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STICKY WICKETS: Custom-Made Trolley Simplifies Penstock Inspections

To inspect the steel penstocks that supply water to two of its hydro projects, Nova Scotia Power developed a custom-made trolley. Using the trolley, which operates on electric power, allows for faster inspections than those done by foot. In addition, it improves worker safety by minimizing the risk of slipping and puts less physical demand on the inspectors.

Why the penstocks needed inspections

The penstocks at Nova Scotia Power’s hydro facilities were old, having been installed between 1922 and 1967. To ensure the continued reliable operation of these facilities, Nova Scotia Power carried out a penstock refurbishment program from 1992 to 2006. The program involved evaluation of the existing structures, including inspections of the penstock interiors. Work performed included repairs to the foundations and steel; replacement of corroded penstocks; and application of interior and exterior protective coatings.1

Choosing the trolley

When inspecting the interior of steel penstocks, certain concerns dictate the method used. These include access issues, entry into a confined space, and rescue planning. For example, the existing manholes on top of the penstocks are small and cumbersome for personnel access. In addition, they make it difficult to retrieve injured workers if necessary.

Historically, Nova Scotia Power personnel inspected the interiors of all penstocks on foot. However, the penstock that supplies water to the 6.8-MW Nictaux hydroelectric project is quite long at 1.04 kilometer. This was a significant concern because of the length of time needed to perform the work, as well as the distance of travel that might be needed to retrieve an injured person at the far, low end of the penstock.


This steel trolley simplifies penstock inspections at Nova Scotia Power’s hydro facilities.
Click here to enlarge image

In the early summer of 1999, Nova Scotia Power sought a safer, more efficient alternative for performing this and other future penstock inspections at its hydro plants.

After some consideration of the alternatives, the utility determined that a custom-made trolley would meet these needs.

Building the trolley

The trolley design selected by Nova Scotia Power consists of a rectangular steel platform on four wheels, about 0.8 meters by 1 meter. The deck frame is made of 3.8-centimeter-square steel tubing, a 3-millimeter-thick steel plate deck with flat plate skirting, and round steel tube rails on the sides. The back end of the trolley contains a raised bench seat with handrails. The back rails were stiffened by addition of plate material, with holes for cable connections. Four roller wheels were mounted at the four corners of the deck frame.

The trolley is powered by gravity while descending and by electrical power while ascending.

A small local machine shop designed and fabricated the trolley at a cost of about C$1,500 (US$1,500). A Nova Scotia Power engineer then reviewed the completed trolley to ensure it was constructed to meet the requirements of the National Building Code of Canada.

Performing the inspections

In July 1999, the Nova Scotia Power inspection team used the trolley to inspect the buried penstock that supplies the Nictaux project. This 1.7-meter-diameter steel penstock is 1.04 kilometer long and leads into a 296-meter-long wood stave penstock that connects to the head pond intake structure. The penstock drops about 122 meters from the head pond to the turbine unit. It has a relatively flat profile and does not contain a surge tank.

After the penstock was dewatered, welders cut the top two-thirds of a pipe section for access, near the high point of the penstock. A steel cable connected the trolley to a cable drum, which was powered by an electrical winch mounted on temporary supports on one side of the pipe. Similarly, a steel cable installed as the safety line was connected to a separate cable drum through another electrical winch mounted on the opposite side of the pipe. The safety line was connected to a full body harness worn by the inspector.

Because the penstock was relatively flat, Nova Scotia Power personnel installed two fans at the generator floor of the powerhouse to supply fresh air to the penstock. Before the inspector entered the penstock, Nova Scotia Power personnel conducted air quality tests using a hand-held Scout MultiGas Monitor from Scott Instruments. With forced air from the fans and the natural “stack effect,” there was more than enough fresh air moving through the penstock.

The inspector was trained to locate corrosion and pitting, as well as gauge the depth of the corrosion. The inspector carried out a visual inspection of the interior surfaces of the penstock as the trolley was lowered down the incline. Radio contact was maintained between the inspector and the safety watch and other team members located at the temporary access opening. This communication was used to identify the inspection locations and report pipe condi- tions. The entire inspection took two days. Overall, the interior of the steel pipeline was found to be in good condition. Neither steel repair nor repainting was required.

Based on the positive results of using the trolley during the Nictaux penstock inspection, Nova Scotia Power decided to use the trolley to inspect the above-ground steel penstock for the 5-MW Paradise hydro facility in November 2000. This 610-meter-long penstock begins at the surge tank and is a continuation of a 1.5-meter-diameter, 3.1-kilometer-long wood stave pipeline that connects to the head pond intake structure. As the steel penstock progresses, its diameter reduces gradually to 1.2 meters at the point where it enters the powerhouse. The penstock is supported by concrete cradles.

All the manhole covers were removed to start air circulation. Welders cut an access hole on the side of the high point of the penstock. The access hole was sized to accommodate the trolley, as well as to allow relatively easy access for workers. An electrical winch was mounted on temporary supports and attached to the trolley with a steel cable. A rope, used as a safety line attached to the inspector’s full body harness, was fed by hand but could be attached to the winch system of a tractor should a rescue operation occur. The four-wheel-drive farm tractor was needed to transport materials for this steep site.

Before the inspector entered the penstock, Nova Scotia Power personnel conducted air quality tests using the hand-held monitor at each manhole and at the access hole. With the steep slope of the penstock (about 20 percent), plenty of fresh air came from the lower manholes, moving upward due to “stack effect.” Personnel determined that no supplemental air was needed to meet air quality standards.

The trolley containing the inspector was lowered by gravity, with speed controlled by the winch operator. The inspector visually assessed the interior surfaces of the penstock. For this operation, two persons followed along the exterior of the penstock as the trolley moved. At areas of the penstock needing further inspection, the inspector wiped the area clean and marked it with a liquid pen designed to write on steel. In addition, the inspector tapped the pipe in this area so the outside persons could mark the locations using a liquid pen. At each manhole, the inspector passed on his report, which detailed the condition of the areas with corrosion, to the outside persons. The corroded areas were mapped and numbered in sequence from the start of the penstock to the end.

The entire inspection took two days. Only one small corroded section required steel repair. The other mapped areas suffered only mild corrosion that required repainting.

Results

For both inspections, the trolley provided safe and efficient access. Using the trolley minimized the slippage hazard that exists during inspections by foot. It also resulted in a faster inspection process and put less physical demand on the inspectors.

Nova Scotia Power plans to continue using the trolley when needed for penstock inspections and repairs, possibly as early as 2008 for the steel penstock that supplies water to the 3.8-MW Dickie Brook project.

– By C. T. Chen, P.Eng., hydro consultant, and Larry T. Fraser, senior construction superintendent, Nova Scotia Power Inc. The authors may be reached at Nova Scotia Power Inc., P.O. Box 910, Halifax, Nova Scotia B3J 2W5 Canada; (1) 902-428-7557 (Chen), or (1) 902-565-8336 (Fraser); E-mail: ct.chen@nspower.ca.

Note

  1. Chen, C.T., Terrence F. MacIvor, and A. Elaine Locke, “Refurbishing Surge Tanks and Penstocks,” Hydro Review, Volume 26, No. 5, September 2007, pages 22-27.


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