GB2149854A - Self priming centrifugal pump - Google Patents

Abstract

The pump for pumping sewage (1) comprises a bladed impeller (3) which is rotated in a casing (2) having an axial inlet, an outlet (7), and a swirl chamber (6) oriented tangentially to the impeller (3). The swirl chamber (6) is arranged so that in the priming stage the liquid enters the chamber (6) where it creates a vortex (18) before re- entering and recirculating around the casing (2). On entering the swirl chamber (6) the liquid is accelerated outwards and then directed radially back inwards towards the impeller (3) by the shape of the chamber (6) so that it penetrates the drag stream (16) of the impeller (3) to strip air bubbles (19) from the surface of the impeller blade (5). The stripped air enters the vortex (18) to escape through the outlet (7). This creates suction at the inlet and continues until the pump is fully primed and liquid delivery commences. <IMAGE>

Description

SPECIFICATION Centrifugal pump This invention relates to centrifugal pumps comprising a bladed impeller which is driven to rotate in a casing, an axial inlet to the casing, and a peripheral delivery outlet, and is particularly concerned with such pumps of the self-priming type which are commonly used for pumping sewage.

The suction capacity, and thus the performance, of such pumps is directly associated with the way in which the air which collects in a cavity at the centre of the impeller can be removed. As soon as the impeller rotates and picks up the quantity of liquid which is initially necessary in every self-priming pump and which fills the pump from its priming chamber to just above the centre of the impeller, a drag stream rotating about the axis of the impeller forms in the annular or spirally shaped casing, the drag stream consisting almost exclusively of liquid in an outer zone and a mixture of liquid and air in an inner zone. There is no sharp boundary between these two zones, and the drag stream which is held in suspension by the revolving impeller, has absolutely no sucking action.

The drag stream and the central cavity are surrounded by a ring of liquid which occupies the remaining part of the spiral or annular casing, and the air-liquid mixture must penetrate through the outer liquid ring during suction in order that the cavity and air accumulated at the centre of the impeller, and also the air bubbles entrained in the liquid stream, can be reduced. This is counteracted, however, by considerable centrifugal forces in the rapidly rotating liquid layer generated by the impeller.

German specification No. AS 1 528 879 discloses a self-priming device for a centrifugal pump in which a tube, open at both ends, is situated in the delivery outlet so that the tube extends into the pump as far as the periphery of the impeller. The tube, which is rectangular, possesses at its inner end an oblique portion which penetrates into the mouth of the revolving drag stream, and is designed to branch off a portion of the internal zone of the drag stream which is heavily laden with air, for conduction to the delivery outlet. The air in the extracted portion can ascend undisturbed with more or less laminar flow in the tube and passes into a pressure line connected to the outlet, whereas the liquid overflows at the outer end of the tube in the region of the pump outlet and returns back into the revolving drag stream.By this separation of air and liquid, the air can be conducted out of the pump while the liquid is retained in the pump. The resulting reduced air volume in the pump chamber creates a suction at the pump inlet, producing a vacuum in the priming chamber for lifting liquid situated below the level of the pump.

Pumps are also known in which separation of the air and water components is achieved by partitions, flaps, or other displacement and guide members disposed in the outlet region and subdividing the mainstream. All the known self-priming centrifugal pumps therefore possess obstructing internal components in the outlet region, which increases the costs of manufacture and assembly. Furthermore, such components in centrifugal pumps used for pumping sewage are disadvantageous because of the risk that they will accumulate dirt and thereby cause a blockage.

The aim of the present invention, therefore, is to provide a centrifugal pump which is simpler and less expensive to construct than the known pumps, and which operates without obstructing internal components. The invention starts from the fact that, as described above, in addition to the central drag stream in a centrifugal pump, a secondary stream also flows around the periphery of the impeller, and the liquid-air mixture which is driven around in the drag stream and which continually renews itself until delivery commences must be continually led away to the delivery outlet through the secondary stream, this being the purpose of the obstructing guide members in the known pumps.The secondary stream comprises an almost airless volume of liquid having a through flow velocity of zero, and keeps the liquid particles emerging from the impeller in motion and ensures that liquid again enters the impeller. The airless liquid particles are flushed back into the impeller by the circulating liquid-air drag stream.

According to the present invention the aim may be achieved by providing a centrifugal pump of the kind described with a swirl chamber disposed tangentially to the impeller between the casing and the delivery outlet.

The swirl chamber preferably diverges in a direction towards the delivery outlet, preferably comprising a lower, initial diverging portion leading into an upper portion of constant cross-section. Such a swirl chamber may have its initial portion funnel-shaped, having the smallest diameter near the lowest point of the impeller when the pump is oriented with the swirl chamber directed vertically upwards towards the delivery outlet, and the maximum diameter approximately level with the centre of the impeller.

By means of the swirl chamber leading to the delivery outlet and having its entering flow tangential to the impeller diameter, the secondary stream can be influenced in such a way that, as a result of the differently oriented centrifugal force resulting from the special funnel-shaped form of the chamber in the zone of the impeller, the liquid otherwise accelerated radially to the shaft of the impeller passes more rapidly back into the impeller and thereby cuts off a portion of the gas or air entrained in the cavity. The form of the swirl chamber does not interfere with the general flow, so that the cut off gas circulated by the impeller blade can enter the region of the secndary vortex and be flushed onwards by the latter; i.e. it is pushed away by the centrifugal force of the rotating, denser liquid into the rotational core inside the swirl chamber.Here the gas ascends together with the upwardly spiralling liquid and passes from the liquid through the delivery outlet into the pump circuit.

