WO1994013403A1

WO1994013403A1 - Apparatus with screw conveyor

Info

Publication number: WO1994013403A1
Application number: PCT/EP1993/003479
Authority: WO
Inventors: Denis Locke; James W. Trueman
Original assignee: Alfa Laval Separation A/S
Priority date: 1992-12-10
Filing date: 1993-12-08
Publication date: 1994-06-23
Also published as: GB9225783D0; GB2273253B; GB2273253A

Abstract

A decanter centrifuge has wear-resistant tiles (10) mounted along the edge of the screw conveyor (8), these tiles having on their rear faces dovetail projections (12) with a width tapering radially outwardly for cooperation with similarly formed grooves (15) provided in backing plates (11) for the tiles. The tiles are held in place by abutment with adjacent tiles whereby only the end tiles require additional retention elements (18).

Description

Apparatus with screw conveyor

This invention relates to apparatus, especially centrifuges, having a rotatable scroll or screw conveyor for moving material towards a discharge port. More particularly the invention is directed to providing wear-resistant elements at the working edge of such a conveyor. In a decanter centrifuge a helical screw is mounted within the centrifuge bowl and is rotated at a speed slightly different to that of the bowl so that the screw conveyor causes solids collecting on the inside surface of the bowl to be conveyed along the bowl to an outlet. Due to abrasion by the solids, the edge of the screw conveyor can suffer rapid wear.

As a solution to the foregoing problem, it is known to mount plates or tiles of wear-resistant material along the edge of the screw conveyor blade. However, the mounting of these wear-resistant members remains a problem. If the members are attached by bolts or other mechanical fasteners, it can have adverse effect on the rotational balance when the conveyor is rotated at high speed. Attempts have also been made to braze or weld preformed wear-resistant members directly to a conveyor edge, which has not proved satisfactory because the high heat input over a wide area sometimes causes serious warping of the conveyor and perfect adhesion is impossible to ensure. An imperfect bond can lead to a member becoming detached and causing further damage to the conveyor. It is also difficult to repair such conveyors by welding or brazing replacement members in place because it requires the application of large quantities of heat to a broad area, which can effect adversely the bond of adjacent members. It is also known to fix backing plates to the screw conveyor, e.g. by welding, and to fix wear-resistant members, such as tungsten carbide tiles, to the respec¬ tive backing plates by means of mechanical interlocks and/or fasteners as described in US-A-4328925,

US-A-4416656 and GB-A-2048728. These constructions are generally satisfactory, but are complicated by the particular methods of fastening the tiles to the backing plates. In particular, tile replacement operations are not facilitated. In GB-A-1492210 there is described a conveyor in which retaining members are welded at inter¬ vals around the conveyor edge and have undercut side edges so that each pair of retainers defines a dovetail slot. Wear resistant tiles of ceramic material shaped by moulding are formed with dovetail projections on their rear surfaces. The tiles are engaged in the dovetail slots in a radially outward direction, and are held against subsequent inward movement by abutment with pins welded to the screw conveyor adjacent the inner edges of the tiles. This arrangement has the disadvantages that accurate setting of the retainers on the blade of the screw is essential and is difficult to ensure. Further¬ more, the dovetail slots are defined to a large extent by the blade of the conveyor and, as the blade is itself cut and formed from sheet material and has a double curvature, great accuracy is needed to ensure adequate location and support for the relatively brittle tiles of tungsten carbide.

The object of the present invention is to improve the tile retaining arrangement described in GB-A-1492210 and, thus, to reduce or eliminate its above mentioned disadvantages considerably. In accordance with the present invention this is accomplished by a screw conveyor comprising a series of backing plates attached to the screw along the edge thereof, and tiles of wear-resistant material secured to respective backing plates by complementary connecting means on the backing plates and tiles, adjacent tiles being in abutment and said connecting means being so arranged that each tile can be engaged directly with its respective backing plate by sliding movement in a direction substantially radial to the axis of the screw. It is, thus, essential that said connecting means for each tile is arranged on the relevant tile and a single backing plate. Preferably, each tile located between two adjacent tiles is retained against radially inward movement by the abutment of said tile against the adjacent tiles.

