Dieter Holzdeppe, Ersin Cetin,
and Sabine Groh, TLT-Turbo,
discuss the research and design
of anti-wear protection coating
systems for cement fans.
Figure 1. Particle flow slot type damage along a crucial
welding seam of a centrifugal fan rotor blade with flow
direction/gas velocity VG.
Figure 2. Particle flow contour damage on radial fan
blade, original contour marked in red.
Figure 3. Schematic wear rates WRC of ductile,
semi-ductile, and brittle materials as function of particle
flow impingement angle α.1
Figure 4. Topology of surface deformation/disruption
resulting from abrasion and erosion caused by contact
with particle flow.3
Figure 5.1. Micro chipping:
aluminium sheet metal
impacted by quartz test
sand particles; θP1 ≈ 20°, VP1
= 160 m/sec.
Figure 5.2. Micro fatigue/
crushing. Aluminium sheet
metal impacted by quartz
test sand particles and with
imbedded cracked quartz
test sand particle; θP1 ≈ 90°,
VP1 = 160 m/sec.1
Figure 6. Welded hard-facing coating with typical crack.1
Cement fans, being part of complex and customised
ventilation systems, require permanent R&D activities
concerning all aspects of design. Wear is one of the
essential challenges in this technically demanding
Accordingly, different solutions of surface
anti-wear protection coating systems have been
investigated and developed by TLT-Turbo within the
last few years. These coatings have been and will be
continuously optimised for specific cement process
TLT-Turbo’s new anti-wear coatings, e.g.
TLT-H-101, are designed to conserve the aerodynamic
geometry of the fan blades for longer than planned.
Consequently, fan efficiency and drive power can be
kept close to the as-planned level.
Finally, these coatings generally allow the
preservation of structural strength of the fan blades
for a comparably longer operational time, while
reducing facility downtime for maintenance and
respective service costs.
TLT-Turbo’s different coatings have already been
applied on aerodynamically and structurally critical
fan components, particularly to impeller blades and
centre discs exposed to strong particle flow.
Fan wear in cement ventilation systems
Air in cement plants is typically loaded with small,
hard, and sharp-edged particles. Additionally, there is
also much bigger, rigid debris of a different kind, but
still transportable by the flow.
Regardless of the component material, the
impeller, particularly the impeller blades, as well as
stator components should be additionally protected.
There are many parameters, such as particle
velocity and mass flow, that can cause severe
damage to fan rotor blades. Strong damage
propagation can be seen within a few months
Wear can partially produce extensive alteration
of the geometry of fan components (Figure 2),
determining the aerodynamic quality of a fan.
This, in turn, results in drastic degradation of the
planned/specified volume flow, pressure rise, and
aerodynamic efficiency, causing an increase of drive
power and operational cost.
A few months after commissioning, fatigue limits
of the fan components concerned can be reached.
That results in an early, unplanned shutdown of the
cement plant with a costly and probably long-term
loss of production. Time and cost consuming repairs
on site or in the workshop are inevitable.
With the help of advanced tungsten carbide
coatings, the consequences of particle flow induced
wear can be minimised.
Particle flow wear
Particle flow wear is an extremely complex and
interdisciplinary issue. Complexity grows if particle
flow wear is additionally superimposed by caking,
which causes additional imbalances for the rotor.
The wear rate (WR) of isotropic materials
due to particle flow impact is a function of more
than 20 parameters and can be roughly described
by an empirical relation. Particle impact
energy/velocity is considered as main wear parameter
In the preceeding equation, C and n are
constants depending on material, impingement
angle, and type of flow particles. The exponent n is
a parameter whose amount strongly varies due to
different material behaviours and failure mechanisms
(e.g. n ≥ 2).
Besides the most important impact energy, the
characteristic wear rate develops as a function of
impingement angle α, and plays a decisive part in the
selection of optimal anti-wear material (Figure 3).
According to Czichos,2 wear is the proceeding loss
of material and the separation of particles from the
surface of a solid body; it is characteried as a contour
and/or material alteration at the active surfaces
of solid bodies being subject to tribological stress.
With regard to the use of the terms ‘wear’, ‘erosion’
and ‘abrasion’ concerning particle flow induced
wear, there seems to be no common definition in
the accessible literature. Therefore, the following
definitions shall be used and proposed for the future:
Any material loss resulting from contact of blasting
particles with the surface of substrates, coatings,
and components, with the specific processes: micro
crushing, micro fatigue, micro ploughing, and micro
Wear shall be the generic term for the mechanical
material loss at coatings and components respective
Any material loss resulting mainly from flat angle or
tangential relative movement of a blasting particle
during contact with the surface of substrates, coatings,
and components (gliding flow and inclined flow area,
see upper part of Figure 4 and Figure 5).
Any material loss resulting mainly from steep angle
or orthogonal impact of a blasting particle onto the
surface of substrates, coating, and components (impact
flow area, see lower part of Figure 4 and Figure 6).
Anti-wear solutions for fans
Typically, metal-based anti-wear solutions, including
expensive special blade materials, welded hard-facing
coatings, or certain high-velocity oxygen fuel
(HVOF)-sprayed tungsten coatings, are the current
standard for centrifugal fans.
