Chemical Engineering September 2014 - 59

150
safeR WoRk enviRonment foR
aluminum manufactuReR
Single
mechanical
sealed pump
100
Double
mechanical
sealed pump
50
Mag drive
Pump size
Discharge Q =
Head
Speed
Power
50-32-160
10 m3/h
H = 28 m
n =
P =
2,900 rpm
1.5 kW
85-50-200
40 m3/h
41 m
2,900 rpm
7.0 kW
Source: Infractor
DEGUSSA-Hüls
Figure 5. This chart compares the lifecycle costs for centrifugal pumps
using various sealing options
and also carries high water-usage
costs. Introducing a sealless pump
to a remote location eliminates the
need for seal checks and flushing,
and provides a reliable system with
minimal maintenance.
By taking the seal out of the system,
a major failure point is eliminated
and reliability is enhanced.
Eliminating bearings is a second
feature mag-drive pumps offer that
improves reliability. Close-coupled
mag-drive designs eliminate the
power end, removing the possibility
of oil-lubricated bearing failure,
coupling misalignment, and in most
cases, the need for a baseplate.
Comparison to other seal options
A mag-drive pump is comparable
in cost to single-sealed and cannedmotor
pump designs, and is less
costly than double-sealed pumps
that utilize barrier-fluid systems.
There is no replacement of seals
and no running of flush lines, leading
to reduced maintenance and
downtime. When evaluating different
pump sizes and duty points one
must look at total lifecycle costs.
The chart shown in Figure 5 provides
a breakdown across the various
pump designs. Canned-motor
pumps require more maintenance
time and dollars, as they are generally
very difficult to work on, and
require a specialized maintenance
shop on site. Also, operators may
need to send the motor out for service
if there is a breach of the primary
barrier to the internal motor
windings. Double-sealed pumps
have higher costs across pump
sizes compared to mag drives. Mag
Canned
motor pump
A
North American architectural aluminum
store-front and door manufacturer used
several anodizing tanks to treat and color
its aluminum products. Each tank contained
17% sulfuric acid that needed to be maintained
at 70°F for optimal results. During the course
of the anodizing process, heat was generated
as the sulfuric acid reacted with the aluminum
drives also deliver optimal
performance, with metallic
mag-drive efficiencies often
equivalent to that of metalsealed
ANSI pumps. Also
worth noting is that nonmetallic
(lined) mag-drive
pumps can be up to 30%
more efficient than metallic sealless
pumps.
products, and the acid was pumped out of the
tanks through chillers. Each of the pumps was
a mechanically sealed ANSI pump with a large
sheet of Plexiglas leaning up against the pump
unit, effectively acting as a " spray shield. " A fine
mist of sulfuric acid would emit from each pump
seal, barely noticed by plant personnel. This
hazardous condition resulted in workers finding
multiple holes in work clothing where the acid
had splashed during the workday. Needless to
say, a change was needed in order to increase
worker safety. Initially, the plant agreed to install
one non-metallic magnetically driven pump as a
test on one of these tanks. The mag-drive pump
cut maintenance, increased safety and reduced
downtime. The replacement was such a success
that mag-drive pumps replaced all mechanically
sealed ANSI pumps, providing a safer and more
efficient work environment for plant personnel. ❏
Lined versus metallic
Lined mag-drive designs (Figure 6)
protect the metal casing from corrosion
that is common in chemical
applications. Some common linings
are PTFE (polytetrafluoroethylene),
PFA (perfluoroalkoxy alkanes),
ETFA (ethylene tetrafluoroethylene)
and PVDF (polyvinylidene fluoride).
All offer corrosion resistance from
chemicals; however, they are not
universally resistant to all media,
which is why various coating options
exist. Some components can also be
fiber reinforced with carbon or glass.
For example, the impeller can often
be reinforced with these materials to
provide greater protection from hydraulic
erosion. In a lined mag-drive
pump there is no eddy-current heat
buildup or potential power loss from
currents as the containment shell is
typically fiberglass-reinforced plastic
(FRP) that is coated with one of
the linings listed above.
One thing to consider with lined
mag-drive pumps is that they are
generally limited in the areas of
temperature and pressure. The
linings can only accommodate
temperatures in the mid-200°F
range, with various casings typically
capable of sustaining design
pressures in the mid to upper 200
psi range. Overall, lined mag-drive
pumps are very good for working
with various acids or bases as long
as they are below certain temperature
and pressure thresholds.
A metallic mag-drive-pump design
(Figure 7) is capable of withstanding
higher pressure and temperatures
limits. These pumps are
well suited for solvents, heat-transfer
fluids and other non-conductive
fluids that typically run hotter.
Some metallic mag drives can easily
handle over 500°F for liquid-service
temperatures. Metallic mag-drive
pumps also are strong solutions
for pumping non-conductive fluids,
such as benzene, that can build up
an electrostatic discharge, which
can be an issue for designs employing
an FRP polymer-lined containment
shell. When using an FRPlined
shell design, the electrostatic
discharge can " arc " through the
nonmetal containment shell, causing
a pinhole leak that will cause
complete pump failure over time,
as well as introduce environmental
and personnel hazards that may be
difficult to observe at their onset.
In a metallic mag-drive pump this
arcing charge will not penetrate the
alloy containment shell.
Finally, metallic mag-drive designs
have better solids-handling
ChemiCal engineering www.Chemengonline.Com September 2014 59
Life cycle costs, %
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Chemical Engineering September 2014

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