With such a screw conveyor almost all of the tiles can be secured onto their backing plates at an adequate location with a great accuracy by a simple operation of slotting the tiles into place, after which they are retained against becoming disengaged by abutment with adjacent tiles. Only those tiles on the end backing plates require additional means to keep the tiles in place. Thus, after one end tile has been mounted in position on its backing plate on the screw, all the intermediate tiles can be positioned in turn on their respective backing plates by a simple slotting operation. Similarly, when one end tile has been removed from the screw conveyor, all the intermediate tiles can be easily removed by sliding them free of their backing plates. Therefore, the invention considerably simplifies both initial assembly of the tiles on the screw conveyor and subsequent tile replacement operations. In a preferred construction the tiles are pressure moulded sintered tungsten carbide material and the connecting means on each tile is formed by an integral part of the tile in the shape of a dovetail projection on its rear surface. The backing plates are metal castings, preferably precision castings manufactured for example by a lost wax process, and are made with dove¬ tail grooves for receiving the tile projections. By these manufacturing methods, the accurate fit and mating between the tiles and backing plates, which is necessary to ensure adequate support for the brittle tungsten carbide material, can be obtained without difficulty.

The dovetail grooves and projections taper in width in the radially outward direction, which serves to ease further tile mounting and demounting operations. Furthermore, the taper ensures that under the centri¬ fugal forces acting on the tiles during conveyor rotation they are pressed into tighter engagement with their respective backing plates. The tiles on the extreme end backing plates may be retained in place by a keeper member fastened, such as by welding, to a related inner end portion of the backing plate so that the keeper member contacts the edge of the tile over at least a major part of its area in order to spread the forces acting between them.

A more complete understanding of the invention will be gained from the following detailed description of an exemplary embodiment, reference being made to the accompanying drawings in which:

Figure 1 is a fragmentary schematic axial section through a centrifuge incorporating a conveyor in accordance with the invention; Figure 2 is a fragmentary elevation showing a series of tiles mounted on the blade of the conveyor;

Figure 3 is a section taken along the line 3-3 in Fig. 2;

Figure 4 is a plan view illustrating the dovetail interlock between a tile and its backing plate.

Figure 1 illustrates a decanter centrifuge of well known basic construction. It comprises an axially elongated bowl 1 within which a conveyor 2 is mounted coaxially. The conveyor includes a hollow shaft the interior of which serves to as a feed chamber 4 for mixture to be separated, the feed mixture being supplied through an inlet tube 5. The mixture passes through parts 6 from the chamber 4 into the separation chamber 7 confined between the bowl and the conveyor. Carried on the conveyor shaft is a helical flight or blade 8 which forms a screw conveyor for conveying solids along the bowl to an outlet port 9 provided in a tapered end portion of the bowl. In operation of the decanter centrifuge, the bowl and conveyor are both rotated at high speed, but the conveyor is driven at a slightly different speed to the bowl so that, due to the relative rotation between the conveyor and the bowl, the screw conveyor conveys the solid material, which collects on the inside surface of the bowl due to the centrifugal forces, towards the outlet. The separated liquid leaves the bowl through another outlet (not shown) at the other end of the bowl. Provided along the edge of at least part of the conveyor blade edge is a series of wear- resistant tiles 10, each tile being mounted on screw blade by a respective backing plate 11 fixed to the blade. Each tile 10 is a unitary member, preferably made of tungsten carbide, and formed on its rear face with a projection 12 of dovetail section extending the full radial length of the tile. The projection 12 tapers in width in the radial direction the angle α of taper relative to a radial line β bisecting the tile being at least equal to and preferably slightly greater than the angle β between the lateral edges of the tile and said radial line.

Each tile backing plate 11 is provided on its rear face with a shoulder adapted to rest on the outer edge of the screw conveyor blade 8. The backing plate is fixed to the blade by welds 14. On its front face the plate 11 has a tapering dovetail groove or slot 15 of comple¬ mentary configuration to the tile projection 12. The backing plate is conveniently manufactured as a precision metal casting, such as by the so called "lost wax" casting technique. The tile can also be manufac- tured to close dimensional tolerances, e.g. by a moulding and pressing process, and in this way a good fit can be ensured between the interlocking dovetail formations of the backing plate and the tile. Should it be thought desirable a filler or grouting material may be applied as a thin layer between the backing plate and the tile to fill any spaces which might otherwise be formed between them. The tile and backing plate are of substantially the same width and, in particular, the tile does not project beyond the edges of the backing plate when properly mounted thereon.

