Non-clogging pump

Abstract: The invention concerns a propeller pump to be used for pumping of liquids containing pollutions such as elongated fibres. According to the invention the hub (1) of the impeller is conically formed and the vanes (2) are given a substantially radial direction meaning that the turning axis (6) of the vane (2) is situated in the center of the vane.

Description: The invention concerns a pump of the roto-dynamic type comprising an impeller rotating in a pump housing. Pumps of this type may roughly be divided into centrifugal pumps and axial-flow pumps. The centrifugal pump comprises an impeller which consists of a hub and at least one cover disc provided with a number of vanes, a so-called open impeller. A so-called closed impeller is provided with two cover discs with intermediate vanes. The liquid is in both cases sucked axially into the center of the impeller and leaves the impeller essentially in a tangential direction from the circumference. The axial-flow pump is of an open type, having a slot between the vane and the housing and differs from the centrifugal pump in that the liquid moves essentially in an axial direction through the pump. The tangentially directed velocity vector which is obtained after the impeller, is reduced in a number of guide vanes in the housing downstream of the impeller. Said guide vanes also normally serve as support elements in the housing construction. When pumping waste water and certain types of industrial process water containing elongated fibres, the operation may be disturbed by rags, fibres etc getting stuck to the leading edges of the vanes on the impeller as well as on the guide vanes in the pump housing. This build up can dramatically decrease the efficiency of the pump. The result will normally be that the flow decreases and the power demand increases. One way to make the objects leave the vanes is to let the impeller rotate backwards at certain intervals, but this is of course not a solution to be recommended. Another way to diminish the risk for clogging can be to provide the pump with a cutting means which comminute the pollutions before they are sucked into the impeller. A device of this sort is shown in the Swedish Patent No 8205774-6. A disadvantage is then that the cutting means are quickly worn out and the clogging problems may become even worse. It is also known to design the vanes with backward swept leading edges in the flow direction, whereby the pollutions more easily glide off. An example is shown in the European published publication 237 921. This impeller has however a design which deteriorates the cavitation abilities. Propeller pumps on the market today are designed with a hub of a spherical shape provided with vanes which are turnable around axes mainly perpendicular to the direction of the rotation axis to adjust the pitch angle. This possibility to control the vanes means that a wide range of flow capacity is covered with one and the same pump. The spherical form also means that a vane may be turned into different angles while keeping the same slot towards the hub thus minimizing losses in the slot When designing an axial-flow pump it is often desired to keep a high specific rotation speed, i e a maximum flow should be obtained at a given speed. This means that the inlet area, the area between the hub and the wall of the housing, should be maximized. As the outer diameter of the housing is limited because of the cavitation problem, there is only a decrease of the diameter of the hub left. Spherical formed hubs always mean problems of a flow technical art when the radius is shortened as the possible geometric length of the connection between the hub and the vanes also shortens. If an acceptable efficiency should be obtained, said connection length must never be less than a certain value and this means that there must be a compromise between the two goals: large flow and high efficiency, respectively. For pump impellers where the vanes should be adjustable to obtain different pitch angles, it is desired that the entire vane is kept collected around the axis around which the vane is turned when adjusted. Then a minimal axial translation movement occurs during rotation and a flow effective change of angles for all profile sections (chords) is obtained. If, for a conventional propeller pump having a relatively high specific rotation speed, the profile sections that form the vane are swept backwards in such away that the leading edges become selfcleaning, the performance of the pump will be almost unchanged, provided the angle is kept. However, short connection line between the hub and the vane, means that also the trailing edge will be strongly swept backwards and this means that there is no optimum turning axis. The swept impeller will thus be less effective after turning to another angle than the non-swept impeller. This means that it is impossible to design a turnable and swept vane having an optimum performance if a spherical formed hub is used. In order to obtain a vane that is sufficiently collected around its turning axis and simultaneously being swept backwards, it is known to shorten the chord lengths in the direction of the periphery of the vane, thus limiting the backward sweep of the trailing edge. This means however, that the cavitation abilities deteriorate. The spherical formed hub could be avoided by moulding hub and vanes in one single piece. However, this does not give the same flexibility and is thus expensive. The problem to obtain a propeller pump which fulfils strong demands concerning flowing abilities, flexibility and competitive costs is obtained by help of the device stated in the claim. The invention is described more closely below with reference to the enclosed drawings. Fig 1 shows a comparison between known technique and the invention in an axial and a radial section. Fig 2 shows an impeller vane according to the invention in an axial and a radial section, while Fig 3 shows an impeller according to the invention in a perspective view. In the drawing 1 and 1' stand for an impeller hub, 2 and 2' an impeller vane having a leading edge 3 and 3' respectively and a trailing edge 4 and 4' respectively. 5, 5' stand for the connection between the hub and the vane and 6, 6' stand for the position of the turning axis of the vane. In Fig 1 a comparison is shown between a conventional impeller vane (dotted lines) and a vane according to the invention (continuous lines) for pumps having the same inlet areas. The leading edges 3 and 3' respectively have been designed almost corresponding while the trailing edges 4 and 4' have been designed differently which is possible thanks to the conically formed hub according to the invention. As can be seen in the comparison a considerably wider vane area in the direction towards the vane connection is obtained by help of the invention as here, the trailing edge is not swept backwards to the same extent. This means that the axis 6, around which the vane is turned at angle adjustment, will have a central position in the vane and thus good flowing conditions are kept at different angles. By a conventional vane on the other hand, the turning axis 6' is not central which means that its abilities deteriorate as soon as the angle is changed. The facts that the connection surface (5) between the vane and the hub (1) is more extended than the corresponding surface (5') for the conventional vane and also more perpendicular to the turning axis (6) and (6') respectively, add to better abilities for the new vane according to the invention.

Claims: A propeller pump impeller comprising a hub (1) and a number of vanes (2) having backwardly swept leading edges (3), as seen in the flow direction, in such way that the leading edges (3) become self-cleaning, characterized in that said vanes (2) are adjustable to said hub (1) at different angles by being turned around a turning axis (6), that the hub (1) has an axial cross section which conically increases in the flow direction, and that the profile section chord, of each vane is constant, the turning axis (6) of each vane (2) being located close to the center of each profile section.

Inventors: ARBEUS ULF, ARBEUS, ULF
Applicants: FLYGT AB ITT, ITT FLYGT AKTIEBOLAG
Application year: 1992
Main classification: F04D29
IPC codes: F04D29, F04D7, F04D11