WO2005100796A1

WO2005100796A1 - Impeller

Info

Publication number: WO2005100796A1
Application number: PCT/SE2005/000541
Authority: WO
Inventors: Axel Fredriksson
Original assignee: Pumpex Production Ab
Priority date: 2004-04-15
Filing date: 2005-04-14
Publication date: 2005-10-27
Also published as: EP1747377A1; SE526557C2; SE0400964L; EP1747377B1; SE0400964D0; DE602005012498D1; ATE421638T1

Abstract

The present invention. relates to an open or half- open channel impeller for a centrifugal pump for waste-water. The channel impeller has a substantially circular periphery and comprises a central hub with a hub axis and a hub plane to which the hub axis extends perpendicularly. The channel impeller further comprises at least one blade which extends from the hub to the periphery. Each blade has a front edge adjacent to the hub, which front edge is continuously curved with an inclination to the hub plane which is 20° minimum and 50° maximum. The invention also relates to a centrifugal pump comprising such a channel impeller.

Description

IMPELLER

Field of the Invention The invention relates to a channel impeller of the open or half-open type for a centrifugal pump for waste- water, said channel impeller having a substantially cir- cular periphery. The channel impeller comprises a central hub with a hub axis and a hub plane to which the hub axis extends perpendicularly. The channel impeller further comprises at least one blade which extends from the hub to the periphery, said blade having a front edge adjacent to the hub. The invention also relates to a centrifugal pump which comprises a pump casing with a channel impeller.

Background Art The invention relates to open or half-open channel impellers. A half-open channel impeller is limited inwards to the drive shaft by a hub plate on which the blade or blades is/are mounted. Outwards to the inlet, the blades are arranged to cooperate with a stationary wear plate. The distance from the blades to the wear plate is often less than 0.5 cm to provide good efficiency. In an open channel impeller the blades are arranged to cooperate with a stationary wear plate also inwards to the drive shaft. In contrast, the blades of a so-called closed channel impeller are mounted on co-rotating hub plates both inwards and outwards. EP 0 406 788 discloses a centrifugal pump which comprises a pump casing with an impeller in the form of a half-open channel impeller. The channel impeller com- prises several blades on which blade projections are arranged. A wear plate is mounted at a constant distance from the blade projections. Such blade projections reduce the flow from the high-pressure side of the blades to the low-pressure side of the blades in operation. As a result, the efficiency of the pump is improved compared with a pump without blade projections. The blades are made of bent sheet metal and mounted on a hub plate of metal sheet. The hub plate has a hub projecting a distance which is the same as the height of the blades from the hub plate. The blades further have a front edge facing the hub. In this prior-art pump, it is a problem that objects, such as rigid objects or flexible elongate pieces of material, if they are entrained in the liquid that is being pumped, tend to get stuck between the front edge, the hub and the wear plate. This increases the risk that additional objects adhere so that pump is finally clogged and only a small flow, or no flow at all, passes through it. Centrifugal pumps with a channel impeller of a type as described in EP 0 406 788 have so poor clogging properties that in practice they can only be used for liquids which do not contain such objects. Alternatively the pump must be stopped and cleaned regularly, which is time-consuming and expensive.

