Chemical Engineering August 2022 - 38

Engineering Practice
Magnetic-Drive Positive-Displacement Pumps
for Chemical Processing Applications
Advances in sliding-vane and internal-gear seal-less pump technologies can provide
many benefits when compared to centrifugal pumps or more well-known positivedisplacement
pump options
Geoff VanLeeuwen and
Michael Coburn, PSG
F
acilities in the chemical process
industries
(CPI) use a
wide array of flowing substances
that must be processed,
stored and transferred during
manufacturing activities. The technology
tasked with facilitating this liquid
movement is the industrial pump. As
befits what has grown into a very large
global industry, a number of competing
technologies are constantly positioning
themselves to grab the attention
- and capital-investment dollars
- of the world's chemical processors.
The oldest of these battles divided
the camps into those that favored
centrifugal pump technology, which
uses velocity, momentum and kinetic
energy to transfer the liquid - but
only if the pump is operating at or near
its best efficiency point (BEP) - and
those with an affinity for positive displacement
(PD) pump technologies,
which capture a fixed volume of liquid
and transfer it " like clockwork. " These
distinctions have led some to call
centrifugal pumps " emotional, " since
changes to their liquid and system environment
directly influence their operational
capabilities, while PD pumps are
seen as more " stoic, " with the pump
steadfastly achieving peak efficiency
at all times, no matter the changes in
operating environment.
In recent years, however, the discussion
has begun to revolve around
another design distinction: is the best
pump - be it PD or centrifugal - for
CPI applications sealed or seal-less?
This article answers that question
while illustrating that seal-less PD
pump technologies, including slidingvane
and internal-gear pumps, can
ultimately be the better choice for certain
applications within the realm of the
PD pump universe (Figure 1).
FIGURE 1. A sliding-vane pump is a type of sealless
positive-displacement (PD) pump that can
offer some operational benefits when compared to
centrifugal pumps or more well-known PD
pump options
38
Un-sealing the deal
The difference between sealed and
seal-less pump technologies is very
simple - sealed pumps use dynamic
seals to keep the liquid contained,
while seal-less pumps do not require
dynamic seals to contain the liquid.
Sealed pumps can accommodate a
constant bleed of trace fluid across
the seal faces (even in double seals),
whereas seal-less pumps offer true
leak-free operation. Although sealed
pumps remain more common, seals
are viewed as the weakest link in a
pump's operational chain, with some
analyses of pumping operations indicating
that upwards of 80% of all
pump failures may originate at the
seal. A quick tour through any CPI facility
will support this hypothesis: look
for wet baseplates and empty baseplates.
Both indicate reduced uptime
related to a failure in a sealing device.
Now, this doesn't mean that all of
these failures are solely the seal's fault.
In fact, there are many operational
occurrences - including pulsation,
vibration, shaft deflection, dry run,
viscosity, temperature and pressure
changes and liquid crystallization, to
name just a few - that can cause
the seal to leak or fail, either of which
will put the pump out of service. Regardless,
a compelling case is created
for seal-less pumps, since using this
pumping technology could theoretically
significantly reduce seal-related
downtime occurrences, ancillary costs
for maintenance and repair and maintenance
requirements - some of the
major headaches that are inherent in
operating a pump.
So, if we can agree that there are
compelling reasons to consider incorporating
seal-less pumps in a
chemical-processing operation, the
next question becomes, " Which type
of seal-less pump? " Which brings the
discussion back to the centrifugal or
PD pump conundrum.
There has been recent growth in
next-generation seal-less designs
that feature a magnetic-drive (magdrive)
coupling that is used to transmit
torque to the pump. In these configurations,
a magnetic coupling connects
the driving shaft to the driven shaft
without use of a dynamic seal to keep
the pumped liquids contained.
Admittedly, mag-drive seal-less
pumps typically have a higher purchase
price than basic sealed pumps,
but as the seals and accompanying
seal-support systems get more
complicated (and remain more prone
to failure), they get more expensive.
For example, a triple-lip seal can cost
$3,000, and if you're repairing or replacing
it every 12 to 18 months,
that cost will add up, along with the
added expense of any maintenance
or repair calls. Or, for double mechanical
seals that require seal-support
systems, the seal-support system
adds $4,000 to $8,000 itself in purchase
cost, plus associated costs for
monitoring equipment.
Financial considerations aside, magdrive
seal-less PD pumps outperform
their seal-less centrifugal cousins by
offering new functionality that is not
possible with centrifugal pumps. First,
seal-less PD pumps are self-priming
and have good suction-lift capabilities,
while mag-drive centrifugal pumps,
just like sealed centrifugal pumps, rely
on an integral reservoir that must be
pre-filled before it is turned on. This
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