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The Practical Pumping Handbook .......................

material, it is also much more brittle and susceptible to thermal and
mechanical shock.
Some designs also provide a flexible mounting for these beatings with
the use of O-tings which can be subject to the same chemical
compatibility issues as they would in a mechanical seal installed in a
conventional pump.
Consequently, while the mechanical seal in a conventional pump tends
to act like a fuse in an electrical system and becomes the first failure
point, the bearings in a magnetic drive pump tend to perform the same
function and become the first point of failure.

9.3.2 Temperature considerations
Magnets are also temperature sensitive and will demagnetize if exposed
to temperatures exceeding their upper limit. This provides yet another
reason to avoid any upset condition that would cause the generation of
heat within this type of pump. Such conditions would include running
the pump dry or against a closed discharge valve.
To provide some degree of protection against this problem, the
material of the magnets should be selected to be able to handle 25 to
50 Fahrenheit degrees above the expected maximum operating
temperature. It should be noted that, of the two most common types of
permanent magnets used in magnetic drives, samarium cobalt has a
higher temperature rating than neodymium iron boron, but has only
60% of the strength. It is also more expensive.

9.3.3 Decoupling
All magnetic couplings are rated for a maximum torque capability
beyond which the magnets no longer operate at the same speed. This is
referred to as 'decoupling' and, if the pump operates in this state for
very long, the magnets will be permanently demagnetized.
Consequently, the magnetic drive pump is particularly vulnerable to any
abnormal operating conditions that might result in an excessively high
torque demand.
An excessive load demand can be created by a variety of hydraulic
conditions or a change in the nature of the liquid. It can also be caused
by running the centrifugal pump at a higher capacity. An increase in
specific gravity or viscosity of the pumpage will also increase the power
draw and the load.

In spite of this, most magnetic drive pumps are normally designed to
operate with a safety factor that is frequently less than the safety factor
of the mechanical shaft coupling or the electric motor driver, thus
leaving the magnetic coupling itself in the most vulnerable position for

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