Abstract:
An inner drive for a magnetic drive pump includes a magnet supported on a yoke. The inner drive is driven about an axis to pump a corrosive process fluid. The magnet and yoke are fully encapsulated during the molding process to completely surround the magnet and yoke in a protective plastic shell. A sleeve is arranged radially outwardly of the magnet to provide further protection. Backing rings are arranged on either side of the magnet. A bonding material joins the plastic shell to the backing rings and sleeve to prevent a space from forming beneath the plastic shell that would become filled with the process fluid once it has permeated the plastic shell. A protective coating is arranged on at least a portion of the magnet to further insulate the magnet from the process fluid.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application relates to a magnetic drive centrifugal pump.  
         [0002]     Magnetic drive centrifugal pumps include a wet portion, which contains the process fluid that is being pumped, and a dry portion having a drive, which provides power to the pump fluid. The dry portion is exposed only to the atmosphere surrounding the pump. In one typical magnetic drive design, an inner and outer drive are separated by a containment shell, which prevents the pump fluid from escaping to the environment. The outer drive, which is usually driven by an electric motor, is located in the dry portion and magnetically drives the inner drive in the wet portion that is attached to a pump impeller. Since magnetic drive pumps are sealless, they are often selected to pump very acidic or caustic process fluids, such as hydrochloric acid, nitric acid and sodium hypochlorite.  
         [0003]     Both the outer and inner drives have a series of magnets mounted around their peripheries. Each magnet is synchronously coupled to a respective magnet that is of an opposite pole on the other drive. The attraction between the magnets results in a magnetic coupling between the two drives causing the inner drive to rotate at the same speed of the outer drive, which is driven by the motor. The inner and outer drives must be located relatively close together for efficient power transmission, which requires a relatively small clearance to be maintained between the containment shell and each drive. In one example, the clearance is approximately 0.060 inch.  
         [0004]     In one type of magnetic drive pump, the inner drive magnets are primarily protected from the corrosive process fluid by a chemically resistant plastic shell, which is typically injection molded around the magnets of the inner drive. Corrosive process fluid eventually permeates the plastic shell, thus attacking the underlying magnets. Once the corrosive process fluid has permeated the plastic coating, the shell swells causing interference between the inner drive and the containment shell and pump failure.  
         [0005]     Therefore, what is needed is an inner drive that is more resistant to swelling once the process fluid has permeated the plastic shell.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a magnetic pumping element, such as an inner drive of a magnetic drive pump, that includes additional protections from corrosive process fluid. The inner drive includes a yoke with multiple magnets supported on the yoke. A protective coating surrounds at least a portion of the magnet, and in one example, extends partially over the yoke. Typically, a metallic member, such as a nickel-based alloy sleeve, is arranged proximate to the magnet. A plastic shell is arranged proximate to the sleeve. In one example, the shell completely encapsulates the yoke and magnet as a result of the molding process so that further operations, such as plastic welding, are not required to encapsulate the yoke and magnet.  
         [0007]     A bonding material is arranged between the plastic shell and metallic sleeve, including backing rings, joining the plastic shell and metallic sleeve to one another. The bonding material prevents formation of a cavity that can become filled with the corrosive process fluid once it has permeated the shell. Additionally, the bonding material prevents the process fluid from reacting with the sleeve and from migrating between the plastic shell and the metallic sleeve/backing rings and into the joints and magnet areas  
         [0008]     Accordingly, the present invention provides an inner drive that is more resistant to swelling once the process fluid has permeated the plastic shell.  
         [0009]     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross-sectional view schematically depicting a magnetic drive centrifugal pump assembly.  
         [0011]      FIG. 2  is a partial cross-sectional view of an integrated impeller and inner drive assembly.  
         [0012]      FIG. 3  is a cross-sectional view of the inner drive shown in  FIG. 2  and taken along line  3 - 3 .  
         [0013]      FIG. 4  is an enlarged view of the area indicated by circle  4  in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     A magnetic drive centrifugal pump assembly  10  is schematically shown in  FIG. 1 . The assembly  10  includes a motor  12  that drives a pump  14 . The motor  12  and pump  14  are supported by a frame  16 . The motor  12  includes a drive shaft  18  that is coupled to a driven shaft  20  of the pump  14 .  