Summary of the Invention An object of the present invention is to provide an open or half-open channel impeller for a centrifugal pump which has improved clogging properties compared with the prior-art channel impeller. According to the present invention, this object is achieved by the> channel impeller of the type mentioned by way of introduction being given the features as defined in claim 1. Preferred embodiments will be evident from the dependent claims. Another object of the present invention is to provide a centrifugal pump comprising an open or half-open channel impeller which has improved clogging properties compared with the prior-art channel impeller. According to the present invention, this object is achieved by a centrifugal pump of the type mentioned by way of introduction which is given the features as defined in claim 11. The invention is thus based on the knowledge that the design of the front edge is important to the clogging properties. In this patent application, the front edge of the blade relates to the edge that divides the liquid flow at the inlet of the pump . The channel impeller according to the present inven- tion and the centrifugal pump according to the present invention are adapted to be used for pumping liquid of a type that consists of wastewater. In this patent application, wastewater relates to contaminated water which is passed to waste. For instance, wastewater from domestic toilets, bath, washing-up and washing, referred to as black water and grey water respectively. Wastewater can also be industrial wastewater from industrial processes. The invention aims particularly at improving the clogging tendency of a channel impeller with regard to wastewater containing flexible, elongate pieces of material, such as elongate textile or plastic threads, dishcloths or residues from the paper industry. Traditionally, channel impellers have been constructed so that a rigid object of a predetermined maxi- mum size should be able to pass through the channels of the channel impeller. If it has traditionally been desired to improve the clogging properties of a channel impeller, the dimensions of the channels have thus been increased so that a larger, rigid object can pass there- through. According to the invention, it has however been realised that it is often longer, flexible objects, such as cloths, bands or fibre material, that get caught by an unfavourably designed front edge and stick thereto. Since other flexible objects then get stuck to the flexible object that has already got stuck, the throughput of the pump will quickly be made smaller. This means that even relatively small, rigid objects will be prevented from passing through the channels of the channel impeller and the flow decreases. If it would be possible to design the front edge so as to reduce the tendency of flexible objects getting stuck, it will also be easier for rigid objects to pass through the channel impeller. It would thus be possible to improve the throughput of a channel impeller without increasing the dimensions of the channels . According to the invention, this is achieved by a front edge which is inclined 50° maximum to a hub plane. A front edge which is inclined 40° maximum is preferred. With a steeper front edge, elongate flexible objects tend to get caught on the edge and get stuck. If the channel impeller has a projecting hub, it is also common for flexible objects to slide down along an excessively steep front edge and wind onto the hub. In this patent application, hub plane relates to a plane including the hub of the channel impeller. The hub is connected to a drive shaft. In use, the channel impel- ler rotates on an axis, referred to as the hub axis, which coincides with the drive shaft. The hub plane constitutes a plane perpendicular to the drive shaft/hub axis. If the hub plate of a half-open channel impeller is flat, the hub plane is parallel to this. If the inner wear plate of an open channel impeller is flat, the hub plane is parallel to this. The front edge further has the shape of an even curve, i.e. the shape of a curve without discontinuities, such as steps or sharp corners on which a flexible object could be caught. The exact shape of a curve of the front edge relative to the hub plane is determined by the shape of the blade in other directions, which is determined by other factors, such as power requirements and degree of efficiency. However, the front edge should not at any point have an inclination that exceeds 50° to the hub plane. According to the invention, the front edge can have a convex or concave shape relative to the hub plane. It can also be curved, for instance in the shape of an S or a double S. It is also possible to have a front edge which is substantially rectilinear. In one embodiment of the present invention, the front edge of the blade extends between a lower inner end and an upper outer end. The inner end of the blade is positioned closer to the hub than the outer end and is the portion of the blade which is closest to the hub. The inner end is further positioned closer to the rear inner wear plate and the hub plate than is the outer end. The front edge of the blade thus extends between the inner end and the outer end and has over the whole of this portion an inclination of 50° maximum. In one embodiment, the inclination (g) of the front edge between the inner end and the outer end in relation the hub plane is described by the formula , _N tan(a)*tan (Jb) , tan(gr)=—, , wherein Vtan²( )+1 a = cone angle of the blade, i.e. the angle in a vertical plane which is perpendicular to the hub plane, through the centre of the axis; b = sweep angle of the blade, i.e. the angle of the blade in the hub plane . In one embodiment of the present invention, the blade further comprises an outer edge side, which is arranged to cooperate with a wear plate. The outer edge side of the blade and the wear plate are adjusted to each other so that the blade and the wear plate have the desired distance from each other as the channel impeller rotates. The wear plate can be flat, cup-shaped or have any other rotationally symmetrical shape that corresponds to the outer edge side of the blade . At the outer end of the front edge of the blade, the front edge passes into the outer edge side of the blade. This portion of the blade constitutes a transition por- tion. The transition portion is limited by the outer end of the front edge and by the outer edge side of the blade from the point where the outer edge side is adapted to cooperate with the wear plate. The wear plate is provided with a central inlet, which is intended as the inlet for the liquid to be pumped. The liquid then flows on into the channels of the channel impeller. In one embodiment of the present invention, the transition portion, as well as the front