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Robotic Cleaning of the Steel Penstock at Glenlee

After 70 years of continuous operation, the interior of the penstock that supplies water to the 12 MW Glenlee hydroelectric project, in southwestern Scotland, needed to be refurbished to extend its working life. This refurbishment included stripping what remained of the original protective coating and removing corrosion.

To accomplish this task safely and effectively, N.E.T. Waterjet Ltd., a subcontractor to Concrete Repairs Ltd., used a 324 robot manufactured by Conjet AB of Sweden. This robot automatically advances through the penstock as it cleans the interior using a high-pressure water jet. Employing this technology, the entire 4,200 m2 surface of the penstock was cleaned in six weeks.

Background on the Glenlee hydro project

Glenlee is one of six facilities in the Galloway hydroelectric scheme, which was commissioned in the mid-1930s. The six plants have a combined output of 106.5 MW. A dam blocks the natural outflow of Loch Doon and acts as the main storage reservoir at the top of the scheme, which has a drop of 210 meters over its length before it enters the ocean. A 60 km-long network of lochs, dams, tunnels, aqueducts, pipelines, and rivers interconnects the six power stations in a cascade system. Each station reuses the water discharged by the one above it to generate electricity.

Use of the Conjet 324 robot allowed subcontractor N.E.T. Waterjet Ltd. to safely and effectively clean the interior of the penstock at the 12 MW Glenlee project in Scotland without exposing personnel to hazardous work conditions.

Glenlee is the fifth station in this cascade. Water from Clatteringshaws Reservoir, impounded by Glenlee Dam, flows through a 6 km-long tunnel to a portal control valve. From here, the water drops 125 meters down the hillside through a 570 meter-long steel penstock to the station's twin 6 MW turbines. Water exits the Glenlee plant and combines with natural river flows to form Loch Ken, the reservoir for the last of the six stations in the Galloway cascade.

After 70 years of operation, the interior of the Glenlee penstock needed to be refurbished because of the breakdown of the existing paint system and the corrosion that resulted. The steel pipeline ranges in diameter from 1.8 to 3 meters and has gradients up to 18 degrees.

In early 2010, project owner Scottish Power Generation Ltd. awarded a contract for this refurbishment work to Concrete
Repairs of Falkirk, Scotland.

Choosing the method

At first, blasting with abrasive grit was considered to remove the old paint and clean the penstock. However, because of the confined working space and the lead dust that would be generated during
removal of the original protective coating, Concrete Repairs decided instead to employ ultra-high-pressure water jetting. The company hired N.E.T. Waterjet of Meigle, Pertshire, UK, as a subcontractor to perform this work. N.E.T. Waterjet specializes in ultra-high-pressure water technology, as well as diamond drilling and sawing.

N.E.T. Waterjet personnel were concerned about the risk to the people who would be operating the water jets during the penstock cleaning at Glenlee because of the confined space and challenging conditions. To overcome this concern, the company began looking into the possibility of adapting a robot normally used for concrete hydrodemolition to perform this cleaning work. Tom Wallace, managing director of N.E.T. Waterjet, had studied robotic equipment and asked Conjet if its 324 robot, which would be small enough to enter the pipe, could be adapted for this project.

Castellan, the UK agent for Conjet hydrodemolition equipment, arranged a factory visit to view the Conjet 324 robot and learn more about its specifications and capabilities. As a result of this visit, N.E.T. Waterjet determined that the robot could be used for this application at the Glenlee plant with the addition of a Hammelmann blast or rotor head instead of the normally applied concrete hydrodemolition lance.

As a result, N.E.T. Waterjet bought a 324 robot. The robot was specified with an 11.5 kW electric drive and a synchronized winch system to help cope with the steep gradient of the pipe. The robot arrived at the Glenlee hydro plant in May 2010.

Performing the work

Concrete Repairs removed the flange-bolted expansion joints and butterfly valves to provide access at several points along the penstock for the Conjet 324 robot. N.E.T. Waterjet used a 250 kW Hammelman 120 high-pressure pump to supply water at a pressure of 2,500 bar and a flow of 25 L/min. N.E.T. Waterjet used the robot to remove the buildup of debris, the old paint coating up to 4 mm thick, and original mill scale back to a clean and smooth metal surface.

The Conjet 324 robot features a
180 mm-diameter blast head that rotates full circle around the inner circumference of the pipe. The robot then advances the head 90 mm, rotates back in the
opposite direction, and blasts the full inner circumference a second time. Thus, each area of the pipe surface gets cleaned twice. Total area to be cleaned was about 4,200 m2, with diameters of 3, 2.7, 2.4, and 1.8 meters.

In addition, it is possible to adjust the blast head settings and moving speed of the robot to deal with the degree of adhesion of the old paint coating. For example, in some areas of the pipe where the coatings and scale were more heavily built up, N.E.T. Waterjet could automatically slow the rotation speed of the blast head and rotating arm to allow greater penetration by the water jet. The company also could adjust the robot's step distance between 0 and 190 mm per cycle to allow for
removal of heavier deposits.

Using this method, the paint removal did not create any dust. The waste water and debris generated during the blasting flowed down the penstock and was collected by Concrete Repairs in a sump in the turbine house. This water was then pumped into tankers for off-site environmental treatment and disposal.

Using the Conjet 324 robot, N.E.T. Waterjet achieved cleaning of the entire inside circumference of the Glenlee penstock with no fatigue or safety issues for the operator of the remote system. The work began in mid-May 2010 and was completed in just six weeks.

Once the penstock interior was clean, Concrete Repairs sprayed on a two-coat glass flake epoxy coating, using a purpose-made gantry. The company also repainted the outside of the pipe with a four-coat epoxy paint system.

Concrete Repairs began the repainting work at Glenlee in February 2010 and completed it in the autumn of that year. Scottish Power Generation has since restarted power production at this hydro electric generation facility.

— By Francis Allan, site manager, Concrete Repairs Ltd., and Thomas Wallace, managing director, N.E.T. Waterjet Ltd.

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