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Pump systems, pipes, valves and seals
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Mechanical Technology — June 2016
11
Metso’s MD pumps are double adjustable:
the front and the back impeller gap can both
be restored at same time.
A typical MDM series pump section summarising some of the pump’s key features.
Metso’s EnviroSet seal has a built-in lantern ring and a helical grooving system that imparts a centrifugal
flow to the flush water, which throws the particles in the fluid to the outer bore of the separation chamber.
Gland flush water savings of over 50% can be achieved.
making the pump significantly cheaper,
easier to handle and to maintain,”
Sedgwick explains.
“Hence the importance of selecting
the appropriate aspect ratio for the slurry
being pumped. It is the starting point in
selecting the most cost effective pump,”
he advises.
Metso’s slurr y pump selection
chart and its pump selector software,
Pumpdim, use the aspect ratio as the pri-
mary variable in organising its range for
different slurry requirements. “The pink
colour (HM and HR range), for example,
represents an aspect ratio of around
2.5. Our slurry pumps are organised on
the selection chart in order of reducing
aspect ratio, that is, from most abrasive
(3) to least abrasive (2), with Metso’s
MD pumps being the most suited to
cope with the highest abrasion levels,”
Sedgwick notes.
Another differentiator for Metso is its
focus on limiting the inlet fluid velocity
of its pumps. “For heavy slurries we keep
our inlet velocity down to below 5.5 m/s
to minimise impact damage to the impel-
ler from sharp, coarse and heavy solids.
From a hydraulic design perspective,
we also try to make sure that the pump
always operates just to the left of the best
efficiency point (BEP), whilst maintain-
ing a relatively low inlet velocity. This
results in the highest possible efficiency,
minimum internal turbulence and the
longest wear life. Wasted energy has to go
somewhere, into vibration or turbulence,
for example, which may cause bearing
failure and accelerated wear,” Sedgwick
points out.
Addressing the design innovations
that improve the hydraulics for abrasive,
applications, Sedgwick says: “A pump
is always going to wear when pumping
slurry. On some applications, the best
products might only last for 400 to 600
hours, which can be less than a month.
This makes it important for operators
to choose pump designs that extend
wear life and make it easier to maintain
hydraulic efficiency.”
As an example, he says that a slurry
pump does not operate well with a big
gap between the impeller and the front
casing liner. “Pumps accelerate fluid
under centrifugal action and convert this
kinetic energy into pressure energy. If
there is a gap between the suction wear
plate liner and the impeller, fluid under
pressure at the outlet is forced back to
the inlet through this gap. This process
is known as recirculation.
“The more high-energy material that is
flowing around the front of the impeller,
the higher the wear rate on the liner – and
the larger the gap the higher the recircu-
lating flow. So this gap must be kept to a
minimum at all times,” says Sedgwick.
“Recirculation also reduces the spe-
cific energy of the pump, ie, the energy
used per m
3
of flow, since some of the
flow is being recirculated,” he adds. “So
this front liner gap has to be adjustable.
The conventional way is to push the
impeller forward on its shaft until it hits
the front liner, then to shift it back a little
for clearance.
“On our MD pumps, however, we
have a front liner that can be adjusted
independently of the impeller shaft. The
front adjustment can be made by mov-
ing the suction wear plate liner forward,
without disturbing any of the shaft
components,” he explains, adding: “Big