Gas turbines

New and improved 7FA - the fifth generation

1 March 2010

The 7FA.05 gas turbine – an upgraded, “5th generation”, version of GE’s 7FA gas turbine – aims to offer bigger output, higher efficiency, greater flexibility and better availability. Shipping of the first 7FA.05 units is expected in 2012.

In support of having the new, Mk 5, version of its 7FA gas turbine – the 7FA.05 – ready for shipping in 2012, GE reports that most component level testing is now complete, with full completion targeted for mid 2010. Detailed engineering of the test stand is complete and its construction is well advanced.

Among the first of the new machines in the field are likely to be those at the planned Oakley generating station in California, due to enter service in 2014.

The basic concept of the upgraded, “Fifth generation”, FA is to provide increased output and efficiency over its predecessor “while maintaining the reliability, availability and the operational flexibility power generators need to achieve greater revenue in cyclic and peaking operation,” says GE, including the high availability needed to capture as much revenue as possible from offering ancillary services.

More MW and faster start

At ISO conditions, the gas turbine simple cycle output has been increased to about 211 MW, an increase of some 36 MW over today’s 7FA, while maintaining essentially the same footprint as the previous machine. In a 207FA combined cycle configuration the output will be about 627 MW, also at ISO conditions, an increase of 98 MW over the existing 7FA in the same combined cycle configuration. The ISO efficiency has been increased to about 38.5% for simple cycle application and about 57.5% in a 207FA combined cycle, assuming ISO conditions and using a cooling tower.

Operability features include: fast start (about 75% power in 10 minutes); fast ramp rates (+40 MW/min); additional turndown capability (+21 MW); introduction of variable stator vanes; and additional capability for fast start (+23 MW).

The new machine will be able to benefit from “GT 10 Minute Start” and “CC Rapid Response”, two existing GE technology offerings.

GT 10 Minute Start combines faster speed ramps and load ramps with an updated purge approach that accomplishes the purge credit during the shutdown process and maintains the purge until the next start-up. The combination of these features can result in a total start-up time approximately 1/3 less than a conventional start-up. CC Rapid Response is a system for reducing start-up times in combined cycle mode. The basic approach is to thermodynamically decouple the gas turbine from the bottoming cycle, thereby allowing the gas turbine to start without the hold times needed to allow the HRSG and steam turbine to heat up. “Both Fast Start and Rapid Response significantly reduce start-up times, start-up emissions, and start-up fuel costs,” says GE.

In the new FA enhanced maintainability will be achieved through: field replaceable blades; improved inspectability; and prognostic anomaly detection.

In terms of emissions the new machine will maintain single digit NOx and CO (see table of main data), achieve improved emissions density (lbm/MBTU), and provide significant NOx and CO reduction per start (90% in combined cycle mode and 47% in simple cycle).

The new machine employs a number of technologies that have been demonstrated in other GE gas turbine platforms and “leveraged for this 7FA application.”

The key technology advancement that enables the improved output is in the compressor, which is a scaled derivative of other proven GE heavy-duty turbine compressors.

The improved efficiency is the result of hot gas path enhancements that have been already demonstrated as a field uprate for 7FA gas turbines in service. The new 7FA gas turbine will continue to use the DLN 2.6 combustor, which is well proven, says the company, with more than

15 million hours of operation and 405 000 starts in the GE fleet.

How does the new FA compare with the FB, introduced a few years ago as an advanced version of the FA, with higher firing temperature, and some technologies drawn from the H? “The current version of the 7FA (4th generation) already uses some of the best technology of the 7FB,” says GE, “and is comparable in terms of output and efficiency operating on natural gas. The new 7FA (5th generation) will have additional output capability due to the compressor. The 7FB is intended to be GE’s primary IGCC turbine, and plans are that it will be sold as the 7F-Syngas turbine for 60 Hz applications.”

The new 7FA will incorporate advanced turbine materials already used in the FB, but in less severe conditions (lower firing temperature) providing greater durability.

Key areas targeted for the new 7FA include the key 60 Hz regions, Latin America, Saudi Arabia, Japan, Taiwan, South Korea, and of course North America, where, as already noted, the first machines are likely to be deployed.

“Power generation in North America is primarily cycling duty,” notes GE, “and this pattern is expected to continue.” The company says “the 7FA has been shown to offer exceptional value in combined cycle duty, and the new the 7FA is expected to continue that trend. In addition, the flexibility of the new 7FA will bring significant value to customers operating power plants with a variety of operating profiles in all 60 Hz regions around the world.”

GE has not yet announced any specific plans for an equivalent new version of the FA for 50 Hz applications (9FA) but says it “continually develops improvements to its products and services and evaluates customer needs in the segments we serve.”

Improved compressor

As already mentioned, it is in the compressor

of the fifth generation FA where the key advancements have been made.

The compressor of the new FA “leverages aircraft engine practices and the experience of other GE heavy-duty gas turbines,” says the company.

It contains 14 stages and has a row of inlet guide vanes at the entrance and three stages of variable stator vanes on the first three stages. These variable geometry vanes increase operational flexibility.

The compressor also incorporates several features aimed at improving maintainability, including field replaceable moving and stationary blades, enhanced borescope coverage and a blade health monitoring system.

The compressor will undergo “a rigorous process in accordance with GE’s product introduction practices”, says the company.

The engineering work will be performed primarily at GE’s Gas Turbine Design Center of Excellence in Greenville, South Carolina. Component and subsystem testing will be performed in parallel with these efforts. The complete compressor will initially be validated in a series of tests in a compressor validation rig, followed by full-speed full-load tests of the complete 7FA gas turbine. The compressor validation rig tests and full-speed full-load tests will be conducted in a new facility being constructed at GE’s Greenville facility and “will be validated before the first unit ships”, says GE.

As reported in Modern Power Systems, March 2009, p 22, cracking of the R0 compressor blade has been a particular issue for existing FA machines. But GE maintains that, according to ORAP third party availability data, the “7FA compressor is a leader in reliability.”

Nevertheless, “based on the extensive fleet experience of the 7FA gas turbine, GE has implemented enhancements to the compressor that should further enhance its reliability,” the company says. GE reports that these enhancements were validated in an instrumented rotor test at a customer’s facility on the US gulf coast and are packaged to fit into the schedule of any upcoming planned outages. The full suite of enhancements has been incorporated into 7FA gas turbine units shipping from the factory beginning in April 2009, GE says, and “the compressor in the next generation 7FA gas turbine shipping in 2012 has incorporated the lessons learned from the reliability enhancement effort.”

Among the compressor blade enhancements are understood to be different materials, improved aerodynamical modelling (drawn from GE’s aviation experience), and polished/ mirrored surface rather than coating.

Testing, testing

Prior to delivering the first unit to a customer, the next 7FA gas turbine will undergo a series of component level, subsystem level, and system level tests, culminating in a full-speed full-load test in the new state-of-the-art test facility at Greenville. This testing will enable GE to fully characterise the 7FA gas turbine beyond real-life operating conditions in a laboratory setting. No specific field testing programme is expected other than the testing normally done during plant commissioning. The lead units will have a remote monitoring and diagnostics programme and borescope inspections performed during planned shutdowns based on the customer’s operating profile.

The primary manufacturing and final assembly facility is also in Greenville.

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