An agitator

Abstract

An agitator (12) for a pump for pumping particulate solids which are settled in and beneath a liquid, the agitator comprising a plurality of vanes on a body and walls of the body defining a hole with a female thread for screwing the agitator onto an end of a shaft (7) of the pump; wherein the female thread is a multi-start thread.

Description

[0001] AN AGITATOR

[0002] This invention relates to an agitator for use in a method of and apparatus for agitating particulate solids which have settled in and beneath a liquid in order to assist pumping of the solids by increasing the solid loading of the liquid, and for assisting in pumping of mixtures, suspensions and viscous liquids. The invention has particular, but by no means exclusive, application to pumping out ponds, reservoirs or lagoons in which have been deposited the products of mineral treatment plants, in which case they contain water together with settled and sedimented tailings and fines. Obviously, generally similar applications such as pumping out sand-bottomed ponds and lakes, and indeed dredging channels, are possible.

[0003] In the particular application mentioned, the situations encountered may include ponds which have for many years stood idle, so that the fines and tailings have deposited and the sediment has become well compacted, and may at the other extreme include reservoirs or bunkers where fines have settled merely overnight. In any case, the sediment has intrinsic value, realisable if it can be recovered reasonably easily, while it may of course be desired to re-use the ponds or fill them and "landscape" the area.

[0004] A problem which is encountered with all types of sediment is that it is not sufficient merely to locate a pump at or just above the top of the sediment, because the pump would only pump water and hardly any of the fines would be entrained. In effect the water above the sediment would be removed and the upper part of the sedimented layers would become dewatered and it would not be possible to lower the pump into the sediment with a view to pumping it out. Thus, ways have been developed for disturbing sediment, particularly in the case of sand, with a view to increasing the solids content of the liquid. One way is to fit, beneath a submersible centrifugal pump which is designed to have a downwardly opening inlet, an agitator which is designed to cause a flow of water in a downward swirling motion so that the flow sweeps across the surface of the fines in the vicinity of the pump with a stirring action to loosen and entrain the fines in the water which is then pumped away. While this type of construction is successful with certain types of settled particles, its area of influence is rather small and its method of operation scarcely encourages induction of material into the eye of the pump. This means that any successful sediment agitation and loading of the liquid with solid particles will only be in the vicinity of the pump, so that it is necessary repeatedly to reposition the pump if any but the smallest area is to be cleared. A pump of the type just described is disclosed in British Patent Specification 2 070 687.

[0005] Alternatively a vaned agitator can be used which is attached to a pump and rotated such that, in use, pressure waves are set up in the liquid and the settled solids, and the solids are loosened so as to be pumpable, the agitator being arranged to impart to its surroundings an axial component of movement towards the pump inlet. Such an agitator is disclosed in EP 0,110,562.

[0006] The present invention relates to an agitator of either of the above described types. Due to the use of such agitators with solid particles, agitators can wear out and therefore need to be replaced at regular intervals. Replacement should be quick to minimise down time.

[0007] The requirement for quick replacement is addressed by the present invention.

[0008] The present invention provides an agitator for a pump for pumping particulate solids which are settled in and beneath a liquid, the agitator comprising a plurality of vanes on a body and walls of the body defining a hole with a female thread for screwing the agitator on to an end of the shaft of the pump; wherein the female thread is a multi-start thread. This arrangement allows for quicker assembly of the pump because a two start thread means that the agitator moves axially at least twice as far for one rotation than for a single start thread.

[0009] Additional optional features deal with the difficulties of combining quick replacement with sufficient strength (to resist shock forces, for example) and ability to operate in a dirty environment and the need to keep replacement parts inexpensive is addressed by the present invention.

[0010] In an embodiment a first thread of the multi-start thread has a different cross-sectional shape to that of a second thread of the multi-start thread. This allows the first and second threads to be tailored to achieve different functions. For example, the first thread can have its cross-sectional shape chosen ease of manufacture, easier and better ability to operate in contaminated environments, and / or for ease of being made up. To this end, the first thread may be a trapezoidal thread with a thread angle of between 61° and 68°, preferably between 64° and 66° at most preferably of 65°. The second thread may be optimised for strength, for example resistance to torque and / or shock forces. The second thread may have a thread angle of between 25° and 35°; preferably between 28° and 31°.

