Small Gas Turbines: Evolving to compete

Demand for small and mid-sized gas turbines is rising, and manufacturers such as GE are developing their machines to meet new challenges.

The worldwide demand for mid-sized gas turbines is expected to remain steady over the next several years. Two-thirds of these machines will be used for cogeneration and small combined cycle applications.

To meet the requirements of this important industry segment, GE continues to evolve its gas turbine technology. The latest example is the MS6001C (Frame 6C), which brings the high efficiency and performance of large, advanced gas turbines into the 40 MW class. The Frame 6C technology was introduced by GE at the Power-Gen Europe 2002 conference and the first two units will be shipped to a customer site in Turkey.

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While the Frame 6C builds on the experience and performance of the GE Frame 6B gas turbine ” which has demonstrated high levels of reliability and availability in more than 40 million hours of service worldwide ” it also represents the flowback of GE’s F technology into the 40 MW class. The Frame 6C incorporates fuel flexibility, reliability/availability and maintenance features from the GE fleet of F class machines, which is approaching seven million hours of commercial operation around the globe.

The new 6C gas turbine is designed for low-cost electricity production and offers a strong product fit for heat recovery applications including industrial cogeneration, process industries, municipalities (district heating), combined heat and power, and mid-sized combined cycle projects.

GE launched the Frame 6C at Power-Gen Europe in 2002. The new machine is aimed at the cogeneration and small-scale combined cycle market
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The development of the Frame 6C was supported by the synergies and the resources of several GE businesses, including GE Power Systems, GE Aircraft Engines and the GE Global Research Center. Materials, analytical methods and design features proven in thousands of heavy-duty gas turbine and aircraft engine applications were integrated into the development of the Frame 6C.

A truly global project, the gas turbine itself was designed by GE Energy Products in Greenville, SC, USA; the compressor testing was carried out by GE Oil and Gas in Florence, Italy; and the gas turbine packaging, accessories and combined cycle power plant were designed by GE Energy Products in Belfort, France.

The Frame 6C will offer simple cycle efficiency of 36.3 per cent, an increase of approximately four per cent over the Frame 6B, with an output of 42.3 MW. Optimized for heat recovery applications, the new machine will achieve 54.4 per cent efficiency and 126.7 MW of output in a GE 206C (two gas turbines, one steam turbine) configuration. Designed to be environmentally compatible, the 6C will limit nitrous oxide (NOx) emissions to 15 ppm when operating on natural gas.

The first two Frame 6C gas turbine units will be installed in a combined cycle power plant in Kemalpasa-Izmir Turkey as part of an equipment engineered power island package
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A key target of the Frame 6C design is easy maintenance, to reduce costs and improve availability. For example, all the compressor rotor and stator blades can be removed on site without removing or unstacking and rebalancing the rotor. Each combustion chamber can be removed quickly without removing the main pipework, turbine shells or any heavy parts such as the rotor itself.

The Frame 6C development was fully supported by GE’s Six Sigma initiative. The new machine will be manufactured in GE’s facilities in Belfort, France.

Single shaft design

The Frame 6C engine is a single-shaft design with a gear-box driven from the compressor or ‘cold end’ of the machine. The rotor speed is 7100 r/min and the unit can be geared for either 50 or 60 Hz applications. At the ISO day design point, the compressor pressure ratio is 19:1 with a total mass flow of 114 kg/s. A turbine exhaust temperature of 574à‚°C combined with an exhaust flow of 422 t/h enables a steam production rate of 86 t/h without duct firing, providing an optimal configuration for cogeneration or combined cycle operation.

The three-stage Frame 6C turbine incorporates many design features from the E and F class machines while also utilizing hot gas path technology from GE commercial aircraft engines. The turbine rotor assembly is a traditional through-bolted configuration and consists of a forward shaft, three turbine wheels, three turbine spacer wheels and an aft turbine shaft, which includes the number two bearing journal. All three turbine wheels are subjected to a hot spinning process to reduce operational stresses at the bore.

The hot gas path components are modelled after the GE Frame 7FB heavy duty gas turbine while building on technology from the GE90 and the CFM56 aircraft engines. The stage-one vane is a single configuration cast from equi-axed GTD-lll. The stage one bucket is cast from single crystal RENàƒâ€° N5, and uses a serpentine design much like that of the 7FB and the GE90. Both the first stage vane and bucket utilize an applied thermal barrier coating to reduce heat flux.

The stage-two vane of the 6C is a doublet configuration with a cooling design similar to many other GE engines, using impingement to cool the outer band and airfoil, and a trailing edge bleed for convection cooling in the aft portion of the airfoil. The Frame 6C uses an unshrouded, serpentine cooled second-stage bucket which allows a reduced bucket count, more efficient cooling and a more uniform temperature distribution in the airfoil.

The third-stage components are uncooled and, similar to the E and F class turbines, the third-stage bucket uses a shrouded airfoil to provide increased damping for the higher aspect ratio airfoil.

All turbine components were designed using advanced design and analytical techniques. Multi-stage CFD analysis was performed on the turbine to ensure a robust and efficient design. Particular attention was paid to the vane designs to optimize flow migration on the turbine buckets. Heat transfer analysis was performed with the latest tools developed by GE Aircraft Engines and the GE Global Research Center for internal cooling circuit design and boundary condition calculation.

