Magnetic Drive Pumps
Magnetic Drive Pumps were originally designed to pump toxic and other dangerous fluids without the use of mechanical seals. This is achieved by retaining the pumpage inside the pump casing and a containment shell, while the impeller shaft is supported on sleeve bearings lubricated by that pumpage. The impeller shaft is driven by a magnetic field passing through the containment shell from the driver shaft.
When a metal containment shell is used, adequate removal of the heat generated by eddy currents is vital, particularly when the liquid being pumped is sensitive to temperature changes.
A major difference between magnetic drive pumps and the conventional style is the location and type of bearings. In conventional pump designs, the bearings are usually located well away from the pumped liquid in a well controlled environment, and the operator has a wide choice of lubricants that can be utilized.
With magnetic drive pumps however, the bearings on the impeller shaft are lubricated by the pumpage that may not be an appropriate lubricant. In addition, when the pump runs dry, or operates at very low flows, the lubricant tends to disappear and the bearings will overheat. More importantly, as the bearings are usually of the sleeve type with slots or grooves to supply the lubricant to the bearing running surface, any solids in the pumpage will be detrimental to the bearing reliability.
Consequently, while the mechanical seal in a conventional pump becomes the first failure point, the bearings in a magnetic drive pump tend to perform the same function.
Magnets are also temperature sensitive and will demagnetize if exposed to temperatures exceeding their upper limit. This provides yet another reason to avoid any upset condition that would cause the generation of heat within this type of pump. Such conditions would include running the pump dry or against a closed discharge valve.
To provide some degree of protection against this problem, the material of the magnets should be selected to be able to handle 25 to 50 Fahrenheit degrees above the expected maximum operating temperature.
All magnetic couplings are rated for a maximum torque capability beyond which the magnets no longer operate at the same speed. This is referred to as “decoupling” and, if the pump operates in this state for very long, the magnets will be permanently demagnetized. Consequently, the magnetic drive pump is particularly vulnerable to any abnormal operating conditions that might result in an excessively high torque demand. Consequently, the use of power monitors are recommended for all applications in which magnetic drive pumps are used.
Many end users approach the magnetic drive pump as a cure-all for all pump ills. In fact, they have less tolerance for misapplication and process upsets than conventional pumps. However, with an understanding of their limitations and unique advantages, they can provide reliable operation.
Author of “The Practical Pumping Handbook” and a specialistr in Pumping Reliability, Ross Mackay can be reached at www.practicalpumping.com or at 1-800-465-6260