MECHANICAL PROCESSINGCHEMICALPHARMA/
BIOTECH

FOOD/
BEVERAGEOIL/GASWATER/WASTEWATERMANAGEMENTDESIGNOPERATION

MAINTENANCE

WEAR PROTECTION

CUBED

Extending lifetime of agitator parts in three different ways — Wear of impellers is aproblem that is often encountered in mixing applications. It is possible, however, tomaximize the lifetime of agitator components by implementing appropriate measures withrespect to their shape, coating or solid ceramic design or a combination of the above.

BENJAMIN MULTNER, WOLFGANG KELLER*Ceramic designCoating40

PROCESS Worldwide

6-2016

Source: Ekato; © sveta/Fotolia.com; [M] GötzelHorn

E

nically, wear can be defined as aprogressive surface material lossof a solid body caused by a relative
motion against yet another solid,
liquid or gaseous media resulting

Impeller Design

One of three possibilities to masterwear: the use ofsolid ceramic impeller/mixer

ven in every day life, wear is
a common phenomenon.
Who has never known thefeeling of mourning over a pair ofworn-out jeans or old shoes? Tech-in grinding, rolling, hitting or thermal stress.
In process plants, rotating equipment and piping components are
subject to wear especially if theprocess media contains solids.
Consequences of wear here are not
only the costs to replace the wornout parts but also the loss in production time. In most industries,
the consequences of wear are an
accepted since principally unavoidable problem. However plant operators demand that suppliers ofcomponents provide the highest
possible service life that can beachieved by implementing various
measures. On the one hand, awear-balanced design of the agitator will be aimed for with respect
to the mixing technology, on theother hand the agitator manufacturer is also requested to develop
and implement wear-optimized
impeller geometries. Additional
indirect wear protective measures
are the use of surface-hardening
processes or special materials. In
this context, ceramic materials
must be mentioned as their processing and manufacturing possibilities have taken great developmental strides particularly withrespect to shape.

B. Multner is Head of Mechanical Development
R&D and W. Keller Senior Process Engineer
R&D at Ekato, Schopfheim.
Phone +49-7622-29-0

Wear is a very complex process
which makes it difficult to provide
a specific description of the wear
process let alone an exact prediction of an expected lifetime. It ispossible to perform tests with original products in laboratory scale tooptimize the agitator design for
mixtures in which the composition,particle size distributions and concentrations are not exactly known.
This empirical approach has proven particularly useful for productswith rheological anomalies, i.e.
mainly non-Newtonian flow behavior with yield stress.
Computational Fluid Dynamics(CFD) can be used to investigate
the flow around the impeller blades
and optimize blade geometries
concerning wear. One result ofsuch an optimization is theEPOX-R. Due to its optimized
shape vortex shedding and therefore wear is suppressed without
losing the performance capability

2. Coatings

A variety of coating methods andmaterials exist in order to increase
the lifetime of equipment components. These materials are so designed to increase the surface
hardness or permit impacting particles to "bounce off". When selecting the coating material, however,boundary conditions such as theoperating temperature, corrosive
characteristics of the medium or
the presence of solvents must all
be taken into consideration. Possible materials and methods for coating agitator components are asfollows:• Metal oxide coatings,• Weld cladding / hard-facing,• Ceramic linings,• Rubber and polymer coatings,• Filled polymers based on epoxy
resins or polyurethanes.
Based on the type of coating selected, typical layer thicknesses lie
within the range of a few micrometers to several millimeters. In determining the scope of coating to
be used, it is often interesting froman economic point of view to only
protect the component areas exposed to wear — for example titanium impellers with a diameter

Source: Ekato

1. Appropriate Design>2 m. Only the darker areas at theouter ends of the blades are hardcoated. To be able to define thecoated areas selectively, thewear-exposed areas must of course
be well known. This can be determined in laboratory scale or byusing CFD studies.

3. Ceramic Components

In addition to the possibilities
described above to increase equipment service life, it is also possible
to manufacture impeller blades orcomplete impellers using solid ceramic components. As opposed tocoatings, a solid construction offers the advantage that the same
surface hardness exists throughout the entire component. Then
once the protective coating is wornoff, the base material will again be
subject to high wear rates.
Complex geometries can bemanufactured thanks to recent developments in the field of ceramicmaterials and their improved processing options. Nevertheless, it isstill especially important to exactly
know the hydraulic and vibrational loads on the agitators when designing impellers using brittle ceramic materials.
These are decisive factors
for an operationally reliable design.
A ceramic-compliant design ofthe composite — metal and ceramic — or solid structures is a criticalfactor for the technical feasibility
and economic viability of the project. When designing industrial
impellers, composite structures areusually used where only thewear-exposed areas are designedof solid ceramic components.
Those areas that are not so affected by abrasion — for example
blade holders, hub and hub connections — are made of metallic
materials.

It is possible to compare uncoated, coated and solid ceramic components with each other in regard
to their abrasion behavior on thebasis of pilot wear tests at the Ekato test center in 1 m3 scale. Basedon measurements, it was possible
to draw conclusions concerning
the relative wear resistance of thematerials and their differences inservice life. A comparison of solid
ceramic components with coatedcomponents revealed that the lifetime can be increased by a factorof 10 to 15 with solid ceramic components. The use of advanced ceramics is not only worthwhile inareas where components wear out
very quickly and plant shutdownscause high costs, but their use isequally attractive if metallic attrition or dissolved metal ions due tocorrosion are not permitted. Highly pure and fine-particle silica
serves here as an example.

An EPAL impeller
coated with metaloxide (left) andsimulation of thevorticity at theblade tip (right).

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SummaryImpeller wear is a frequently encountered problem in mixing applications. The lifetime of agitator
parts can be extended using themeasures described. The increasein cost and time pressure during
maintenance work can make an
investment using impeller coatingsor ceramic materials profitablewithin a short period of time. Apartfrom the direct investment forparts to be replaced, downtime
costs can be reduced and unplanned breakdowns caused bywear issues can be avoided. Ekato
offers for this purpose individual,
custom-made solutions. These
range from an optimization of mixing processes based on tests inpilot plants and the scale-up to industrial scale as well as an operationally safe design of all agitator
components and the vessel.

6-2016

PROCESS Worldwide41