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Refurbishing Surge Tanks and Penstocks

Ten of Nova Scotia Power’s 33 hydroelectric generating facilities feature steel surge tanks and penstocks to supply water to the turbines. As part of a recently completed refurbishment program, the utility evaluated the condition of these structures and made improvements where necessary to extend their lives.

By C.T. Chen, Terrence F. MacIvor, and A. Elaine Locke

At ten of Novia Scotia Power’s 33 hydroelectric plants, water is conveyed via pipeline from the reservoir to a steel surge tank, then to a penstock for the descent to the turbine.

Eleven of the 12 tanks (two facilities have two) are of differential design. This design consists of an elevated tank, with internal and external risers. These risers connect the penstock to the bottom of the tank. The tanks have conical roofs, can-shaped main sections, and elliptical bottoms. They are supported by four-column towers braced by diagonal tension rods and horizontal struts. All the loads eventually go down to the concrete column foundations.

The twelfth surge tank, the one for 3-MW Avon #2, is of the differential design and sits directly on a concrete foundation that contains the penstock. The concrete foundation and surge tank are located on top of a high cliff.

The penstocks at nine of the ten facilities are made of steel. At the tenth facility, 18-MW Weymouth, the two penstocks are 11-foot-diameter wood stave pipe.

All of these surge tanks and penstocks were old, having been installed between 1922 and 1967. To ensure their continued reliable operation, Nova Scotia Power carried out a surge tank and penstock refurbishment program from 1990 to 2006. The program involved evaluation of the existing structures, repairs to the foundations and steel, installation of bubbler systems, and application of interior and exterior protective coatings.

Assessing existing structures

To evaluate the existing surge tanks and penstocks, Nova Scotia Power conducted design reviews and physical inspections one or two years before the construction work was scheduled to begin. The design review of surge tanks revealed some of the foundations were constructed from standard drawings supplied by the tank manufacturers. The foundations were undersized for certain conditions, such as high wind load when the tanks are dewatered. For the steel penstocks, the design review revealed a lack of expansion joints and undersized thrust blocks.

To perform the physical inspections of the tanks and penstocks, Nova Scotia Power formed teams consisting of engineers, iron workers, utility workers from the plants, and, when required, technicians from material testing companies. Before inspecting each site, the team prepared a work plan to ensure a safe and effective inspection. Issues covered included access, fall protection, confined space entry, communication, and the rescue plan.

First, the anchor points to be used for fall protection while working around the surge tanks were inspected. Nova Scotia Power personnel performed visual inspections and a technician from a material testing company performed ultrasonic thickness measurements to adequately determine the degree of steel deterioration. Because of the advanced ages of the structures erected before the 1950s, material testing companies performed sample analyses for weldability to facilitate the remedial work.

Physical inspections revealed several common problems with the surge tank foundations, including deterioration from freezing and thawing, concrete spalling, and cracks. For penstock cradles, signs often indicated accelerated concrete failure due to the rocking effects from thermal movement of the penstocks.

Common problems for the steel structures were paint failures and pitting of the interior surface steel. In both surge tanks and penstocks, rivet heads had eroded and steel members had deteriorated, especially at connections.

Table 1 on page 26 lists work performed at each facility.

Refurbishing concrete structures

Several surge tank foundations required substantial reinforcement to meet the current requirements in the National Building Code of Canada for resistance to uplift and lateral loading. For the undersized concrete foundations of the tower column surge tanks, contractors installed rock anchors by placing re-bar in grout-filled holes drilled into bedrock, then enlarged these foundations by adding a reinforced concrete encasement.

Almost all the concrete cradles supporting the steel penstocks needed to be replaced or partially reconstructed. To perform this work, Nova Scotia Power personnel dewatered the penstocks. With the penstock emptied, the contractor could fully or partly remove and replace the existing cradles at alternate locations. The reconstructed cradles had a friction-reducing ultra-high-weight-polyethylene plastic seat lining attached at the top. This high-density polyethylene material has high abrasion resistance and impact strength and a lower coefficient of friction than regular polyethylene.


One step in the refurbishment of steel surge tanks at Nova Scotia Power hydro projects consisted of installation of new spiral-welded pipe to replace severely corroded external risers.
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Finally, several of the existing thrust blocks that were undersized under large waterhammer loadings were reinforced with the addition of rock anchors and concrete encasement to strengthen resistance to overturning and sliding.

Repairing steel on the surge tanks

The next step was to repair deteriorated steel on the surge tanks. Nova Scotia Power replaced severely corroded internal and external risers of the surge tanks with spiral-welded pipes. Spiral-welded pipes were selected over longitudinal seam pipes based on price and availability. Erection of the risers involved temporary removal of the conical tank roof so the pipes, supplied in 40-foot-long sections, could be hoisted in place by a crane. Often, an auxiliary crane was required to aid the lateral movement of the pipe section. Several of the elliptical tank bottoms were reconstructed using pie-shaped plates on top of the existing bottoms, bent to fit and joined together by butt welds.

Eroded rivets were either ring welded or replaced with high-strength bolts. New safety cable systems were added to the access ladders. Finally, new internal access platforms were installed to facilitate future inspections and maintenance work.

Installing bubbler systems

Almost all the old surge tanks had timber frost casings around the tanks and external risers to prevent ice formation in the winter. The casings – two layers of æ-inch- or 1-inch-thick creosoted timber vertical sheathings – were connected to the steel by horizontal timber nailers and steel needles, with fiberglass insulation in between. After years of service, even the creosoted timber started to decay and rot.