[0011] The number of threads per inch is chosen to provide a balance between strength and speed of assembly. It has been found that the best balance between strength and speed of assembly is achieved when the female thread has more than 12 threads per inch and fewer than 14 threads per inch. Having fewer than 12 threads per inch results in low thread strength, whereas having more than 14 threads per inch results in slow make-up. Having more than 12.5 threads per inch and less than 13.5 threads per inch is found to be particularly beneficial with the number of threads per inch preferably being 13.

[0012] The invention will be more clearly understood from the following description which is given by way of example only with reference to the accompanying drawings, in which:

[0013] Fig l is a vertical part sectional view of a centrifugal pump provided with an agitator below its downwardly facing inlet;

[0014] Fig 2 is an expanded perspective view of the shaft assembly of the pump of Figure 1;

[0015] Figs 3 and 4 are side views of the agitators suitable for use with the invention;

[0016] Fig 5 is a plan view and end view of an end of the shaft;

[0017] Fig 6 is detail of area A from Figure 5; and

[0018] Fig 7 is a cross-section through line of B-B of figure 5.

[0019] Shown in Figure l is a submersible centrifugal pump having a casing 100 containing a motor 50 from which extends a drive shaft 7 on which is mounted a pump impeller 1 having vanes to urge liquid from an inlet centrifugally outwards and then upwards through water channels in the outer part of the casing 100 to emerge from an outlet from the pump. Attached on the shaft 7 at an end portion of the shaft 7, is an agitator 12 which as shown has a (cylindrical) boss or body on which generally radially extending vanes are mounted.

[0020] In the Fig 1 embodiment the agitator 12 extends downwards within a protective apertured skirt comprising a circular rim 15 supported on legs from the bottom of the pump. The rim 15 is designed to rest on a hard surface to protect the agitator 12. The skirt may assist in propagating shock waves, e.g. by itself resonating. The blades / vanes of the agitator 12 are inclined at a relatively small angle to the axis of the agitator 12 and the inclination is such that the axial component of thrust induced in liquid upon rotation is away from the pump to assist in fluidisation of the surrounding material with the shock waves created enabling it to be pumped.

[0021] The impellor 1 has a through-hole which allows the shaft 7 to pass all the way through the impeller 1. This allows further components such as a lock nut 40 and the agitator (sometimes called an inducer) 12 to be attached to the end of the shaft 7 and to be rotated along with the impeller 1. This is advantageous particularly for a submersible slurry pump because the agitator or inducer 12 can agitate the slurry at the entrance to the pump thereby sending shock waves down below the pump making settled solids start to flow, in a way similar to tapping one’s foot on the sand on a beach.

[0022] In an embodiment, the impeller 1 is tightened up against a boss on the shaft 7 such that it abuts the boss. The shaft 7 may include a male spline comprising a one or more teeth or grooves to interact with grooves or teeth in the through hole of the impellor 1 to prevent relative rotation. The shaft 7 comprises an engaging means at a first location for an impellor (an impellor engaging means) and a thread at a second location nearer the end of the shaft 7 (an agitator thread). The impellor engaging means may be different to the agitator thread. For example, the impellor engaging means may comprise a spline or a keyway that provides an antirotation function. Alternatively, the impellor engaging means may comprise a thread that is different to the agitator thread, for example, by comprising any one of a different pitch size, a different threading direction, and a different number of starts (i.e., it can be a single-start thread or a multi-start thread comprising two or more thread starts). The agitator thread engages with the agitator 12 on an end of the shaft. The impellor engaging means engages with the impellor 1.

[0023] The impellor 1 is surrounded on all but the side comprising the vanes by a lower housing 20. The shaft 7 enters the lower housing 20 through a shaft sleeve for reducing leakage of slurry between the shaft and the housing 20. The impeller 1 sits inside the housing 20 on the shaft 7 as described above. A wear plate 30 encloses the impeller 1 within the housing 20. The impellor may comprise vanes that are 0° to the impellor axis and that urge liquid from an inlet centrifugally outwards and then upwards through water channels in the outer part of the casing 100.

