In the abstract, most everyone knows California is a tough place for power generators to do business. Up close, it’s even tougher, as a visit to NRG Energy’s 560-MW El Segundo Energy Center (Fig 1) revealed. What was also evident is how the business of generating power continues to change, with implications for how combined cycles are designed to respond to grid requirements.
In a nutshell, the value of operating flexibility continues to grow. In California, however, flexibility has to be juxtaposed to the equally important objective of avoiding every ppm of air emissions and every droplet of discharge possible.
Opportunities in the market and restrictions in emissions led NRG to a unique plant design, the Siemens’ Flex Plant 10 in a two-unit 1 × 1 configuration. The cycle design (sidebar) received broad exposure in 2010 when it was selected by NRG, and again in 2013 when the plant was commissioned (August 1 of that year). Less has been reported on operating experience.
Ken Riesz, general manager, noted that the plant meets its flexibility performance requirements “with no problem.” El Segundo is obligated to deliver 150 MW from each unit to the California ISO within 10 minutes, to meet (and get paid for) the requirement for “non-spin reserve.”
Each gas turbine, rated at 192 MW (without steam power augmentation), takes five minutes to synchronize with the grid and another five minutes to get to 150 MW. “We only have about 90 seconds of margin within this start window,” Riesz said. Start reliability for both units combined is 99%.
The plant has met this obligation about 10 times since commissioning. Twice since commissioning, the plant has started up [IT] twice [RM] in one day. Most of the time, however, the plant operates in the CAISO day-ahead market, the exception being summer months when the units have functioned more as baseload capacity. Siemens originally described its Flex series of combined cycles as something between peak- and intermediate-load plants. At El Segundo, it appears to be living up to that billing.
Riesz reports that it takes a total of 45 minutes to achieve full steam-cycle output from a warm start condition, the plant’s normal protocol.
Keep in mind that the plant also has only one hour after the start signal to be in compliance with its 2-ppm stack NOx limit (10 ppm in the GT exhaust). The facility also is regulated by an aggregate annual limit for pounds of NOx discharged.
In the scheme of contemporary combined-cycle designs, the Flex 10’s 49% cycle efficiency (8900 Btu/kWh heat rate) is not that high. But El Segundo wasn’t designed for baseload-like efficiency, but instead for robust cycling and dispatch while meeting the state’s onerous emissions limits. For example, the stack temperature is higher than for the latest combined cycles, which makes for a less efficient cycle, but this lessens back-end corrosion and minimizes issues with the selective catalytic reduction (SCR) unit.
Steam-cycle lessons
Much of the 1% start unreliability is attributed to steam-cycle challenges. “It’s easy to make a mistake on startup with a small single-pressure drum,” Riesz said, “and then hot water flashes to steam, and you trip the steam turbine and the gas turbine.” He cautioned that you have to be careful controlling drum level (Fig 2) and even more careful during a hot restart.
Steve Petenbrink, maintenance manager, noted one maintenance issue the plant had to solve was failing baffle plates in the area where the GT exhaust rises to meet the first panel of HRSG tubes. “We conduct annual weld repairs of baffles which maintain the mixing and reduce bypass flow past the tubes.”
Water-chemistry control also is different for the El Segundo boilers compared to most HRSGs. Example: Conductivity is higher. Plant staff installed “little degas scrubbers” to remove CO2 and attain a conductivity level that would meet spec. On the other hand, this boiler does not require a condensate polishing unit.
Raw water source for the plant is the West Basin Municipal Water District, which provides grey water via a four-mile pipeline.
In this boiler, supplied by NEM, a Siemens company, far more water resides in the tubes than in the drum—but the drum has a much thinner wall than a conventional HRSG so it can heat quickly. Thermal growth is rapid initially, but then stops, consistent with the need for fast steam-cycle starts and full steam-cycle output (70 MW) in 45 minutes. The HRSG design makes one ask whether differential thermal expansion among the bottles, drum, tubes, risers, and downcomers would be an issue, but Riesz and Petenbrink have observed no evidence of this.
Another area to watch with this HRSG design is the steam bypass system. The plant has a condensate receiver tank but no conventional hot well separate from the deaerator. Because of the positive-pressure steam-turbine exhaust (it’s a backpressure unit, not condensing), the condensate temperature is higher than one might expect and, if not managed properly, can trip the condensate pump.
