Patent Application
20130106018
This application relates to a magnetic drive centrifugal pump.
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.
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.
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.
Therefore, what is needed is an inner drive that is more resistant to swelling once the process fluid has permeated the plastic shell.
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.
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
Accordingly, the present invention provides an inner drive that is more resistant to swelling once the process fluid has permeated the plastic shell.
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.
A magnetic drive centrifugal pump assembly 10 is schematically shown in
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.
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.
In the example shown in
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.
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%.
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
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.
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.
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.
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:
Formulation 1:
Formulation 2:
Formulation 2 results in a lower viscosity, and is preferably sprayed on as opposed to application by brush or pad.
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.
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.
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.
This application is a divisional application of U.S. application Ser. No. 12/718,236, filed on Mar. 5, 2010, which is a divisional of U.S. application Ser. No. 11/009,613, filed on Dec. 10, 2004.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12718236 | Mar 2010 | US |
| Child | 13715292 | US | |
| Parent | 11009613 | Dec 2004 | US |
| Child | 12718236 | US |