[0024] An optional lock nut 40 is provided on the shaft 7. The agitator or inducer 12 is attached to the end of the shaft 7 and sits outside of the lower housing 20. Thus, the agitator or inducer 12 is differentiated from an impellor (such as impellor 1, the impellor disclosed in CN 202520631 U, and the impeller disclosed in CN 106151318 A) in that the agitator or inducer 12 can be in direct contact with solids and other materials to perform its function of helping to fluidise settled solids, whereas an impellor is within a housing and is thus not capable of helping fluidise settled solids. The agitator 12 has a hole defined by walls of the body. The hole has a female thread for screwing the agitator 12 onto an end of the shaft 7 which itself has a male thread on an end of a peripheral surface. The agitator 12 has a closed tip and the hole is not a through hole like the impeller 1 has (or like the impeller described in CN106151318A). The closed tip provides a sacrificial contact surface on the agitator 12 which will wear off during agitation instead of the main components of the pump wearing off (such as the shaft, if an impeller with a through hole as in CN 106151318A is used for agitation). Advantageously, only the agitator 12 has to be replaced when it is worn. Furthermore, the agitator 12 can be securely fastened to the shaft 7 without requiring a nut or other component to hold it in place. The agitator or inducer 12 agitates slurry which is present under the pump to encourage the slurry to flow and thereby enter the casing 20 and be pumped by the impeller 1 through the housing 100.

[0025] In an embodiment, two or more vanes can be provided on the agitator 12, the preferred numbers being three or four. Fig 3 shows a side and end view of one possible form of agitator and Fig 4 shows side and end views of another form, the latter having a tapered nose projecting beyond the ends of the vanes which is found significantly to increase the agitating effect and thus the effect of the overall apparatus in pumping solids entrained in the liquid.

[0026] It may be noted that the agitators shown are quite small. This ensures that the desired effect is obtained without consumption of excessive energy, and that the energy which is used is effectively employed in agitation. Further, the inclined blades will not display any significant tendancy to propel the apparatus in any direction as they exert little axial thrust. It is desirable to design and run the apparatus so that maximum resonance is observed. This appears to increase the fluidising effect. Indeed trials have indicated that a large percentage weight of solids, in fact 65% or more, can be pumped and can be agitated over a wide area. The agitators are not, therefore, primarily propellors or impellers, and they do need to be clear, or largely clear, of surrounding structures or casing which resist shock wave propagation. However, the structures shown which allow such propagation may in fact assist it.

[0027] In an embodiment the agitator 12 is so designed and rotated that it generates pressure waves and these can be particularly effective in loosening the solids over a wide area. One way to achieve this effect is to mount the agitator 12 adjacent the pump inlet and to incline the vanes on the agitator 12 and rotate it so that the agitator 12 imparts an axial component of movement to the liquid / solid mixture which is away from the inlet to the pump. The effect of generating pressure waves can be remarkably effective in agitating and loosening the solids so that they begin to behave rather as if in a fluidised bed to which fluid is supplied from beneath. It is believed that the pressure waves set up by the agitator are in effect shock waves passing through the liquid and through the sediment which prove most effective in loosening the sediment. The reason for this may be that the shock or pressure waves are effective to disturb the surface tension forces which tend to hold the particles together. Thus, it is known that in sediments the passages and cavities between particles attract and retain liquid by capillary action, and the liquid once present holds the particles together by surface tension. The disturbance of the shock waves may well upset this arrangement and cause release of adjacent particles from each other, whereon they will be mobilized and free to "fluidize" and allow the pump to sink down amongst them. The agitator is most conveniently provided attached to a pump such as a submersible centrifugal pump. In, for instance, the primary application of pumping fines sediment, the sediment is mobilized and loosened so that the pump can be immersed in the sediment for most effective pumping thereof. When the agitator 12 is mounted below the pump inlet, it will initially be located just above the surface of sediment, but as agitation proceeds it will be able to sink down into the sediment. The pump will follow it down and thus be more efficiently in contact with the material which it is to move.

