Magnetic Drive Pumps Section 3

Recirculation Circuit
All magnetic drive pumps circulate some of the process fluid to lubricate and cool the bearings supporting the inner rotor.

Magnetic drive pumps with metal containment shells, also require a circulation of some process fluid through the containment shell to remove heat generated by eddy currents. For pumps with metal containment shells, the fluid recirculation path must be carefully engineered to prevent vaporization of the process liquid necessary to lubricate the bearings. A pressurized circuit as shown in Fig. 4 offers excellent reliability for pumps with metal containment shells.

Magnetic drive pumps with electrically non-conductive containment shells, such as plastic or ceramic have no heat generated by eddy currents. Since no heat is required to be removed from the containment shell, a much simpler recirculation circuit can be used.

sectb 4b fig04
For liquids near vaporization, a calculation must be made to ensure the process fluid does not vaporize at the bearings. This calculation includes the effects of process fluid specific heat, vapor pressure, drive losses, recirculation flow, etc. This calculation procedure can be found in the GOULDS PUMPS HANDBOOK FOR MAGNETIC DRIVE PUMPS. An external cooling system can be added to the recirculation circuit to prevent vaporization.

Fail Safe Devices

Condition monitoring of the pump is a "key objective" and provides the user with an assurance of safety and reliability.

System and pump malfunctions can result from the following:

  • No-flow condition through the pump
  • Dry running as a result of plugged liquid circulation paths in the pump bearing and magnets assembly section
  • Cavitation due to insufficient NPSHA
  • Uncoupling of the magnetic drive due to overload
  • Temperature and pressure transients in the system
  • "Flashing" in the pump liquid circulation paths due to pressure and temperature transients.
These malfunctions can contribute to:
  • Overheating of the drive and driven magnet assemblies
  • Overload of drive motor and drive magnetic assembly
  • Extreme pump bearing load conditions
  • Damage to pump due to extremes in temperatures and pressures due to transients that exceed normal design.
Various fail safe devices are available with the pump to control malfunctions and provide safety and reliability including:
  • thermocouple / controller
  • low amp relay
  • liquid leak detector
  • power monitor