Hydro Review

Replacing an Aged Woodstave Penstock

The 40-plus-year-old penstock supplying water to the 29-MW Rocky River pumped-storage plant was replaced with a smaller-diameter pipe that actually increases water flow to the plant, boosting power generation.

By Erin Boudreaux and Stuart H. Piermarini

New Milford, Conn., is home to the 29-MW Rocky River Hydroelectric Power Plant, the first large-scale pumped-storage development in the U.S. The plant was completed in 1928, and the American Society of Mechanical Engineers declared it a National Historic Mechanical Engineering Landmark in 1980. The plant is owned by FirstLight Power Resources, which operates 22 conventional hydro stations, as well as Rocky River and the 1,080-MW Northfield Mountain pumped-storage plant.

The original penstock, which operated from 1928 to 1965, was made of Douglas fir that was held in place by iron rings. In 1965, the penstock was replaced by another made of the same materials.

The penstock transports water between Candlewood Lake (upper reservoir) and the plant's surge tank above the Housatonic River (lower reservoir) by means of the Rocky River plant. Candlewood Lake was named for nearby Candlewood Mountain and is the largest man-made lake in the state.

An 8-square-mile storage area is filled by pumping water from the river during periods when electricity is least expensive. When electricity demand reaches a peak, water is released through the same penstock, and the motors driving the pumps reverse to become generators to produce electricity.

The existing wood penstock supplying water to the 29-MW Rocky River pumped-storage plant had outlived its useful life, as can be seen by the water leakage into the drainage ditches that run parallel to the pipeline.
The existing wood penstock supplying water to the 29-MW Rocky River pumped-storage plant had outlived its useful life, as can be seen by the water leakage into the drainage ditches that run parallel to the pipeline.

"The statement that a hydroelectric plant can pump its own water supply sounds absurd on the face of it, yet this is virtually what happens in the case of the Rocky River Hydro Plant ...," said chief engineer Paul Heslop, at a Connecticut Society of Civil Engineering meeting in 1928.

Problems emerge

As the 1965 penstock aged, it began to sag and leak into the drainage ditches that run parallel to the pipeline. In fact, the leakage was substantial enough to create a wetland area adjacent to the pipeline.

"The wood penstock had clearly outlived its useful life," said Richard Laudenat, Connecticut hydro plant manager for FirstLight Power Resources. "To limit the amount of water leakage and extend its useful life, a second set of bands was installed around its circumference, and joints around the pipe were also fitted with wedges. Both of these maintenance activities were to no avail, as the woodstave pipe aged and the sagging continued, which opened it to increased leakage." Leakage was substantial enough that continued maintenance was not useful. Replacement was required.

Kleinschmidt Associates, a firm that specializes in hydro generation and water resource projects, was hired to design a replacement penstock that would be reliable, cost-effective and, as much as possible, maintenance-free. The new penstock would have a diameter of 10 feet, as opposed to the 15-foot diameter of the existing penstock, because the material used would reduce head loss. Most of the replacement penstock would be half-buried to the center line, except for two 20-foot-long sections at the upper end of the penstock. This 40-foot section, supported on concrete cradles, allows for surface water drainage from a hill above the penstock. Steel transition sections were designed at the upper and lower end of the new penstock to allow for the new 10-foot pipe to connect into the existing 15-foot steel sections. The existing sections at the bottom would be connected to the new penstock using a steel reducer.

Kleinschmidt had conducted a feasibility study for replacing the penstock at the 3.5-MW Jackman project and considered five pipe materials: woodstave, concrete, HDPE plastic, steel and fiberglass-reinforced polymer. Based on the corrosion resistance and relative stiffness of the FRP pipe, which meant it could be supported on the existing grade and half-buried, the owner chose FRP for that project. For Rocky River, Kleinschmidt evaluated three pipe materials: plastic, steel and FRP. FirstLight Power Resources was involved in the purchase decision and inspected the Hobas facility to ensure quality of manufacture and suitability for the project.

A new 10-foot-diameter fiberglass pipe was installed at the 29-MW Rocky River pumped-storage hydro plant to replace the existing 15-foot diameter wooodstave penstock.
A new 10-foot-diameter fiberglass pipe was installed at the 29-MW Rocky River pumped-storage hydro plant to replace the existing 15-foot diameter wooodstave penstock.

This $3 million project went out to bid in January 2012. Hobas Pipe USA of Houston was the lowest cost supplier and after review by the owner was awarded the contract. The company supplied about 950 linear feet of 120-inch-diameter centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) piping that included three miter joints. It was manufactured with a stiffness class of 36 psi and pressure class of 26 psi. The line operates under a nominal pressure of 22 psi.

The Hobas material allowed for easy connection into the steel transition pieces and is UV resistant. In addition, because the Hobas CCFRPM pipe exhibits a lower friction factor than other piping material of the same diameter, more flow will enter the turbine; therefore, more electricity is generated due to reduced head losses.

The FWC coupling joint used is commonly used in direct bury applications and also for above-ground installations such as penstocks. It is a structural filament-wound sleeve overwrapped and mechanically locked to an internal full-faced elastomeric membrane. The sealing design includes both lip and compression elements so the joint is suitable for non-pressure and pressure service. Hobas supplied the FWC couplings with a UV coating that will protect the joints from long-term sun exposure.

Quick installation

The installation contractor, Blakeslee Arpaia Chapman (BAC) of Branford, Ct., took 11 days to remove the penstock. "We cut the existing penstock into 20-foot sections, and as these sections were removed we dropped them onto timbers, causing them to implode," says Joseph DelVecchio, construction superintendent for BAC.

BAC then installed the new penstock, which had about 50 joints. "Our best production rate was 14 joints in one day. The pipe joined easily and when honing the pipe we used less tonnage [the amount of force used to push the pipes together] than was specified for a pipe of that size," explains DelVecchio.

BAC evaluated two orientations of the replacement penstock. The FWC couplings are flexible and allow for a half degree of angular deflection per joint. However, for the orientation chosen, in the three areas of directional change that exceeded this amount, mitered fittings were necessary. Hobas provided three mitered elbows of 9, 18 and 20 degrees, manufactured from the same materials as the pipe. Because it was a pressure application, BAC encased the elbows in concrete to resist deformation.

With the line installed, the contractor performed visual inspection and testing of the joints. "The FWC coupling proved to be an effective and reliable method to join the sections of Hobas pipe, and everyone was very pleased that no leaks were identified and no joint repairs were required," explains Harold Thompson, senior consulting engineer with Kleinschmidt. The work was complete and the penstock returned to service in December 2012.

"The new penstock continues to meet all of our expectations after its first winter under severe New England weather conditions," states Laudenat. "The hydraulic performance of the very smooth interior of the fiberglass pipe allowed for a reduction in the diameter of the pipe. This smaller size reduced the project's capital costs and made shipping by truck a much more viable alternative."

Erin Boudreaux is marketing manager for HOBAS Pipe USA, which supplied the new penstock for the Rocky River plant. Stuart Piermarini is engineering and compliance manager with FirstLight Power Resources.

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