Chemical Engineering December 2011 - 32

Dickow Pumps, USA
Cover Story
pump application can be combined
into one pump.
This article concentrates on the
single fluid pump, because it has the
most demands placed on it due to the
fact that it is in a constantly changing
temperature environment. The items
that should be taken into consideration
when specifying a single fluid
pump are explained here.
Thermal stress: As the pump changes
temperature, the metal components of
the pump will expand or shrink, according
to the direction of the temperature
change. Different parts may
change at different rates, depending
on the weight of the particular casting
and the percentage of its surface directly
in contact with the pumped liquid.
Rapid temperature changes can
cause different components to press
against each other unevenly. Over
time, this will result in maintenance
problems or out-and-out failures.
Materials of construction: Construction
materials are important in
specifying a single fluid pump. While
cast iron may be adequate for water
pumps and even some light-duty
chemical pumps, it is too brittle for
heat transfer duty, as rapid temperature
changes could cause a cast-iron
casting to crack. Ductile iron, cast steel
or stainless steel are better choices for
materials of construction.
Alignment: Mechanical alignment is
critical for long bearing and mechanical
seal life. If a (end suction) pump
has a conventional foot mount under
the casing, the expansion of the casing
under heating can push the centerline
of the pump out of alignment with the
motor. This allows vibrations that can
shorten bearing and seal life.
To counteract these problems,
pumps are available with " centerline
mounts " , where the pump casing is
supported from the horizontal centerline.
This supports the casing at
the shaft centerline and maintains
alignment by allowing the casing to
expand both up and down when temperature
increases.
Mechanical seals: Mechanical seal
specification and installation is deserving
of considerable attention in
single fluid pumps (Figure 4). Most
pump " failures " exhibit themselves
as mechanical seal failures, although
the failure may not be
the sole fault of the seal.
If - when you remove a
mechanical seal - the
carbon face (or other rotating
face) is not worn
back to the shoulder, then
the failure may not be the
fault of the seal. Seal failure
on hot pumps is often attributable
to oxidation of the fluid
as it migrates across the seal
faces. The fluid degrades
to carbon particles that
either score the seal faces
or agglomerate and force
the faces apart, allowing
the fluid to leak. There
are many methods for addressing
this problem, but one of
the simplest and most-often used
is to apply an inert gas quench to
the back of the seal. The inert gas
can be steam, nitrogen, carbon dioxide
or another inert, non-oxidizing
gas.
Seals that operate at cold temperatures
(below 0°C) can leak as well, but
not from fluid oxidation. Water can
condense, and ice can form at the seal
faces of cold seals, which can eventually
force the seal faces open and allow
a fluid leak. This problem can be a
challenge to troubleshoot,
because
when the pump is removed from service
for maintenance, the ice in the seal
melts and the faces go back together.
As with hot seals, an inert gas quench
goes a long way toward mitigating the
problem. Obviously, steam is not a good
choice here, but nitrogen is used successfully
in many cases.
Seals that operate near the pumpedfluid
temperature can also wear abnormally
if the temperature range of
the system is large. This problem can
be mitigated by environmental controls
on the seal that stabilize the seal
temperature, or by choosing a pump
that locates the seal away from the
pumped fluid.
Flange leaks: Low viscosity, high
vapor pressure and low surface tension
cause thermal fluids to be prone
to leaking. A well-thought-out piping
materials specification will go a long
way toward reducing the possibility of
leaks. One way to minimize leakage
opportunities is to weld valves and fit32
CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011
tings into the pipe, and to use bellowssealed
valves. Threaded connections
on pipe and fittings are the least acceptable
type of pipe joining in these
systems. Where connections cannot be
welded, as with pumps, the specification
of 300-lb raised face flanges and
the use of carbon- and stainless-steel
spiral-wound gaskets significantly reduces
the possibility of fluid leakage.
Sealless pumps
Mechanical seal problems can be
avoided by using sealless pumps.
The rotating assembly of a sealless
pump is completely sealed from the
outside, in a canister at the rear of
the pump. The rotating-assembly
bearings are lubricated and cooled
by the pumped liquid.
Magnetic drive pumps employ a rotating
magnet outside the can that is
driven by a conventional electric motor.
(Figure 5) The magnetic field of the
driving magnet penetrates the metal
of the can and couples with an internal
magnet, which is part of the rotating
assembly and drives the pump.
Canned motor pumps use a rotating
assembly similar to those in magnetic
drive pumps, except the internal
magnet is replaced with a motor
rotor winding (Figure 6). A threeFIGURE
4. The pump in this image
is fitted with a double mechanical
seal and a reservoir for the barrier
fluid. The barrier fluid circulates by
natural convection in this pump
(although there are methods to
force the circulation), and helps
to protect the seal from extreme
temperatures. The hot
fluid is also protected from
contact with atmospheric
oxygen, mitigating fluid oxidation
at the seal
xi
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Chemical Engineering December 2011

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