Introduction
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
environment.
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
requirements.
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
(Figure 1).
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).
Definitions
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:
Wear
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
chipping.
Wear shall be the generic term for the mechanical
material loss at coatings and components respective
substrates.
Abrasion
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).
Erosion
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
Standard solutions
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