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Design Review of
Magnetic Drives versus Canned Motor Pumps
Courtesy of Teikoku Electric Pumps
Much has been written about the differences between canned motor pumps and
magnetic drive pumps. However, careful attention must be paid to
exaggerated claims between competing manufactures.
Both products use product lubricated bearings and are subject to high
failure rates if solids or abrasives are present in the fluid. Both
require that fluid is being circulated continuously through the drive
section to cool and lubricate the bearings. Each product offers one unique
feature that differentiates it from the other. Canned motor pumps offer
true second containment that minimizes the possibility of leaks during a
containment shell failure. Magnetic drives, on the other hand, offer
standard electric motors.
The Canned Motor Pump
Canned motor pumps employ a combined rotor/impeller assembly driven by
the magnetic field of an induction motor. The stator windings of the motor
are prevented from contacting the pumped or process fluid by a corrosion
resistant “can”, that completely seals the windings. A portion of the
pumped fluid is circulated through the motor section to cool the rotor and
lubricate the bearings. Canned motor pumps can be reinforced to handle
operating pressures up to 100 MPa.
The Magnetic Drive Pump
Magnetic drive pumps require more power to overcome the drag from
rotating the inner magnetic coupling and the pump shaft in the process
fluid. Some power is also lost from hysteresis when flux energy is
absorbed in the containment shell. A magnetic drive pump tends to occupy
more space because extra bearings are needed to support the spinning
magnet and the coupling. The smaller canned pump can be configured as
vertical in-line to accommodate minimal space requirements.
High viscosity fluids, such as Alkylphenols, require that a certain amount
of inertia be overcome. After pumping these fluids for extended periods,
magnetic-driven pumps tend to develop starting problems. The magnet
eventually decouples or spins, causing serious mechanical problems. In
canned motors excessive viscosity simply causes the pump to shut off,
thereby preventing any damage. Further, the fluid in a canned pump is
warmed as it passes through the motor section, enabling the pump to handle
higher viscosity materials.
Bearing and Rotating Elements
A major problem for sealless pumps is vaporizing of fluid at hot spots
inside the bearings. To eliminate this flashing, pump manufacturer Teikoku
increases uses a special design which directs the flow through the
bearings proportionate to the motor horsepower. The pressure drop and
temperature rise in the motor is calculated and compared with the pumped
fluid vapour pressure curve. If a sufficient margin does not exist, the
flow can be increased to reduce the temperature difference and/or the
pressure caused by using an orifice on the pump’s discharge side. In low
viscosity application (less than 0.8 cP), this manufacturer recommends
that pumps are installed in vertical orientation to reduce radial bearing
loads. Another method to improve bearing life is to dynamically balance
all rotating parts to ISO 1.0 and the rotating assembly to 2.5. Teikoku
also employs precession design standards in the manufacture and assembly
of the rotating components, where pinpoint balance is absolutely critical
for top performance. In addition, all pumps shall be performance tested,
including a vibration test to insure a safe operation and that all
parameters are within the tolerance.
2nd Containment
Most magnetic drive pumps do not offer secondary containment. Assuming
the primary containment shell is penetrated due to a mechanical failure,
the canned motor pump is designed to seal the liquid or gas inside the
secondary containment. On failure, no fluid can escape to atmosphere. This
is a very important safety aspect compared to the magnetic drive.
Circulation Flow Rate
Most sealless pumps only offer small flow paths for fluid used for
cooling. This causes the motor to frequently overheat. The Teikoku pumps
are designed to allow larger flows mainly through circular flow paths.
Even in high-temperature application, process fluid flows through the
bearings for cooling and lubrication. The flow rate is 3-5 times larger
than that offered by other manufactures.
High Temperature Application
Centrifugal pumps in high temperature service require special
consideration, and many different pump designs are in used on the market.
The effect of high temperature on the pump, however, raises several
considerations that may help to narrow the actual pump selection. Proper
alignment, pipe stress analysis and proper materials of construction are
vitally important to the successful long-term operation of high
temperature frame-mounted pumps.
Following is a list of desirable attributes for a high temperature pump:
§ Casing centreline supported to maintain alignment over temperature
§ Casing with raised faced flanges for adequate gasket sealing at high
temperatures with thin fluids
§ Casing manufactured from cast steel rather than cast iron
§ Impeller manufactured from non-thermal-shock-sensitive materials
§ Impeller fully enclosed to eliminate the need for impeller adjustment
Review of Conventional Shaft Seals
For many years pumps used shaft packings as shaft seals. However,
packing imposed many problems because they are required to leak to work
efficiently. This picture changed with the introduction of mechanical
seals. For some applications, the temperature inside a pump is above the
maximum operating temperature of available elastomers. The practice of
using O-ring type mechanical seals with cooled seal chamber and/or cooled
seal flush is seen less and less since it depends completely on the
environmental controls on the seal. Mechanical seals are now available in
which all components of the seal are rated for temperatures above that of
the fluid. With such seals, a cooled bearing housing and/or cooled seal
flush is still recommended to enhance seal life. Loss of cooling will not
necessarily cause seal failure.
In some cases it is appropriate to use double or tandem mechanical seals.
As mechanical seal proved reliability and more maintenance-friendly,
particularly with the introduction of metal bellows seals that eliminated
the need for water cooling, they gradually replaced packing as the shaft
seal of choice. But increased use of mechanical seals also increased the
awareness of possible danger that a catastrophic failure could allow large
amounts of media to leak and to injure plant personnel.
Conclusion
Several studies by major chemical manufactures covering 400.000 pumps
for more than 10 years showed that revealed that standard chemical pumps
with mechanical seals have the most expensive life cycle costs, followed
by magnetic drive units with flange mounted motors. Canned motor pumps
were shown to have the lowest total cost in use. Every end-user must
carefully evaluate the particularly plant and process requirements when
deciding what type of pump and sealing system is best for his service.<<
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