Processing, October 2018

The heat generation from dry running conditions is the number one mode of failure for magnetic drive pumps OCTOBER 2018 www processingmagazine com 19 torques Even in extreme situations it will act as a fuse Magnetic couplings can be broken temporarily if the load of the pump is too great In practice it means the pump does not overload and get damaged Magnetic drive pumps are available in most metallurgies and materials metallic and nonmetallic Polymer lining pumps are also used because they offer improved corrosion resistance Polymer coating options include polytetrafluoroethylene PTFE perfluoroalkoxy alkanes PFA and polyvinylidene fluoride PVDF These lined or non metallic options are usually employed for ordinary temperatures below 90 C as a rough indication Metallic magnetic drive pumps have been used for even higher temperatures Bearings for magnetic drive pumps Because a magnetic drive pump is an enclosed piece of equipment lubrication oil or grease cannot be used for bearings Therefore pumped liquid is used for the lubrication of bearings as well as for cooling needs A portion of pumped liquid is usually taken from the discharge of the pump this flow is often known as recirculation flow and is usually employed for the cooling of the magnetic system lubrication of radial and thrust bearings and for other similar applications where lubricant or cooling fluid is needed Magnet drive pumps are usually equipped with sleeve bearings which use the pumped liquid for hydrodynamic lubrication A sleeve bearing is one of the simplest types of bearing comprising just bearing surfaces and no rolling elements Therefore the sleeve or journal slides over the bearing surface where pumped liquid is used for its lubrication These bearings are also compact and lightweight and have a high load carrying capacity and longer life compared to many rolling element bearings The materials of construction of these and the required clearances of the sliding surfaces may restrict the liquids and services for which this kind of pump may be used Traditionally magnetic drive pump bearings have been made from silicon carbide which has been an excellent bearing material under normal operation however it can suffer during some abnormal conditions such as dry running dynamic loading and potential impact Some modern materials such as advanced ceramics have been used in these with great success Another proposed material is a fiber reinforced silicon carbide ceramic composite with bearing properties similar to silicon carbide but with high impact resistance and excellent tolerance of dry running These modern options offer great benefits such as better impact resistance excellent wear resistance and generally longer life and better performance Modern sleeve bearings for magnetic drive pump applications should feature smooth surfaces whether by a dedicated surface treatment on sleeve surfaces or a coating to sufficiently reduce the coefficient of friction which enables longer periods of safe operation under dry run conditions Dry running can occur from a wide range of malfunctions and maloperations The heat generation from dry running conditions is the number one mode of failure for magnetic drive pumps Modern bearing materials are usually up to two times harder than conventional silicon carbide ensuring they wont be compromised in even the harshest operational environment To successfully perform its task of lubrication and cooling the recirculated flow should remain in the liquid phase at all points at all times An area for concern is the magnetic coupling area where the intense heat could boil the liquid Therefore a robust design and sufficient flow rate are needed for these areas The determining factors for preventing phase change are the mass flow rate heat capacity and localized pressure at any point within the system The details related to recirculation flow will affect the variables of mass flow rate and localized pressure To act as a lubricant the pumped liquid should maintain a proper film thickness at least a few microns at operating temperature and load or else the sleeve bearings will experience severe wear Many liquids such as hot water and most solvents do not have this property and cannot act as lubricant On the other hand if sufficient pressure or flow rate is not provided for the liquid through the bearings and other critical areas it leads to overheating of the area which can become a serious problem The result can be flashing of the liquid and a potential loss of lubricating ability as the liquid increases in temperature and decreases in viscosity Disadvantages limitations Because magnetic drive pumps utilize magnets to transfer the torque and power from the drive assembly to the impeller assembly there are some limitations with these pumps For example when magnet materials are exposed to temperatures above their threshold they can begin to lose their magnetism Therefore temperature characteristics of each service are a critical factor Some energy is lost in the magnetic coupling This is primarily due to some magnetic resistance As the result of this and some other effects a typical magnetic drive pump is usually less efficient than conventional centrifugal pumps There are some limitations in power rating as a very large or powerful magnetic coupling is neither feasible nor cost effective Power and torque limitations should always be considered for the selection of these pumps One of major limitations of magnetic drive pumps is the risk associated with running dry Because the pumped liquid will act as lubricant and coolant in case of dry run the bearing and some other parts could overheat and eventually become damaged In services and applications with risks of dry running magnetic drive pumps should not be used Magnets should be sized correctly so magneticcoupling breakaway torque is not exceeded during startup or in all other potential transient operational cases If breakaway torque is exceeded the magnetic coupling between the drive and the driven assemblies is lost and the impeller will cease to spin which means the pump should be tripped to allow the magnets to recouple These pumps are also extremely sensitive to transient cases such as low flows and operation near shutoff head conditions Magnetic drive pumps for liquids containing solids Magnetic drive pumps have very limited solids handling capability The pumped liquid should be clean or An example of a magnetic drive pump Courtesy of Amin Almasi