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Unconventional design
Alternative sealless pump technologies
Dennis Heath, Wanner International
Many pump specifiers in the process industries and elsewhere have
difficulty in finding a good solution for a particular application
because the combination of features they seek is not easily found in one
and the same pump. Such difficulties are currently intensified by safety
concerns, new environmental regulations and the need to contain costs.
This article reviews some of the strengths and limitations of pump types
commonly used in non-straightforward applications, and explains why a
pump of unconventional design may offer an increasingly interesting
alternative.
In a document released to the press by DECHEMA for the opening of the
Achema 2003 Fair, its authors made a prediction. “Visitors to the fair
will be asking one key question: how can operators of a process
technology facility handle material safely without emissions and, above
all, what is the most cost-effective way of doing this?” One
conclusion reached in DECHEMA’s Trend Report NO.2: pumps/fittings/seals,
was the increasing importance of sealless pumps. “The focus is back on
units that do not leak,” said the authors. Reporting on actual and
projected pump sales (in this case for centrifugal units) they noted
that “it is not surprising that sealless pumps show the highest growth
rates.” Commenting on the potential for magnetic drive pumps, and
recognising present limitations, the report mentioned that many
observers were expecting ‘quantum leaps’ in innovation in the not too
distant future.
Dale Moore, of Dow Chemical-Midland, in a paper for Seal Forum,
reprinted in Pump Engineer, August 2003, puts forward a rather
different view. “Why has the use of sealless pumps levelled off?” he
asks. “Why don’t we expect to specify a major increase of this design
versus conventionally sealed pumps?” For his company the answers are
apparent: comparative total life cycle costing and product reliability.
He goes on to explain why sealless pumps, though giving excellent
service when properly applied, have not been a cure-all.
Hydraulic diaphragm pumps
In the terminology of many pump users, a ‘sealless’ pump is taken to be
a magnetic drive or canned motor pump. The merits and limitations of
these types of pump are generally known. Their popularity has tended to
increase alongside growing concern with industrial safety and the
imposition of more stringent environmental regulations. However, in
their present state of development, such pumps are more commonly
successful on applications where they do not have to contend with
non-lubricating fluids, suspended particles or crystal formation, and
are never allowed to run dry. If problems do occur, repairs and downtime
can be significant.
If service conditions preclude the use of mag drive/canned motor types,
does this effectively oblige the pump user to choose a pump with seals
and accept the likelihood of wear, leading to possible problems of
leakage, seal replacement costs, downtime and maybe even replacement of
the whole pump? Not necessarily, because there is, of course, another
form of high performance sealless pump -the hydraulic diaphragm type.
In these pumps, hydraulically actuated diaphragms pump the liquid, and
also isolate it from the drive mechanism. As compared with mag drive
pumps, hydraulic diaphragm pumps can safely handle a wider range of
difficult media, including non-lubricating liquids and those with
suspended solids. They have higher pressure capability and superior
linearity; while of course they share with the mag drive type the
advantage that they are not subject to the consequences of seal wear.
In fact though, ‘hydraulic diaphragm’ is a broad label that can be
applied to two quite different types of pump. In varying degree, all
the features mentioned in the previous paragraph apply to both, but
there the similarity ends. On the one hand are the slow-running,
elaborately engineered pumps manufactured in conformity with API 675
requirements for metering pumps. (In practice the pumps have a wider
field of application and are often used on other duties.) They are made
to tight specifications and must incorporate extra safeguards such as
double diaphragm construction and built-in leak detection systems. In
consequence, even models in the flow ranges below l0m³/hr tend to be
massive in construction. Size, combined with sophistication and a high
specification, makes them expensive to buy. They can also be costly to
repair if problems do occur. Pumps of this type are available over a
wide flow range, with some models capable of 50m³/hr or more.
However, many pumping applications in the process industries call for
relatively modest flow capability. It is at these levels, where the
flow requirement from a single pump is no more than about 7m³/hr, that
a second category of hydraulic diaphragm pump comes into the reckoning
and can offer an increasingly interesting alternative, not only to the
bigger diaphragm pumps, but to many other types of sealed and sealless
pump. These alternative diaphragm pumps are from one manufacturer,
Wanner Engineering. They must be considered as a separate category
because they are not properly classifiable under any regular type. Table
1 gives a general indication of their characteristics in relation to
some other types of commonly used pumps.
Hydra-Cell pump
Visually, the immediately obvious thing about a Wanner Hydra-Cell pump
is its size in comparison with a typical API 675 unit of very similar
flow and pressure rating (See Figure 1, originally a photograph on
which the larger pump has been converted to a silhouette). Compactly
constructed, and not obliged to incorporate the extra devices built into
pumps conforming to API 675, these pumps are nevertheless well
engineered, robust and of proven reliability over a wide range of
applications. In the design, positive displacement pumping is provided
by diaphragms flexed from behind by hydraulic fluid: each diaphragm
closes off a hydraulic cell (single-cell, 3-cell and 5-cell
arrangements are used) and separates drive fluid from pumped fluid. The
drive end operates in a lubricating oil bath. Liquid pressures on either
side of the diaphragm are automatically held in balance, so that the
diaphragms operate without stress, even at high pressure levels. Unlike
API 675 type pumps, which are slow moving and deliver a large volume on
each stroke, Hydra-Cell pumps work at high speed, delivering a small
volume from each cell but at high frequency. As well as reducing
pulsation, this is one reason why the pumps can be physically small in
relation to flow capability. Another reason is their exceptional
efficiency (80%) which also reduces power requirement and energy costs.
