MAN Introduces New Industrial Gas TurbineTwo-shaft GT6 gas turbine rated 6.2 MW, one-shaft version planned
by roberto chellini
MAN Diesel & Turbo has introduced its new GT6 industrial gas turbine, developed for mechanical drive of its compressors and for power generation applications. The GT6 has been completely developed in-house during the last six years and is now available in the twoshaft configuration. The one-shaft version will follow in one year.
The introductory rating is 6.2 MW, but the program is to go up to 8 MW and subsequently uprated to cover the power range from 6 to 10 MW. MAN’s THM models will continue to cover the 10 to 13 MW power range, while a scaled up version of the new GT6 could cover the 15 to 20 MW range further in the future. This two-shaft machine features an 11-stage compressor, with a 1:15 pressure ratio, driven by a two-stage, air-cooled, high-pressure turbine. The second shaft features a two-stage low-pressure power turbine. In the single-shaft configuration, the turbine section will feature three stages, with the first two air cooled. The generator will be coupled on the cold-end side to allow an axial exhaust duct to the stack or to the HRSG.
The combustion system features six cans fitted with dry-low emission burners to minimize NOx and CO emissions. This combustion system has been designed from the very beginning as DLN for natural gas-fueled units; a system for liquid fuels will be implemented further on. The machine is built up in mod- ules for easy assembly and maintenance operation.
The auxiliary gearbox carrying the electric starter motor and the main lube oil pump also serves as front support for the whole gas turbine. The turbine air intake casing, on the
auxiliary gearbox, contains the first journal bearing and the double-acting axial thrust bearing, both of the tilting pad type. Both bearings are easily accessible without the need to open the horizontally split casing, said MAN.
The drum-type shaft features a front hub section followed by three rotor discs and a rear hub. Hirth serrations ensure correct axial positioning and centering of the rotor components once the central tie rod is tightened; they also absorb the torque transmitted by the HP turbine. The 11 blade rows of the axial compressors are mounted on the rotor drums. The stator vanes are mounted on the vane carrier, and the inlet guide vanes and the first three rows of stationary vanes are of the variable geometry type. A high accuracy electric servo motor, connected to the turbine digital control system, actuates the four variable geometry rows from the completely closed position at start-up, until 75% of speed to the fully open position at 95% of speed and full load.
The discs of the two-stage HP turbine are mounted on the rear hub before the second journal bearing. The unshrouded first stage and the shrouded and interlocked secondstage turbine blades are anchored to the discs by fir-tree roots. Both discs and blades are cooled by a stream of air from the axial compressor diffuser.
The intermediate turbine casing, as well as hosting the compressor diffuser, serves as support to the six combustion chambers, the hot gas transition ducts and the two nozzle guide vane carriers. At the end of the intermediate casing, six struts support the rear bearing casing and serve as lube oil ducts for bearing lubrication. The combustion chambers, positioned with 35° inclination relative to the center line, feature a vortex-stabilized burner section, a large volume for combustion and supply through the transition ducts, and hot gases at uniform temperature to the first turbine nozzles. Both combustion tube sleeves and transition ducts are impingement cooled with air from the compressor outlet.
At start-up and low load, the burners operate in the diffusion mode and shift automatically to the premix mode as soon as the turbine picks up load to ensure minimum production of NOx and CO. The two-stage power turbine has been optimized for a high efficiency within a wide speed range, it can operate between 45 and 105% of nominal speed (12 000 r/min). The two discs of the low-pressure turbine are bolted, with Hirth serrations, at the inner end of the power shaft in an overhung position. The shaft is supported by two tilting pad journal bearings — the rear one being combined with a pivot shoe-type thrust bearing. The bearing casing is placed in the center of the low-pressure turbine casing.
Boroscope access is provided all along the air and hot gas path to ensure quick inspection and reduced maintenance time. After assembly, the prototype was placed on a specially built test stand at the Oberhausen, Germany, facilities for a series of tests. First firing took place on Nov. 19, 2010. With newly designed digital control system and intensive component testing, it was possible to reach full load only one week after first ignition. Tests of the prototype are still under way at the time of writing. At the end of the testing program, the GT6 will be available in the package form complete with all its auxiliaries for mechanical and generator drive applications.