shaft 101 is not likely to occur and alter clearances, since the pump is cooled by the effluent being pumped, and because all parts are made of the same or similar metal as described below. Clearances within the stack of impellers along the shaft, in relation to adjacent diffusing plate surfaces that are clamped to the housing 105 by tension bolts 111 rely on control of impeller thickness, and control of diffuser periphery thickness by grinding or other form of machining carried out after casting of the parts. Cast iron or more preferably "SG" iron (spheroidal graphitic iron) is preferred for the impellers and diffusers. It has a good surface hardness; greater than cast iron. The cast and machined or ground surfaces become passivated during use, so that corrosion is slow. Stainless steel parts may be used 220 for the impellers and diffusers in appropriate cases, although it is more costly and close clearances may lead to galling of stainless parts and seizure. Fig 2 shows an external view of a pump with a motor provided as an integrated module, according to this example. 201 represents an AC induction motor rated at about 1kW. 202 is a bell-like connector between the motor and the bearing housing 102A pump, and preferably includes a flexible coupling between the motor shaft and the pump shaft, in case of any misalignment between the two shafts. The bearing assembly 102/102A could be used as the bearing for the motor as well, if convenient. 203 is a base or mounting plate. Fig 3 shows the obverse side of a cast impeller plate or disk 120 which rotates anticlockwise. Incoming effluent is directed near the boss 301. This side has a small number such as 3 or 4 curved 230 impeller vanes; one labelled 117 rising from a surface 303, which, when rotating, fling the liquid or suspension being pumped towards the periphery. All vanes are typically included in the mould used for casting. Each vane originates near the impeller plate near the central boss 301 and reaches a maximum height 302, shown hatched, at which it is ground or otherwise shaped into the same tangential plane as the other impellers. The central boss has a keyway 304 to lock the impeller plate to the shaft 101. The plate cannot move far laterally along the shaft because its vanes on the obverse 8 WO 2012/169904 PCT/NZ2012/000085 side share a flattened surface 302 which skims over the notched but otherwise flat surface of the diffuser of Fig 6; except in the case of the last impeller 122. Fig 4 shows the reverse side of the impeller 120. This side comprises a flat surface 401, shown hatched, which when in use skims over the vane-bearing side (shown in Fig 7) of the next diffuser 240 plate along the multi-stage pump. The flat surface carries a small number, such as 4 comminuting slots 402 the edges of which are intended to shear any fibre that finds its way into the space. These, as shown at 402 in the edge view of Fig 5, should have a relatively sharp trailing edge in order to shear any suspended fibres, although this might wear with use. It is desirable to use different numbers of vanes and slots, such as 4 vanes against 3 slots for example, to avoid noise and vibration that would occur if the counts of slots and vanes were the same. The central boss is the same as that of Fig 3. Fig 6 shows one non-rotating diffuser plate 109 of the pump, in reverse view. The plane surface 604 is skimmed over by the vanes of the adjacent impeller. The two slots 605 have sharp or relatively sharp trailing edges (in relation to the direction of rotation) in order to macerate any 250 fibrous material that becomes trapped or wrapped over the impeller vanes during use. The slots may be formed during casting and the edges are preferably finished off with an angle grinder or the like. Lug 601 is used at the time of assembly to line up the individual parts and to aid disassembly. These lugs can be knocked with a hammer to assist separation of the casings (diffuser external parts) during maintenance. Casings can tend to "glue" together over time. Fig 7 shows the six fixed vanes of the preferred diffuser 109. There are three high vanes 119, and three low vanes 701. The high vanes share a common plane surface 704 (hatched) which, when in use, is skimmed over by the plane surface 401 of the impeller of Fig 4. The six vanes are intended to return the liquid or effluent being pumped towards the central shaft and on to the next stage; at the same time decreasing any rotation that was imposed by the rotation of the impeller. All vanes 260 are formed by casting in the preferred manufacturing process, and the plane surface 704 of the high vanes is finished by grinding or turning or any other appropriate process. The outer shell of the diffuser 109 includes sealing means such as a stepped periphery that can accommodate a rubber ring or other seal for the stage, as is known. A person skilled in the relevant art will appreciate that a four stage pump, for example, is made by simply adding an extra diffuser plus housing, and an extra impeller, to a longer shaft. 