Innovative engineering and design is turning a 30-year-old flood storage dam into a hydroelectric power producer at Jordan Dam.
By Paul Cyr, James Price and Andrew Feimster
The 4.4-MW Jordan Hydroelectric Project, installed on the discharge tower of a U.S. Army Corps of Engineers flood storage dam, is truly the first of its kind. The project involves the installation of two conventional vertical Kaplan turbine-generators located on the upstream side of the discharge tower.
Each turbine-generator is installed in an enclosure that seals (like a headgate) to the upstream side of the tower's intake. While the two turbines have a total discharge capacity of 1,100 cubic feet per second (cfs), the hydroelectric project controls flow releases of up to 3,100 cfs through project equipment. Above 3,100 cfs, the units are raised to allow flows to pass underneath while continuing to generate power. When flows exceed the hydraulic capacity of the hydro project, the equipment is raised to restore the full discharge capacity. The project required negligible modifications to the discharge tower and does not affect the Corps' operation of the tower or control of flow releases.
The first turbine became commercially operational in January 2012 and the second in July 2012. Many design features implemented in the Jordan Hydroelectric Project could be used at other dams that contain a similar discharge tower.
|The discharge tower at Jordan Dam is the site of the Jordan hydroelectric project, where two turbines installed in power modules provide a total generating capacity of 4.4 MW.|
Jordan Dam in Moncure, N.C., was built in 1982 and is a rock-filled structure 113 feet high and 1,200 feet long. It is owned and operated by the Corps for flood control and water quality for the Haw River downstream. The dam is typical of many flood storage dams, with flows released from a multi-gated, rectangular concrete discharge tower and returned to the river via a non-pressurized outlet conduit. Under normal pond levels, the dam creates a 15,000-acre impoundment, with a gross head of 57.5 feet at normal pool. With the impoundment at flood control level, the tower's discharge capacity is 17,000 cfs.
Since its construction, Jordan Dam has released enough water to have generated more than 500,000 MWh of electricity. The Jordan Hydroelectric Project began to harness that energy on January 20, 2012, when the first of two turbine-generators became commercially operational.
The Jordan Hydroelectric Project consists of two vertical Kaplan turbine-generators (65-inch runner), each with a capacity of 2.2 MW under a gross head of 57.5 feet and a flow of 550 cfs, for an estimated combined annual generation of 16,900 MWh. Each turbine-generator is installed in a 180-ton steel enclosure - the power module - located on the upstream side of the discharge tower, sealing off the 12-foot-wide by 30-foot-high intake opening. The modules are 12 feet square, 77 feet tall from invert to generator floor, and 120 feet tall overall (see Figure 1 on page 26). Each module contains two 3-foot-wide by 4-foot-high spillgates to discharge flow above the turbine's capacity. The vertical synchronous generators are direct-connected, 327 rpm air-cooled units located inside an enclosure about 10 feet above normal pool level.
Licensing of the project with the Federal Energy Regulatory Commission began in 1993, with a license received in 1997 and amended in 2006 for the two-unit project. Design and construction documents were submitted to the Corps for review between October 2008 and December 2009, and the 408 Permit from the Corps was received in November 2010. On-site work began immediately with the construction of a 23-kVA transmission line to connect the facility to the local utility (Progress Energy Carolinas).
The project team included the Corps as dam owner; developer Jordan Hydroelectric Limited Partnership; structural designer Kleinschmidt Associates; designer, fabricator and lead contractor North Fork Electric Inc.; consultant Diehl Engineering Co. as marine engineer for shaft analyses and bearing requirements; turbine-generator manufacturer China Huadian Engineering Corp.; and fabricator and underwater diving contractor Grainger Underwater Services.
|One of the two turbines at the Jordan hydroelectric project is shown being installed into the tower in its power module. Each unit has an installed capacity of 2.2 MW.|
The modules are located upstream of the discharge tower's emergency and service gates, in the maintenance bulkhead slots. Under normal operating conditions, the head differential can vary from 57.5 to 64 feet. The modules are designed to the same criteria as the Corps' service and emergency gates and are capable of withstanding the hydrostatic loading associated with a flood pool at elevation 240 feet and the tower dewatered (a 90-foot differential).