[0028] Generally, the vanes of the agitator 12 will make only a small angle with the agitator axis, for instance an angle of about 10° for example, at an end closest to the hole of the agitator 12, but in certain circumstances much larger angles of up to 30° or even up to 80° for example, at an end closest to the hole of the agitator 12may prove appropriate. In an embodiment the vanes of the agitator 12 make an angle with the agitator axis of 40° or less for example, at an end closest to the hole of the agitator, preferably 30° or less for example, at an end closest to the hole of the agitator, more preferably 25° or less for example, at an end closest to the hole of the agitator and even more preferably of 20° or less for example, at an end closest to the hole of the agitator and most preferably 15° or less for example, at an end closest to the hole of the agitator. Because the vanes are intended to provide an upward impetus to the liquid in order to perform the agitating (for example in contrast to the impellor of CN202520631U which provides an outward impetus to the liquid), preferably the vanes make an angle of 2° or more to the agitator axis for example, at an end closest to the hole of the agitator 12, preferably at least 5° for example, at an end closest to the hole of the agitator 12. Thus, the agitator 12 is distinguished from a scroll type impeller which may comprise scroll vanes that are at 0° to the impeller axis such as the scroll impellor of CN202520631U. Thus an angle between 2° and 40° for example, at an end closest to the hole of the agitator 12, preferably between 5° and 20° for example, at an end closest to the hole of the agitator 12is generally desirable or between 5° and 15° for example, at an end closest to the hole of the agitator 12, depending on the inductive effect desired and its effect on the wave generation and consequent mobilization of the particles. Therefore, the agitator 12 is distinguished from a rotor comprising a plurality of blades that are at an inclination of 47-50° to the shaft (such as disclosed in CN 106151318A). By virtue of the inclination of the blades a shockwave is generated helping to fluidise any settled solids. Furthermore, the agitator 12 is distinguished from a thread type impeller which may comprise only one vane in the shape of a screw thread (such as disclosed in CN 211549995U) as opposed to the plurality of vanes of the agitator 12. Further still, the agitator 12 is distinguished from a rotor comprising a plurality of pump blades.

[0029] In some embodiments, the acute angle measured towards the tip of the agitator and at an end radially closest to the hole made by the vanes relative to the agitator axis is positive and that of the acute angle measured towards the tip of the agitator made by the thread of the agitator 12 to the agitator axis is positive, or vice versa. This means that in use during pumping the rotation of the shaft tends to tighten the agitator to the shaft, cause the vanes to push fluid and / or solid materials axially towards the hole and shaft and cause the agitator to have a downward impetus (such as a digging force) and propagate shock waves below the pump making settled solids start to flow, in a way similar to tapping one’s foot on the sand on a beach.

[0030] This is different to vanes of the impellor 1 which can be angled in parallel to the impellor axis (i.e. at 0° to the impellor axis) so that thrust is induced in a direction away from the shaft 7 (i.e. orthogonally to the shaft 7), such as the impellor 11 shown in CN202520631U.

[0031] The provision of the agitator described above is quite different from the provision and function of inductors which have been known to assist in feeding material to the impeller of a centrifugal pump, to which the present invention can also be applied. In fact, such inductors have inevitably been provided in a conduit so that they operate somewhat like an Archemedean screw. The present invention is applicable to any type of agitator, but is most suited to the type of agitator which sets up pressure waves.

[0032] The agitator 12 is a replicable part. This is because due to the pump being used to pump slurry, particles wear the vanes, meaning that the effectiveness of the agitator 12 over time is decreased. Therefore, it is necessary to replace the agitator 12 from time to time.

[0033] The agitator 12 has a female thread formed on walls of the body defining the hole. The female thread engages with the agitator male thread formed on an outer peripheral surface of the shaft 7. Therefore, the agitator 12 is differentiated from a propeller for chopping that does not have a hole and female threads within the hole (such as the propeller disclosed in US 3973866 A). In some embodiments, the agitator 12 may be monolithic such that the female thread formed on the walls of the body and the plurality of vanes are all formed of a single piece of solid material (e,g, cast, forged or machined). A monolithic configuration is advantageous over an arrangement comprising multiple components because it is easier and cheaper to manufacture a component from a single material compared to manufacturing multiple components that have to interact with each other. Additionally, a monolithic configuration drastically reduces the time and complexity of replacing the agitator 12 in comparison to an agitating arrangement comprising multiple components.

