Patent Publication Number: US-11384764-B2

Title: Containment shell for magnetic pump

Description:
This invention relates to a containment shell for a magnetic pump and, in particular, a containment shell that is easier and quicker to manufacture. 
     Historically the construction of a seal-less magnet-driven centrifugal pump has relied upon a thin walled tube arrangement known as a containment shell or ‘can’ forming part of the pump pressure vessel to provide the boundary between the magnetic coupling parts. A standard magnetic coupling has two parts, one of which is inside the pump (the driven part) on the wet side, and the other side of which is outside of the pump (the driver) on the dry side. 
     The reasons for this sort of coupling are typically related to the material which is desired to be pumped as this type of pump is typically used to pump highly corrosive, toxic or otherwise dangerous or hazardous fluids, for which it is vital that the risk of leakage is minimised. Thus, a seal-less magnet-driven centrifugal pump provides an arrangement in which there is no moving seal through which leakage might occur, but rather provides a contactless transfer of motion by way of the magnetic coupling across a static physical barrier, which can more easily be sealed (than a moving seal) to prevent leakage. 
     The containment shell must therefore function in conjunction with the magnetic coupling and has historically been a thin walled containment shell of a metallic, non-magnetic, construction, compatible with the liquid being pumped. However, one of the downsides with a metallic construction is that eddy currents are created and this leads to eddy current losses that increase with shell diameter, increasing rotational speed and increasing wall thickness (due to increasing internal pressure requirements). 
     The eddy current parasitic loss has limited the range of application opportunities for seal-less, magnet drive, centrifugal pumps. However, in recent years, alternative materials of construction such as carbon fibre composites and ceramics have been used for the containment shell. The primary technical and operational advantage of these materials is that they reduce, or in the case of ceramics completely eliminate, the eddy current loss. However, alongside the technical and operational advantage, there is a significant downside. The use of these materials results in high set-up costs, unit manufacturing costs and costs associated with the yield of the parts produced. 
     In particular, alternative composite material constructions, for example, when using standard carbon fibre mats in a laying up process, is timely, labour intensive and therefore relatively expensive. For example, each layer of carbon fibre mat has to be positioned accurately to align the woven fibres correctly and this process is timely and therefore relatively expensive. 
     According to the present invention there is provided a containment shell for a magnetic pump, the shell comprising: a body section having a continuous side wall defining a chamber, and an end wall closing the chamber at one end, the chamber being open at the other end, wherein the body section and the end wall are integrally formed from a matrix material in which chopped carbon fibre material is distributed. 
     Thus, the present invention provides a containment shell which can be manufactured by an injection moulding process, because it uses chopped, short strand carbon fibres as the material reinforcement. This is in contrast to the carbon fibre construction discussed above. Injection moulding allows individual shells to be formed quickly and accurately, and with minimal human interaction, meaning that the cost per unit is significantly lower than that which can be achieved by alternative more manually intensive manufacturing processes. 
     The random alignment of the present invention does result in a lower internal pressure capability for the part, when compared to an alternative composite material configuration for a similarly sized structure, but importantly it maintains the elimination of eddy current losses. The lower internal pressure capability is still acceptably high, for certain magnetic drive pump types. The injection moulding process is faster, allowing a higher rate of manufacture, eliminates quality and yield issues associated with for example a ‘fibre lay-up’ composite manufacturing process, results in better component repeatability and is generally a more cost-effective manufacturing solution. 
     In a preferred example, the shell is formed constructed from a composite material, which may include PEEK (Polyetheretherketone, a semi-crystalline organic polymer thermoplastic exhibiting a highly stable chemical structure) and randomly aligned carbon fibre strands. Preferably, the carbon fibre strands are between 35 and 45% by volume of the composite, more preferably 37.5 to 42.5% by volume and most preferably 40% by volume. 
     PEEK is beneficial as it displays high resistance amidst a wide range of chemical environments, and at elevated temperatures. It can only be dissolved by certain materials including some acids, so permits many highly corrosive fluids to be pumped. It also provides good friction as well as wear properties, and can for example, be exposed for a long period of time to high pressure water and steam without exhibiting any serious degradation. 