edge, has the shape of an even curve. The transition portion has a radius of curvature relative to the hub plane of at least 20 mm and suitably at least 30 mm. A radius of curvature of at least 50 mm is preferred. The radius of curvature should be 200 mm maximum, preferably less than 150 mm, and a radius of curvature of less than 100 mm is preferred. Thus, the transition between the front edge and the outer edge side will be smooth so that the risk of flexible objects getting stuck is minimised. The objects can instead slide into a channel or leave through the gap between the blade and the wear plate. In one embodiment of the invention, the transition portion of the blade with the outer end of the front, edge of the blade is positioned at the periphery of the inlet of the wear plate. In this way, the entire inlet area of the front edge of the blade is utilised, thereby making it possible to achieve a minor inclination of the front edge relative to the hub plane. In one embodiment of the invention, the hub extends in the hub axis direction, i.e. the hub projects outwards to the inlet. Then the hub has an outer terminal edge which is positioned at a distance from a rear plate, i.e. the rear wear plate of an open channel impeller or the hub plate of a half-open channel impeller. In one embodiment with a projecting hub, the blade has an inner edge side which is opposite to the outer edge side facing the wear plate. The inner edge side of the blade follows the rear plate from the periphery of the channel impeller to the hub and then abuts against the hub. This means that a portion of the inner edge side of the blade is mounted on the hub. The inner end of the front edge of the blade will then be placed at the outer edge side of the hub. Thus, the inner end of the front edge of the blade is positioned closer to the inlet than the -inner edge side of the blade adjacent to the rear plate. This means that it is advantageously possible to have the inclination of the front edge relative to the hub plane smaller than in an embodiment where the inner end of the front edge of the blade is positioned adjacent to the rear plate . In one embodiment, the projecting hub has the shape of an even curve to obtain good flow- properties and also to prevent flexible objects from getting stuck on the hub or in the contact points of the blades. For a half-open channel impeller, the hub has, for example, a rotationally symmetrical shape with an increasing diameter towards the hub plate, so that the hub will be levelled adjacent to the hub plate. In one embodiment of the invention, it is thus possible to affect the inclination of the front edge by changing the size of the inlet, thereby moving the position of the outer end of the front edge, and by changing the extent of the hub towards the inlet, thereby moving the position of the inner end of the front edge. A moderate size of the inlet limits the inclination of the front edge to at least 20° or 25°, preferably at least 30°. In other words, the front edge does not at any point have an inclination which is less than 20°. The choice of the size of the inlet is limited, however, by power and efficiency requirements for the pump, and therefore it is not possible to choose any size. Moreover it is usually not possible to let the hub project too far towards the inlet of the wear plate, since this would make the passage between the hub and the blade too narrow. The channel impeller would then not satisfy the flow requirements even if the dimensions of the channels are sufficient. In one embodiment of the invention, the outer edge side of the blade is provided with a blade projection. A blade projection is a flange-shaped component which projects from the principal plane of the blade and which mainly has its extent in a hub plane. Thus the blade projection has a width perpendicular to the blade. The blade projection is arranged to cooperate with the wear plate in the same way as the outer edge side in an embodiment without blade projections. This means that the blade projection can be flat in order to cooperate with a flat wear plate or have any other shape adapted to the wear plate. However, the blade projection and the wear plate are adjusted to each other so that the blade projection and the wear plate have the desired distance from each other as the channel impeller rotates. Open or half-open channel impellers are provided with blade projections for better efficiency. The blade projection makes it difficult for the pumped liquid to pass through the gap between the blade and wear plate. The blade projection extends substantially along the entire outer edge side from the hub to the periphery. At the end of the outer edge side which is closest to the hub, the width of the blade projection decreases down to the thickness of the edge side and connects to the front edge with the shape of an even convex curve. This reduces the risk that a flexible object gets stuck to the inner end of the blade projection. An embodiment of the present invention which, in addition to having a front edge which is inclined 50° maximum, also has a transition portion with a radius of curvature relative to the hub plane of at least 20 mm and a blade projection which connects to the transition por- tion with the shape of an even convex curve, thus advantageously has no sharp edges oriented towards the flow direction, to which a flexible object could get stuck. The blade projection can project in both directions or only one direction from the outer edge side of the blade. In one embodiment of the invention, the blade projection projects from the outer edge side only on the high-pressure side of the blade. The channels of the channel impeller are limited on the one hand by the rear plate and the wear plate and, on the other, by at least one blade. There is a distance from the edge side of the blade projection to a blade projection on an opposite blade portion belonging to the same or an adjoining blade. If the blade projection is of a type which projects only on the high-pressure side of the blade, there is a corresponding distance between the edge side of the blade projection and an opposite blade portion belonging to the same or an adjoining blade. In one embodiment of the invention, said distance is constant or increases from the hub to the periphery. As a result, there is no projecting portion to which a flexible object could get stuck. In one embodiment of the invention, the channel impeller comprises a plurality of blades, for instance two to five blades. One embodiment comprises two blades. The blades can be uniform or be differently dimensioned. The wear plate must then, of course, be adjusted to the maximum axial dimension along each circumference. In one embodiment, the blades are arranged in an angularly symmetrical manner.

