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National Environmental Services Agency (NESA). Tom addresses the issues that bug you the most. And Tom knows!! With 35 years experience in providing environmental support services to public and private sector clients on a wide range of environmental issues. Tom has also co-authored and presented training courses on wastewater treatment systems.
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Hose Pumps : Ideal for
Abrasive Applications
Guest article by Chuck Treutel P.E., Watson-Marlow Bredel Pumps
Printer friendly PDF
 Originally
conceived as a cement pump, the hose pump is fast becoming the
technology of choice for transferring and metering abrasive
fluids in many industries.
In 1971, engineers at
Holland-based Bredel (now Watson-Marlow Bredel Pumps, the world
leader in peristaltic pumps) were busy developing a machine to
spray a cement mixture. When the time came to put a positive
displacement pump in the new device to move the concrete,
company engineers were underwhelmed by the choices in the
marketplace. Consequently, the decision was made to internally
create a brand new pump—one that could effectively pump the
concrete through the machine and also tolerate the material’s
abrasive properties.
This revolutionary cement pump
worked so well, it spawned new research and development at
Bredel that ultimately yielded longer hose life, broader flow
ranges, and greater chemical compatibility. It was not long
before the hose pump transformed from a purpose-built cement
pump into the technology it is today—a worldwide solution for
pump users looking to increase productivity and minimize their
life cycle costs when moving abrasive fluids.
When it comes to handling
abrasive materials, hose pumps deliver a significant advantage
from both a performance and financial perspective. Before we
consider the issues inherent to pumping abrasive materials, it
is beneficial to recognize and understand the differences in the
various types of pumps.
Positive Displacement Pumps
 Unlike
constant speed centrifugal pumps, which predominantly find use
in transferring thin nonabrasive fluids, positive displacement
pumps (PD pumps) were created to meter or transfer
hard-to-handle fluids like corrosive, viscous, shear sensitive,
or abrasive slurries at various speeds without pressure—induced
flow drop. Within the realm of PD pumps, there are reciprocating
and rotary pump technologies: diaphragm pumps, which are the
most commonly used reciprocating pumps, and progressive cavity
pumps, which are commonly used rotating pumps. (Photo left:
Thickener underflow installation)
Other PD technologies include
gear pumps, piston pumps, and rotary lobe pumps. Hose pumps are
a classification of peristaltic pumps, rotary style PD pumps
that take their name from the biological process of peristalsis:
muscular contractions that move mixed phase fluids (solids,
liquids, gases) throughout the digestive system.
Although PD pump technologies
differ, all positive displacement pumps incorporate moving parts
that come in contact with the material being processed, a
reality that is critical to life cycle costs when pumping an
abrasive fluid. For example, diaphragm pumps (typically air
operated or electro-hydraulic) use a reciprocating diaphragm to
induce flow between two internal ball check valves. Abrasive
fluids inevitably cause erosion or clogging of the valves,
requiring frequent rebuilds of the pump’s wetted end.
Progressive cavity pumps move
fluid along the successive cavities formed between the meshing
of a fixed stator and rotating rotor. Erosion from abrasive
fluids widens clearances between the rotor and stator, causing
internal slip which requires the user to speed up the pump in
order to maintain capacity—accelerating wear until rotor and
stator need to be replaced, normally four stators to every rotor
replacement. With many PD pumps, abrasive fluids can cause
problems beyond the normal wetted end of the pump. For example,
on a progressive cavity pump it is only a matter of time until
universal joint (gear or pin type) seals fail, allowing abrasive
slurry to erode the numerous parts within each joint, including
the ends of the connecting rod.
The negative ramifications of
wear on common PD pumps from abrasion cannot be overstated. When
repairing a progressive cavity pump, for instance, it is
necessary to disassemble the entire apparatus, replace the
stator and sometimes the rotor as well—a repair cost that often
represents over 75% of the initial purchase price of the pump.
This does not take into account the lack of productivity that
results from significant downtime, since the complexity of a
progressive cavity pump normally requires the pump to be removed
from the installation site for service in a maintenance shop.
The Hose Pump Difference
 Hose
pumps are unique in that there are no seals, valves, or moving
parts in the product stream, making the pump ideal for abrasive
slurries. The pump’s operation is elegantly simple: a hose
element positioned along stationary pump housing is compressed
from the outside by shoes that are mounted to a rotor.
Fluid is pushed towards the discharge as the rotor slides the
shoes along the hose element while the restitution of the hose
element behind the shoe allows more fluid to be drawn into the
pump. (Photo left: Simple hose pump construction)
This design means the abrasive
fluid is completely contained within the hose element; the rotor
remains outside the pumpage zone and never actually touches the
product being moved. The complete closure of the hose element
gives the pump its positive displacement action, preventing flow
drop or erosion from backflow and also eliminating the need for
check-valves.
