Pumped-storage power plant upgrades penstock

New Milford, Conn., is home to the Rocky River Hydroelectric Power Plant, the first large-scale pumped-storage development in the United States. The American Society of Mechanical Engineers declared it a National Historic Mechanical Engineering Landmark.  

The original wooden and steel penstock transported water back and forth between Candlewood Lake and the plant’s surge tank above the Housatonic River by means of the Rocky River Hydroelectric Power Plant. The station’s upper reservoir, Candlewood Lake, was named for nearby Candlewood Mountain and is the largest manmade lake in the state.

An 8-square-mile storage area is filled by pumping water from the Housatonic at periods when electricity to run the pumps is least expensive. When demand reaches a peak, water is released through the same penstock, and the motors driving the pumps reverse to become generators to produce electricity. Power generated by releasing the same water can then be sold at higher rates during periods of peak demand.

“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.

This 47-year-old penstock was made of Douglas fir that was held in place by iron rings. It was originally built in 1928 and replaced in 1965. As it aged, it began to sag and leak into the drainage ditches that run parallel to the pipeline. 

“The wood penstock had clearly outlived its useful life,” said Richard Laudenat, CT Hydro plant manager for FirstLight Power Resources, the owner and operator of the Rocky River Hydroelectric Power Plant. “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.”

Kleinschmidt Associates, specializing in hydro-generation and water resource projects, was hired to design a replacement pipeline. 

Design considerations

The replacement penstock was designed to be a reliable, cost-effective design and, as much as possible, maintenance free. Most of the replacement 

penstock was half-buried to the center line, except for two, 20-foot 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 at the upper and lower end of the new penstock allow for the new 10-foot fiberglass pipe to connect into the existing 15-foot steel sections. 

This $3 million project went out to bid in January 2012 and was completed in December 2012. The 15-foot-diameter woodstave penstock was replaced with 10-foot-diameter fiberglass pipe. Hobas Pipe USA supplied approximately 950 linear feet of 120-inch piping that included three miter joints of centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe. 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.

Kleinschmidt evaluated three pipe materials for this project: plastic, steel, and fiberglass. Hobas was the lowest-cost supplier and after review by the owner was awarded the contract. Even as the lowest-cost option, Hobas technically exceeded the other materials in design and installation. It allowed for easy connection into the steel transition pieces and is UV resistant. In addition, the hydraulic capacities create more flow going into the turbine; therefore, more electricity is generated due to reduced head losses. As compared with other piping materials, the Hobas CCFRPM pipe exhibits a lower friction factor, and for pipes of the same diameter, this will mean less head loss in the penstock piping. 

The FWC coupling joint utilized on this project is a pressure joint. It 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 both 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), took 11 days to remove the existing woodstave 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,” explained Joseph DelVecchio, construction superintendent, BAC. 

BAC then efficiently installed the new penstock. “Our best production rate was 14 joints in one day. The pipe joined easily and when honing the pipe we used less tonnage than was specified for a pipe of that size,” DelVecchio said. BAC evaluated two alternative orientations of the replacement penstock and decided to utilize mitered joints to accommodate the change in the pipe direction.

Hobas provided fittings manufactured of the same materials as the pipe, three mitered elbows of 9, 18, and 20 degrees. Because it was a pressure application, BAC decided to encase the elbows in concrete to resist deformation.

With the line installed, the contractor performed visual inspection and testing of the joints. “We felt confident in the material since we have used it 

previously on other projects, particularly the Jackman Penstock, and are familiar with its performance. 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,” explained Harold Thompson, senior consulting engineer, Kleinschmidt. 

“The new penstock continues to meet all of our expectations after its first winter under severe New England weather conditions,” Laudenat said. “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 installation project’s capital costs and made shipping by truck a much more viable alternative.”

Posted in Uncategorized | April 14th, 2014 by

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