Around 1990, a bubbler system, designed by a Nova Scotia Power maintenance engineer, was installed at the 5.4-MW Hollow Bridge project. The system consists of a compressor that supplies air, through a half-inch air line attached to the interior of the penstock, to the top of the external riser’s bottom T-section. The air bubbles move upward effortlessly, expanding into bigger circles toward the water surface of the surge tank. The constant agitation on the surface of the water caused by eruption of the bubbles prevents ice from forming.

Installation of this system was considerably less expensive than replacing the two-ply timber frost casing. With good operational experience at the Hollow Bridge project, the bubbler system became the standard anti-icing equipment to replace old frost casing for the surge tanks.

Applying protective coatings

In general, the conditions of the interior surfaces of the surge tanks and penstocks subject to water movement were much worse than the exterior surfaces. The least-deteriorated locations were the interior surfaces of the tanks above the splashing zone, exterior surfaces of the tanks, and external risers protected by the timber frost casing. Contractors used sandblasting and high-water-pressure blasting to remove old paints from surge tank coatings.

Contractors applied surface-tolerant high-performance epoxy coating – such as Bar-Rust 235 from Devoe Marine Coatings and Amerlock 400 from Ameron Coatings – in two or three coats for a total of 12 to 14 dry film thickness. Where surface pitting was severe, such as on the interior surfaces of the surge tank at the 4-MW Sandy Lake project, contractors used Urylon 201-15 from Urylon Plastic Inc., a polymer blend plastic liner type coating applied at a thickness of 20 to 30 mils or more.

For the control measures and testing and disposal procedure for the waste material generated during removal of the old paints, Nova Scotia Power followed the “Guidelines for the Application and Removal of Structural Steel Protective Coatings” issued by Nova Scotia Department of the Environment. Because of the close proximity of this work to water, Level 3 protection in the form of full enclosure was required. The full enclosures were erected with vertically hung tarps laced together on all sides of the surge tanks and penstocks, as well as ground sheets.

Because a drinking water well was less than 300 meters from the surge tank at the 4-MW Tidewater project, a Level 4 full enclosure would be required if sandblasting were used. A Level 4 enclosure requires fully sealed joints and sealed entryways, complete with negative pressure by forced air flow. This would be a difficult and expensive provision. As an alternative, Nova Scotia Power proposed high-pressure water blasting with a Level 3 full enclosure. The Department of the Environment approved this proposal. The result was as good as the commercial sand blasting for surface preparation.

Old lead paint chips and debris from high-pressure water blasting were collected in drums and transported to a disposal site for dangerous waste.

Replacing corroded penstocks

Three steel penstocks were severely corroded and were replaced with new penstocks. Nova Scotia Power replaced the penstocks for the 3-MW Avon #2 and 3.4-MW Hell’s Gate #1 projects with spiral-welded pipes. These pipes were supplied in sections and joined in place on the steep hills using full-strength butt welding.


Table 1: Work Performed during Nova Scotia Power’s Surge Tank and Penstock Renewal Project
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However, the 10-foot-diameter steel penstock at Tidewater was replaced with a fiberglass-reinforced plastic (FRP) pipe. The 293-foot-long penstock has a flat configuration closer to a pipeline than a steep penstock, making it a good candidate for FRP pipe. The 95-foot head at Tidewater was well within the usual pressure range for large FRP pipe, which Nova Scotia Power had used to replace pipelines at several other hydro facilities.


Before beginning penstock inspections, Nova Scotia Power developed a safe work plan for each site. Workers inspected penstocks using a custom-made trolley operated with a cable and winch.
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Reinforced Plastic Systems Inc. of Mahone Bay, Nova Scotia, supplied a 10-foot-diameter FRP pipe, perhaps the largest-diameter FRP pipe ever installed. The company set up a custom-designed moulding form to produce the penstock, typically in 20-foot-long sections with bell and spigot ends. The new penstock was shipped to the site in sections and joined in place with O-rings.

Conclusion

With ongoing maintenance, Nova Scotia Power’s surge tank and penstock renewal program will extend the lives of these important water conveyance structures for several decades. Thanks to proper planning, good engineering practice, cohesive team work, and construction management that fostered cooperation between owner and the contractors, all the projects were carried out in a timely manner and were cost effective.


Nova Scotia Power installed a friction-reducing pad protected by a high-density polyethylene sheet (black) to protect the penstocks at its 5-MW Paradise facility.
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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), (1) 902-478-6174 (MacIvor), or (1) 902-428-7546 (Locke); E-mail: ct.chen@nspower.ca, tmac@ eastlink.ca, or ann.locke@nspower.ca.

Acknowledgments

The authors wish to thank Jim Gordon for wise counsel received over the years on water conveyance, among other things, and Gordon State, a retired welder and gentleman who inspected most of the surge tanks and repaired several tanks. Last but definitely not least, we thank Eric Brown, “Mr. Hydro” for Nova Scotia Power from 1982 until his retirement in 2007, whose dedication to hydro engineering inspired the authors to prepare this article. He was the main thrust behind the work described in this article and all other major Nova Scotia Power hydro programs in the areas of facility renewal, dam safety, and water management.

C.T. Chen, P.Eng., is hydropower consultant, Terry MacIvor is senior construction superintendent, and Elaine Locke, P.Eng., is senior civil engineer for Nova Scotia Power Inc.


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http://www.hydroworld.com/content/hydro/en/articles/hr/print/volume-26/issue-5/feature-articles/water-conveyance/refurbishing-surge-tanks-and-penstocks.html