Rock Products

FEB 2018

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Page 40 of 85 ROCK products • February 2018 • 37 specific areas where increased flow rates are expected or at points where impact damage is expected, such as 90-degree bends. A hard layer is usually applied using a spray coating method such as Air Plasma Spraying (APS), or High Velocity Oxygen Fuel (HVOF) – which one will depend on the required coating thickness and composition. Carbide coatings, which are deposited by using a high velocity oxy-flame, are extremely wear-resistant. Tungsten car- bides in combination with cobalt, nickel or cobalt-chrome matrices are preferred. Thanks to improvements in the powder and the thermal spray processes, the materials combine high wear-resistance and toughness with good corrosion resistance. Thermal spray coatings can be applied to most substrates, but it is a "line-of-sight" process that makes the coating of complex shaped components, such as impellers, difficult. CVD processes, which are used for complex components that are difficult to coat with thermal spray create very hard surface layers, but are conducted at temperatures in excess of 850 C. These high temperatures limit the selection of possible substrates, because structural changes and partial deformations can take place during the cooling stage. Not All Coatings Are The Same The improvements in performance and durability afforded by coating systems have given rise to a number of busi- nesses offering the service. The raw materials and the basic equipment can be acquired relatively easily and used to apply coatings to a range of equipment. However, the quality of an HVOF coating, for example, depends predominately on the spraying parameters, such as the material temperature, application velocity, application rate and the quality of the equipment used. Coatings such as these take time to apply correctly, which will inevitably impact on the final cost of the refurbishment. However, increasing the deposition rate will increase the stresses within the coating and over time this can cause the coating to degrade and fail prematurely. The procedures and settings used by companies such as Sulzer and its coat- ing suppliers have been developed over many years; applying extensive knowl- edge and experience to the process is the only way to improve it. The final proce- dure for each coating is closely guarded, proprietary information ensuring that every client will receive the same qual- ity of coating across the world. To illustrate the importance of these procedures, especially in pump applica- tions, consider the process of installing and removing an impeller. In many sit- uations, the impeller is heated to allow it to be installed or removed from the drive shaft. This shrink-fit procedure can cause inappropriate coatings to be damaged during a routine maintenance operation. Sulzer has ensured that its coating technologies can withstand this thermal shock and continue to deliver long-lasting corrosion protection. Maintaining Legacy Equipment Modern coating technology can be applied to legacy equipment as part of a refurbishment program that will extend the service life of a pump. Implementing a new coating as part of a refurbishment project can significantly improve the performance and reliabil- ity of existing equipment. Ultimately, the key to a successful cor- rosion prevention scheme is to fully understand the application and to use all the available information to deter- mine the most appropriate action. Working closely with experienced mate- rials engineers enables the end user to achieve the most appropriate solution. For those looking to refurbish an exist- ing asset there are a number of potential improvements that can extend the ser- vice life and improve the performance of a pump. If a new pump design is required, there is an opportunity to establish not only the most appropriate base material, but also the best coating system for extended durability. The Future As coating technologies continue to advance, end users will be able to select bespoke coatings that can be applied during the manufacturing process. However, even with the most advanced coating, there is a need to develop an application process that can be used to apply the coating to the complex internal surfaces of a cast impeller and volutes. As such, this remains the "holy grail" of pump design and once this chal- lenge is overcome, the reliability and service life of industrial pumps will be further improved. Improvements in service intervals means reduced maintenance costs and reduced costs attributed to lost produc- tion. Together with improved efficiency, these costs of ownership can be mini- mized through the appropriate use of base materials, protective coatings and the implementation of better pump design to deliver a comprehensive and cost-effective pump solution. Information for this article courtesy of Sulzer,, a worldwide, independent service provider for the repair and maintenance of rotating machines including turbomachinery, pumps and electro-mechanical equipment. With a global network of over 150 technically advanced manufacturing and test facilities, Sulzer offers a collaborative advantage that delivers high-quality, cost-effective, customized and turnkey solutions, provid- ing its customers with the peace of mind to focus on their core operations. Working closely with experienced materials engineers enables the end user to achieve the most appropriate solution.

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