These pumps are characterised by an unusual combination of features,
namely sealless design; isolation of pumped fluid; high flow and
pressure capability in relation to size and energy input; ability to
handle hot, cold, thick or thin liquids; whether non-lubricating or not;
tolerance to solids in suspension and crystals; tolerance also to
chemicals; and through the availability of optional materials for pump
head and for diaphragms. Moreover they are positive displacement pumps.
Flow is linear, that is, it relates directly to pump speed and is
little affected by changes in pressure.
Cost saving applications
The effect of combining so many features in one pump is to make it more
versatile, and enable it to replace more conventional units over a
considerable range of applications. For example, when a Swedish paper
mill installed a Hydra-Cell G25 pump on a filter cleaning system that
had previously relied on a multi-stage centrifugal pump, the mill began
to save energy costs calculated at EUR 6500 per year. Multi-stage
centrifugal units are commonly used in the industry on this type of
work which involves the removal of residual lime mud from the filters,
using re-circulated water. However, high energy consumption and
maintenance costs, mainly through problems with seals and bearings, had
prompted mill engineers to reassess the situation when planning the
replacement of the previous pump 4 years ago. With power consumption of
30kW and assuming energy costs of 260 euro per kW per year, the cost of
running a centrifugal pump for one year worked out at 7.800 euro. That
was for energy alone. By contrast, the Hydra-Cell G25 unit draws only
3kW of power and in every respect is more than equal to needs of the
mill on this application. The initial cost of the pump was some 25% to
50% less than multistage centrifugal pumps originally under
consideration. Energy consumption has been reduced by 90% and ongoing
service costs have been reduced. Performance is also improved. Mill
engineers were able to raise the pressure of the cleaning water to
50bar, and so increase filter capability.
In a German chemical plant a similar Hydra-Cell G25 pump replaced a
hydraulic diaphragm pump of the API 675 type on a spray drying system,
delivering a suspension with 40% solids content to the spray nozzles at
a working pressure of 70 bar and flow rate of 5m³/hr. Service life of
the larger pump had been only ‘modest’ on this application. Taking into
account its procurement cost (about three times higher than the
Hydra-Cell pump) along with relatively high energy and maintenance
expenses, total life cycle costs of the original pump were considerable
and well above those of its replacement. Checked after two years of
service on another chemical process in a German plant, a larger
Hydra-Cell pump (G35) showed no discernible wear at the valves or
diaphragms. No spare parts had been fitted in that period. The unit’s
function is to transfer chemical products from a storage tank to a
production line over a distance of several kilometres, a task
complicated by the tendency of the chemical to coagulate at temperatures
above 600°C. Other pumps considered for this duty included a
hermetically sealed canned motor centrifugal pump, over which the
Hydra-Cell unit had a clear price advantage. Other points in its favour
against multi-stage centrifugals were higher efficiency, smaller motor
and a notably lower drag-in of energy to the medium when the system
operates in bypass mode. It was also noted that the linear
characteristics of the pump made it easy to control in process. Flow
rates (up to 7.5m³/hr) could be reproduced at will. A specialised, but
very well established, application for Hydra-Cell pumps is in the metal
manufacturing industry, where the pumps are used to deliver coolant at
70 bar pressure to the workface during high-speed machining and grinding
operations. The introduction of high pressure in place of gravity feed
cooling systems allowed faster feed and cutting speeds, reduced tool
wear and boosted productivity. But recycled coolant carries swarf and
other abrasive particles and needs ultra-fine filtration before it can
be handled by the piston pumps at first favoured for this work.
Hydra-Cell pumps combine the necessary pressure capability with the
ability to handle small particles. They do not need expensive ultra-fine
filtration. One early customer, a component manufacturer in the UK, was
able to save GBP 10,000 on filtration costs when equipping two CNC
lathes with a high pressure coolant system. Another unpromising
candidate for pumping is Xylene, produced as a byproduct from coal
processing and used as liquid fuel in cement plants. It is
nonlubricating, toxic and contains particles. It must be delivered to
injector nozzles at 25 bar pressure. In one plant, a gear pump, with
seals of good quality, lasted 1 week on this duty. Piston pumps are not
suitable, peristaltic pumps cannot deliver enough pressure. A big
hydraulic diaphragm pump could do the job, but is ruled out on economic
grounds by its initial cost. Hydra-Cell G25 pumps are now performing
this operation in plants in Belgium, France, Switzerland and the Czech
Republic, with no reported problems.
Finally, another recycling story. Management at a small Czech
converting mill, where recycled paper is made into toilet rolls, was
struggling with problems of seal wear and frequent replacements on a
multi-stage centrifugal pump delivering recycled water at 70 bar
pressure to spray manifolds for cleaning the transport belt. The
manager liked the idea of using a Hydra-Cell G35 pump, but demurred at
the cost. “Tell you what,” said the pump salesman, “You can have the
pump and motor for one Czech crown, but you must give me whatever you
save in energy cost over the first year.” The mill manager thought it
over for a few seconds, then decided to pay the full price. Energy
savings paid for the new pump unit within 6 months.
Kel-Cell
Hydra-Cell pumps have a good track record for reliability, but they have
sometimes been damaged through system problems, poor system design,
faulty installation or some operational incident that has not been
allowed for. The KelCell innovation safeguards the diaphragm in the
event of abnormal or fault conditions that would cause the diaphragms to
operate out of hydraulic balance. For example, an inlet valve shutting
off, an inlet filter blocking, or continual operation of the pump at
zero outlet pressure could upset the balance, with the effect that
diaphragms gradually deform and may eventually rupture. Kel-Cell DPC is
designed to stabilise the diaphragms in all such conditions and so
prevent incidental failure. Not a substitute for good system design (standard
installation guidelines still apply) Kel-Cell is a significant advance
for the extra protection it gives, especially on critical applications.
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