Thermaspray, a company specialising in refurbishing, manufacturing and repair services for power plants, recently refurbished 2way-3way valves at one of South Africa’s coal fired power generation plants. These valves had been in operation for many years, with many companies affecting repairs on them.

Depending on plant size, up to 15 000 valves can operate at a single power generation plant. In the steam cycle, components such as steam generators, pumps and turbines that handle steam and water require a variety of control, safety and shut off systems. High pressure steam, high temperatures and metal-to-metal wear at seating areas are the main contributors of wear in steam and water valves.

Thermaspray overcomes power plant valve refurbishment challengesAs a result of the various repairs done over the years, Thermaspray conducted a complete inspection of the valves. This included non-destructive testing on all the valve parts such as material analysis on the valve seat face as well as the outer diameter to establish what welding material was used for the previous repair and what the substrate material is. Analysis was also performed on the outer body, inner body and spindle.

According to Philip van Wyk, Engineering Support Manager at Thermaspray, while not all the repair methods used are known, Thermaspray was able to determine that thermal spray coatings and welding processes were used on some of the valves. “Subsequent to material analysis it was found that a variety of different materials was used on the valve seats which were not necessarily the recommended Stellite 6 coating. Furthermore the split rings were manufactured from a range of materials and not the specified cast iron.”

He adds that of major concern was the preparation method used for the spraying of the spindles and piston bores as well as the required seat face weld thickness required to refurbish the parts to original specification. “Historically, it seems that the minimum weld thickness was applied on the seat faces and the seat’s overall width was never refurbished to the original size. This meant that in some cases, welding thicknesses of more than 10mm were required. In addition, previous thermal spray repair preparation made use of a ‘thread finish’, underneath the coating, which had to be completely removed, resulting in coating thicknesses of more than 2,0mm.”

To enable a thermal spray coating and PTA repair, all the old coatings, whether thermal spray, plated coatings or welding, had to be removed by machining and grinding. Non-destructive testing was repeated on the parts after coating removal to ensure substrate material integrity.

All areas to be sprayed were then prepared using international best practice techniques. Locating diameters were sprayed using a metal alloy. Where wear resistance was required, carbide coatings were applied, and were final-ground using diamond wheels. In most cases, due to the multiple repairs and techniques used for the past repairs on the valves, the bores and the piston spindle outer diameter required material build-up in excess of 1,0 mm. A metal alloy build-up layer was applied first, followed by the application of the wear resistant coating.

Repair of the seat faces were done using Stellite 6 material applied with the PTA welding process. In some cases, up to 10mm thick layers were applied to restore the seating area to the correct dimension.

Thermaspray also refurbished the valve spares to a standard set of dimensions to ensure interchangeability if required. These sizes were developed together with engineers from the power station.

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