With the impoundment at flood pool level, the generators would be 6 feet underwater. Designing the modules to withstand flood pool loadings continues to provide the tower with three levels of water shutoff. The modules are also suitable as maintenance bulkheads during repairs to the tower and its emergency and service gates.
Each turbine has a hydraulic capacity of 550 cfs, and each of the modules has two spillgates each with a hydraulic capacity of 500 cfs. The total 3,100 cfs capacity of the two modules will control the flows out of Jordan Dam 86% of the time. When discharge exceeds this capacity or the impoundment reaches the uppermost grease-lubricated bearing at elevation 223 feet, the modules are raised up to 46 feet, permitting the tower's service gates to control the flow.
Based on historical flow records, it is estimated the modules will be raised five times a year for flood releases. Initial commissioning tests have shown the turbines will operate with the modules raised up to 5 feet while passing water underneath. This will extend the generation of the units up to flows of 8,000 cfs, a flow that is exceed only 5% of the time.
|The 2.2-MW Kaplan turbine is shown prior to installation within a power module for ease of attachment to the headgate.|
The 180-ton modules are fabricated of 50 ksi structural steel, hot-dipped galvanized for corrosion protection. A module is assembled in four distinct sections (turbine, flume, intake and generator). The turbine section acts as a headgate with the turbine strapped to the upstream side, sealing against the tower's 12-foot-wide by 30-foot-high intake opening.
A structural tubing cage encloses the lower part of the turbine section to guard the draft tube from damage in the event the module is lowered onto debris sitting on the tower sill. This cage also protects the draft tube from debris when flow is discharged beneath the module in the raised position. The upstream portions of the module's turbine section are enclosed by removable steel plating to prevent the wicket gate operating mechanisms from being damaged or jammed by debris. The module bears on the tower's invert only along the downstream edge seal, 4.5 inches wide, resting on the existing steel gate sill. The module's intake section is enclosed within fixed trashracks that bear on a concrete slab and forms the roof of the existing concrete grizzly racks
In the normal operating position, the Kaplan runner is 5.5 feet above tailwater. The module's two spillgates are located immediately above the elbow-shaped draft tube and are operated by hydraulic cylinders located above the normal pool level just below the generator floor. The spillgates open automatically after a generator load rejection to maintain flow discharges. The gates are also used to increase each module's discharge capacity to a total flow of 1,550 cfs. Each module contains a hydraulic power unit in the generator enclosure to operate the wicket gates, runner blades, spillgates and other auxiliary systems.
A programmable logic controller inside the generator enclosure monitors and controls the various systems and may be operated locally via a touchscreen or remotely via computer control.
Each module has six electrically operated screw jacks at the generator floor that are extended to "lock" the generator floor to the tower slots. These jacks resist the generator's normal running torque of 51,256 foot-pounds and short circuit torque of 243,000 foot-pounds. The jacks are extended before operating the turbine and retracted before raising or lowering the module.
Maintenance and repair of the module is accomplished by raising or lowering as required to provide access to the various components from either the upper or lower work platforms. Major repair of the turbine components requires that the module be disassembled and removed from the bulkhead slots.
The module is raised and lowered only under balanced head conditions, with the tower's service or emergency gate in the closed position and the downstream chamber between the module and gate flooded. Each module is raised and lowered by a hydraulic cylinder located within the discharge tower with a maximum lift rating of 200 tons. In an emergency condition, the cylinder and associated mechanisms are designed to apply 50 tons to assist in closing the module. The cylinder engages the module via a lifting carriage that has two rotating cam arms that engage a series of steel lift blocks located on the downstream face of the module, similar to a forklift.
For maintenance operations, the module can be raised 67 feet to position the turbine runner at the level of the lowest access platform. The lifting cylinder and associated carriage raises and lowers the module in increments of 10 to 12 feet.
