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By Douglas J. Smith IEng, Senior Editor
Recent-model large-frame gas turbines have experienced some growing pains, but modifications and upgrades have increased their reliability substantially.
Due to low natural gas prices, low capital costs, ease of permitting, quick installation, and the need to add capacity, the 1990s saw a dramatic increase in the market for gas turbines. To meet the demand all of the major gas turbine manufacturers developed gas turbines with larger capacities, higher efficiencies and low NOx emissions. Although some of these heavy frame gas turbines did not initially perform to the manufacturer's specifications and customer's expectations, subsequent design and operational changes have made them reliable elements of the turbine fleet.
After going into commercial operation the large Frame F and G gas turbines experienced a number of problems. These included:
- Turbine blade failures
- Compressor disk cracking
- Humming/Flashback
- Vibrations—rotors, compressor diaphragms
To overcome the initial design and operating problems with their large frame gas turbines the OEMs developed a range of design improvements to resolve the problems.
GT24 and GT26 Gas Turbines
The first ALSTOM GT24 gas turbine, installed in GPU's Gilbert station, went into commercial operation in 1996. Reliant Energy now owns the plant. Unlike other gas turbine designs where the OEM increased the firing temperatures to increase efficiency and capacity, the GT24 gas turbine uses sequential combustion. With sequential combustion, the fuel is injected twice into the gas turbine, and the capacity and efficiency are increased without significantly increasing the firing temperature. The GT24 60 Hz gas turbine is rated at 179 MW and the GT26 50 Hz machine is rated at 262 MW.
A German utility, EnBW Kraftwerke AG, repowered Unit 4 at its Rheinhafen power plant with a GT26 gas turbine. However, because of previous problems with blade rubbing of the high-pressure section of the compressor, the OEM recommended operational changes to the turbine to prevent the problem. Nonetheless, during startup of the plant in 1997, compressor blade rubbing still occurred.
The problem of blade rubbing was rectified by changing the blade clearances and adding abrasive heat shields in the 17th compressor blade row area. In addition, the bleed air segments of the 17th blade row were modified. According to the utility these modifications have been successful.
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By 2000, ALSTOM had developed an upgrade improvement package for the GT 24 and GT26 gas turbines and although the Rheinhafen unit was operating with few problems, the utility decided to upgrade their GT26 gas turbine. The upgrade package included a new combustor rear wall design that resolved several inadequate operating characteristics in the first combustion stage of the gas turbine. Some of the other modifications included temperature monitoring of the EV burner shells, improved blade cooling and modified flushing for the EV burner system.
The first GT24 and GT26 gas turbines with the new design enhancements were installed in combined-cycle power plants in Agawam, U.S. and Enfield, England. Shortly after start up of the Agawam facility a boroscope inspection revealed that the locking piece on row 16 of the compressor had become loose. ALSTOM resolved the problem by modifying the locking pin so that it dovetailed into the first and last blades of row 16. This modification is now standard on the GT24 and GT26 gas turbines.
Another boroscope inspection, conducted at the Enfield plant after only a few hundred hours of operation, found that cracking had occurred in the EV outer liner. ALSTOM determined that the design changes made to the liner for the installation of the modified burner was the cause. According to ALSTOM, strengthening the components has rectified the problem.
In 1999, ALSTOM launched the more advanced GT24B gas turbine. Unfortunately, two problems occurred during the initial operation of the GT24B gas turbine. These were: Cracking of the EV combustor liner in early 2000 and deformation of the low-pressure turbine row two blade shroud in mid 2000.
According to ALSTOM, modifications to rectify the EV combustion liner problem were made to three field units and 32 units that were in the process of being manufactured. Since being modified no problems have been reported or detected with the liners.
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Lack of impingement cooling from the heat shield, precipitated the deformation of the low-pressure turbine blade shroud. After drilling additional impingement cooling holes in the stationary shield the overheating problem has been resolved. All GT24B gas turbines, in the field, or ready for shipping, are being modified. The units that have been modified have run trouble free since 2000.