     The shell of the present invention is more robust when dealing with disrupted operation as well. The use of PEEK as the matrix material within which the carbon strands are distributed does not require cooling, in the way that certain metallic shells do—the lack of eddy currents in the invention ensure that the shell is not being heated in operation and increases the robustness of the pump to process upsets. Further advantages of fact that injection moulding can be used as a manufacturing method include (i) that the amount of post formation machining to achieve the desired product finish is reduced or indeed eliminated, and (ii) the side wall of the shell can be formed with parallel surfaces more easily, when compared to other composite material configurations or metallic shells formed over a mould from which the shell must be removed. For this to happen, the sides of the metallic or alternative composite configuration shells may taper inwards slightly towards the closed end of the shell. Any form of adverse taper, i.e. where the open end is narrower than the closed end of the shell, would prevent the shell being removed from the mould, hence the standard practice of creating a slight taper towards the closed end. 
     The shell is preferably a pressure containing structure which is preferably able to withstand a pressure of at least 25 bar. 
     The shell may comprise a flange extending radially outward from the body section. The radial flange length may be less than 10% of the height of the shell. The height of shell is the distance from the open end to the closed end. If the closed end is domed, the height is typically measured from the open end to the furthest point on the closed end, usually the centre of the dome. 
     A vortex breaker may be provided on the inside of the end wall of the chamber. The vortex breaker may be integrally formed with the end wall. 
     The side wall and/or end wall thickness is preferably between 2 and 4 mm. 
     The shell may include a curved section or chamfer between the side wall and the end wall. 
     The side wall of the body section is preferably cylindrical and circular in cross section. 
     The matrix material of the shell is preferably PEEK (Polyetheretherketone). The carbon strands are preferably randomly aligned. 
     Carbon fibre strands may comprise 40% of the material by volume of the shell. 
     The invention also provides a magnetic pump comprising: a pump body supporting an output shaft on a wet side of the pump; a drive shaft on a dry side of the pump, at least one magnet for coupling the input and output shafts such that motion of the input shaft causes motion of the output shaft, and a pressure containing structure mounted on the pump body for separating the dry and the wet sides, wherein the pressure containing structure is formed from a matrix material in which chopped carbon fibre material is distributed. 
     The pressure containing structure is preferably a shell as described above. The shell may be formed only from the matrix material in which chopped carbon fibre material is distributed. 
     The pressure containing structure preferably allows magnetic coupling between the input and the output shafts. 
     The pump may further comprise at least one magnet on each of the input and output shafts. 
     The pressure containing structure preferably passes between the input and output shafts. 
     In a pump incorporating the shell of the present invention, it is preferable that the shell is the only pressure containing structure. By this, we mean that the composite material and randomly aligned carbon strand structure is able to withstand the operating pressures without support from other structures. Typical operating pressures can be up to 25 bar, so the shell is preferably able to withstand such pressures without other structures, such as bands or loops or additional layers of strengthening materials being applied. 
     The containment shell may include an integral vortex breaker feature on its inner surface. The vortex breaker may take the form of a cross or other cruciform shape and is preferably located in the centre of the inner surface of the closed end of the shell. The vortex breaker has two functions. Firstly, by being integrally formed with the shell, it strengthens the end closure part of the containment shell and, secondly, the projection of the vortex breaker away from the inner surface prevents liquid inside this end of the containment shell from swirling in a fixed location, thereby reducing wear or erosion damage to the inner surface of the shell. Given that the fluid to be pumped could be under extreme pressure and/or moving at significant velocities and/or maybe corrosive and/or toxic, the integrity of the shell is paramount to safe operation of the pump. 
    
    
     
       The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: 
         FIG. 1  shows a magnet driven pump; 
         FIG. 2  shows the internal flow of process fluid through a magnet driven pump; 
         FIG. 3  shows a schematic representation of eddy current generation; 
         FIG. 4  shows a containment shell with a vortex breaker; 
         FIG. 5  shows the internal flow of process fluid using the bush holder of  FIG. 6 ; 
         FIG. 6  shows the bush holder from  FIG. 5  in more detail. 
     
    
    
       FIG. 1  is an axial section through one example of a magnetic seal less pump  10  incorporating a shell  22  according to the present invention. 
     The pump  10  includes a casing  11  and an impeller  12  that form what is typically described as the hydraulic  13 . In common with all other types of centrifugal pump, the hydraulic  13  includes a suction nozzle  14  through which liquid is drawn into the hydraulic and then by virtue of the rotation of the impeller and the design of the casing volute  15 , is expelled at a higher pressure, through the perpendicular, discharge nozzle  16  (at the top of the illustration). The casing volute  15  is a curved funnel that increases in area as it approaches the discharge port  16 . The volute of a centrifugal pump is the part of the casing  11  that receives the fluid being pumped by the impeller  12 , reducing the velocity of the fluid and converting kinetic energy into pressure head, as the fluid is directed through the discharge nozzle  16 . 
