Conjet cleans up at Glenlee power station

The refurbishment of a giant penstock at the Glenlee hydro power station required specialist technology from Sweden. Lars-Göran Nilsson, executive vice president of Conjet tells the story

The 12MW Glenlee Hydro-Electric Power Station in Scotland is part of the 106.5MW Galloway Hydro-Electric Power Scheme, which was built in the 1930s. A 60km long network of lochs, dams, tunnels, aquaducts, pipelines and rivers interconnect six power stations in a cascade system, which reuses the water several times for power generation.

Glenlee is the fifth of the six power stations in the interlinking renewable energy scheme that covers a large area of Galloway and South Ayrshire. Water gathers in Loch Clatteringshaws and flows through a 6km tunnel to a portal control valve above the power station. From here water plunges 125m down the hillside through a 570m long steel penstock of varying diameter to the station's twin 6MW turbines and out of tailrace valves and into a spillway, for discharge into the River Dee. The discharged water combines with natural river flows to the Loch Ken, which acts as the reservoir for the last power station in the scheme at Tongland.

The Glenlee penstock needed cleaning and repainting and Scottish Power Generation awarded a contract to Concrete Repairs Limited (CRL). The flange bolted steel pipeline had not been cleaned and painted internally for over 70 years, since it was installed. CRL believed the internal cleaning and paint removal could be done with high-pressure water jetting and contacted NET Waterjet, a contractor specialising in ultra high pressure water technology and diamond drilling and sawing.

"I believed it was initially feasible to use handheld high-pressure water jetting lances in the penstock's varying diameters and gradients to remove the old paint coating and peat lying in the bottom, but there was a risk for the operators," says NET managing director Tom Wallace. "So I looked at the possibility of adapting a robot, normally used for the hydrodemolition of concrete, and contacted Castellan, the UK agent for Conjet hydrodemolition equipment in Sweden.

"Castellan's managing director, Colin Jailler, arranged for me to visit the Conjet factory and as soon as I saw the compact 324 Robot I knew it would work, using the Hammelmann blast or rotor head instead of the normal concrete hydrodemolition lance. I ordered a Conjet 324 Robot from Castellan and it arrived on site in May."

NET used its new Conjet 324, with optional rotor head, to remove the build up of debris, old paint coating up to 4mm thick and unexpected original mill scale, back to a clean and smooth metal surface. The company had about 4,200m2 to clean from the inner surface of the penstock in diameters from top to bottom of 3m, 2.7m, 2.4m and twin 1.8m and on varying gradients from 1:100 up to the steepest 18 degrees.

CRL removed the flange bolted expansion joints and butterfly valves in the penstock to provide NET access at several points for the Conjet 324, which was supplied with water at a pressure of 2,500 bar and flow of 25l/min from one of NETs existing 250kW Hammelmann 120 high-pressure pumps. "We are working a single shift seven days a week and averaging to clean about 200m2/shift, but our best has been 320m2/shift," said Wallace. "The surface of the pipe gets cleaned twice as the 180mm diameter blast head is rotated full circle round the inner circumference of the pipe, then advanced 90mm by the Robot and then rotated back in the opposite direction.

"The Conjet Robot has been superb and worked very well and is a lot safer and about three times faster than using hand lancing." There is no dust from the paint removal and the wastewater and debris from the Robot cleaning process flows down the penstock and is collected by CRL in a sump in the turbine house. It is then pumped into tankers for off site environmental treatment and disposal.

CRL is following on spraying on a two-coat glass flake epoxy coating from a purpose made gantry that will travel down the inside of the pipe.

CRL is also repainting the outside of the pipe with a highways bridge specification four-coat epoxy paint system. CRL started on site in February and Scottish Power Generation brought Glenlee Power Station back into operation in October.

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