[0034] The requirements of the female thread of the agitator 12 and the corresponding thread of the shaft 7 are very demanding. There are several conflicting requirements including, but not limited to, the following: primarily the thread must be such that it is possible quickly to change the agitator 12 to reduce down time of the pump. The thread must also have sufficient strength to resist the forces on the impellor due to the torque transferred between the shaft 7 and the agitator 12 and possibly also shock waves which the agitator 12 is intended to impart to the slurry. When the pump starts, a great deal of torque can be generated and the threads must be strong enough to resist the forces resulting from that torque. Such forces can be significantly higher on pump start-up than in normal operation if the pump were to be started in settled slurry. Due to the working environment of the pump, contamination of the threads by particles is quite likely. Therefore, the threads are ideally one which handle contamination well.

[0035] To meet the conflicting requirements of the female thread of the agitator 12 and the corresponding male thread of the shaft 7, both formed of a multi-start thread, particularly a two-start thread. A multi-start thread is a thread which has more than one-thread start. Most threads are single-start threads. In a multi-start thread, the thread has several starts and each individual thread has the same pitch as the other individual threads. That is, in a multi-start thread, several threads run parallel to each other. Multi-start threads are different to a single thread in combination with a guide groove on the shaft that extends in a direction parallel to the shaft and substantially orthogonal to the threads (for example as shown in CN 208911864U). Instead, multi-start threads (for example, the multi-start threads on the shaft 7 and / or the multi-start threads on the agitator 12) are threads which are made up at the same time and by the same relative rotational movement of the two components being joined. In an embodiment the plurality of individual threads of a multi-start thread are in parallel with each to allow the agitator 12 and the shaft 7 to be made-up.

[0036] Multi-start threads are particularly suited for the agitator because more travel is achieved with fewer turns of a multi-start thread compared to a single-start thread so that it is quicker to remove an agitator 12 and replace it with a new one. Additionally a multi-start thread is particularly suited to power transmission. Therefore, the use of a multi-start thread is ideally suited for the agitator 12.

[0037] In an embodiment, the multi-start thread comprises a first thread and a second thread. In an embodiment the cross-sectional shape of the first thread is different to a cross-sectional shape of the second thread. This is illustrated in Figures 5-7. This configuration is advantageous because the characteristics of the first thread and second thread can be optimised to provide different properties, all of which are needed by the thread in order to ensure optimum performance.

[0038] For example, the first thread is a thread adapted for coping with contamination, ease of make-up and ease of manufacture. Conversely the second thread is adapted for strength. Replacement in non workshop conditions where mechanical equipment and resources are limited is enhanced by this development. However, the functions of the different threads are not limited to these examples and the threads may have different characteristics, for example, anti-galling properties, properties to reduce the chance of thread crossing, anti-fatigue properties and so on. In an embodiment, the multi-start thread is a two-start thread comprising a first thread and a second thread that run parallel to each other, as is described below with reference to Figures 5 to 7. The manufacture of multi-start threads, such as multi-start threads comprising three or more threads, can be very demanding due to the high complexity involved in the machining process. The configuration of a two-start multi-start thread is advantageous because the benefits of a multi-start thread are achieved, while reducing the complexity of manufacturing. This reduces overall cost of production for both the shaft 7 and the agitator 12.

