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Valves utilised by the chemical, petrochemical and mining sectors are exposed to corrosive fluids containing wear particles which, when trapped between rotating and stationary surfaces, cause abrasion wear damage. Coating technology specialist Thermaspray has therefore introduced a High Velocity Oxy-Fuel (HVOF) thermal spray process to help extend the service lifecycle and enhance the performance of metal seated ball valves.

Thermasprays HVOFIn the thermal spray process, a coating, most often metal alloy, carbide or ceramic that may vary in thickness, is applied onto a substrate through the projection of a molten stream of the necessary material. HVOF is a flexible dry-coating technology which constitutes two main carbide materials used in thermal spray processes in order to improve wear resistance and decrease coefficient of friction between various sliding components.

HVOF exhibits higher densities and superior bond strength (typically greater than 80 MPa) and less decarburisation than many other thermal spray processes, making it possible to produce very dense and hard coatings with excellent coating adhesion. This is due to the higher particle impact velocities and relatively low peak particle temperatures associated with the HVOF process.

One of the many advantages of thermal spray technology is the ability to produce carbide based coatings, says Paul Young, Spray Shop Manager at Thermaspray. Carbides, also known as tungsten carbides (WC), are a unique family of materials that combine the hardness of a ceramic with the toughness of a metal. Carbide coatings combine excellent wear properties with good corrosion resistance. Carbide particles are glued together with a metal matrix.

In the HVOF process, particles travel faster than the speed of sound, generating the best bond strengths, density and hardness compared to other thermal spray processes, Young says. “We apply the state-of-the-art HVOF coating process to protect all components of metal seated ball valves.”

Because spraying parameters like standoff distances and spray angles are fundamental to controlling coating quality and uniformity, robotic manipulation is used for spraying complex components like ball valves. Thermaspray coating processes are applied at the company’s facility in Johannesburg, using 6-axis robotic gun manipulation, and extend to the coating of new and worn rotary, butterfly and gate valve components.

Thermaspray’s extensive portfolio of thermal spray coatings includes thermal spraying, Plasma, HVOF and Arc Wire Spraying. These are used to protect new and refurbished components against wear, corrosion, oxidation of cavitation and deliver cost saving benefits as a result of enhanced performance, extended valve service life through restoration of worn components to precise OEM dimensions, increased wear resistance against erosion, abrasion, increased traction, and increased resistance to chemical and high temperature corrosion.

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