Timothy Huskey
GE Power Systems, USA

The 9FB is the latest addition to GE’s F technology gas turbine family. It builds on the design of the FA machine, and incorporates some of the latest developments in turbine technology and materials.

Many gas turbine applications today require the gas turbine to run almost continuously. With this increase in operating hours, the cost of fuel has assumed greater significance in optimizing machine design. As operating (or fuel) cost has become more important, technology development has been focused on improving efficiency, primarily through increased firing temperatures. But higher operating temperatures can drive design engineers to use more expensive parts that may affect operating and maintenance practices.

In this environment – with gas turbines in widespread use as base load machines in combined cycle configurations for power generation and cogeneration applications – optimizing gas turbine design requires balancing multiple objectives of low first cost, fuel cost, and operation and maintenance costs over the life of the machine.

Challenges

To address these challenges, GE Power Systems continues to evolve its FB gas turbine product line. The latest example is the Frame 9FB, introduced at Power-Gen Europe 2002 as the world’s most advanced air-cooled, 50 Hz gas turbine. This machine will be available for shipment starting in early 2004. The 60 Hz member of the FB family, the Frame 7FB, was introduced in 1999 and the first unit has been installed at a Reliant Energy project in Hunterstown, Pa., USA


Figure 1. A Frame 9FA rotor is lowered onto the half-shell. The 9FB is the latest addtion to GE’s F technology fleet of gas turbines
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The Frame 9FB, configured with GE’s new High Efficiency Advanced Technology (HEAT) steam turbine in gas-fired, combined cycle operation, is designed to produce more than 412 MW and achieve a nominal, net plant efficiency of 58 per cent. By comparison, GE’s 50 Hz Frame 9FA gas turbine has a combined cycle output of 390.8 MW with net plant efficiency of 56.7 per cent.

The 9FB is the latest evolutionary step for GE’s F technology and is based on continuing advancements in turbine technology and materials. GE’s installed fleet of F technology gas turbines has compiled more than six million fired hours, and the company expects this advanced technology operating experience will translate into high performance and reliability for the new 9FB machine.

Higher temperatures

From its Aircraft Engines business, GE had the enabling material and cooling technology to go to high firing temperatures with the early F machines. The original Frame 7F machine, introduced in 1989, fired at 2300°F (1260°C); today’s Frame 9FB is a 2500+°F (1370°C) class machine.

The firing temperature of the F and FA machines was limited by the desire to avoid cooling the third (last) stage turbine blades, and the desire to keep the gas turbine exhaust temperature in the realm of conventional steam turbine admission temperature. Previous uprates also have been accompanied by airfoil materials changes or pressure ratio increases. Firing temperatures in the F series have consistently increased since the technology was introduced. For the FB machines, the firing temperature has increased more than 100°F over the FA gas turbines.

By leveraging the current FA compressor’s generous stall margin, it is possible to significantly increase firing temperature while limiting exhaust temperature increase. Further increases in firing temperature are possible by applying bucket alloy developments. The increase in firing temperature is significant. It drives output up, fundamentally changing combined cycle output and efficiency potential. The increase in pressure ratio changes the flow vectors in the turbine, which necessitates aerodynamic redesign of nozzles and buckets (vanes and blades).

Compressor design

Incremental improvements in the F series compressor have been incorporated throughout the evolution of F technology. In 1998, an FA compressor test was performed to revalidate the compressor’s capability. The objectives of the test were firstly, to thoroughly map the FA compressor’s aerodynamic and aeromechanical behaviour, and secondly to characterize the thermal behaviour of a high-radius rabbet (HRR) compressor rotor structure.


Figure 2. GE’s installed fleet of F technology machines has accumulated a total of over six million fired hours world-wide
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One key result of the test was the establishment of compressor surge and stall characteristics, which demonstrated a compressor operating limit that would allow significant pressure ratio growth. Another major result was the empirical determination of rotor thermal transients, which was used to validate analytical predictions for the HRR rotor structure.

The FA compressor test helped pave the way for the evolution to FB technology. Retaining the basic design used for the FA machines, the compressor for the FB gas turbines is an 18-stage, axial flow unit with variable inlet guide vanes to maintain high part-load efficiency and low emissions over a wide operating range. Variable vanes also improve low-speed surge characteristics, make startups easier, and provide better part-load performance in combined cycle applications. Closing the vanes keeps exhaust temperature up at reduced loads, thus retaining steam-raising capabilities if the gas turbine is running at less than 100 per cent load.

Throughout the compressor blade path, all airfoil material is high chromium-stainless steel. While the FA units operate at compressor pressure ratios of 15.5 to 1, the FB machines run at 18.5 to 1. The compressor rotor tie bolts material is changed to IN718 alloy to provide improved clamp load for the higher torque margins of the FB.

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The inlet casing on the FB also has been modified to improve aerodynamic performance at the higher mass flow. Compressor bleed air extraction manifold configurations cast into the casings have been modified to provide better transient clearance control to the outer compressor flow path.

GE made a lot of detail design changes. The relocation of lifting lugs, optimization of false flanges, judicious use of casing insulation, and other changes were incorporated to keep casings round during engine startup and shutdown.

Turbine modifications

An advancement for the FB compared to the FA series comes in the design of the three-stage turbine. With the increase in firing temperature, the gas path airfoils for the FB were redesigned and new materials were specified. Advanced, three-dimensional aerodynamics were applied to the turbine design, using computational fluid dynamics to optimize gas flow, resulting in higher efficiency and lower cooling air consumption.


Figure 3. Like the FA, GE’s Frame 9FB gas turbine uses an 18-stage compressor and a 3-stage turbine
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The higher firing temperature required the application of single crystal N5 in the first stage bucket, with thermal barrier coating. Second and third stage buckets are comprised of directionally solidified GTD444 materials, replacing the DS GTD111 material of the FA units. First and second stage nozzles for the FB utilize GTD111, while earlier F units used the older, cobalt-based FSX414.

The GTD111 alloy, originally developed for buckets, offers higher creep strength and is used in the FB to maintain the same hot gas path inspection intervals of the 7FA. Utilizing high-pressure air supply bled from the compressor, the FB turbine bucket rows 1 and 2 are fully air cooled, using a combination of impingement, convection and film cooling. A similar system is used to cool nozzle rows 1 and 2.

Advanced combustion

Addressing the growing need for clean power, the 9FB gas turbine will be equipped with GE’s advanced, dry low NOx2+ combustion system, which is designed for less than 25 ppm NOx (vppmd@ 15% O2) when burning natural gas without water or steam injection. This combustion system features 18 can-annular type combustors arrayed at a shallow angle around the engine casing, and is similar to the system used for the Frame 9FA and Frame 9H gas turbines. The combustor offers proven, dual fuel capability for operation firing of gaseous and/or low sulphur fuels.


Figure 4. The 9FB will be equipped with GE’s dry low NOx2+ combustion system
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Firing temperature is the key to combined cycle efficiency and, consequently, to minimizing fuel cost. GE’s growing experience with advanced technology gas turbines has enabled a significant advance in firing temperatures for the FB product line. Developments continue to provide valuable contributions to product technology and design and manufacturing techniques, to further enhance performance and reduce overall power plant costs.

However, designing gas turbines in today’s deregulated world requires balancing all three major elements of life-cycle cost: capital cost, operation and maintenance cost and fuel cost. Determining the optimum design solution – deciding which advancements to incorporate – is a very complex exercise in designing for multiple objectives.