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.