South Africa’s Thermal Spray and Plasma Coating technology specialist, Thermaspray, delivers world-class refurbishing, manufacturing and repair services to numerous power stations around the country.

Equipment and components such as steam generators, pumps and turbines that handle steam and water in the steam cycle operate in extremely harsh environments, including high pressure steam, high temperatures and metal-to-metal wear at seating areas.

“These are the main contributors of wear in steam and water valves,” says Thermaspray Spray Shop Manager, Paul Young. He adds, “These components require a variety of control, safety and shut off systems and this is where a lot of wear occurs. Furthermore, the surface of valve seats and spindles need to inhibit oxidation, which can lead to adhesion damage at high temperatures.”

“Our thermal spray and Plasma Transferred Arc (PTA) (figure 1) capabilities enable us to refurbish and repair a wide range of components used by local power stations. These include fan blade and steam side spindles, servo motor and rack spindles, stator pump bearing and accumulator housings, fan blade shafts and liners, pump impellers, nose tip liners, deflector rings, control casings, sleeve and rotation plates, sleeves and bushes as well as a range of valves.”

Thermal spraying provides increased resistance to high temperatures, oxidation, traction, cavitation, chemicals and corrosion as well as wear resistance (resulting from erosion, abrasion and sliding). Thermal spraying is the method of applying materials onto a prepared base material by heating particles in the stream of a heat source to create a semi-molten state. Particles are then propelled at high velocity onto a prepared substrate where they adhere via a mechanical bond; particles continue to build until a specified thickness is attained.

This spray method, which is considered the ‘cold method of welding’ where high temperatures can be achieved in the heated pocket while the temperature of the part itself usually remains under 100°C, restores worn components to precise original dimensions. “The result is an extension of component service life, and the subsequent increased uptime and improved production translate to significant cost savings for the end-user,” notes Young.

PTA weld / clad hardfacing offers the ideal solution for applications where severe impact and corrosion are prevalent. Here coatings that can withstand such conditions by providing the necessary protection to the substrate, are required.

“This welding process is used to produce high-quality weld hardfacings (figure 2) of Ni, Co and Fe alloys, as well as tungsten carbide containing grades of the nickel alloy,” continues Young. ”The PTA hardfacings that are metallurgically bonded to the parent material, are able to handle impact, point and/or line loads that a thermal spray coating cannot tolerate.”

The scope of work for various repairs and refurbishments conducted by Thermaspray at eight power stations include:

• Complete refurbishment of Semple Valves, which involved stripping of the valve, pre-machining, stress relief, thermal spraying, PTA welding, NDT, final machining, final grinding, and assembly.  Thermaspray also manufactured rings and bushes for the Semple valve from Mehanite material to the client’s specification.

• The refurbishment of valve spindles, in partnership with a business partner, included pre-grinding, thermal spray coating, final grinding and NDT (DPI) on the coating. The repair and refurbishment conducted on gland boxes demanded pre-machining, thermal spray coating, final machining and final grinding.
• Refurbishment of valve seats included pre-machining, PTA welding with a Co alloy, and machining to the required dimensions. The job included PTA welding on new seats.
• Pump refurbishment, which was completed in conjunction with a business partner, included machining, grinding, thermal spray coating, and final grinding. Electrical run-outs have also been conducted on these shafts ensuring the quality required. Thermaspray also coated butterfly and ball valves to increase wear and corrosion resistance.

Thermaspray, in a joint venture with Cape Town-based Surcotec, offers an extensive portfolio of engineering and thermal spray coating solutions that extend component lifecycles to assist OEM and end-user clients across southern Africa in reducing costs and increasing production.

The companies’ world-class quality wear- and corrosion-resistant thermal spray coatings, Plasma Transferred Arc (PTA) cladding and Polymer coatings (in partnership with Plasma Coatings USA and Diamant Metallplastic Germany) are augmented by a host of specialised allied services.  /ends