Continuous Improvement In Medium-Sized Gas Turbines: Siemens presents its Medium-Sized Gas Turbine Continued Product and Operation Improvement program, aimed at reducing life-cycle cost

Life-cycle cost is one of the most important parameters for power plant owners, having significant influence on power plant profitability. Recognizing the criticality of it, Siemens
continuously runs a product and operation improvement program aimed at addressing all performance aspects of its medium-sized gas turbines (MGT) over the entire life cycle.

Siemens focuses on continuous improvement and life-cycle costs of its aging medium-sized gas turbines, with a focus on super-alloy materials and thermal barrier coatings.

The turbines included in this plan are the four Siemens Industrial Turbomachinery AB medium-sized types SGT-500, -600, -700 and -800 with outputs from 17 to 47 MW all manufactured at Siemens in Finspong, Sweden. These turbines have a long history and, over time, have already been upgraded in various phases. The SGT-500 is the oldest industrial gas turbine currently under production at the facilities in Sweden, in commercial operation since 1955. The turbine started with 9 MW of power output and through several upgrades has recently reached a power output of 18.5 MW.

The other most recent upgrades in the Finspong medium-sized line are the SGT-800, brought from 45 to 47 MW in 2007 and the SGT-700 from 29 to 31.2 MW in 2008. Vladimir Navrotsky, product development manager at Siemens Industrial Turbomachinery AB in Finspong, said there are several ways for the OEM to reduce the life-cycle cost of a gas turbine. Two areas with a major impact are product performance enhancement and maintenance cost reduction.

“Gas turbine performance enhancement includes gas turbine efficiency improvement, power output increase and lifetime extension. Maintenance cost reduction is primarily achieved through a new approach in the repair and refurbishment of expensive gas turbine components and via a reduction of maintenance scope or staff, and finally a reduction of the number of periodic overhauls,” said Navrotsky.

In the case of medium-sized gas turbines, the development programs within Siemens include upgrade and repair solutions, life extension programs, new inspection technology and tools, as well as remote monitoring and diagnostics. “We are, in particular, actively addressing the extension of the scope of repaired components in our development programs,” said Navrotsky. “The replacement of expensive components is one of the dominating costs in the maintenance of a gas turbine-based power plant. “This is why a repair clause is now included in the  maintenance contract, which means that costs for an unexpected repair intervention are included, depending on the maintenance contract specifications,” he said. “The repair service is also being regionalized with several workshops located around the world, and the repair process is being simplified.”

At the same time, remote monitoring is being addressed as a fundamental support to repair interventions and to a reduced downtime, as well as a key tool for condition monitoring and a diagnostic program. Navrotsky said, “Siemens’ Oil & Gas division decided three years ago to develop a common remote monitoring platform for industrial applications, meaning compressor, steam turbines and gas turbines. This program in now in its implementation phase. “Since 2009, all rotating equipment delivered for industrial and oil and gas applications have remote monitoring capability. Moreover, for our mediumsized gas turbines it is mandatory to have remote monitoring, at least during the warranty period,” he added.

Navrotsky said Siemens has a solid process and tools in place to follow up and analyze gas turbine operating experience and statistics.  At the end of December 2009, Siemens’ data for the SGT-600 fleet showed that 260 units of this type ave been produced and more than 60% of them are in commercial operation. The total accumulated operating exerience is more than 5.5 million equivalent operating hours (EOH) and over 71 000 starts.

The operating statistics show a reliability factor of 99.1% and an availability factor of 96.5%, said Siemens. From the point of view of performance improvement, enhancements that influence lifecycle costs include power output uprates, efficiency improvements, increased fuel flexibility and lifetime extension. In this field, Siemens recently carried the power uprate of type SGT800, previously rated at 45 MW power and
37% electrical efficiency.

“Operating records and a detailed mapping of the hot section of the turbine evidenced that the amount of the cooling air required for blades and vanes could be redistributed and even saved,” said Navrotsky. “These savings, combined with an increase in compressor airflow of 1.5%, have been the basis for a power output increase to 47 MW and an efficiency increase to 37.5%. Together, the increase in mass flow, combined with an increase in exhaust heat temperature, has increased combinedcycle efficiency
from 53% to over 54%.”

Another recent enhancement to the SGT800 was a modification to the DLE combustion system, with the introduction of a passive damping of pulsations. The combustor modification, combined with the reduction of thermal stress in key components (blades and vanes) has led to a further increase in reliability and intervals between maintenance, which in turn lowers the lifecycle cost of the gas turbine.

On the other hand, the continuous development toward higher efficiency and thus higher firing temperatures has resulted in higher thermal loads on the hot gas path components. In parallel to continuous progress in materials and cooling technology, increased efforts have been made for the maintenance of these components to ensure their proper function, reliability and reasonable cost over the whole lifetime of the engine. Siemens accelerated the development in reconditioning methods for superalloy materials and thermal barrier coatings.

To this extent, Magnus Hasselqvist, specialist in Life Assessment of Hot Parts at Siemens in Finspong, represents the company’s research and development efforts in this area. He was named “Inventor of the Year 2009” by the company for his development of a highly heatresistant nickelbased superalloy for gas turbine blades in a remarkably short period of time. The new alloy will be used by Siemens for repairs of these components. Gas turbine components are subjected to extreme forces: temperatures of roughly 950°C at the metal, a pressure of roughly 20 bar, and the mechanical loads due to centrifugal force are unrelenting. For this reason constant research is being done to improve the resistance of the blades, vanes, heat shields and burner components.

“Primary criterion for determining the suitability of superalloys for new gas turbine components is their resistance to deformation during operation at high temperatures and high loads. However, the metal used for the repair of oxidation damage needn’t have a particularly high creep strength because the loading tends to be moderate in the areas experiencing the highest temperatures,” said hsselqvist. “Much more important is a combination of good oxidation resistance and good welding properties.” Based on these criteria, Hasselqvist began his research into suitable filler lloys a mere three years ago. He said it was possible to achieve results in this relatively short period of time because he had all the experts onsite.

Siemens’ developments have been driven by operator demands and an aging of the fleet. In fact, the company reported that a significant number of the engines in the SGT500 and SGT600 fleet are approaching their design life of 120 000 EOH. The latest design modifications of MGT fleets and positive operating experience gave Siemens the opportunity to consider the extension of the life cycle of the engine beyond 120 000 EOH (up to 180 000 EOH, dependent on the previous operation profile and history) and to extend the maintenance intervals.

“Life cycle was expanded to 160/180 000 EOH in the SGT500 and ­600, while a study is currently on its way to extend life cycle in the SGT700 and ­800,” said Navrotsky. “It has been calculated that the extension in the SGT600 from 120 000 to 180 000 EOH enables the operator to reduce the lifecycle cost by approximately 10%,” he said. At the same time, maintenance intervals have been extended for SGT600 and SGT800, whereas the SGT500 and SGT700 maintenance interval extension development is ongoing.

In the case of the SGT800, the modifications in hot gas path components have enabled not only the enhancement of power output and efficiency, but also the extension of components’ life and, as a result, extension of the time between overhauls from 20 000 to 30 000 EOH. “This enables the operator to reduce planned outage hours by more than 30% for the whole life cycle,” concluded Navrotsky.


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