Enhancements to W501F Gas Turbines
Since its introduction in the early 1990s, Siemens Westinghouse has continued to improve the design of its W501F gas turbine. Over a ten-year period Siemens Westinghouse has improved the efficiency of the W501F gas turbine by 6 percent, increased power by 27 percent and reduced NOx emissions by 67 percent.
Improved efficiency of the compressor has been achieved by increasing the diameters of the first and second stages as well as incorporating new 3-D controlled diffusion blade path technology to optimize the air flow. To reduce corrosion, Sermatel 5380DP coating has been applied to some compressor components. However, the disc and rotor construction and the compressor blade and vane material has not been changed or modified.
 Figure 3. Mitsubishi steam cooled combustor. Source: Mitsubishi Heavy Industries |
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To meet the challenges of increased firing temperatures and reduced emissions, the W501F uses 16 individual Dry Low NOx (DLN) dual fuel can-annular combustors. Improvements in airflow management, coatings, fuel/air premixing and dynamic monitoring of the combustion system have enhanced emissions, operation and performance of the gas turbines.
The combustor air bypass system, used on the initial version of the DLN combustor, has been eliminated in the new design. Removing the bypass system has helped to improve the reliability, availability and maintainability of the combustion system.
Although the design changes to the turbine were minimal some enhancements have been made. In order to reduce wear, the turbine ring segments have been coated with a rub-tolerant coating. As a result, air leakage has been reduced and the efficiency and output of the gas turbine has been improved. Similarly, improvements have been implemented to reduce leakage and wear, Figure 1. In addition, the fourth stage turbine blade was redesigned to maximize the gas turbine's output during cold ambient temperatures. According to Siemens Westinghouse, the W501F gas turbines shipped since 2000 have a maximum power output of 215 MW.
Efficiency and emissions improvements were achieved in part due to cooling air optimization. With the original W501F design, the amount of cooling air for vane cooling was fixed, and therefore could not be optimized for specific site operating conditions. However, with the new design the cooling air is automatically modulated, thus optimizing the consumption of the cooling air.
Commercial Operation of W501G
In 1999, Unit 5 at Lakeland Electric's McIntosh facility, Florida, put the first Siemens Westinghouse W501G gas turbine into operation. During the commissioning period the unit experienced problems with compressor rubbing, high frequency dynamics and transition wear.
Since initial operation, Siemens Westinghouse, at their expense, have upgraded and made modifications to rectify the problems. According to Ed Colter, plant superintendent, McIntosh, the high frequency dynamics have been resolved by the installation of resonators on the transitions of the gas turbine. In addition, by changing the style of coatings, the life of the transitions has been extended.
After modifications and upgrading, the McIntosh gas turbine was put into simple-cycle operation in April 2001. Colter reports that since that time the unit has met the specifications for capacity but NOx emissions still need some improvement. Colter is also not convinced that the fixes to rectify the operating problems are long term. Overall Colter says that the gas turbine is operating very well and the only issue that still needs to be proven is the life of the components.
Over Seven Million Hours
As with the other major gas turbine manufacturers, GE experienced some problems with their large frame units. Early 7F (60 Hz) and 9F (50Hz) gas turbines had incidences of cracking in the aft compressor rotor structure in the turbine stage three-spacer disk. According to GE, the cracks were related to thermal stresses induced during cold rotor start-ups and during rapid start-ups following a trip. Since design modifications no cracking of the rotor has occurred.
According to a recent GE news release, the installed fleet of over 500 F technology gas turbines has reached 7.1 million hours of commercial operation worldwide. Since the F technology was introduced a decade ago GE has incrementally improved the efficiency and output of the units. When first introduced in 1986, the Frame 7F had a simple-cycle rating of 135 MW. The more recent model, the 7FA, is rated at over 170 MW in simple-cycle operation.
One of the first upgrades to the 7F gas turbine involved improving the machine's performance through higher firing temperatures, higher cycle pressure ratios, reduced leakages and increased turbine cooling. With this upgrade the limits on metal temperature for the un-cooled last stage turbine bucket and exhaust frame were maintained.