9 WO 2012/169904 PCT/NZ2012/000085 EXAMPLE 2 Fig 8 shows an external view of a vertical sump pump 800 in which the motor 801 is located at some distance above the normally immersed pump assembly 804. The entire assembly may be mounted on a pontoon or on a jetty, as required with reference to the safety of maintenance 270 engineers. The paired ball or roller bearing assembly, internally configured as described in Example 1, is inside the housing 802. A cylinder of indefinite length 803 encloses a stout drive shaft and reaches into the liquid to be pumped. No further bearings are used. One advantage is that if the sump runs dry, no damage will occur. When in use, liquid enters the pump 804 at 805 and exits at 806. Not shown in Fig 8 is the use inside pump 804 of multi-stage comminuting parts including rotatable impellers and non-rotating diffusers as described in Example 1 with reference to Figs 1, 3, 4, 5, 6 and 7. EXAMPLE 3 Fig 10 shows a self-priming pump. The inlet 901 is located above the axis of the pump, and is provided with a flap valve 902 leading to inlet cavity or suction box 903, which has an inspection 280 hatch 904. For priming purposes there will always be some liquid in the cavity 903. This pump has two impellers; first stage 910 and second stage 911, together with comminuting diffusers and stators as described with reference to Figs 1, 3, 4, 5, 6 and 7. The wear ring 912 that serves as a sealing collar on the second stage has typically 0.25 mm clearance around its periphery within the wear ring aperture 913 included as part of the bell housing 914. During operation, an optional bleed pipe 905 of restricted aperture sucks any leaked fluid from the bell housing chamber 906 back into the suction box 908. A flap valve may be included at the exit or vent 907 of pipe 905, to prevent back flow. An advantage of this Example is that it may use parts from the multi-stage pump as previously described in this section, along with parts of an existing single stage production pump, thus reducing overall production and stocking costs when producing a variety 290 of models. EXAMPLE 4 Fig 11 shows a venturi nozzle arrangement mounted coaxially with the axis of the shaft at the input end of a multi-stage pump 1100, similar to that of Example 3. A suction is created as a result of flow through the venturi eductor 1104 when a jet of liquid or mixed liquid and suspended solid matter ejected from venturi nozzle 1105 creates a partial vacuum and assists in 10 WO 2012/169904 PCT/NZ2012/000085 driving liquid to be pumped toward the first impeller of the pump 1100. As a result, the pump exhibits a significantly greater suction, applied through input port 901, than would a pump relying solely on impeller suction. Longer suctions lines may be used. Although some liquid is bypassed from the output through aperture 906 and hose 1102, the overall effect is to raise the 300 pump output. The large size of the suction box 903 also assists with priming and flow. Venturi jet and eductor assemblies have not previously been used on effluent containing fibrous material, perhaps because support webs or struts comprise obstacles to flow on which suspended fibres may become entangled. RESULTS Fig 9 indicates performance of a two-stage (YM53-2stg), and a three-stage (YM53-3stg) pump according to this invention as solid lines, as compared with Reid & Harrison (Yardmaster, Matamata, New Zealand) single-stage effluent pumps type RH-10 (220) and RH-10 (215) shown here as dashed lines. A first set of curves shows dependence of output flow in litres per second along the horizontal axis against head of pressure in metres of water on the vertical axis at the left 310 side. The maximum head against which flow is pumped is larger. The vertical axis on the right shows power consumed in kilowatts. The pumps according to this invention always show a lower power consumption. Flow is more directly dependent on pressure as shown by the gradients of the solid lines as compared to the gradients of the dashed lines, but occurs against a greater head especially for the three-stage version. These single stage pumps inherently cannot pump against a head of over about 80 metres water. ADVANTAGES The target of pumping against at least 100 metres head of water is met by a multi-stage centrifugal pump at good efficiency. A large single-stage pump could not achieve the same pressure. Fibrous material is comminuted. Since the entry side of all vanes is angled, any fibrous material 320 reaching an impeller tends to move axially, to within the space between the flat surface and adjacent vanes. The flat surfaces carrying slots effectively break down any fibrous material which is caught within the space. The pump cartridge, a name used for the entire assembly of impellers and diffusers, can be replaced as a whole from time to time on account of wear, corrosion or damage. The sealing