“We have to maintain suitable pressure in the deaerator to get proper pressure to the condensate/feedwater pump,” Petenbrink said, “and this can be more of an issue on hot restart because the pressure tends to die off.” Riesz added “we just needed the right procedures.” No issues were reported with the attemperators on the bypass and main steam lines. The plant features a large attemperator for use during startup and a smaller unit to control peak steam temperatures.
Another interesting operating nuance is sequencing the four steam turbine inlet valves (Fig 3) regulating flow into the steam chest. Valves inside the steam chest regulate flow to different areas of the turbine. Steam enters at the middle of the unit and is split between the high-pressure turbine and the intermediate/low-pressure turbines, both running on one shaft with the generator.
During plant startup, water level in the HRSG drum is maintained relatively low to accommodate the pending swell that occurs as the water is heated. As pressure builds in the drum, the steam bypass valve opens to maintain pressure by allowing steam to bypass the turbine and flow directly to the air-cooled heat exchanger (ACHE) while the steam turbine is warmed up.
Once the ACHE is properly vented and conditions have reached saturation, ACHE fans are started to control pressure in the exhaust steam duct. Condensate is collected in the condensate return tank, where it is stored until being pumped back to the HRSG via the condensate and/or boiler-feed pumps. As the steam turbine comes up to temperature, the bypass valve is closed and the unit is slowly brought to its valves-wide-open load point.
Flex 10 comprises a 501FD3, unique HRSG design, workhorse steam turbine
Except for the gas turbine, you’re not likely to see El Segundo’s principal components at another US combined-cycle plant. The heat-recovery steam generator (HRSG) is a hybrid of once-through and drum boiler designs, the steam turbine is a design largely used for robust mechanical-drive requirements at process facilities, and the backpressure turbine and unique site required an air-cooled heat exchanger (ACHE) rather than an air-cooled or surface condenser.
In other words, it’s not only the breathtaking seaside setting (and beach just over the retaining wall), the shoehorned plot between a major coastal highway and the ocean, or the quaint beach front town just south of the plant which make El Segundo an atypical generating station.
The SGT6-5000F(3) GT, an upgraded version of the Westinghouse 501F introduced in 1993 (at that time with 150-MW nominal output), features a 13-stage compressor and four-stage turbine, 16 annular combustor cans, single-digit NOx emissions, and 192-MW output at El Segundo without power augmentation, 210 MW output with it.
Features important to the flex operation and reduced emissions include the following:
- Dry low-NOx combustor (DLN) with four stages, each of which comes online independently, with continuous fuel injection through the pilot burners and swirler fuel injection.
- Static frequency converter instead of a mechanical starter motor.
- Turning gear speed of from 3 to 120 rpm, which allows the generator rotor wedges to lock up faster during starts and the unit to cool down faster during shutdowns.
- Supplemental cooling air bypassed around the combustor to reduce CO emissions (less than 10 ppm down to 40% load).
- Turbine outlet temperature control based on compressor inlet temperature, rather than combustor shell pressure.
Thus far, experience at El Segundo confirms that the unit meets expectations for longer intervals between hot-gas-path inspections than the originally planned 12,500 equivalent operating hours/900 equivalent starts. El Segundo is permitted for a maximum of 400 starts annually, 200 for each GT. Both units together have seen 10,000 hours of operating duty and over 500 starts.
The SST-800 steam turbine (figure) is actually one of a family of turbines which can be built up from modules for outputs from less than 50 MW to 250 MW (with dual-casing design). It is described by the OEM as a single-pressure, non-reheat unit with single casing, center steam admission, and reverse-steam-flow inner casing. A combination of impulse and reaction blading is used; all blades are fixed in blade carriers (that is, they are not free-standing). Nominal steam inlet conditions are 1450 psig/940F; steam flow is around 620,000 lb/hr.
Blades in the inner casing are structured to enable steam to flow in the opposite direction towards the middle zone. The middle zone is configured for direct flow towards the exhaust (as in El Segundo’s case) or up to seven controlled or uncontrolled extractions, which may be common in mechanical-drive service.
The turbine’s base is fixed at the exhaust end. At the high-pressure end, the rotor is supported by independently accessible bearings. This alignment allows a wide degree of thermal and mechanical flexibility. At El Segundo, the four steam inlet valves can be throttled for additional flexibility.
HRSG. The single-pressure, non-reheat HRSGs, branded DrumPlus™ by OEM NEM, were first described in CCJ (1Q/2014) as part of its coverage of a Combined Cycle Users Group meeting. Summary: A conventional steam drum is replaced by a knockout vessel, external separator bottles, and a much smaller drum. The best way to think about the design is that the drum provides primary water/steam separation while the bottles accomplish secondary water/steam separation.