     An output drive shaft  17  is connected to, or formed integrally with, the impeller  12  and extends axially along the pump  10 , passing through bush holder  18  which is mounted on/connected to the casing  11 . One or more axial and radial bearings  19  fix the radial axial and radial position of the output drive shaft with respect to the bush holder and permit rotation of the shaft relative to the bush holder. 
     The output drive shaft includes an inner rotor  20  which retains one or more inner magnets  21 . A containment shell  22 , having an open end  23 , a closed end  24  and a side wall  25 , passes over the output drive shaft  17 , the bearings  19 , the bush holder  18  and the inner rotor  20 . In this case, the containment shell  22  has, at its open end, a flange  26  which is mounted and sealed to the bush holder  18 . The sealing is achieved by way of one or more seals  27 , typically one or more gaskets. The flange is relatively short (in the radial direction) when compared to the height of the containment shell. By height of the shell, we mean the distance from the open end to the closed end. 
     The containment shell  22 , the bush holder  18 , the output drive shaft  17 , the impeller  12  and the casing  11  thereby define the wet output side of the pump. There are no moving seals, thereby reducing the risk of leakage of the pumped fluid. 
     The impeller  12  is caused to rotate due to application of an input rotation on a dry input side of the pump  10 . The input rotation is provided in this case by an input drive shaft  30  which is coupled with an input drive element, in this case an outer rotor  31  which includes one or more outer magnets  32 . The outer rotor  31  is positioned around the outside of the containment shell so that the outer magnet(s)  32  are aligned with the inner magnet(s)  21  such that rotation of one of the rotors causes the magnetic attraction between the inner and outer magnet(s) to create motion in the other rotor. The outer magnets  32  and the inner magnets  21  are typically aligned and spaced only a small distance from the containment shell. 
     In use, the input shaft  30  is rotated by some form of drive means such as a motor (not shown) which transfers rotation to the outer rotor  31 . The magnetic attraction between the outer  32  and inner  21  magnets causes the output drive shaft  17  to be rotated, thereby rotating the impeller. This causes fluid to be drawn axially into the hydraulic  13  via the suction nozzle  14 . Continuing rotation of the impeller draws fluid through the impeller and increase the pressure of the fluid to drive it out of the discharge port  16 . 
     Thus, the magnetic coupling is comprised of the outer magnet ring (OMR) formed by the outer rotor  31  and the outer magnet(s)  32 , an inner magnet ring (IMR) formed by the inner rotor  20  and the inner magnet(s)  21 , and the containment shell  24 . The OMR which is supported by a rolling element bearing assembly  35 , spins in air outside of the shell  24  and is driven by a motor (via the drive shaft  30 ). The bearing assembly ensures that the outer rotor  31  runs concentric to the inner rotor and containment shell. The containment shell  22  is a thin shelled pressure boundary, containing the process liquid, through which the magnetic flux between the OMR to the IMR travels, enabling the synchronous rotation of the OMR and IMR (or magnetic coupling). The IMR is connected to the impeller  12  via the output shaft  17  to form the pump rotor. 
     The axial and radial bearing(s)  19  may include sleeves on the output shaft  17  and provide radial bearing support to the IMR and output shaft  17 . These typically run against static bushes fitted to the bush holder  18  to radially support the pump rotor. Thrust bearing parts are fitted to the IMR and the impeller and react to pump rotor thrust loads. The bearing parts may be formed from any suitable bearing material. 
     The dry input side of the pump is covered by an outer coupling housing  45  which ensures that the outer rotor  31  is enclosed, locates the bearing housing  46  and limits outer rotor  31  excursion, in the event of bearing failure 
     Magnetic drive pumps are characterised by their use of the magnetic coupling as described above and this necessitates process liquid lubrication of the pump rotor bearing system. This enables highly corrosive/toxic process liquids, or very high temperature/pressure process liquids, to be pumped but yet maintain a safe pressure boundary. 
       FIG. 2  illustrates the process liquid flow direction through the pump and how the process liquid itself is used to lubricate the bearings  19 . 
     At the LHS of  FIG. 2 , the bulk flow of the process liquid into the pump  10  through the suction nozzle  14  is shown. This flows through the impeller  12  (by virtue of impeller rotation), is pressure recovered by the casing volute  15  and then exits through the discharge nozzle  16  at the top of the illustration. 
