By Siân Green

Introducing advanced gas turbine technology to the market is a challenging task. PEi looks at the track record of the W501G – Siemens Westinghouse’s latest offering – developed under the USA’s Advanced Turbine Systems programme.

In April 2001, the first power plant based on Siemens Westinghouse’s advanced W501G combustion turbine entered commercial operation at Lakeland Electric’s McIntosh Unit 5 in Florida. There are now around W501G units in commercial operation, and another ten are scheduled to start up in the next 12 months.

In introducing the 501G to the commercial power market, Siemens Westinghouse has succeeded where others have encountered trouble. Developing advanced gas turbine technology is a complex task in itself, but many manufacturers find that their biggest challenge comes in the first few months of commercial operation, and in persuading the market that their new product will be reliable and will meet all performance expectations.

There are now seven W501G units in commercial operation in power plants in the USA.
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The W501G was developed under the US Department of Energy’s Advanced Turbine Systems (ATS) programme. Running for a period of some ten years, the programme’s aims were to develop technology that would bring a decrease in the cost of electricity, achieve fuel flexibility, achieve NOx emissions of less than 9 ppm, and boost system efficiency to 60 per cent for utility-scale combined cycle systems.

The 60 per cent efficiency level has long been the ‘four minute mile’ barrier for combined cycle gas turbine systems. Since the technology steps to achieve these targets require verification, Siemens Westinghouse chose to introduce part of the technologies into the intial W501Gs with the capability to mature the product to the 60 per cent efficiency. This strategy has allowed Siemens Westinghouse to achieve commerical experience with new technologies such as steam cooling.

GE Power System’s H System, also developed under the ATS programme, is slated as being the first combined cycle system able to achieve 60 per cent efficiency levels and the market is eagerly anticipating the start-up of the first H System plant later this year in Baglan Bay, Wales.

The W501G reaches a net combined cycle efficiency of 58 per cent, and the 60 per cent barrier is something that Siemens Westinghouse plans to attain through the evolution of Siemens and Westinghouse gas turbine technologies, according to Prof. Klaus Riedle, president of Siemens Power Generation’s gas turbine division.

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“With the initial version of the W501G, we expect to demonstrate efficiencies of 58 per cent,” said Riedle. “We expect to get up to 59 per cent, and 60 per cent is [a goal] for the next stage [of development].”

Advanced features

The W501G is the world’s largest and most efficient 60 Hz power unit in commercial operation. It is a 3600 r/min, 2600°F (1427°C) class turbine tailored for combined cycle operation. It consists of a 16-stage high efficiency axial compressor, a combustion chamber comprising 16 dry low emission combustors, and a four stage reaction type turbine.

One of the machine’s advanced design features is the integration of the gas turbine’s cooling systems and the plant’s steam cycle. This has allowed thermal efficiency to be increased over other machines in the W501 family, on which much of the W501G design is based.

The W501G rotor design was taken from the 501 series and consists of a single rotor supported by two tilting pad bearings. The rotor has a bolted design and is made up of the compressor and turbine discs, bolted together with through bolts.

Compressor: The compressor section is based on the compressor design of the 501 series and incorporates technology that was developed and validated under the ATS programme. The disc and rotor construction was scaled up from the 501F engine, and is designed to achieve the flow and efficiency required to achieve the necessary power and heat rates. The compressor has a pressure ratio of 19:1.

Combustor: The combustor consists of 16 individual can-annular combustors, and is equipped with Siemens Westinghouse’s dry low NOx (DLN) system, designed for the increased firing temperatures.

The 501G was tested for 2000 hours and underwent 150 start-stop cycles at the Lakeland Electric site
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Multiple fuel stages provide flexibility for lowering emissions, combustor dynamics, and metal temperatures. The initail W501G was designed to achieve 25 ppm NOx. Further potential NOx reduction improvements are being pursued by Siemens Westinghouse.

Each of the 16 burners has a transition, which ducts the hot combustion gases to a corresponding segment of the turbine. The transitions are capable of operating at 2600°F (1427°C) firing temperatures while maintaining the use of conventional materials as they are closed-loop steam cooled.

High temperature thermal barrier coatings (TBC) are used to protect the transitions and improve service life. The steam-cooled transitions allow the gas turbine to operate at high rotor inlet temperatures while maintaining essentially the same burner outlet temperature as the W501F gas turbine by eliminating the dilution cooling of the transitions.