At least the two backing plates for the two end tiles include inwardly directed extensions 16 of reduced thickness so that their front surfaces are substantially level with the bottom surfaces of the dovetail slots 15. Fixed onto the extension is a keeper 18 having the form of a rectangular metal bar which can be welded in place so that it extends over the full width of the dovetail slot. The keeper is preferably of substantially the same thickness as the tile so that it abuts against the tile over substantially the entire surface area of the inner edge of the tile in order to spread any forces which may be encountered tending to push the tile inwardly against the keeper.

In the manufacture of the centrifugal conveyor the tile backing plates can be positioned and welded in succes¬ sion along the edge of the screw blade 8, these plates being positioned against one another essentially without intervening gaps so that their location prior to welding is easily determined once the first plate has been attached to the blade. To ensure the required accuracy of fitting the backing plates, a simple shimming or spacing technique may be employed to ensure the correct relative positioning of adjacent backing plates. The tiles 10 can be inserted onto the respective backing plates in turn, starting from either end of the series of plates. By virtue of the tile being sector-shaped, each tile located between two adjacent tiles is restrained against radially inward displacement, and thereby disengagement from its backing plate, by the abutment of the tile against the laterally adjacent tiles. The tapered configuration of the dovetail projection and slots facilitates the mounting of the tiles by limiting the radial displacement necessary to bringing them into interlocking engagement and thereby avoiding sliding movement between close-fitting parts. For the same reasons removal of the tiles e.g. for replacement is also facilitated. Only the two end tiles require additional means, i.e. keepers 18, to maintain them in engagement with the backing plates. These end tiles are obviously engaged with the respective backing plates before the keepers are welded into position. This operation can be per¬ formed either before or after the backing plate has been welded on to the blade of the screw conveyor.

When one or more tiles need to be replaced e.g. due to damage or wear, the keeper 18 is detached from one of the end backing plates and the tiles 10 can then be removed in turn by displacing each tile substantially radially inwardly to disengage the dovetail projection from the dovetail slot. Remounting of the tiles, whether the same tiles or replacements, is effected exactly as described above.

Claims

1. A screw conveyor comprising a series of backing plates (11) attached to the screw along the edge thereof, and tiles (10) of wear resistant material secured to respective backing plates by complementary connecting means (12, 15) on the backing plates and tiles, adjacent tiles being in abutment, c h a r a c ¬ t e r i s e d i n that the connecting means (12, 15) are so arranged that each tile (10) can be engaged directly with its respective backing plate (11) by sliding movement in a direction substantially radial to the axis of the screw.

2. A screw conveyor according to claim 1, wherein the connecting means comprise a projection (12) of dovetail cross-section and a slot (15) of complementary shape in which the projection engages.

3. A screw conveyor according to claim 2, wherein the dovetail projection (12) is provided on the tile (10), and the slot (15) is formed in the backing plate (11).

4. A screw conveyor according to claim 2 or 3, wherein the dovetail projection (12) and slot (15) taper in width in the radially outward direction.

5. A screw conveyor according to any one of claims 1 to 4, wherein the lateral edges of each tile (10) are substantially aligned with and do not project beyond the lateral edges of the backing plate (11) carrying said tile.

6. A screw conveyor according to any one of claims 1 to 5, wherein each tile located between two adjacent tiles is retained against radially inward movement by the abutment of said tile against the adjacent tiles.

7. A screw conveyor according to any one of claims 1 to 6, wherein the backing plate at each end of said series of backing plates has a keeper member (18), fastened thereto to prevent radially inward movement of the tile carried thereon.

8. A screw conveyor according to claim 7, wherein the keeper member (18) is arranged to abut the inner edge of the tile over at least a major part of the area of said edge.

9. A screw conveyor according to claim 7 or 8, wherein the keeper member (18) is fixed to an extension (16) of the backing plate, the extension having a forwardly directed surface against which the keeper member is secured.

10. A screw conveyor according to any one of claims 1 to 8, wherein each backing plate (11) is a metal casting.

11. A screw conveyor according to any one of claims 1 to 10, wherein each tile is a pressure moulded unitary member of tungsten carbide material.

12. A screw conveyor according to any one of claims 1 to 11, wherein the connecting means on each tile is formed by an integral part of the tile.

13. A decanter centrifuge having a screw conveyor as defined in any one of the preceding claims.