Brief Description of the Drawings Preferred embodiments of the invention will in the following be explained in more detail with reference to the accompanying schematic drawings, in which Fig. 1 is a perspective view of a half-open channel impeller with blade projections according to a first embodiment of the present invention, Fig. 2 is a perspective view of a half-open channel impeller with blade projections according to a second embodiment of the present invention, Fig. 3 is a perspective view of a half-open channel impeller with blade projections according to a third embodiment of the present invention, Fig. 4 is a perspective view of a half-open channel impeller without blade projections according to a fourth embodiment of the present invention, Fig. 5 is a perspective view of a half-open channel impeller with blade projections on the low-pressure side of the blade according to a fifth embodiment of the present invention, Fig. 6 is a perspective view of a half-open channel impeller with blade projections and three blades according to a sixth embodiment of the present invention, Fig. 7 is a perspective view of an open channel impeller without blade projections according to a seventh embodiment of the present invention, Fig. 8 is a perspective view of a half-open channel impeller with blade projections according to an embodiment of the present invention and shows particularly different planes and angles, and Fig. 9 is a perspective view of a pump casing according to the present invention with a half-open channel impeller according to the first embodiment of the present invention.

Description of Preferred Embodiments Fig. 1 shows a half-open channel impeller which is intended for use in a pump for pumping of wastewater. The channel impeller has two blades 1 and a circular hub plate 2 with a periphery 3. Two channels 17 are formed between the blades 1, the hub plate 2 and a wear plate not shown in Fig. 1. Centrally in the hub plate there is a hub 4 to be mounted on a drive shaft which coincides • with 3. hub axis 5. The hub plate 2 is in the shown embo- diment flat and located in a hub plane, which is a plane perpendicular to the hub axis 5. The blades 1 extend from the hub 4 to the periphery 3 and are swept back. The blades 1 thus do not extend in a straight radial direc- tion but curve away from the hub 4 to the periphery 3 like a part of a spiral. Each blade 1 comprises a web 6 projecting from the hub plate 2 and having a high-pressure side 7 and a low- pressure side 8. The blade 1 further has an outer edge side which faces away from the hub plate 2 and is provided with a blade projection 9. The blade projection 9. projects from the web 6 of the blade on the high-pressure side 7 only. The hub 4 projects from the hub plate 2 on the side of the blades 1 toward the hub plate 2 a distance that corresponds to a part of the height of the web 6. The hub 4 has an outer terminal edge 10 facing away from the hub plate 2. The hub 4 forms a rotationally symmetrical body and has an increasing diameter from the terminal edge 10 towards the hub plate 2. Each blade 1 comprises a front edge 11, i.e. the portion of the blade 1 that divides the flow when using the impeller in a pump. The front edge 11 extends between an outer end 13 adjacent to the blade projection 9 and an inner end 12 adjacent to the hub 4. The web 6 of each blade 1 has an inner edge side which is connected to the hub plate 2. Adjacent to the hub 4, the inner edge side follows and abuts against the rotationally symmetrical body of the hub 4. The inner end 12 of the front edge 11 is thus located at the terminal edge 10 of the hub. The front edge 11 forms between its inner end 12 and its outer end 13 a three-dimensional continuous curve. The front edge 11 further has a substantially constant inclination of 33° to the hub plane. Fig. 8 illustrates the inclination g of the front edge 11 to the hub plane. The inclination g of the front edge 11 between the inner end 12 and the outer end 13 in relation to the hub plane can be calculated by the formula w,herein