Low levels of NOx

The Dry Low NOx (DLN) combustion system was developed by GE to achieve low levels of NOx output without the addition of water or steam, while operating on gas fuel. This technology has been continuously enhanced since its introduction, and provides the basis of the DLN-2.5H system on the Frame 6C.

The Frame 6C combustion system utilizes a can-annular design with six cans linked by cross-fire tubes. The transition pieces are backside impingement cooled by a perforated cooling shell. This helps to decrease carbon monoxide (CO) and NOx emissions compared to the film cooling method.

In support of the DLN development programme, a new test stand has been built at the GE Gas Turbine Technology Laboratory in Greenville, SC, USA. The 6C test programmes include combustor cooling validation, emissions optimization, and combustion dynamic pressure minimization.

Since a complete combustion system can be represented by one can and it only takes 1/6 of the compressor flow to operate it, the complete combustion system can be fully tested ” full temperature, pressure and flow ” in the laboratory.

Aircraft engine technology

The 12-stage axial flow Frame 6C compressor incorporates technology and experience from GE Aircraft Engines. A scaled compressor test vehicle is being assembled at the GE Oil and Gas facilities in Florence, Italy and will be used to validate the 6C compressor design.

The compressor rotor configuration includes a forward stub shaft carrying three blade rows, five blade/wheel/spacer assemblies, three further wheels and an aft stub shaft. The forward stub shaft has axially oriented broached slots for blade retention. Two sets of removable ‘T’ fairings are located between the blade rows and allow the airfoils to be removed or replaced without unstacking the rotor.

The first two 6C units are being manufactured for Turkey’s Akenerji
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Circumferentially oriented blade attachment features are used for stages 4-12. Blades are assembled by inserting them into a loading slot and then sliding them circumferentially into position. As with the forward shaft, this design allows field removal of the blades.

The rotor assembly is tied together by Inconel 718 tie-bolts and utilizes a high rabbet radius design to provide inherent dynamic stability.

A conventional stator design is used for the Frame 6C, and consists of 11 stages as well as the inlet and exit guide vanes. Interstage air is extracted at the seventh and tenth stages to provide buffer and cooling air to the turbine.

Modular plant arrangement

GE has adopted a modular approach for the 6C plant arrangement and accessory systems, providing easy and quick installation and improved maintainability compared to the 6B. Each module has been optimally located with respect to the gas turbine, to reduce site footprint, interconnecting pipework and electrical cabling. Factory pre-assembly and testing will reduce the installation and commissioning time required on site.

All the modules feature acoustic packages capable of meeting 85 dB(A) at 1 m as a standard, with an option for 80 dB(A) if required. Doors in the packages and internal overhead manual cranes facilitate rapid maintenance during outages.

The air inlet system features a high efficiency static filter and downstream silencer similar to the 6B gas turbine. Using CFD analysis, the silencer location and duct shape have been optimized to minimize pressure loss and increase gas turbine performance.

The 6C will attain efficiencies of 54 per cent in combined cycle configuration
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The axial exhaust system has been developed from the system used on the F-class gas turbines to provide high static pressure recovery in a short axial length. Extensive CFD analysis and model testing have demonstrated a high quality of pressure recovery and temperature distribution into the downstream heat recovery steam generator.

Accessory Systems

The lubrication system for the 6C gas turbine, the load gear and the generator are incorporated in a common lubricating oil module mounted in-line with the gas turbine and generator.

The main lube oil pump is mechanically driven from the free end of the low-speed gear shaft and offers the same high level of reliability that has been demonstrated by GE’s Frame 5, 6, 7E and 9E gas turbines. The lube oil system includes fully redundant filters and coolers plus a full-capacity, AC-driven auxiliary lube oil pump for start-up and shutdown. A DC-driven emergency lube oil pump ensures essential lubrication during and after shutdown in the event of a complete loss of AC power.

The gas fuel control module is located adjacent to the gas turbine to provide a minimum of pipework between the control valves and the combustors. As with previous GE DLN systems, sonic-type control valves are used to ensure accurate relative flows to the nozzles of the DLN combustion system, resulting in minimum NOx emissions even at part loads. For the Frame 6C, these valves have been specially designed to provide a high-pressure recovery and thus minimize the gas fuel supply pressure required at the inlet to the module.

Duel fuel capability is provided by an optional module combining the liquid fuel, water injection and atomizing air systems into a single package placed adjacent to the gas turbine.

Quick installation

A factory-assembled and tested turbine control compartment next to the gas turbine contains the GE Speedtronic Mark VI control system, generator control and protection panels, motor control centres and battery systems for essential supplies. This arrangement ensures quick installation and a minimum of on-site connections. If required, the control units can be installed in a separate control room or building.

The Mark VI system features triple redundant microprocessor-based computer control, similar to the previous Mark V system used on FA-class gas turbines. It offers high reliability with redundant power supplies, and the ability to continue providing safe control while a failed controller is replaced without stopping the unit.

The Mark VI system provides greater I/O capability than the Mark V, more powerful software and hardware diagnostic capabilities, and a high-quality operator graphical interface called the Human Machine Interface (HMI). An Ethernet network capability allows communication with a DCS or other control systems and full control from a remote location if required.

The first two Frame 6C machines are being manufactured in Belfort, France for Akenerji, a leading electricity producer in Turkey. GE will ship the two machines as part of an equipment engineered, power island package for a combined cycle plant at Kemalpasa-Izmir in Turkey.

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