Stefan Irmisch, Alstom, Switzerland
|Alstom GT13E2 gas turbine rotor|
For many decades Alstom Power Thermal Services (APTS) has continuously developed its heavy-duty gas turbines and implemented improvements to upgrade customers’ machines. This has given customers greater flexibility of operation of their machines and has raised the turbines’ output and efficiency.
Typically these developments have allowed turbine operators to choose between greater power output or extended lengths between service intervals, a philosophy that has enabled customers’ to extend their plant lifetimes and raise the productivity of their plants, thus reducing overall lifecycle costs. APTS calls this process its continuous improvement cycle and has employed it in the development of its products such as the GT11N2 M gas turbine and the more recently introduced GT13E1 MC compressor upgrade. In both cases APTS has used stable and proven development processes, and has cooperated closely with suppliers to significantly reduce time to market.
Cutting outage time
An upgrade of the GT13E2 has cut the gas turbine’s outage time by an average of 36 hours, allowing customers with installed turbines to increase their revenues.
When APTS applied its process of continuous improvement to the machine in 2008 it eliminated the need to perform multiple installations and removals of major engine parts, and hence reduced the time required for their assembly.
The upgrade involved three changes: validating a modified concept for adjusting the alignment of the turbine’s combustor, new jacks to lift the combustor assembly and a new tool to measure clearances between the GT13E2’s inner liner tipping segments and its compressor diffuser.
Accurate alignment of the turbine’s annular combustor is essential. In particular, the combustor liners must be properly aligned between the turbine and compressor to accommodate the large thermal deflections these components experience during operation.
|Cutaway of Alstom’s GT13E2 gas turbine|
A misalignment of the combustor can result in hard contact between the outer liner and the turbine vane carrier and/or between the inner liner and the compressor diffuser, which impose high stresses and adversely affect their lifetimes.
To allow fine adjustment of the alignment shims are included in the combustor’s supports, which can be changed as required to realign the combustor axially and radially during assembly.
But the GT13E2’s original design made them inaccessible while the combustor was in the engine. The entire combustor assembly, rotor, compressor diffuser and turbine vane carrier had first to be removed.
Also, the measurement of the alignment was only possible when the combustor, turbine vane carrier, and compressor diffuser were reinstalled into the gas turbine casing.
This meant the alignment procedure involved multiple cycles of trial assembly and disassembly of the components to change shims and verify alignment prior the final assembly.
The modification of the GT13E2 combustor assembly introduced shim drawers containing combustor support discs and shims. During an outage these drawers are removable, which allows adjustment of the shims without the need to remove the combustor from the turbine thermal block assembly.
The modified combustor is installed only once during assembly. If realignment is necessary a set of specially-designed lifting jacks can raise the combustor assembly a few millimetres off its supports to allow the withdrawal of the shim drawer assemblies for adjustment and reinstallation while the combustor, turbine vane carrier, diffuser, and rotor remain in the engine.
Upgrade of the GT11N2 M gas turbine
A service upgrade programme for the well-respected GT11N2 gas turbine was launched in 2004 when a market demand for increasing its performance, efficiency and operational flexibility became apparent.
One objective of the programme was to develop a fully retrofitable upgrade package that would increase engine power output and efficiency by using the latest available technologies, already proven in other Alstom gas turbines. A second objective was to allow for flexible operation modes, which allow the operator to run the engine with the highest power output and efficiency and with unchanged length of the service interval or with increased length of the interval and a smaller increase in output and efficiency.
To meet these objectives, the upgrade package comprised three parts: complete redesign of turbine blading, retrofitable for higher engine performance and longer lifetime; modifications to the combustor, hot-gas casing and structural components for longer lifetime and new operation logic for flexible operation.
The upgrade ended in early 2008 with the successful validation of the first implementation in a customer engine. The two modes of operation are called ‘performance’ and ‘lifetime’. The GT11N2 turbine first appeared in the mid-1990s, primarily for the 60 Hz market.