a = cone angle of the blade 1, i.e. the angle in a vertical plane which is perpendicular to the hub plane, through the centre of the axis 5; b = sweep angle of the blade 1, i.e. the angle of the blade in the hub plane . At the outer end 13 of the front edge 11, there is a transition portion 14 to the blade projection 9. This transition portion 14 forms a continuous, convex, three- dimensional curve. The transition portion 14 has a sub- , stantially constant radius of curvature of 75 mm relative to the hub plane. The blade projection 9 connects with the shape of a continuous curve to the transition portion 14. Each blade projection 9 extends in a plane which is parallel to the hub plane and substantially perpendicular to the web 6. The blade projection 9 has an edge side 15 facing away from the web 6. The blade projection is dimensioned so that a distance 16 from the edge side 15 to the web of the opposite blade increases from closest to the hub 4 to the periphery 3. The channel impeller is adapted to cooperate with a wear plate 23 not shown in Fig. 1, cf. instead Fig. 9. The wear plate 23 is provided with a circular inlet with a radius which is the same as the distance from the hub axis 5 to the transition portion 14. Thus the front edge 11 of the blade 1 is designed in such a manner that, in consideration of the inlet of the wear plate 23, there is no steep edge or no projecting portion in the flow direction when the channel impeller is used in a pump, and therefore elongate flexible objects do not get stuck but slide into one of the channels 17. Moreover there is sufficient space between the blades 1 and the hub 4 for rigid objects, that are usually present in wastewater, to pass. Owing to the distance 16 increasing along the channel 17, objects cannot be wedged between the blade projections 9 and an opposite blade 1 either. Figs 2-7 illustrate alternative embodiments of the present invention. These embodiments are designed in essentially the same way as the one described with reference to Fig. 1 and only the differences will in the following be explained in more detail. In Figs 2-7 equivalent components are provided with the same reference numerals as in Fig. 1. Figs 2 and 3 shown half-open channel impellers of the same type as the one described with reference to Fig. 1. The channel impeller according to Fig. 2 has, however, a steeper front edge 11 than does the channel impeller according to the first embodiment in Fig. 1. The front edge 11 shown in Fig. 2 has an inclination of 45° relative to the hub plane. The channel impeller according to Fig. 2 is adapted to cooperate with a wear plate 23 whose inlet has a smaller diameter than the wear plate 23 with which the channel impeller according to the first embodiment is adapted to cooperate. The channel impeller according to Fig. 3 has a less steep front edge 11 than does the channel impeller according to the first embodiment. The front edge 11 shown in Fig. 3 has an inclination of 25° relative to the hub plane. The channel impel - ler according to Fig. 3 is adapted to cooperate with a wear plate 23 whose inlet has a greater diameter than the wear plate 23 with which the first embodiment of the channel impeller is adapted to cooperate. Fig. 4 shows an embodiment of a half-open channel impeller without blade projections 9. Thus, this embodiment, of course, also lacks a connecting portion for a blade projection 9 adjacent to the transition portion 14 of the front edge 11. The front edge 11 and the transition portion 14 are designed in the same way as in the first embodiment of the channel impeller. Fig. 5 shows a fifth embodiment of a half-open. chan- nel impeller according to the invention. This differs from the channel impeller according the first embodiment • by each blade projection 9 projecting from the respective blades 1 on the low-pressure side 8. Thus the blade projections 9 connect to the respective transition portions 14 from the low-pressure side with the shape of an even curve. Also in this embodiment, the distance 16 between- the edge side 15 of the blade projection of one blade 1 and the web 6 of the opposite blade 1 increases from the hub 4 to the periphery 3. Here is meant the distance from the high-pressure side of the web 6 of the opposite blade 1. Fig. 6 shows a sixth embodiment having three blades 1. Each blade 1 is designed in the same way as the blades 1 in the first embodiment. Also in this sixth embodiment, a distance 16 between the edge side 15 of a blade projection 9 and the web 6 of the opposite blade 1 increases from the hub 4 to the periphery 3. Fig. 7 illustrates a seventh embodiment of the present invention in the form of an open channel impeller with two blades 1. This channel impeller lacks blade projections 9 and a hub plate 2. However, the channel impeller has a hub 4 which resembles the hub 4 in the first embodiment . Furthermore the hub is positioned in a hub plane which is a plane perpendicular to a hub axis 5. Each blade 1 has a web 6 with an inner edge side 19 and an outer edge side 18. Each blade 1 also has an end 20 facing radially away from the hub 4. The inner edge side 19 is adapted to cooperate with an inner wear plate, a rear plate, and the outer edge side 18 is adapted to cooperate with an outer wear plate with an inlet (not shown) . Two channels 17 are formed between the wear plates and the blades 1. Also this channel impeller can be said to comprise a periphery 3, viz. the circumference of a circle with its centre on the hub axis and extending through the ends 20 of the blades 1. That part of the inner edge side 19 of each blade 1 which abuts against the hub 4, the front edge 11 and the transition portion 14 of each blade are designed in the same way as in the first embodiment. Fig. 9 shows a pump casing 24 according to the present invention with a half-open channel impeller accord- ing to the first embodiment of the present invention. The pump casing 24 accommodates a wear plate 23 with an inlet for the entering flow 21. The channel impeller is provided with two blades 1, which each have a blade projection 9 projecting on the high-pressure side. The channel impeller is arranged in the pump casing so that the blade projections, have a distance of less than 50 mm to the wear plate 23. The pump casing also comprises an. outlet for the escaping flow 22.