         [0015]     An outer drive  22  is supported by the driven shaft  20 . The outer drive  22  includes magnets mounted on a periphery of the outer drive for magnetically driving an inner drive  28 , which supports magnets having an opposite pole of the magnets on the outer drive  22 .  
         [0016]     The pump  14  includes a housing  24  that supports the driven shaft  20  and outer drive  22  in a dry portion  26  of the pump  14 . A pump case  34  provides a wet portion  36  for holding the process fluid, which is separated from the dry portion  26 . The pump case  34  houses the inner drive  28 , which is coupled to an impeller  30 . The impeller  30  rotates about a stationary shaft  32 . The process fluid is pumped from an inlet  38  to an outlet  40  by the impeller  30 .  
         [0017]     In the example shown in  FIG. 2 , the inner drive  28  and impeller  30  are formed in such a way so as to provide an integral, or separable, impeller and inner drive assembly  42 . A typical inner drive  28  includes a yoke  44  that supports multiple magnets  46  about its outer periphery. The yoke  44  is typically constructed from a magnetic conductor, such as ductile iron, to absorb the magnetic flux lines behind the magnets  46 . Front and/or rear backing rings  48  are arranged on the yoke adjacent to either side of the magnets  46 . The backing rings  48  are typically constructed from a non-magnetic material such as stainless steel so that they do not interrupt the magnetic flux lines on the working side of the magnets.  
         [0018]     A sleeve  56  is arranged radially outboard of the magnets  46  to protect the magnets  46  from process fluid. The sleeve  56  may be constructed from a nickel-based alloy such as Hastelloy or Inconel. The sleeve  56  may be a thin can that is pressed over the magnets  46 . Alternatively, the sleeve  56  may be a machined enclosure that is integral with and extends axially from one of the backing rings  48 .  
         [0019]     A shell  60  is molded about the yoke  44 , magnets  46 , backing rings  48  and sleeve  56  to protect the components from the process fluid. The shell  60  may be constructed from a fluoroplastic such as Ethylene Tetrafluoroethylene (ETFE). Other melt processible fluoropolymers may also be used, such as Perfluoroalkoxy (PFA). The resins may also be glass or carbon fiber reinforced. Fibers in the range of 10-35%, for example, may be used, and in one example, 20%.  
         [0020]     In the prior art, only the shell  60  and sleeve  56  protected the magnets  46  from the process fluid that permeated the shell  60 . However, increased protection from the corrosive process fluid is desired. To this end, the inventive inner drive  28  also includes a powder coating  52  arranged over the magnets  46 . The powder coating  52  may extend from one axial end of the yoke  44  to the other end of the yoke  44  providing a barrier that seals the magnets  46  relative to the yoke  44 . The powder coating  52  is arranged between the backing rings  48  and the yoke  44 , in the example shown. Referring to  FIG. 3 , generous fillets  50 , currently made using potting material  54 , are provided in gaps  49  between the magnets  46 . The fillets  50  provide a smooth transition between the magnets  46  and yoke  44 , which creates a smooth, continuous coating that is free of pits and cracks. Potting material  54 , which is typically used in inner drives, fills the rest of the gaps  49  between the magnets  46  and sleeve  56  in order to prevent sleeve rupture as a result of injection molding.  
         [0021]     One suitable powder coating is an epoxy polyester hybrid, which has a low cure temperature (250-275° F.). One example hybrid has approximately 50% epoxy and 50% polyester. The powder coating preferable has good adhesion, chip resistance, and chemical resistance. More than one coat may be desirable. The coating must withstand the molding temperatures of the shell  60  (over 600° F.). A table of the properties of examples suitable potting and powder coatings materials follows.  