II

WO 2012/169904 PCT/NZ2012/000085 means 125 and 126 is replaceable at the same time. Use of a rigid shaft and an absence of shaft bearings within the pump itself removes the problem of their sealing and maintenance. Finally it will be understood that the scope of this invention as described and/or illustrated herein is not limited to the specified embodiments. Those of skill will appreciate that various modifications, 330 additions, known equivalents, and substitutions are possible without departing from the scope and spirit of the invention as set forth in the following claims. 12

Claims (7)

1. A multi-stage comminuting centrifugal type pump having a housing and an inlet for receiving a liquid to be pumped and an outlet for pressurised liquid and within pump which a rotatable shaft is made to rotate by a turning force provided by a motor, characterised in that the pump includes at least two stages: a. each stage including an impeller having a preferred direction of rotation mounted on to the rotatable shaft and confined within a housing, and a diffuser mounted on to the housing; where both the impeller and the diffuser are made from a hard material 340 capable of retaining an edge, b. and wherein each stage includes comminution means each comprising a first radial surface, perpendicular to an axis of the shaft, including spiral vanes; said surface being placed close to a second radial surface including at least one comminution slot comprising a shallow radially or tangentially directed groove cut into the radial surface having at least one sharp or cutting edge on a trailing edge between the groove and the radial surface; comminution thereby occurring within all stages during relative movement of the first and second radial surfaces, c. and wherein each stage receives the liquid to be pumped in a sequence, commencing at a first stage, so that the pressure differential across all stages is 350 added.

2. A multi-stage centrifugal type pump having a housing and within which a shaft is made to rotate, characterised in that: a. the rotating shaft has a base and a free end extended into the housing and engaging with each impeller; b. the rotating shaft has a relatively large diameter and is stiff; c. the rotating shaft is rotatably supported from near the base only, by bearing means including more than one rolling bearing (102) spaced apart and protected by sealing means between the bearings and the space containing pressurised liquid.

3. A pump as claimed in claim 2, characterised in that: 13 WO 2012/169904 PCT/NZ2012/000085 360 a. each impeller comprises a disk having an obverse face directed forwardly toward the inlet and a reverse face directed away from the inlet; b. each obverse face includes a plurality of curved vanes extended forwardly from the obverse face; each curved vane commencing near a central shaft mount of the impeller and terminating at an outer periphery of the impeller; c. a forward edge of every vane is finished with a flat radial surface in a common radial plane; d. each impeller has a smooth radial surface in another radial plane on the reverse face of the impeller save that the smooth surface is interrupted by a plurality of radially extended grooves, the trailing edge of each of which is provided with a cutting 370 edge; e. each diffuser has an obverse face includes a plurality of curved vanes extended forwardly from the obverse face; each curved vane commencing an outer periphery of the diffuser and terminating near a central aperture of the diffuser, f. at least some of the curved vanes are finished with a flat radial surface in a common radial plane, g. each diffuser has a smooth surface in a radial plane on a reverse face directed away from the inlet that is placed close to an obverse face of an adjacent impeller; the smooth surface including at least one comminution slot; h. thereby allowing a pump having more than one stage to be constructed and wherein 380 comminution means are provided at the obverse face and at the reverse face of each impeller so that fibrous material does not accumulate within any stage of the pump.

4. A pump as claimed in claim 3, characterised in that the last impeller of a multiple set comprises a disk of greater diameter than that of the other impellers, and when in use pressurised liquid is permitted to access a space behind the reverse face of the last impeller where back vanes and wear rings on the reverse face of the impeller reduce the pressure exerted by the liquid so that an axial force applied along the shaft by the developed liquid pressure is at least partially compensated. 14 WO 2012/169904 PCT/NZ2012/000085

5. A pump as claimed in claim 1, characterised in that the pump has a suction box comprising a closed cavity sealed around the first impeller, and an inlet pipe is connected on to the 390 suction box at a position above the axis of the shaft, so that when in use a pool of liquid will collect within the suction box and allow the pump, when operation commences, to prime itself and commence suction.

6. A pump as claimed in claim 5, characterised in that an assembly comprising a venturi eductor and venturi jet is mounted coaxially adjacent the free end of the shaft and directed toward the first impeller; the eductor substantially enclosing the input to the impeller, an inlet to the venturi jet is connected by a pipe to the outlet of the pump, so that when in use the venturi jet is supplied with a proportion of the pressurised and comminuted liquid at the outlet of the pump and ejects the liquid through the eductor, so that a greater amount of suction is developed within the suction box. 400

7. A pump as claimed in claim 1, characterised in that the number of slots and adjacent vanes between which comminution may take place are not the same number, so that vibration and noise are reduced. 15