SELECTING SEALLESS
MAGNETIC DRIVE PUMPS
ith the increasing
awareness in industry
of new environmental
legislation, product liability,
operator safety and the
tightening of international standards
governing these areas, there is an urgent
need for industry to review and, where
necessary, to revise its selection process
for pumps.
W
There is also a growing need for engineers
to appreciate the relative merits of the
available design options.
The end suction mechanically sealed
pump has been the workhorse of industry
for decades. It is available in a range
of specifications from the European
ISO 2858 to the American ASME / ANSI
B73.1 standards. These standards have
all evolved to reflect the vast quantity
of pumps installed, standardisation and
good engineering practice.
Sealed pumps are fitted with a variety of
sealing systems such as single mechanical
seal, double mechanical seal, tandem seal
and the associated seal support systems.
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PECM Issue 32
Some of the features of a sealed
centrifugal pump are that it is usually a
low unit cost, has commonality of design
across product ranges and familiarity of
the product design when maintaining the
pump. Seal manufacturers have simplified
the sealing system of such pumps
by introducing replaceable cartridge
designs that require less arduous set-up
procedures, although such designs do
command a higher financial outlay.
A point, which should always be
considered, is that whatever seal system
is chosen, there is one inherent weakness
– the seal has to leak to lubricate the seal
faces and when the seal fails the product
being pumped will leak to atmosphere,
possibly endangering plant personnel,
resulting in lost production, loss of
product and potentially damaging the
environment. In comparison a magnetic
drive sealless pump, properly applied,
will eliminate the effect of leakage. The
basic principle of a magnetic drive sealless
pump is that the internal pump shaft and
supporting journal bearings rotate within
the process fluid.
This shaft is supported by internal plain
bearings located in a rigid bearing holder.
The shaft has an inner magnet ring
connected to one end and the impeller
at the other. The shaft and bearing holder
are inside a can or containment shell that
contains the process fluid and is rated for
the same process conditions as the pump
casing in which the impeller is located.
Engineering improvement in containment
shell design has enabled designs that can
withstand temperatures from -100 ºC to
+450 ºC and pressures from full vacuum
up to 185 bar or higher.
Typically the material of construction of
the containment shell would be a high
nickel alloy, providing excellent chemical
compatibility. The design of the shell
will vary depending upon the pressure
rating and size. Shell thicknesses range
in sizes down to 1mm depending on the
application.
Rotating around the outside of the
containment shell is a corresponding
magnetic ring, the outer magnetic ring.