Repairs were recently carried out at the Dinorwig pumped storage plant in Wales, avoiding the need for lengthy shutdowns and subsequent lost generation
A purpose-built machining solution, developed by Furmanite, has successfully repaired a 5.5m diameter pump-turbine at one of Europe’s largest pumped storage power stations. The high tech solution avoided shutting the plant down for several weeks, which would have led to substantial generation losses.
The specialist circular self-levelling milling machine was developed for the Dinorwig power station in North Wales, operated by First Hydro Company and owned by Edison Mission Energy of the US. A damaged bearing track needed to be repaired and the grease lubricated bronze had to be replaced with a self lubricating bearing – First Hydro Company was keen to move away from grease for environmental reasons and to improve working conditions.
Upon inspection the phosphor bronze regulating ring bearing was found to be worn. Where the bearing’s retainer screws had worked loose, there was damage to the top cover vertical and horizontal journal faces. The damage had to be repaired before replacing the bearing, but stripping down the turbine and removing the top cover (5.5m in diameter and weighing 100 tons) to a workshop, would have entailed several weeks’ shutdown, and transportation of the sizeable cover.
In situ solution
First Hydro approached Furmanite to find an in situ solution. Based on its previous experience with similar types of repair, Furmanite developed its circular self-levelling milling technology to re-machine the top cover journal faces, and fit stainless steel liners as a counterface for the self-lubricating bearing. The equipment was developed, based on existing technology and adapted to the First Hydro Company’s requirements, including the fact that the machine had to be designed to allow it to be assembled round the main drive shaft.
To guide the milling machine a special split bearing sleeve was designed on the shaft. This was set concentric to the top cover outside diameter using a clocking arm. The flatness of the top cover’s horizontal bearing was maintained by the machine’s novel self-levelling technology, which controls the milling cutter height via an electro-hydraulic interface from the pre-set datum ring. A special split datum ring was manufactured for the purpose, to go around the drive shaft and mount on the top cover. The datum ring flatness was set using Furmanite’s precision scanning laser system.
The work was successfully carried out in September 2000, removing approximately 3mm of material from the journal faces to allow the stainless steel wear plate sections to be fitted. The vertical bearing face diameter was measured accurately during machining using a precision circumference tape, while the flatness of the horizontal bearing face was within 0.15mm and was independently verified using the precision scanning laser system.
The set-up, machining, and fitting of the stainless steel liners and the replacement of the bearing took nine days of 24-hour shifts to complete.
Commenting on the work, First Hydro Company’s mec- hanical engineering manager, Bill Moss said: ‘We called on Furmanite because they did excellent work for us in the
past on the bottom cover on one of the units. Again, in this instance work was completed quickly and efficiently, we encountered no problems and the machining was extremely accurate. We are delighted with the results.’
The work has now been carried out on three of the power station’s six turbine units.
Dinorwig power station is built beneath the old Dinorwig slate quarry in Snowdonia, North Wales. It was commissioned in 1983 and is among the largest pump storage power plants in Europe. It was specifically designed to meet sudden increases in electricity demand and can generate up to 1200MW of station output within approximately 12sec. The station is one of two power plants operated by First Hydro Company and owned by Edison Mission Energy in California.
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