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

coating (Figure 6) has been welded on the affected fan component. 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 generated. 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 the coating. Beyond that, a rough coating surface augments the risk of caking on fan components, impacting the aerodynamic efficiency, which can result in strong unbalance. 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.

Advanced solutions

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.

Conclusion

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.