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Silverhawk: Sound design, good equipment, top people

To earn a lifetime availability in the high 90s, a power­plant must be well-designed and -constructed, have good equipment, and be operated and maintained by first-rate personnel who work as a team. Tom Price, regional director of NV Energy’s Arrow Canyon facilities (Lenzie, Silverhawk, and Allen), emphasized the people part of the equation.

First thing he showed the editors when they visited Silverhawk Gen­erating Station was the framed Sil­verhawk Constitution that faces the receptionist’s desk; it was signed by all employees on the founding staff (sidebar). You don’t get into the plant without reading it—at least when Price is present.

“People are critical to our suc­cess,” he said proudly. “They devel­oped the constitution to define the thinking, the attitude, the mindset important to making this the best plant possible. We show it to all pro­spective employees and suggest that if they would have difficulty embrac­ing this philosophy somewhere else would be a better place for them to work.”

Background. Silverhawk was under construction at the same time as Duke Energy North America’s Moapa facility (now Lenzie), located a mile or two to the East. When the gas-turbine bubble burst, Duke couldn’t get out of town fast enough. By contrast, the Silverhawk own­ers—GenWest LLC, an unregulated generation unit of Pinnacle West Capital Corp, which owns Arizona Public Service Co, and Southern Nevada Water Authority (SNWA)—didn’t flinch, at least not publicly.

The project continued moving forward because there was commit­ment from the owners, EPC con­tractor CH2M Hill, equipment sup­pliers, labor, and plant employees. The 520-MW 2 × 1 combined cycle, powered by Westinghouse (now Sie­mens Energy Inc, Orlando) 501FD2 gas turbines (GTs), began commer­cial operation in May 2004—one month ahead of schedule. Construc­tion personnel logged more than 1.1 million man-hours with no lost-time accidents.

CH2M Hill Project Manager Jim Williams remembered Silver­hawk as one of the best experiences in his professional career. “It was a team effort,” he said. “We worked directly with the owners on all facets of the project. Together we fine-tuned the design to achieve the optimum schedule, cost, operability, and main­tainability. The owners ordered the gas and steam turbines, air-cooled condenser (ACC), and heat-recov­ery steam generators (HRSGs); we bought the remainder of the equip­ment.

“Our contract had fixed and vari­able components,” Williams contin­ued. “For example, engineering, pro­curement, commissioning, and a few other things were ‘fixed’; subcontrac­tors and labor, mostly variable. We and the owners arrived at a ‘target’ price for the variable components. If the job came in high, it cut into our fee; if we completed work below esti­mate, the saving was shared with the owners.

“Our companies jointly executed the project and the result was a true ‘win-win’ conclusion: under budget and a good operating plant. Many of our people and GenWest’s people had come from a previous project we completed—West Phoenix 5—and we built off that experience.

“To streamline construction, we ‘bathtubbed’ the site and put all the electrical cable and piping we could underground. This way we wouldn’t be insulating pipe while the electri­cians were pulling cable.

“One of the reasons Silverhawk got off to the great start that it did, Wil­liams added, was Plant Manager Bill Simko (now NV Energy’s director of generation engineering). We upload­ed on GenWest’s documentation sys­tem the final as-built drawings and documentation, and provided some additional operator training materi­als. Bill and his management team used their skills and that information to develop an excellent O&M staff. I recall that the plant availability for the first three years was 98.5%.

“Bill was proactive and always willing to share lessons learned. After the first year of operation, he invited us back to Silverhawk. We brought six design and construction personnel to a meeting that proved a valuable learning experience.”

Others thought highly of the proj­ect as well. It was selected by Power Engineering magazine as a “Project of the Year” in December 2004.

Nevada Power Co bought Gen­West’s 75% ownership share in Sil­verhawk in January 2006; SNWA retained its 25% share to assure a reliable source of power for the authority’s water treatment facilities and pumping stations.

With SNWA as an owner, Silver­hawk is particularly parsimonious regarding water consumption. The authority said this air-cooled facil­ity uses 90% less water than a typi­cal powerplant with a water-cooled condenser. Furthermore, the plant’s brine concentrator recycles about 75% of the water withdrawn from onsite wells.

Price and Operations Manager Forrest Hawman told the editors that evaporative and blowdown losses from the GT inlet-air evap coolers, and from the wet cooling tower used to back up the fin-fan cooler in the closed cooling-water circuit, is 200 gpm on a peak day. Wells, rated at 600 gpm, and a 2-million-gal storage tank, assure sufficient cooling water to handle any process upset imagin­able.

Industry experience indicates that brine concentrators (BCs) work best when they are supplied wastewa­ter at a relatively consistent chemis­try and flow rate. Designers acknowl­edged this by bringing all blowdown, drains, etc, to a small surge pond (about 2 million gal). Take the “plant tour” in the sidebar to see the plant arrangement and major equipment.

The surge pond allows time for the water to achieve a degree of equi­librium—particularly with respect to total dissolved solids—before it is withdrawn at about 50 gpm and sent to the brine concentrator. The plant DCS controls BC operation at near steady-state conditions with essen­tially no operator assistance—except for testing water once a shift—un­til the surge pond runs dry. Then the brine concentrator is put on hot standby.

BC distillate quality is excellent—a conductivity of 1 microSiemens/cm or less. Typically, BC distillate is mixed 50/50 with well water and used for the GT evap coolers. It also can be polished and used as boiler makeup. When the BC is out of service a single-pass RO trailer processes well water to about 30 microSiemens/cm; that permeate is polished in demineral­izers. Of course, using makeup water of such quality increases the cost of demin water significantly.

The waste stream from the BC is 2 gpm of a 35% slurry. It is routed to one of two evap ponds located along side the surge pond. Each evap pond is designed to hold all the solids col­lected over a 15-year period. Plus, one pond can retain all the wastewa­ter produced by the plant for about a week if the BC is out of service. This is a ZLD facility.

The BC, supplied by Aquatech International Corp, Canonsburg, Pa, is of the mechanical-vapor-com­pressor-driven falling-film seeded-slurry type. Seeded-slurry evapo­ration technology overcomes the limitation imposed on conventional evaporators by the saturation limits of low-solubility scaling compounds present in Silverhawk well water.

The seeded-slurry process estab­lishes and maintains a slurry of calci­um-sulfate seed crystals in the evap­orator recirculating brine solution. Careful design and process control allow the calcium sulfate and silicon dioxide to co-precipitate preferentially on the recirculating seed crystals and much more slowly on the BC’s heat-transfer surface.

Design of the Silverhawk evapo­rator precludes the need for chemi­cals to maintain the seed population inside the brine concentrator. How­ever, seeding is needed for initial startup or for restarts after pro­longed shutdowns.

End notes. Maintenance of Sil­verhawk’s GTs is managed under a long-term service agreement with the OEM. Both units, now approach­ing 35,000 hours of operation, had hot-gas-path inspections at 24k.

All plant equipment has per­formed reasonably well given the high availability. One annoyance with the HRSGs has been floor panel cracking near the GT inlet that allows insulation to be sucked out of the GT-to-HRSG transition duct and blind catalyst. ccj

 

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