[0039] In practical use, the agitator 12 can be of varying sizes and weights. Typical agitators 12 are relatively heavy and spin at a relatively high speed and with high torque to ensure effective pressure waves are set up in the liquid and the settled solids. During use, the large reaction force caused by the pump shutting down can cause the agitator 12 to briefly spin in an opposite direction than intended for use (i.e., the agitator 12 can briefly spin in reverse). This can cause the agitator 12 to become loose or fall off during use. In addition to providing high torque during operation, multi-start threads can be used to quickly replace an agitator 12. However, multi-start threads, such as three and four-start threads typically have a high thread rise angle which can cause the agitator 12 to fall off during use as described. A two-thread start provides a make-up arrangement for an agitator 12 and a corresponding shaft 7 that has a smaller thread rise angle and a reduced lead of the thread than a multi-start thread comprising three or more threads, thereby leading to a decreased risk of the agitator 12 becoming loose or falling off during use. However, maintaining a two-start thread as opposed to a single start thread allows for a relatively quick replacement time of an agitator 12 in comparison to a single start thread. The inventors have found that by using a two-start thread in use, a balance is struck between maintaining high torque capabilities and ensuring improved fastening capabilities of the agitator 12 when the pump shuts down (in comparison to other multi -thread starts) and a quick replacement time of an agitator 12 (in comparison to a single thread start). Figure 5 illustrates the end of the shaft 7. A male thread is formed on the end of a peripheral surface as illustrated. The male thread of the shaft 7 is formed to make-up with a matching female thread of the agitator 12 for screwing the agitator 12 onto the end of the shaft 7. A detail of the male thread is shown in Figure 6. Here it can be seen that there is a first thread 200 which has a different cross-sectional shape to a second thread 300. The first and second threads 200, 300 run parallel to each other in the two-start thread which is illustrated. In an embodiment, the agitator 12 comprises walls defining a hole with a female thread for screwing the agitator 12 onto an end of the shaft 7. The female thread of the agitator 12 comprises a first thread that engages with the first thread 200 of the shaft 7. The female thread of the agitator 12 comprises a second thread that runs parallel to the first thread of the agitator 12 and engages with the second thread 300 of the shaft 7. Accordingly, the two-start thread of the agitator 12 is formed to make-up with the two-start thread of the shaft 7. Reference to the first thread and the second thread below refers to both the thread patterns of the shaft 7 and the thread patterns of the agitator 12.

[0040] In an embodiment the thread shape of the first and second thread is trapezoidal, but that is not necessarily the case. The embodiment of the figure 5-7 uses a trapezoidal thread shape for the first and second threads as an illustration.

[0041] The difference in cross-sectional shape between the first thread 200 and second thread 300 can most clearly be seen in Figure 6 by comparing the width of the root of the threads. That is, the root 200 A of the first thread 200 is much narrower than the root 300A of the second thread 300. Because the pitch of the threads is constant, the different width of the root 200A, 300A is the result of a different thread angle 0 being used for the first thread 200 and the second thread 300. The thread angle 9 is defined as the angle made by the thread flanks, measured in a plane containing the thread axis. In other words the thread angle is the included angle of the groove between the thread flanks. The thread angle 9 is illustrated in Figure 7. The thread angle 9 is measured between the load flank 200C, 300C and the stabbing flank 200D, 300D of a single thread. The surfaces forming the thread flanks are extended past the radial position of the root 200A, 300A until they meet. Where those imaginary lines meet, the angle at which they meet is the thread angle 9.

[0042] As is most clearly seen in Figure 7, the thread angle 9 of the first thread 200 is greater than the thread angle 9 of the second thread 300. This means that the efficiency of the first thread 200 is lower than that of the second thread 300. This is because any force in the axial direction on the load and stabbing flanks is applied at a less efficient angle in the first thread 200 compared to the second thread 300. However, a greater thread angle 0 is easier to manufacture, easier to make-up, less likely to gall and is less sensitive to contamination than a thread with a lower thread angle 9. In this way, the two threads of the male and female thread have different properties and the threads are optimised to achieve different properties but as a result, the overall thread properties can be precisely tailored to the requirements of the agitator 12.

[0043] As illustrated in Figure 7, the flank angles (the angle the flank surface makes to the plane perpendicular to the thread axis) of the load and stabbing flanks 200C, 200D of the first thread 200 are the same. The angles of the load and stabbing flanks 300C, 300D of the second thread 300 are the same. However, this is not necessarily the case. For example, in an embodiment the load flank could form a lower angle to the plane perpendicular to the thread axis than the stabbing flank. This would result in a stronger thread as the flank angle is more efficient in resisting axial loads. This may be preferable because the major load the agitator needs to resist is the torque applied through the shaft 7, meaning that the force needed to be resisted by the load flanks is greater than that of the stabbing flanks. However, such a cross-sectional shape is more difficult to manufacture than a thread shape in which the flank angles of the load flanks and stabbing flanks are equal.