     There are two mechanisms of feeding the internal flow system. These are typically described as ‘external feed’ and ‘internal feed’. 
       FIG. 2  illustrates an ‘internal feed’ system, symbolised by arrowed line  40 , whereby a hole and flow path through the bush holder  18  takes process liquid at close to the impeller  12  discharge or outside diameter into the back of the pump. The back of the pump is the region enclosed by the containment shell  22  and the bush holder  18 , and containing the bearings  19 , the output drive shaft  17 , the inner rotor  20  and inner magnet(s)  21 . 
     Either way (whether external feed or internal feed), the flow into the back of the pump flows into a central region of the bush holder  18 . At this central region  50 , the flow splits in three directions. 
     The first direction is a flow path  52  through the front radial and axial bearings  19   a  (i.e. to the left in  FIG. 2 ), lubricating these bearings and returning to the casing/impeller, bulk process liquid flow via one or more impeller balance holes  56 . 
     The second direction  53  is a flow path through the back radial and axial bearings  19   b  lubricating these bearings and then returning to the casing  11  by the arrowed path shown at the bottom of the illustration. 
     The third direction  54  is through cross-drillings in the output shaft  17 , down the centre of the output shaft and out (at the right of  FIG. 2 ) along a small annular gap  55  between the IMR and the containment shell. This flow would cool the containment shell if necessary and centralises rotation, and the flow then returns to the casing  11  by the arrowed path shown at the bottom of the illustration. 
     The containment shell  22  is formed from a composite material made up of a bulk matrix with chopped carbon fibre strands randomly arranged therein. One of benefits of such a construction over an alternative composite material configuration is now described with reference to  FIG. 3 . 
     Consider an alternative composite material configuration, typically a carbon fibre/PEEK composite material as a plain weave, fibre lay-up, flat plate as shown ‘magnified’ in  FIG. 3 . If the magnetic flux passes perpendicularly through the magnetic field (i.e. in to the paper of the page) and the composite material moves upwards due to motion of the magnetic coupling parts, then an eddy current is generated, in accordance with Fleming&#39;s right hand rule. 
     If the magnetic flux alternates as it passes through the composite material, that is by passing magnets of alternate north-south-north-south polarity, then the magnetic field direction reverses and the composite material (which is static) will see a rotation of the eddy current, which will cross the warp  61  and weft  62  of the composite weave, as shown by the arrows  60 . 
     Although carbon fibre is electrically conductive, because the eddy current is trying to rotate and pass through the warp and weft of carbon fibre weave, which is to an extent insulated by the composite matrix, the eddy current formation is reduced when compared with a metallic shell for example. 
     If this construction is used as the pressure vessel between the magnetic coupling parts of a magnetic drive pump, then the albeit reduced eddy current formation in the carbon fibre/PEEK composite material will still result in a magnetic coupling loss that can affect the operation of the pump. 
     If, instead of the regular lay-up shown in  FIG. 3 , the composite material is then changed to a short strand injection moulded carbon fibre/PEEK composite, then the carbon fibre strands are randomly aligned, in three dimensions, within the matrix material. The strands may be less than 1 mm in length. 
     Using this material as before and passing an alternating magnetic field (or flux) whilst the material is static, the formation of a rotating eddy current in a matrix of randomly aligned, short strand carbon fibres is reduced to zero or very close to it. In practice, a pressure containment shell for a magnetic drive pump, constructed using such a technique, would have a magnetic coupling loss of zero. 
       FIG. 4  shows one example of a containment shell  22  having a vortex breaker  70  integrally formed therewith. The vortex breaker has a cruciform shape, that is a cross with four arms  71 ,  72 . The number of arms can be more or less than 4. In this example, arms  71  are shorter than arms  72 , such that one dimension of the vortex breaker along the closed end wall of the containment shell  24  is longer than the other. The cruciform shape may project between 2 and 6 mm from the inner surface of the shell. Where the end wall of the shell is domed, the vortex breaker may extend over an arc of between 30 and 45 degrees of the radius of curvature of the dome. 
     By virtue of the structure of the containment shell, that is a composite material which is preferably injection mouldable and formed of a matrix material and short randomly aligned carbon fibre strands, the vortex breaker can be integrally formed with the shell. This provides numerous benefits including increased strength to both the vortex breaker itself and also to the shell, as the projections of the vortex breaker away from the surface of the shell braces the end wall. 