Other aspects of the redesign of the GT11N2 involve the use of 3D aerodynamic design, optimization of turbine sealing and reduction of consumption of cooling air.
APTS used 3D design principles and methodologies in the upgrade of the GT11N2. The application of a well-validated 3D Navier-Stokes flow solver and 3D design principles based on proven designs allowed further optimization of the turbine’s aerodynamic design.
An example is the minimization of the losses caused by secondary flows and the avoidance of local overexpansion zones on the airfoils. Furthermore, adopting a compound lean design approach for Vanes 3 and 4 was beneficial from an aerodynamic performance point of view, providing substantial growth in stage reaction of turbine stages 3 and 4, a noticeable reduction in outlet Mach number at the inner diameters of Vanes 3 and 4 and an increase in inlet flow angles to Blades 3 and 4.
To further increase turbine efficiency, APTS introduced modifications to minimize cooling air leakage through the stator parts, i.e. vanes and stator heat shields. These modifications were derived from an analysis of the improvement potentials of the existing design and the application of experience with other designs. They include state-of-the-art seals, an improved fixation of stator heat shields in the vane-carrier and a reduction of clearances between the stator parts.
A major contributor to minimizing aerodynamic losses was the reduction of the hot gas leakages at the turbine inner diameter, using proven design features. The introduction of platforms on the inner diameter of Vanes 2 and 3 and adding staggered honeycomb labyrinth seals with fins on the rotor and axially stepped honeycombs on the platforms of Vanes 2, 3 and 4 significantly reduced the leakage flows.
Eliminating the damping bolts on Blade 4 via the introduction of an interlocking full shroud also made a significant contribution to improved engine performance. Similar shrouded designs had been used for many years in other gas turbines, such as the GT24/GT26 and GT13E2.
A major contributor to the improved performance of the GT11N2 is the more efficient use of cooling air in the turbine. This was achieved by the application of state-of-the-art proven cooling technology. For the redesigned Vane 1, a cooling scheme consisting of two cavities was chosen.
Blade 1 is convectively cooled with a separate channel in the leading edge, which has a robust cooling design. The rear part of Blade 1 has a state-of-the-art multi-pass cooling system with two turnings, providing highly effective cooling with a relatively small cooling mass flow. By optimizing the cooling schemes of all the cooled turbine parts, a significant reduction in cooling air can be achieved.
Faster to market
Another challenge faced in the development programme of the GT11N2 was how to get all the new parts through the supply chain on time while simultaneously meeting the cost and quality targets. Previous development projects clearly demonstrated that lead times and product costs could be significantly reduced by optimizing the way the design team, logistics and suppliers interact. Thus, processes were adapted to reflect this.
One improvement measure was to have the key suppliers selected and involved in the design at the earliest stage. This permitted the consideration of manufacturability aspects and supplier capabilities early in the design, avoiding late design iterations, poor quality, low yield rates, and therefore runaway costs and schedules.
At the end of this development programme, APTS delivered all parts on time and the whole design and manufacturing throughput time was one of the fastest achieved to-date. In early 2008, the first prototype of the GT11N2M upgrade package was installed and successfully validated at a customer site.
GT13E1 MC compressor upgrade
APTS’ development processes for compressor upgrades employ available field experience, build on proven technologies and streamline design and manufacturing activities during the development phase. The latest result of this process is the GT13E1 MC compressor upgrade, successfully introduced to the market in early 2008.
Alstom GT13 gas turbine
Based on market needs, the main targets for compressor upgrades developed by APTS were: an increase in mass flow and subsequently in plant power output; increased compressor efficiency; extended part-load operation; and operational stability.
The typical constraints for an upgrade however are: ‘retrofitability’ into existing hardware (such as rotor and vane carrier); the implementation must not extend the outage time; must be implementable on-site; and must be based on proven technology to avoid implementation risks.