Introduction of the 7FB
GE introduced the 7001FB, a 60 Hz gas turbine optimized for combine-cycle duty, in November 1999. Although the gas turbine package is basically unchanged from the 7001FA, the buckets and nozzles have been completely re-designed. According to GE, they have been able to increase the firing temperature while maintaining exhaust temperature at 7001FA levels.
Because of the increased pressure ratio and firing temperature of the 7001FB the compressor rotor bolting and turbine rotor disks were upgraded to a higher strength material. However, the compressor blading aerodynamic design remains essentially unchanged from the 7001FA.
To minimize thermal distortions, and to prevent problems with blade clearances and rubbing, the compressor and turbine casings were modified. These changes included improved air extraction geometry optimization, relocation of lifting lugs, the addition of false flanges to add symmetry to the split casing and the judicious use of insulation. Figure 2 summarizes design changes made to the 7FB gas turbine.
M501G in Operation Since 1997
The first Mitsubishi 254 MW M501G gas turbine was installed in Japan at Mitsubishi Heavy Industries' in-house power plant in 1997. Although the unit has been in commercial operation since that time, supplying power to a local utility, the plant has served as a test-bed to verify the long-term reliability and performance of the M501G gas turbine technology, says Vinod Kallianpur, vice president, Mitsubishi Power Systems.
In addition to utilizing many of the design features of the F series of gas turbines the G heavy frame gas turbine utilizes advanced profile airfoils to increase the airflow in the compressor. This increased airflow has been achieved by incorporating multiple circular airfoils in the first four stages of the rotating blades and controlled diffusion airfoils in the rest of the rotating blades and in all stages of the stationary vanes.
Because of the increased firing temperature, and in order to keep the same NOx levels as the F series, a pre-mixed combustor with a closed steam cooling system was added to the M501G, Figure 3. The previous F series used a dry low NOx air-cooled combustor. To prevent overheating of the turbine blades and vanes, Mitsubishi has applied advanced cooling technologies. These included full coverage film cooling, thermal barrier coating, new heat resistant materials and directionally solidified casting technology.
The rotating blades are made from MGA 1400, a nickel based super alloy. Another nickel based super alloy, MGA2400, is used for the stationary vanes. The MGA2400 super alloy has excellent resistance against thermal fatigue, oxidization and hot corrosion. It also has high creep strength and is easy to weld.
In October 1997, the M501G was removed from service for its first inspection. Except for some minor cracks in the combustor transition piece, the steam cooled combustor, air cooled vanes and blades were found to be in good condition. Subsequent inspections carried out in March 1998, November 1998, March 2000 and October 2000 found no problems with the gas turbine.
 M501G compressor and rotor assembly. Photograph courtesy of Mitsubishi Heavy Industries. |
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During operation at the in-house power plant, Mitsubishi has continued to improve its operating performance. To optimize the flow pattern and reduce aerodynamic losses, the turbine's aerodynamics have been improved in the upgraded M501G1 gas turbine. The upgraded turbine has a capacity of 264 MW. Similarly, because the aerodynamic modifications required upgrading of the blades and vanes, their cooling system also required modification. In addition, an advanced lower NOx combustor was installed on the gas turbine.
Mitsubishi has recently completed the testing of an ultra dry low NOx combustor in Japan. During field testing, the gas turbine was able to operate with NOx emissions of 25 ppm at a firing temperature of 2732 F. According to Kallianpur, the ultra low NOx combustor is now available for the F and G machines at 15 ppm and 25 ppm, respectively.
Durability Still an Issue
According to a WEFA Inc. report "Banking on Advanced Gas Turbines: Prospects for a Financial Meltdown," around 60 to 70 percent of the non-fuel cost of F class gas turbines is consumed in the repair and replacement of hot gas path components. The report goes on to say that the worldwide OEM hot section replacement business exceeds $1 billion annually and the profit margins for the parts are reported to be greater than the margins on the original gas turbine.
The WEFA report states that hot gas path components giving the most trouble are:
- Turbine blades and vanes
- Combustion liners
- End caps
- Fuel nozzle assemblies
- Turbine stationary slides.
Although OEMs have been very supportive and responsive in resolving the initial design problems with their gas turbines, end users must take a long-term view and will be closely watching the extended durability and maintainability of these complex machines.