     In operation of the pump, process fluid is caused to flow in the gap between the containment shell  24  and the output shaft  17  and IMR  20 , and in particular can be ejected axially from the centre of the output shaft. The vortex breaker aims to disrupt this flow and prevent the formation of a vortex (or swirling liquid) adjacent the containment shell end. This ensures that the internal flow is maintained, but eliminates the liquid swirl that has potential to cause wear degradation, especially if any unwanted debris was entrained in the flow. 
     The pump assembly shown in  FIG. 5  has much the same configuration as the pump  10  of  FIGS. 1 and 2 , so like features have been labelled with the same reference numerals. 
     The internal feed starts at port  81  which receives a portion of the pumped process fluid from impeller  12 . The separated process flow is fed along inlet passageway  82  to the central region  50  of the bush holder  18 . The flow is then directed as described in  FIG. 2  around the three separate flow directions. Flows  53  and  54  recombine at point  83  where the process flow then passes along outlet passageway  84  and out of the bush holder  18  and outlet port  85 . 
     On either or both of the inlet and outlet passageways, one or more sensors  86  may be provided. The sensors may be used to measure various properties of the process fluid including, but not limited to: flow rate, temperature or pressure. This can help to ensure correct operation of the pump, for example ensuring that the pump is primed with process fluid or some other fluid ahead of initiating operation, thereby minimising wear and ensuring a smooth start to the pumping process. 
     Access for measurement to either the inlet or the outlet passageway can be either intrusive or non-intrusive. “Intrusive” would be a hole through the wall of the bush holder  18  to fit a sensor directly into the process liquid stream within the passageway. With hazardous liquids, an intrusive sensor has to be sealed into position and there is therefore an ongoing risk of leakage. A “non-intrusive” sensor relies on typically an ultrasonic signal that transmits through the wall of the inlet/outlet port, senses the liquid properties, but doesn&#39;t have the risk of liquid leakage. 
     The sensors could be wireless in that they transmit any signals using a wireless data protocol, or they may be wired, as in  FIG. 5 , thereby necessitating a further pathway  87  to wire-out the sensor(s). This is typically managed by a radial feed-through arrangement in a flange  88  of a coupling housing  89 . 
     The radial position of the inlet/outlet sensor(s) may be dependent on the property being measured. An outlet flow sensor would for example need to be further inboard (i.e. radially more inward) than currently shown in  FIG. 5 , although the more outboard location shown may be suitable for a pressure or temperature sensor. 
     A suitable bush holder  18  for use in  FIG. 5  is shown in greater detail in  FIG. 6 . The bush holder performs a multitude of functions in the pump:
         Retains system pressure at the back of the hydraulic part of the system within the containment shell  24     Locates the seal(s)  27  and  28  between the bush holder  18  and containment shell  22  and the bush holder  18  and casing  11 , respectively   Locates to the casing  11 , pump casing, bearings  19  and containment shell  22     Allows process liquid into and out of the back of the pump to cool and lubricate bearings  19         

     The bush holder  18  has a main body  100  formed from a disc shaped section  101  and a hollow tube section  102  projecting away from the disc section  101  at its centre. The disc has a central hole aligned with the hollow tube  102  to define a passageway  104  from one side of the bush holder to the other. This passageway, in use, accommodates the output drive shaft and the associated bearings. 
     The disc section  101  includes an inlet passageway  80  extending between an inlet port  81  and the central section  50  of the bush holder. The disc section further includes an outlet passageway  84  extending from the recombination point  83  to an outlet port  85 . As described with reference to  FIG. 5 , the bush holder helps to distribute a portion of the process flow which is delivered into inlet port  81  around the bearings  19  and inner rotor, the output drive shaft and into the space defined by the containment shell  24 . 
     The inlet and outlet passageways may be opposite each other across the disc section, or may be off set as shown in  FIG. 6 . The outlet passage way is shown at approx. 135 degrees from the inlet passageway. 
     The disc section  101  of the bush holder includes various pockets  105  on the inner face  106 . These pockets allow ready access to the inlet  80  and outlet  84  passageways and/or the inlet  81  and outlet  85  ports. For the non-intrusive sensing, the pockets enable the sensors to be placed close to the monitoring points, thereby minimising the amount of material that the signals need to penetrate. For intrusive sensing, the pockets enable the sensors to be located with the outline of the bush holder meaning that no other components need to be adapted. 
     The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.