[0044] As noted above, in the illustrated embodiment the cross-sectional shape of the threads is trapezoidal. However, this is not necessarily the case and other thread forms are possible including, but not limited to, triangular threads and square threads. In the case of unequal angles between load and stabbing flanks, the thread shape is sometimes called a buttress thread.

[0045] As can be seen in figure 7, the thread crests 250A, 250B of the first and second threads have a width which is equal. However, this is not necessarily the case. In an alternative embodiment the root 200 A, 300 A of the first and second threads 200, 300 could have the same width and the different thread angles are accommodated by variations in the width of the crests 250A, 250B of the first and second threads 200, 300. In a further possibility, the widths of both the roots 200A, 300A and crests 250A, 250B could be different.

[0046] As illustrated in Figure 7, the threads are non-tapered and are parallel. However, this is not necessarily the case and the threads could be tapered, with a taper angle of between greater than 0° and less than 20°, preferably less than 10°.

[0047] As illustrated in Figures 5-7, the surfaces of the thread roots 200 A, 300A and thread crests 250A, 250B are parallel to the axis of the threads. However, this is not necessarily the case and the root and / or thread crest of one or both of the first and second thread could be at an angle to the longitudinal axis of the thread. For example a Dardelet type thread has a root with an angle to the longitudinal axis of the thread of 6°. Such a thread is self-locking. The crest also / alternatively has a 6° taper. This allows the parts to screw together easily until seated.

[0048] Bearing all of these considerations in mind and through testing, the inventors have found that the best mix of properties is achieved when the first thread 200 has a thread angle of between 61° and 68°. This is slightly greater than a standard UNS thread, which has a thread angle of 60°. The greater thread angle results in easier manufacturability as well as being less sensitive to contamination and more galling resistant. Preferably the thread angle is between 64° and 66° and most preferably is 65°. This has the best balance of thread strength (which is improved with a lower thread angle) and machinability, galling resistance, ease of make-up and resistance to contamination (all of which are improved with a greater thread angle).

[0049] The second thread 300 is based on an ACME thread and has a thread angle of between 25° and 35° preferably between 28° and 31° and most preferably of 29°. This type of thread is often used in lead screws where transfer of torque is important.

[0050] A standard thread normally has 12 threads per inch or 14 threads per inch. More threads per inch result in greater strength but slower speed of assembly as more turns are required for a given axial displacement. The present inventors have found that for the agitator 12 the best balance between strength and speed of assembly is achieved when using more than 12 threads per inch and less than 14 threads per inch. The best balance is achieved when using between 12.5 threads per inch and less than 13.5 threads per inch and the inventors have settled in an embodiment on 13 threads per inch to achieve the best balance between strength and speed of assembly.

[0051] One specific example will be described in detail.

[0052] Beneath a 40 horse power (30 kilowatt) centrifugal pump having a bottom inlet and a 5.2 inch (13.2 cms) outlet is attached an agitator having three vanes on a shaft. The agitator shaft has a 2 inch (5 cms) diameter and the vanes extend radially outwards to a maximum of 1.57 inch (4.0 cms) from the shaft surface but taper towards the bottom and have a length of 3.8 inches (9.6 cms) with the overall length of the agitator (i.e. including the nose of the agitator) being about 5 inches (127mm). The pump is equipped also with a skirt such as is shown at 15 in Figures 1 and 2, this having a depth of 5.5 inches (14 cms) so that the agitator is protected when the pump is stood on a hard surface and a diameter of about 2 feet (60 cms). When rotated at about 1440 rpm the apparatus is exceedingly effective at agitating sedimented particles over a wide area, and pumping them, with solids contents up to and in excess of 65% being noted in the pumped material. The agitating effect is a marked improvement over that offered by prior apparatus.