To realise a limited extension of fan operational
time under highly particle induced wear process
conditions, some standard anti-erosion measures
are applied: until now, wall thickness of critical fan
components has been increased or thick hard-facing
coating (Figure 6) has been welded on the affected fan
Alternatively thick and heavy protection plates,
armoured by hard-facing coatings of about 10 mm
thick, are screwed to fan blades.
These coatings typically show a very rough surface
with numerous cracks that partially reach the surface
of the fan component material/substrate or the
protection plate material.
A rough surface constricts aerodynamic fan
efficiency. Cracks always provoke excessive local
particle flow wear, as well as direct access of
corrosive media to the unprotected surface of the fan
component material/substrate or to the protection
plate material. Local but propagating corrosion of
the substrate, as well as flaking of the coating, is
Flaking and blistering of the coating opens a
‘window’ to the unprotected surface of component
substrates, causing local strong particle flow wear,
possibly corrosion, and in turn accelerated flaking of
Beyond that, a rough coating surface augments
the risk of caking on fan components, impacting the
aerodynamic efficiency, which can result in strong
In addition, such standard coating measures results
in a partially significant increase of fan component
weight. In case of centrifugal fan impellers, a
resulting weight increase can amount up to 30%. As a
consequence, bearings and shafts will become bigger
and heavier and the respective costs will rise.
To reduce the coating-produced weight increase,
TLT-Turbo has undertaken strong research and
development efforts to protect fan component
surfaces of different materials with innovative, thin
anti-abrasive coatings, which significantly decrease the
specific volumetric erosion rates (Figures 7 – 9).
The typical maximum thickness of these
HVOF-sprayed coatings will be ≤ 500 µm. Besides
weight reduction, TLT’s new coating TLT-H-101 features
a hydraulic smooth surface for superior aerodynamic
efficiency as well as extreme low, specially treated
porosity to protect the coating from being corrosively
“undermined”. Furthermore, its low, small-scale
porosity avoids early blistering or flaking of coating.
Finally, it can be assumed that the hydraulic smooth
coating surface diminishes the risk of caking adversely
affecting the aerodynamic efficiency.
Basically, TLT-Turbo’s anti-abrasive TLT-H-101
coating is based on a tungsten carbide powder
with some special additives. It is sprayed onto fan
components using HVOF coating techniques.
The obvious improvement in particle flow wear
resistance (Figures 7, 8, and 9), compared to standard
and even benchmark coatings, does not only result from
the special composition of coating materials but also
from high supersonic spray velocity (significantly beyond
Mach = 4.0).
The special spraying process produces very low
residual tensile stresses within the coating, reducing the
risk of cracks including crack propagation.
TLT-Turbo has developed and continues to create advanced
anti-wear and anti-corrosion coatings to increase the
operational lifespan of heavy-duty fans working in highlyloaded
particle flow processes.
Being compared to the market standard of welded
coatings and to a HVOF-tungsten carbide benchmark
coating, the HVOF-sprayed tungsten carbide composite
TLT-H-101 coating shows a significantly lower wear rate.
The thin coating will be sprayed directly on the
critical fan components, which can also be designed to be
repairable, according to process demands. Furthermore, its
surface is aerodynamically very smooth. These features
allow the design of more advanced anti-wear fans.
TLT-Turbo’s anti-wear fans are built to operate longer
with less downtime time for wear-caused maintenance,
consuming less drive power for longer periods of time,
and featuring less weight.
It is reasonable to compare the coating/material cost
to downtime costs, including the additional maintenance
costs of facilities/processes, by calculating the commercial
benefit. Depending on the wear process, the cost for
advanced coatings amortise within months.
Figure 7. Extreme particle flow loaded stiffening rod in
a heavy-duty centrifugal fan is generally destroyed after
approximately six months of operation; rod coated with
welded hard-facing, VG ≈ VP
≈ 210 m/sec.1
Figure 8. Relative volumetric wear rate of new
TLT-H-101 HVOF-coating, compared to standard
benchmark HVOF-coating and welded hard-facing
coating, as function of abrasive particle jet flow
impingement angle α; quartz sand test particles, VP ≈
Figure 9. Stiffening rod (Figure 7) being repaired with
TLT-H-101 after more than about 24 months of operation.
Coating roughness Ra < 7 µm, VG ≈ VP
≈ 210 m/sec.1
HOLZDEPPE D., Particle Flow Erosion at Fan Components,
2nd Ed. (TLT-Turbo GmbH; December 2015). Available at:
2. CZICHOS H. and HABIG K.-H., Tribologie Handbuch
(Handbook of Tribology), Vieweg+Teubner. Auflage
‘Zum Gahr K.-H.: Reibung und Verschleiß bei
metallischen und nichtmetallischen Werk-stoffen’ (Friction
and Wear of metallic and non-metallic Materials),
Informationsgesellschaft Verlag (1986).
FINNIE, I. Erosion of Surfaces by Solid Particles Wear, Vol.
3, No. 1 (1960).
About the authors
Dr.-Ing. Dieter Holzdeppe is the Senior Technology Engineer at
TLT-Turbo, based in Zweibrücken.
Ersin Cetin is the Regional Sales Manager for Germany
and Turkey After Market Services at TLT-Turbo, based in
Sabine Groh is the Product Manager Industrial fans and R&D
Engineer at TLT-Turbo, based in Bad Hersfeld.