Claims

1. A channel impeller of the open or half-open type for a centrifugal pump for wastewater, said channel impeller having a substantially circular periphery (3) , comprising a central hub (4) with a hub axis (5) and a hub plane to which the hub axis (5) extends perpendicularly, and at least one blade (1) which extends from the hub (4) to the periphery (3) , said blade (1) having a front edge (11) adjacent to the hub (4) , c ha r a c t e r i s e d in that the front edge (11) of the blade (1) is continuously curved with an inclination to the hub plane which is 20° minimum and 50° maximum.

2. A channel impeller as claimed in claim 1, wherein the blade (1) further comprises an outer edge side, which is arranged to cooperate with a wear plate (23) , and wherein the front edge (11) of the blade (1) comprises an outer end (13), and wherein there is a transition portion (14) between the outer end (13) of the front edge (11) and the outer edge side.

3. A channel impeller as claimed in claim 2, wherein the transition portion (14) is continuously curved with a radius of curvature relative to the hub plane of at least 20 mm.

4. A channel impeller as claimed in claim 2 or 3 , wherein the wear plate (23) has a central inlet, and wherein the transition portion (14) is positioned at the periphery of the inlet of the wear plate.

5. A channel impeller as claimed in any one of claims 2-4, wherein the hub (4) extends in the hub axis direction, and wherein the blade (1) has an inner portion abutting against the hub (4) so that the inner end (12) of the front edge (11) of the blade (1) is positioned at the outer terminal edge (10) of the hub (4) .

6. A channel impeller as claimed claim 5, wherein the blade (1) defines at least one channel (17) between a rear plate and the wear plate (23) , said channel (17) extending radially and circumferentially from the hub (4) to the periphery (3) , and wherein the hub (4) only projects so far and the channel (17) has such a dimension that a ball with a diameter of 60 mm is able to pass through the channel (17) . ^'"7. A channel impeller as claimed in any one of claims 2-6, wherein the outer edge side is provided with a blade projection (9) , which is arranged to cooperate with the wear plate (23) and which with the shape of a continuous curve connects to the transition portion (14) . 8. A channel impeller as claimed in claim 7, wherein the blade projection (9) has at least one edge (15) , each edge (15) being continuously curved and dimensioned so that the distance (16) from said each edge (15) to an edge (15) or surface of an opposite blade portion is constant or increases along the blade (1) from the hub (4) to the periphery (3) . 9. A channel impeller as claimed in claim 7 or 8 , wherein the blade projection (9) projects only on the high-pressure side (7) of the blade (1) . 10. A channel impeller as claimed in any one of claims 1-9, wherein said at least one blade (1) consists of two or three blades (1) , which are uniform and uni- formly rotationally symmetrically distributed. 11. A centrifugal pump for wastewater comprising a pump casing (24) with a channel impeller as claimed in claim 1.