[0053] A pump for slurry has very specific operating requirements. In particular, because a slurry contains solid particles, the material of the agitator 12 should be of a high wearing material and / or be coated with an abrasion resistant material (one with a higher abrasion resistance than the base material, an example being tungsten carbide). In an embodiment the material of the agitator 12 has a Brinell Hardness of between 380 and 710 as measured under ASTM E10-15a . Example materials are martensitic precipitation hardened stainless steel (which does not necessarily need an abrasion resistant coating) or softer steels of any type or an iron material (e.g. cast irons (iron with 1.8-4.0% carbon)). Steels and iron may optionally alloyed be with other elements including chromium and / or nickel as found for example in BS EN 12513:2011. Such cast irons are known as white cast iron. Such materials compared to steels and aluminium, for example, are non-ductile and very hard. An impeller for a turbo-charger, for example, has a Brinell Hardness of 70-180 and would typically be made of carbon steel which is iron with a carbon content of about 0.15% carbon and optionally other alloying elements, or aluminium for example with small additions of copper and perhaps magnesium, iron and nickel. The above embodiments have been described in relation to centrifugal pumps. However, the pump may be any kind of pump including but not limited to a positive displacement pump, for example a gear pump or a screw pump.

Claims

CLAIMS1. An agitator for a pump for pumping particulate solids which are settled in and beneath a liquid, the agitator comprising a plurality of vanes on a body and walls of the body defining a hole with a female thread for screwing the agitator onto an end of a shaft of the pump; wherein the female thread is a multi-start thread.

2. The agitator according to claim 1, wherein the acute angle measured towards the tip of the agitator and at an end radially closest to the hole made by the vanes relative to the agitator axis is positive and that of the acute angle measured towards the tip of the agitator made by the thread of the agitator to the agitator axis is positive, or vice versa.

3. The agitator according to claims 1 or 2, where the multi-start thread is a two start thread.

4. The agitator according to claim 1 to 3, wherein a first thread of the multi-start thread has a different cross-sectional shape to that of a second thread of the multi-start thread.

5. The agitator according to claim 4, wherein a thread angle of the first thread is greater than the thread angle of the second thread.

6. The agitator according to claim 4 or 5, wherein the first thread has a thread angle of between 61° and 68°; preferably between 64° and 66°; most preferably of 65°.

7. The agitator according to claim 4, 5 or 6, wherein the second thread has a thread angle of between 25° and 35°; preferably between 28° and 31°.

8. The agitator according to any of claims 4 to 7, wherein at least one of the first thread and second thread is a trapezoidal thread.

9. The agitator according to any preceding claim, wherein the female thread has more than 12 threads per inch and less than 14 threads per inch, preferably more than 12.5 threads per inch and less than 13.5 threads per inch; most preferably 13 threads per inch.

10. The agitator according to any preceding claim, wherein the agitator is configured to rotate in a first rotational direction around a longitudinal axis of the female thread, and the female thread is formed so as to allow the agitator to be mounted by rotating the agitator in a second rotational direction which is opposite to the first rotational direction.

11. The agitator according to any preceding claim, comprising two to four vanes.

12. The agitator of any preceding claim, wherein each vane is at an angle of 40° or less to the axis of the female thread preferably less than 30°, more preferably 25° or less, even more preferably of 20° or less, even more preferably 15° or less for example, at an end closest to the hole of the agitator and most preferably of between 5° and 15° to the axis of the female thread.

13. The agitator of any preceding claim, wherein each vane is at an angle greater than 2°, preferably more than 5° to the axis of the female thread.

14. The agitator of any preceding claim, wherein the vanes are constructed and arranged such that they do not flex in normal use.

15. The agitator according to any preceding claim, wherein the agitator is monolithic.

16. The agitator of any preceding claim, wherein the agitator is formed of a martensitic precipitation hardened stainless steel.

17. The agitator of any preceding claim, wherein the agitator is formed of steel or stainless steel or iron material and is coated with an abrasion resistant coating, preferably wherein the abrasion resistant coating is tungsten carbide.

18. The agitator of claims 1 to 17, wherein the agitator has a closed tip and does not have a through hole.

19. A slurry pump comprising a rotatable shaft; wherein the shaft includes a male thread on an end of a peripheral surface, the male thread being a multi-start thread.

20. The pump according to claim 19, further comprising an impellor.

21. The pump of claim 20, wherein the impellor is mounted on the shaft in a pump chamber and the agitator is outside of the pump chamber.

22. The pump according to any of claims 19 or 20 or 21, further comprising the agitator of any of claims 1 to 14, wherein the male thread and female thread are engaged.

23. The pump according to any of claims 19 to 22, further comprising an apertured skirt beneath the end of the shaft.

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