Because it carries more water than a conventional unit, this NEM boiler is said to provide added thermal stability while the system undergoes temperature transients during starts, stops, and ramps. On the other hand, the holdup time of water in the drum is less than normal. This means that the system is more sensitive to, say, a trip of the feedwater pump, and, in general, less forgiving during startup and transient conditions. The large attemperator for startup is rated at 25% of the steam flow. Full steam bypass is allowed by design.
El Segundo employs an ACHE rather than a condenser because the “footprint” and profile of the ACHE could be made smaller. Steam at a few pounds pressure takes up much less volume than steam under vacuum. The ACHE is quieter, too.
Water everywhere but. . .
A quick tour of the facility with Petenbrink revealed other O&M nuances. Standing underneath the air-cooled heat exchanger, he said, “by permit, not a drop of wash water is allowed to spill to the ground; when we wash these fan blades (Fig 4) to remove salt buildup, we have to collect every drop in plastic containers, treat it, and recycle it.” Apparently, the same stuff that’s in the air and must fall to the ground when it rains isn’t allowed to touch the grounds of a power station. This is what zero discharge means at this location.
The oceanside location also makes for interesting tradeoffs in gas-turbine air filters—with financial consequences. The plant has fine HEPA-grade particulate filters preceded by coarse filters. The former cost five times as much to replace. “We’ve learned to financially optimize how and when we replace each type,” Petenbrink said.
The plant did observe salt deposits on compressor blades the first time one of the GTs was opened. To avoid problems with deposition, the plant conducts on-line water washes every other day and an off-line wash with soap and water monthly. The coarse pre-filters are generally replaced semi-annually (online), the fine filters (replaced offline) annually.
Finally, Petenbrink pointed to the protective cover installed over the boiler-feed-pump motor to prevent moisture and salt intrusion (Fig 5) and he indicated where platforms and maintenance doors were added or modified to facilitate maintenance access (Fig 6).
Conduct of operations
Riesz has high praise for NRG’s formal operations program, called “Conduct of Operations.” He thinks it’s “the best ops model out there.” Since inception of the program, brought to the company by Fran Sullivan, NRG’s senior VP of operations, NRG’s aggregate plant reliability is up, human error by operators has been reduced, and pride of ownership is evident.
Complementing the ops model at El Segundo is a full simulator for training, which uses the current control configuration in the T3000 automation software. “The simulator wasn’t cheap,” Riesz said, “but it allows us to test control modifications and new strategies ‘virtually’ before we apply them to the physical units.”
He also lauds the safety culture at El Segundo. The plant (recall that oil/gas-fired thermal units operated at the site before the combined cycles replaced them) has had no lost-time reportable accidents in 10 years, and that includes the construction of the combined cycles and the dismantling/retirement of the old units.
Every operator is empowered to shut down equipment over safety concerns. Safety comes first. “For that matter, environmental compliance comes before ops, too,” Riesz added.
To the extent possible, the plant staff tries to find problems before they occur. Conventional predictive maintenance techniques are used, like vibration and oil analysis. The plant is currently experimenting with real-time data analytics. They “trend lots of stuff in PI.”
Internally driven excellence is even more paramount at El Segundo. “We’re the only site which has deployed the Flex 10, so there aren’t many lessons learned we can bring from the outside.” Plus, because the plant isn’t a baseload-type facility, there’s only one boiler-feed pump and one condensate pump for each combined cycle (Fig 7).
The plant keeps full spares on site for swap-outs. “We plan our outage time well, nighttime and weekends, to minimize any impact on availability.” During longer planned outages, the goal is to restore the equipment to the most reliable state possible.
To support the 47-person staff, El Segundo has full LTSAs with Siemens—one for the T3000 control system, and one for the gas turbines, which are monitored 24/7/365 by the OEM. Plant staff describes the T3000 as “a lot easier to use than the Bailey Infi 90 serving the old plant,” but that it is graphics intensive, and they struggled with the “not-so-user-friendly” alarm configuration out of the box.
On tap. . .
At some point, the plant has to relocate the control room. Originally, it was easiest to put it in the space of the existing control room serving two of the three original units which have yet to be dismantled. “We have lots of decisions to make regarding replacing servers, what kind of architecture we want, and so on,” Petenbrink said.
Riesz noted the plant is also considering modifications to significantly turn down the gas turbines below 150 MW without violating the site’s CO emissions limits. “It’s a Siemens proprietary approach to air/fuel mixture control in the combustors,” he added. CCJ