 A
hose element has a serviceable life before fatigue requires
replacement—it is predominantly dependent on the pump speed and
compression forces on the hose element, but not influenced by
the abrasiveness of the fluid that is pumped. With pumps that
can deliver a range of 0.005 – 400 gpm and 0-240 psi discharge
pressure, hose pump manufacturers will typically size pumps to
run at speeds to deliver thousands of hours of hose life. (Photo
right: Hose construction)
High quality hose pump
manufacturers machine their hoses to maintain tight tolerances
and utilize adjustable shoes to set the perfect compression
force for specific process conditions. Such steps optimize hose
longevity, maintain flow stability over the life of the hose,
eliminate the potential for abrasion wear from slip and ensure
repeatable performance from hose to hose.
The stark contrast in maintenance
and installation simplicity versus other PD pumps shows why hose
pumps offer the lowest cost of ownership in abrasive
applications. While initial capital cost of a hose pump can be
higher than other positive displacement pumps (a typical hose
pump may cost about $14,000, while a progressive cavity of equal
capacity costs $12,000), the subsequent costs associated with
repair, downtime, and ancillary items quickly tip the life cycle
cost calculation in favor of the hose pump.
Hose element replacement on the
largest hose pump models takes about one hour and is performed
at the installation site. To replace a hose element, simply
remove the flanges from the pump and jog the motor to expel the
old hose and feed in a new one. Replacement hose elements costs
are approximately 5% of initial pump price, compared with
progressive cavity pumps where wetted end replacement parts cost
75% of the pump’s price and are laborious to replace.
Peristaltic pumps also do not
require the ancillary equipment commonly used with a progressive
cavity pump in abrasive applications, such as double mechanical
seals, seal water flush systems, run dry protection systems
(hose pumps can run dry without damage) and in-line check
valves. For ancillary equipment, a hose pump may require a
pulsation dampener in installations with long pipe runs and very
high fluid velocities; however, normally, pulsation is
eliminated without a dampener through minor pipe changes or use
of flexible lines.
Abrasive Applications
 There
is a myriad of abrasive fluid applications for which hose pumps
are ideal. Because hose pumps have a non-slip positive
displacement design, they give repeatable flow per revolution
along their entire speed range regardless of discharge pressure.
This makes them inherently excellent metering pumps, though they
are commonly applied in fixed speed transfer applications as
well. In slurry applications hose pumps are fully reversible,
self priming, and can run dry without damage. (Photo left:
Abrasive mining installation)
These pumps also add the benefit
of being able to blow out blockages or drain process lines of
high settling solids between batch runs. Perhaps the most common
hose pump application is moving hard-to-handle lime slurry,
which is used across many industries: flue gas scrubbing of
sulfur dioxide in power plants, biosolids stabilization in
wastewater treatment plants, mineral filler in hot mix asphalt,
and as a reagent for acid neutralization in multiple processes.
 Similarly,
hose pumps are taking on a greater share of applications in
paint, printing and paper production, moving highly abrasive
pigments like titanium dioxide and iron oxide. Hose pumps are
also commonly used in arduous mining operations to handle
thickener underflow and various metallurgical acid slurries.
(Photo right: Lime slurry pumping)
To reduce maintenance costs while
moving thickened solid laden slurries, market sectors like
waste-water treatment regularly select hose pumps to feed
mechanical solid-liquid separation and filtration equipment like
belt presses, plate and frame filter presses and centrifuges.
Beverage and potable water treatment plants use hose pumps to
meter gritty, powder-activated carbon (PAC) which is used to
purify water.
In fact, beer lovers will be
comforted to find hose pumps are used in many breweries, pumping
shear sensitive yeast and also diatomaceous earth, a naturally
occurring, chalk-like, sedimentary rock that is easily crumbled
into a fine powder and used as a filter in the brewing process.
Other Uses
Hose pump usage is not limited to
abrasive applications. The low operating speeds of a hose pump
make them naturally low shear and perfect for kaolin clay,
yeast, starch, food products and polymers. The ability to pump
mixed phase fluids efficiently and run dry make hose pumps ideal
for draining tanks or pumping “off-gassing” fluids such as
sodium hypochlorite. Corrosive and caustic fluids are also easy
to handle, because there is no metallic contact—the fluid is
contained within the hose element.
Despite all of these advantages,
hose pumps represent a modest percentage of the positive
displacement market in the U.S. This is primarily because
peristaltic technology is relatively new in the U.S. market,
whereas diaphragm and progressive cavity pumps have a lengthy
history. However, with the pressure on plant managers to reduce
life cycle costs of their pumps, the functionality and benefits
of peristaltic hose pumps are becoming more widely known, making
peristaltic hose pumps the fastest growing pump type in North
America.
About our author:
Our author,
Mr. Chuck
Treutel, is a registered professional engineer and the Marketing
Director for Watson-Marlow Bredel Pumps. For more
information, you can contact him here:
Mr. Chuck
Treutel, PE
Watson-Marlow Bredel Pumps
37 Upton Technology Park
Wilmington, MA 01887
Telephone: 978-658-6168
Fax: 978-658-0041
Web site:
http://www.watson-marlow.com/
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