The modules are tended by operating personnel while being raised and lowered, but the controls can be automated to allow an individual to raise a module 12 feet (one stroke of the lifting cylinder) with minimal intervention. This initial lift is accomplished by entering a command into one of the touchscreens or the local control computer. The control system automatically shuts down the turbine and applies the generator brakes and retracts the generator floor stabilizing screwjacks. The tower's service or emergency gate is then closed and the module's spillgates opened to flood the chamber downstream of the module. Once the hydrostatic pressures have equalized, the lift system activates to raise the module.
The tower's service gate is then opened as directed by the Corps to control the release of flow from the discharge tower up to the capacity permitted by the 12-foot opening. Higher flows require the module to be dogged off and the lift carriage lowered down to the next set of module lift blocks, then the module is raised again.
|Both generators are covered by 180-ton steel enclosures, as shown here.|
Two steel platforms were constructed to support the installation, operation and maintenance of the module and its equipment, and a tower control booth was attached to the side of the discharge tower to house the operating controls and electrical switchgear. A 4.16-kV to 23-kV enclosed pad-mounted step-up transformer was installed inside a concrete containment on the crest of the dam, with all power conductors entering and exiting underground.
In the event of an electrical shortage, an automatic transfer switch inside the tower control booth transfers all station service power to a separate electrical service provided by another utility. If both utility services are lost to the tower, a propane-powered standby generator installed on the west end of the dam provides emergency power.
Tower modification and loads
To accommodate the module's installation, modifications to structural elements of the discharge tower were limited to removal of a concrete beam at the top of the tower along the upstream side and two 13-foot-square by 3-foot-thick underwater sections of concrete that formed the top of the concrete grizzly racks. Structural analyses of the access bridge from the dam crest to the top of the discharge tower indicated the bridge has a structural capacity of 250 tons, more than sufficient to support a maximum load of 42 tons when driving the installation crane onto the tower.
To ensure the modules would fit into the bulkhead slots, a survey of the plumbness and squareness of slots was taken at 1-foot increments over the height of the tower from invert to roof. The survey revealed the distance between piers was narrower than shown on record drawings and the slots were neither plumb nor square to the tower, as a 120-foot-tall structure was never intended to fit into slots. Modifications were made to the module design to accommodate the survey results.
Start-up and operation
The first unit began generation on Jan. 12, 2012, and reached rated capacity at the design head and flow. Unit 2 became operational in early July 2012. Through Aug. 1, 2012, Unit 1 has generated 6,623 MWh. The first unit has operated over a range of flow from 100 to 8,000 cfs, having operated for more than three days with the module raised 3 to 5.5 feet above the tower invert, allowing the release of about 1,600 to 3,000 cfs while still providing a capacity of 1.8 MW.
The Corps expressed early concern about vibrations that could be produced during turbine operation and their possible impact on discharge tower integrity. Vibration is monitored continuously by the control system, and the levels have remained low. Displacement measurements and values aside, vibrations are low. Actually, higher vibrations are recorded when the unit is offline and water is discharged through the existing tower gates. There are no increases in vibration when the unit is operated with discharges through the tower gates.
The lifting system has operated more smoothly than expected. The module has been raised and lowered more than 20 times, equivalent to four years of operation. It takes one person 60 minutes to raise the module 46 feet and 40 minutes to lower the module the same distance. Unit output when operating while discharging flow through one or both spillgates is not affected, rather it is improved due to increased submergence on the draft tube. Since put into operation, Unit 1 has had a down time of about 15 days due to various start-up commissioning and troubleshooting activities.
Two turbine-generator equipment-related design problems were encountered during start-up. An over-vibration occurred after 19 days of operation, and inspection showed five of 10 bolts had broken in coupling located just below the lowest grease-lubricated bearing. Early design work on the modules indicated that accurate alignment of the turbine, shaft, bearings and generator was impossible. Every time the module is raised and lowered, the entire shaft/bearing assembly shifts out of alignment.
