Page 178 -

Special Pumps

-]

iill

, ff

!

.... i.i.i.ii : -

Figure 9.14: Magnetic drive pump (Reproduced with permission of Magnatex Pumps Inc.)

The containment shell may be constructed out of metal or nonmetallic
materials. However, when a metal containment shell is used, adequate
removal of the heat generated by eddy currents is an important
consideration. As this heat generation is proportional to the square of
the speed of the pump, it becomes even more vital when the liquid
being pumped is sensitive to temperature changes.

.3.1 Pump bearings
A major difference between magnetic drive pumps and the
conventional style is the location and type of bearings. In conventional
pump designs, the beatings are usually located well away from the
pumped liquid in a well controlled environment. In addition, the
operator has a wide choice of appropriate lubricants that can be
utilized. With magnetic drive pumps however, the beatings 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 bearing will
overheat. Even if the pump survives that abuse it may still not last very
long if such a condition is followed by a cool liquid entering the pump.
Under these conditions the beatings will be subjected to a thermal
shock which could cause cracking or total destruction.
It is also important to note that, as the beatings 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. The material of these beatings is usually either Silicon
Carbide or Carbon, and while the Silicon Carbide is a longer wearing

157