Sealless Centrifugal Pumps

Tutorialpg. 115Magnetic drive and canned motor pumps have many common features including bearing and rotor designs, internal cooling, and lubrication. The primary differences center on the manner in which the rotating magnetic field is created and the makeup of the stationary isolation device. Proper bearing lubrication is essential to long, reliable operation. Fluid film journal bearings are used to support the rotor and flatfaced thrust bearings to limit its axial travel. Hydraulic balance can be used to limit the loads transmitted to the thrust bearings. This can accomplished by either balance holes and set clearance throttling devices or pumping vanes, as in standard pumps or by variable clearance throttles that automatically adjust to maintain axial balance. A rotating magnetic field is generated by either an electrical stator coil or a rotating permanent magnet assembly. This drives an inner rotor consisting of induction motor laminations or permanent magnets. The barrier between the inner rotor and the atmosphere contains the process, but has nonmagnetic properties to allow the passage of the field undisturbed. Eddy currents are set up in the barrier and generate heat that must be removed. A clean cooling stream with sufficient margin from vaporization is required to carry of the heat generated by magnetic hysteresis and bearing friction loses. The pressure/temperature characteristics of the process dictate the type of circulation needed to maintain a liquid state. Instrumentation is offered to help monitor the condition of the cooling stream and avoid deadheaded and dry-running operation