                                       Property   Fillet and Potting Material   Powder Coating                   Product Name   3M epoxy 1 part adhesive 2214 HD   Sherwin Williams Powdura Powder           PMF   Coating - Epoxy Polyester Hybrid       Base   Modified epoxy base   Polyester (80%), epoxy (20%)       Major Ingredients   Epoxy resin, aluminum pigments,   polyester and epoxy           synthetic elastomer       Adhesion       ASTM D-3359 - No failure with 1/16″               squares (cross hatch)       Environmental Resistance   ASTM D-1002 - 1910 psi steel overlap   ASTM D-B117 - passes 500 hr min           shear 365 days in 100% RH   salt fog test       Outgassing   Minimal   NA       Flexibility   See hardness and strength data   ASTM D-522 - pass on ⅛″ mandrel               bend       Density   1.5 g/ml       Impact Resistance       ASTM D-2794 - 100 lbs direct &amp;               reversed - excellent performance       Viscosity   &gt;1,000,000 cps-Brookfield (paste).   Powder consistency prior to oven           Heated to thin for potting fill   bake       Hardness   85 Shore D hardness (approx)   ASTM D-3363 (for thin coatings) - 2H               Pencil hardness       Ultimate Tensile Strength   10,000 psi       Modulus of Elasticity   750,000       Coeff. of Thermal Expansion (cured)   49 × 10 − 6 in/in/C. (0-80 C.)       Cure Temp or Coating Temp   2 hrs @ 225 F. cure temp   275 F. coating temp       Steel T-Peel (ASTM D-1876)   50 lbs per inch of width                  
 
         [0022]     It has been discovered that the process fluid reacts with the sleeve  56  once it has permeated the shell  60  resulting in salts and other compounds that create a build up of solid material under the plastic shell  60 . This build up of material often results in localized swelling of the shell  60  that leads to failure of the pump  14 . Additionally, process fluid that has permeated the shell  60  may be subjected to a pumping effect by the flexing of the shell  60 . This agitation of process fluid that has permeated the shell  60  accelerates corrosion of the sleeve  56  and forces product into joints and magnet areas.  
         [0023]     To address this problem, the inventive inner drive  28  also employs a bonding interface between the sleeve  56  and any other potentially reactive material, such as the backing rings  48  and the shell  60 . This prevents the formation of a cavity that can fill with solid material or process fluid.  
         [0024]     The bonding interface  58  is provided by a suitable bonding material capable of joining the material of the shell  60  to the material of the sleeve  56  and/or backing rings  48 . In one example, the bonding material may be a bonding primer that is a blend of a polymeric adhesive and a fluoropolymer. The bonding primer, in one example, is stable up to 550° F. with negligible to zero out gassing. Two examples of suitable formulations are:  
         [0025]     Formulation 1: 
        PelSeal PLV2100 VITON elastomer, 33% solids-13 grams     PelSeal accelerator no. 4-0.5 milliliters     DuPont ETFE powder 532-6210-4.5 grams        
 
         [0029]     Formulation 2: 
        Methyl ethyl ketone-13 grams     PelSeal PLV2100 VITON elastomer, 33% solids-13 grams     PelSeal acceleration no. 4-0.5 milliliters     DuPont ETFE powder 532-6210-4.5 grams        
 
         [0034]     Formulation 2 results in a lower viscosity, and is preferably sprayed on as opposed to application by brush or pad.  
         [0035]     The yoke  44 , magnets  46 , backing rings  48 , and sleeve  56  are typically assembled into a unit and the shell  60  molded about the unit. A typical molding process results in a void in a molding support region  62 . The molding support region  62  results from a support  64  used during the molding process that locates the unit in a desired position as the shell is molded about the unit. This void in the molding support region  62  must be filled by a secondary fusing operation, such as plastic welding. The fusing creates a boundary interface where poor bonding between the base material and weld material can exist. This frequently results in a weakened area, which can provide a premature leak path for the corrosive process fluid to enter and attack the magnets  46 .  
         [0036]     The present invention utilizes a molding process resulting in shell  60 , fully encapsulating the unit. The support  64 , which may be multiple pins, are retracted at a desired time during the molding process so that the material forming the shell  60  fills the mold support region  62  during molding. The formulations of plastic used for the shell  60  better enable the flow fronts of material within the mold to quickly fill the molding support region once the supports  64  have retracted.  
         [0037]     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. In particular, the materials disclosed and their properties are exemplary only and are no way intended to limit the scope of the invention. For these and other reasons, the following claims should be studied to determine the true scope and content of this invention.