Cooling the transitions using a closed-loop steam circuit is advantageous for the gas turbine and the combined cycle. The gas turbine benefits because less air is removed from the compressor, and can be used to produce power as well as additional premix to reduce NOx. The combined cycle benefits by transferring the heat into the hot reheat steam system.

Turbine section: The W501G turbine is based on the four-stage design of the W501F, although the W501G has fewer parts. The blading was designed using advanced three-dimensional flow analysis, resulting in sophisticated airfoil designs that attain high aerodynamic efficiencies. The rotor cooling system maintains the NiCrMoV turbine discs at a temperature sufficient to keep the disc below the creep range of the material.

System integration

In combined cycle operation, the W501G uses a highly integrated cycle designed to maximize plant output and efficiency while maintaining operational flexibility and satisfying cooling requirements.

The W501G’s steam cooled combustor transitions require a reliable source of steam supplied from the intermediate pressure system and returned to the steam turbine as hot reheat. The balance of the intermediate pressure steam developed in the heat recovery steam generator (HRSG) is not used for gas turbine cooling. This steam is mixed with the cold reheat steam prior to entering the reheater section of the HRSG. The cycle has normal make-up water quality requirements and can quickly produce acceptable gas turbine steam purity levels during start-up; even when starting with relatively impure water for initial fill.

Interstage compressor bleeds are used for cooling stationary turbine components as well as assisting the gas turbine’s startup. As with other W501 frames, the W501G cools the extracted air with a heat exchanger in the cycle then filters the air prior to returning it to the gas turbine. The energy captured from the cooling air is transferred into the cycle to generate additional steam turbine power.

Operating experience

The prototype W501G machine at Lakeland Electric’s McIntosh Unit 5 power plant in Florida started operating in simple cycle mode in 1999. The machine underwent about a year and a half of testing and validation before entering commercial operation and conversion to combined cycle.

As a simple cycle plant, the W501G installation at Lakeland included a once-through steam generator to produce steam. The plant is now a one-on-one combined cycle plant and has accumulated around 2000 fired hours.

Testing of the W501G at the McIntosh plant was carried out under contract with Lakeland Electric. Over this period, the machine was fired for 2000 hours and underwent around 150 start-stop cycles. Testing included operating the unit on heated fuel gas, fuel oil, with steam power augmentation and in a cyclic, load following mode. The unit was assessed on its overall performance, efficiency, and environmental performance, while components exposed to dynamic loads were examined to validate life expectancy and determine fatigue.

“Out of these tests, and also some tests on later units, certain simplifications were implemented,” says Riedle. These, he says, included the elimination of the combustor bypass system and the simplification of the steam-cooled transitions. “This makes the machines more reliable,” adds Riedle.

“All W501Fs and W501Gs with a Dry Low NOx system used a so-called combustor bypass system which was used to change the air-fuel ratio during start up. Stability enhancements to the combustion process have resulted in the elimination of the bypass system in the 501F as well as the 501G.”

The W501G has steam cooled transitions, which take hot gases from the combustor to the turbines, and to bring the steam to this area is very challenging because conditions go from ambient temperature to close to 1000°C within less than a minute. Siemens has therefore simplified the design by reducing the number of steam feeder lines.

Market influence

According to Riedle, other modifications have been made to the W501G to take into account changes in market conditions in North America. The W501G was originally designed to be the largest, most efficient baseload engine on the market. However, deregulation across some parts of the US – the largest 60 Hz market in the world – means that new demands are being placed on power plants. In particular, engines are now exposed to daily start-stop cycles .

“So we looked at what we could do to simplify daily starts and stops, and we modified the design specifically to address that issue,” comments Riedle. “We opened up some clearances so that quick shut down and immediate re-starting can be done without rubbing of compressor or turbine blades.”

Verification testing at the Lakeland site, and at the PG&E National Energy Group-owned Millennium power plant – the second site to enter commercial operation with a 501G – is now complete.

Future moves

Siemens is already planning and working on the future development of the W501G and its gas turbine family. Initially, the company will look to increase the output of the W501G by increasing mass flow and improving aerodynamics, and also improving heat rate by optimizing the cooling system.

Siemens is also working on further developing its high efficiency gas turbines, which will include the combined technologies of the Siemens and the Westinghouse gas turbine product designs. This new family, says Riedle, will be configured for both 50 and 60 Hz markets, and will achieve the 60 per cent combined cycle efficiency levels.