Choosing the right pump: magnetic drive pumps vs electric diaphragm pumps

Magnetic drive pumps are a good transfer solution as regards to leak prevention, maintenance requirements and can handle corrosive, toxic, or flammable liquids. But much like standard centrifugal pumps, magnetic drive pumps are incompatible with many fluids. Furthermore, their magnetic mechanism can cause overheating and even alter the fluid’s composition. Graco’s QUANTM electric operated double diaphragm (EODD) pumps address all of these issues while also providing some unique features to enhance the transfer process.

Magnetic drive pumps

While magnetic drive pumps eliminate the problems associated with mechanical seals, they have a number of limitations that make them less than ideal for a range of industrial applications.

• An unsatisfactory solution for abrasives and solids
  Magnetic drive pumps are primarily designed for pumping clean liquids that do not contain solids. Solids in the transfer material can cause quick failure of the close tolerance sleeve bearings and thrust surfaces inside the pump. These issues interfere with the pump’s performance and will accumulate, eventually causing complete failures of the pump and motor drive. Although some magnetic drive pumps are capable of moving heavier or more viscous fluids, they are best used for applications that need to transport clean, low viscosity fluids. Ideally, they should not be used for heavier applications that process solids containing fluids such as sludges, slurries and blends.

• Narrow preferred operating range and best efficiency point
  Magnetic drive pumps are similar to their cousin centrifugal pumps with a specific impeller diameter, meaning they only operate at optimal efficiency at a specific flow. Also, just as with centrifugal pumps, the operating range is narrow. Not only will moving outside the preferred range significantly reduce the pump’s efficiency but it will also, eventually, cause cavitation, vibration, impeller damage, suction and discharge recirculation, or reduced bearing and seal life.

• Deposits because of magnet overheating
  The coupling action of the magnets can generate a lot of heat. Heat given off by surfaces warms the liquid in the pump and is passed into the process.  If the material is not evacuated efficiently, the heat may accumulate enough to bake constituents of the process liquid onto the impeller magnet hub, resulting in build-up of a deposit and ultimately catastrophic failure of the pump itself. Furthermore, the magnets in a magnetic drive pump can demagnetise when exposed to temperatures above their upper limit. Dry-running mag-drive pumps exacerbate and speed up these types of premature failures in the pump system.

• Sensitivity in low flow or near shut-off head conditions
  Magnetic drive pumps are extremely sensitive when in low flow operation or near shut-off head conditions because the impeller is working against a higher head pressure. The magnetic coupling breakaway torque should not be exceeded. If this is not observed, the magnetic coupling between the drive and the impeller axis is lost, causing the impeller to stop spinning, thereby damaging the pump or system.

• Sensitivity to variations in viscosity during operation
  Liquids can vary in viscosity based on temperature or chemical reactions. The viscosity of the pumped fluid affects the required input power and magnetic torque required for transfer. All magnetic couplings are rated for a maximum torque; and beyond this point, the magnets operate at reduced speeds (decoupling). Operation in this state can permanently de-magnetise the magnets, making these pumps especially vulnerable to variable operating conditions and resulting in high power supply requirements. The integration of power monitors into the process should be included in the total investment cost for this type of pump.

• Not self-priming
  Most centrifugal pumps are not self-priming. For the pump to work properly, its casing must be filled with liquid before start-up. When the casing fills with vapours or gases, the pump impeller becomes gas-bound and incapable of pumping. To make sure the pump remains primed and does not become gas-bound, centrifugal pumps need to be installed below the fluid level from which the pump takes its suction. Alternatively, the pump can be primed by supplying liquid under pressure through another pump placed in the suction line.

• Unable to run dry
  Because the pumped liquid  acts as a lubricant and coolant, in the event of running dry, the bearing and some other pump head parts will overheat and eventually become damaged requiring servicing or replacement. Magnetic drive pumps should not be used in services and applications where there is a risk of running dry.