Chemical Engineering September 2014 - 60

Source: ITT Goulds Pumps
Source: ITT Goulds Pumps
Feature Report
capabilities compared to their lined
counterparts. Metal is more resilient
to solids erosion than a lined pump,
although only up to a point. Also,
with metal-mag-drive designs there
are typically options for providing
additional internal bearing flush
since concerns over breaching and
compromising polymer linings are
eliminated. Metal mag drives also
offer more options for monitoring and
controls. Instrumentation devices,
such as thermocouples, resistance
temperature detectors (RTDs), level
switches and temperature switches
are all more readily available to use
on a metallic mag drive, once again
because concerns about breaching
lined components are eliminated.
Avoiding failure modes
When using mag drives, one can
encounter failure modes not present
with a sealed-pump application.
The most common mag-drive
failure mode is dry run. Simply
put, dry run occurs when a pump
is run without fluid. Mag-drive
designs employ process lubricated
bearings often made of carbon or
silicon-carbide material. During dry
run, the lack of fluid in the system
causes the bearings to lose lubrication,
which can result in cracking,
fracture and eventual failure due to
the nature of the bearing material.
Another cause of dry run is when
fluid overheats, boils and flashes off
in the lubrication circuit, effectively
removing process lubrication. This
can lead to chipping, cracking and
increased temperatures that can
melt plastic linings and lead to a
pump seizing.
Some solutions to avoid dry run
are to attach a power monitor to the
pump, which can alert the operator
or control system of the condition,
or just automatically shut the pump
down after a short period of time.
There are also specialized methods
of treating or coating rotating and
stationary silicon-carbide component
surfaces that reduce their coefficient
of friction and can, for a
short time, minimize the effects of
dry run.
The other common failure mode
of mag-drive pumps is dead headFigure
6. Mag-drive pumps can be
supplied with polymer linings for corosion
protection
ing, which can occur by running the
pump against a closed discharge
valve. Fluid temperatures begin
to rise quickly because of energy
buildup of impeller rotation in a
closed system, leading to boiling,
cavitation and bearing damage.
Dead heading can be avoided by
flow and temperature monitoring
and controls.
Processing solids can also provide
a set of challenges due to erosion or
clogging. Mag-drive pumps are not
made to handle solids. Introducing
solids will cause accelerated erosion,
especially within lined magdrive
pumps. Solids can also collect
in low-pressure areas in the bearing
circuits, typically at the rear
corners of the containment shell,
causing accelerated erosion. Clogging
of process lubrication and recirculation
circuits can also occur
where the bearings can lose lubrication
and fail. A primary solution
in metallic mag drives is a bearing
flush where added pressure is
introduced to a pump system and
can help to keep solids from building
up in these circuits. Improved
pump designs in both lined and
metallic mag drives can also lead
to more fluid movement (and thus
easier solids removal from " problem
areas " ), achieved by introducing
more channels or adding pumping
actuators in the lubrication circuit.
A final important consideration is
magnetic decoupling. In mag-drive
pumps, the magnetic field between
the magnets spins the impeller.
The magnetic coupling has a torque
limit, so the pump must be properly
sized, and designs must take
into consideration the proper magnetic
materials as well as sizing. If
magnet materials and size are not
properly specified, the torque limit
is exceeded and the magnetic cou60
ChemiCal engineering www.Chemengonline.Com September 2014
Figure 7. Metallic mag-drive pumps
are more suited for higher temperature
and pressure applications
pling connection can be broken, resulting
in downtime and possible
unit failure. Some common causes of
magnetic decoupling are hard starts
with higher rates of start-up torque
than seen during steady operation,
wet end clogging due to an unexpected
high shear or fibrous fluid, or
a process upset, such as encountering
colder temperatures than anticipated,
which causes a spike in fluid
viscosity. Employing a power monitor
can detect magnet decoupling
and mitigate these effects.
Final remarks
CPI plant operators continue to gain
a deeper understanding of the latest
magnetically driven pump technologies,
which offer simpler and more efficient
fluid-process solutions. Using
magnetically driven pumps can
yield significant benefits by improving
safety, reducing plant downtime,
slashing maintenance time and associated
costs, cutting onsite parts
inventory and eliminating potential
failure points through a hermetic
sealless design. Today's mag-drive
pumps have ideal processing applications
in industries that handle
hazardous fluids. Mag-drive pump
designs offer distinct advantages
over traditional sealed models when
used in the proper environments
and appropriately specified.
■
Edited by Gerald Ondrey
Author
Richard Tym is global
product manager for the
non-metallic and mag-drive
product lines at ITT Goulds
Pumps (240 Fall St., Seneca
Falls, NY 13148; Phone: 315568-7378;
Fax: 315-568-7076;
Email: richard.tym@itt.com).
During his six years with ITT
Goulds Pumps, Tym has held
various positions in engineering
and product marketing.
He earned a bachelor's degree in mechanical engineering
from Wilkes University and an MBA
from the Rochester Institute of Technology.
http://www.Chemengonline.Com
Chemical Engineering September 2014

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