An example of a centrifugal pump in accordance with the invention will now be described with reference to the accompanying schematic drawings, in which: Figure 1 is a vertical section through the pump in a plane perpendicular to the impeller axis, showing the impeller in a first position during its rotation; Figure 2 is a view similar to that of Fig. 1 showing the impeller in a second rotary position relative to the casing and swirl chamber; Figure 3 is a view similar to that of Fig. 1 and showing the impeller in a third rotary position; and Figure 4 is a view similar to that of Fig. 1 and showing the impeller in a fourth rotary position.

The pump 1 comprises a casing 2 and an impeller 3 having a drive shaft 4 and a blade 5 mounted to rotate in the casing. The annular or spirally shaped casing 2 leads into a swirl chamber 6 which is funnel-shaped from near the lowest point T reached by the impeller to a region M approximately level with the centre of the impeller and acts in the manner of a cyclone. From this region M the swirl chamber 6 continues with a uniform crosssection as far as the delivery outlet 7. Figs. 1 to 4 show in dot-and-dash lines the crosssection of the spiral and of the transition to the swirl chamber in the region of the outlet end, as viewed in the direction of flow, of the annular spiral casing 2, the free cross-section of which at the transition is equal to the width of the impeller 3.

Axially of the impeller 3, the casing 2 has inlet connections, not shown, to which a priming chamber (also not shown) is attached by means of flanges. The side walls of the casing 2 have only a slight clearance relative to the impeller 3 and its blade 5, whereas the outer wall of the casing 2 is spaced from the path of the trailing edge 5a of the impeller blade 5 by a distance which is not less than the width of the impeller 3. The circular cross-section of the casing 2 likewise positively influences the forming of the secondary stream.

After switching on, the pump 1 first sucks empty the priming chamber, and the impeller 3 fills the surrounding casing 2 with liquid.

During the first revolutions of the impeller 3, no delivery takes place, but a liquid ring 8 forms in the casing 2 with a large centrifugal force directed radially outwards, so that the liquid flows tangentially laterally of the wall of the swirl chamber 6 as indicated by the arrows 9. From the funnel-like lower portion of the swirl chamber, which has a circular cross-section at each horizontal section, further centrifugal forces are produced which cause the liquid to flow back into the impeller 3 as indicated by the arrows 1 2. The general flow in the chamber is not however disturbed.

The gas and air particles 1 3 contained in the liquid can ascend in the large, cylindrical portion, acting as a balancing vessel, following the funnel-shaped part of the swirl chamber 6 and can leave through the delivery outlet 7.

During suction, very large velocities become established, which correspndingly influence the centrifugal force of the liquid ring 8 and of the funnel-shaped portion of the swirl chamber 6 supplied from it. The rotating impeller 3 with the central, air-filled or gasfilled cavity 14, drags a body of air 1 5 behind it from the trailing edge 5a of the blade. The drag stream 16, likewise produced by the impeller 3, contains centrifuged air particles circulating as satellites in the liquid drag stream, and frequently also contain air in a large bubble cavity 1 7 on the surface of the blade 5.

With increasing length of the distance travelled, the liquid-free dragged body of air 1 5 lengthens as shown in Figs. 2 and 3, this body being determined largely by the rotational speed of the impeller 3, the angular velocity of the liquid ring 8, the sum of the liquid columns suspended at the suction side and supported at the pressure side, and the form of the impeller blade 5. In this way the air enclosed in the cavity 1 7 arrives in the region of the secondary vortex, which flushes the air onwards and, due to the centrifugal force of the revolving denser liquid, separates it off into the rotary core denoted by the arrows 18, where the air, together with the ascending liquid and the other air particles 13, flows to the delivery outlet 7. The air constituents in the liquid decrease with increasing number of revolutions of the impeller, the kinetic energy of the liquid decreasing only slightly due to the centrifugal forces, so that the liquid passes back again into the liquid ring 8 in the casing 2 from the swirl chamber 6.

Figure 4 illustrates the position where the blade trailing edge 5a has entered the region of the secondary flow 9. Promoted by the special funnel-like form of the swirl chamber 6, liquid from the flow 9 penetrates more rapidly into the impeller 3, due to the differently directed centrifugal force, and encloses a portion of the dragged body of air as an air bubble 1 9. The impeller blade 5 in the vicin ity of its leading edge 5b prevents the liquid swell extending towards the centre of the impeller from penetrating into the cavity 14, and the enclosed portion 1 9 of the dragged air body 1 5 is thus cut off from the interior of the impeller. The rotating impeller blade 5 drives the air bubble 1 9 in front of it and flushes it into the rotary core 1 8. This repeats itself at each revolution of the impeller 3, and with increasing suction and delivery head the drag angle in the region of the drag stream 1 6 decreases. Simultaneously therewith, the velocity of the liquid increases there and in the rotary core 1 8. From this relationship, the pumping performance can be explained in that during suction the head is produced by the liquid ring, and in delivery the head is produced by the impeller blade, the velocity of the liquid ring 8 decreasing according to the flow rate.

Claims (5)

1. A centrifugal pump comprising a bladed impeller which is driven to rotate in a casing, an axial inlet to the casing, and a peripheral delivery outlet, characterized by a swirl chamber between the casing and the delivery outlet and oriented tangentially to the impeller.

2. A pump according to Claim 1, in which the swirl chamber diverges in a direction towards the delivery outlet.

3. A pump according to Claim 2, in which the swirl chamber comprises an initial diverging portion which leads into a portion of constant cross-section.

4. A pump according to Claim 3, in which the initial portion of the swirl chamber is funnel-shaped and has its smallest diameter near the lowest level of the impeller when the pump is oriented with the swirl chamber directed substantially vertically upwards towards the delivery outlet, the largest diameter of the funnel-shaped portion being approximately level with the centre of the impeller.

5. A pump according to Claim 1, substantially as described with reference to the accompanying drawings.