Operation and analysis by Diehl Engineering showed that the long, relatively slender shaft will bend and deflect without overstressing the shaft steel or creating excessive guide bearing reactions.
One oversight was the strength of the coupling bolts supplied by the turbine manufacture. The shafts have typical one-piece forged ridged couplings supplied with ten 43 mm ASTM A668 Class J bolts. These bolts would have been adequate in a traditional hydro installation with a perfectly aligned, stationary shaft. At Jordan, bending and flexing of the shaft during operation caused cyclical loading and unloading of the coupling bolts at each revolution that resulted in the premature failure. The solution was to replace all the coupling bolts with 12.9 class socket-head bolts and applying a preload torque equal to 70% of yield.
The second equipment problem was with wicket gate operation, with the gates moving hesitantly in increments of 20% gate opening. Torsional backlash in the 63-foot-long wicket gate operating shaft caused a problem due to insufficient rigidity. The solution was to remove the 7.625-inch-diameter shaft and operating cylinder and replace them with an underwater hydraulic cylinder attached to the wicket gate operator ring.
Key development factors
The following key factors were identified in the design and construction of the Jordan Hydroelectric Project as being critical to the ability and success of installing a similar hydroelectric project at other discharge towers:
- Modifications to or removal of any part of the concrete grizzly racks must not be required. The racks are massive and located well below the impoundment's water surface and are extremely difficult and expensive to modify.
- The discharge tower's bulkhead slots must be located upstream of the tower's upstream face, eliminating any need to remove any part of the upstream wall to accommodate a module.
- The spacing between piers containing the bulkhead slots must be reasonably consistent, and the bulkhead slots must be reasonably plumb and square to the tower.
- The tower's roof and access bridge must have the structural capacity to support the loads needed to install the modules, or means be implemented to redistribute or redirect the loads so as not to overload the affected structure.
- Upfront communication with the owner of the discharge tower regarding any possible temporary and/or long term impacts of the hydro project on the structural integrity and operation of the tower is a vital aspect for success.
- Support from the owner of the discharge tower and their belief that the project can be constructed and operated successfully is an important component of a successful project.
- Respecting the discharge tower and dam as the property and "home" of the owner contributes greatly to good relations throughout the project.
- The installation of a hydroelectric project on a tower results in a long-term partnership between the developer and owner. Mutual respect and excellent communication must always exist, during construction and throughout the life of the partnership.
Factors of success
Relations with the Corps have been outstanding. Their review personnel and oversight teams have been cooperative and supportive of the project. Throughout the design and construction process, the development team made Corps personnel welcome in the design review, at the fabrication shops, and on site. The design and construction teams have been upfront with Corps personnel, taking the philosophy that the discharge tower and dam is their house. Project field personnel freely submitted diver plans and critical lift plans to the Corps and worked closely with Corps operating personnel in communicating and coordinating the timing and control of flows released from the dam. The Corps personnel who operate and manage the dam along with the district commanders have all been impressed and pleased with the level of communication and project personnel doing what they said was going to be done.
The Jordan Hydroelectric Project did not utilize any proprietary or patented design, equipment or operating systems. However, the concept and many of the operating systems were custom-designed to fit the site, accessibility and operating requirements. Many of the turbine-generator and associated components can be adapted or installed at other dams that contain a discharge tower.
The owner of the Jordan Hydroelectric Project is proceeding with a second hydroelectric development at a Corps dam, the 3.7-MW Gathright Project in Virginia, which will contain a single vertical Francis turbine. The unit will be installed in a 175-foot-tall, 250-ton module that will be placed on the upstream face of the discharge tower, against the 15-foot-wide by 70-foot-high intake opening. This project will be "a turbine strapped to the back of a floating bulkhead." Stay tuned.
Paul Cyr, P.E., is senior engineering consultant with Kleinschmidt Associates. James Price, PhD, is president of W.V. Hydro and Noah Corporation. Andrew Feimster is president of North Fork Electric Inc.