Wear Protection Stirred, Not Shaken: Three Ways to Maximize the Lifetime of Agitator Parts
Wear of impellers is a problem that is often encountered in mixing applications. It is possible, however, to maximize the lifetime of agitator components by implementing appropriate measures with respect to their shape, coating or solid ceramic design or a combination of the above.
Even in every day life, wear is a common phenomenon. Who has never known the feeling of mourning over a pair of worn-out jeans or old shoes? Technically, wear can be defined as a progressive surface material loss of a solid body caused by a relative motion against yet another solid, liquid or gaseous media resulting in grinding, rolling, hitting or thermal stress.
In process plants, rotating equipment and piping components are subject to wear especially if the process media contains solids. Consequences of wear here are not only the costs to replace the worn out 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 of components provide the highest possible service life that can be achieved by implementing various measures. On the one hand, a wear-balanced design of the agitator will be aimed for with respect to the mixing technology, on the other 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 with respect to shape.
Measure 1: Appropriate Design
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 is possible to perform tests with original products in laboratory scale to optimize 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 products with 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 of such an optimization is the EPOX-R. Due to its optimized shape vortex shedding and therefore wear is suppressed without losing the performance capability
Measure 2: Coatings
A variety of coating methods and materials 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 the operating 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 as follows:
- 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 from an economic point of view to only protect the component areas exposed to wear — for example titanium impellers with a diameter >2 m. Only the darker areas at the outer ends of the blades are hard coated. To be able to define the coated areas selectively, the wear-exposed areas must of course be well known. This can be determined in laboratory scale or by using CFD studies.