The aerodynamic design of APTS’ compressor upgrades is based on modern controlled diffusion airfoils (CDA) technology, individually optimized for each blade row. The CDA technology is characterized by a controlled flow along the blade surface, which keeps the boundary layer as thin as possible and prohibits flow separation over a wide range of operation. In combination with mechanically optimized, thin blades of this design approach results in increased compressor efficiency compared with the previous designs and a sufficient surge margin.
The design tools used during the process are integrated into a design system based on a configuration management and variant control logic and feeds directly into a CAM system. Having the fundamental dimensions, material selection and design features available at an early stage allows early ordering of raw material. This shortens the overall development cycle, especially for items with long lead times or time-critical manufacturing processes and therefore reduces time to market.
The extensive experience in compressor design and upgrades and use of proven technology allows the minimization of the amount of instrumentation requirements and the related extra work typically needed for the first implementation of an upgrade.
Most of the additional pre-implementation and post-implementation validation test runs can be performed within the time frame of a standard C inspection outage.
During the validation tests, the new compressor showed smooth operational behaviour, including the start-up phase. Evaluation of the validation measurements confirmed the expected efficiency improvement as predicted during the design. Additionally the measured mass flow improvement exceeded predictions.
Finally, the surge margin was measured to be very close to the existing old compressor and confirmed that the improved compressor can be safely operated in all GT13E1 units throughout the world.
Extending its continuous improvement process to include key suppliers APTS has further reduced lead times and improved production yield rates and product quality. And by incorporating new technologies for use in the development of new products after they have been validated in existing products, APTS is continuously creating value for its customers over the entire lifecycle of the power plant.
The author would like to thank Wolfgang Kappis and Marcelo Rocha of Alstom (Switzerland) Limited, Robert B. Davis of Alstom Power and Sergey Vorontsov of Alstom Power Uniturbo Limited for their contributions.
ALSTOM’S Continuous improvement cycle
Six partially overlapping phases make up the continuous improvement cycle. Each product goes through its own cycle, whether it is a complete engine or an upgrade to an individual component.
Phase 1: Prototype manufacturing
The cycle starts with manufacture of the first prototype of a newly designed product. Even at this stage, valuable information is collected and used to improve the future serial production of the product, for example, to reveal shortcomings and bottlenecks in the manufacturing processes. APTS works with suppliers to implement these first improvement measures, examples of which are the optimization of individual manufacturing steps or slight adjustments to the design.
Phase 2: Prototype implementation
The implementation of the new product for the first time demonstrates whether it has met the aims of the design with respect to serviceability.
APTS engineers determine what measures to take to ease assembly and later disassembly. This reduces assembly and outage times, and the associated expense. Typical measures include the design of new tools, procedures and methods.To get feedback from the first assembly at the earliest opportunity, APTS uses the virtual assembly tools of CAD applications in the design phase.
Phase 3: Prototype validation
The validation test of the first implementation of a new product plays a central role in the continuous improvement cycle. Here APTS demonstrates that the product has matched its design aims, which can include engine power output and efficiency, emission levels or product lifetime depending on the product.
At this stage the product runs under various operational conditions alongside sensors and visualization technologies. Visual inspections between test runs detect any issues the instrumentation may not capture.
Phase 4: Serial production
Commercial production begins after a successful validation of the product and its release for sales. This phase stretches over the remaining life of the product. Continuous monitoring of quality indicators, throughput times and yield rates at all stages of the supply chain allows the identification and elimination of any remaining process instabilities, unnecessary cost-drivers and quality issues.
Phase 5: Product care, repair and reconditioning
Long-term validation of the product and the optimization of its life and serviceability occur as more products are implemented and more operational experience accumulated.
If necessary, APTS now introduces small design improvements, depending on long-term experience and observations during outages. These improvements may involve measures that allow for easier assembly and disassembly and shorten outage times. APTS also develops and validates repair and reconditioning processes at this stage, based on the analysis of the real wear and tear of parts that servicing has highlighted.
Phase 6: Upgrades
APTS regularly evaluates the upgrade potential of existing products by closely monitoring market needs, evaluating customer feedback, considering emerging legislation and looking at advances in technologies.
The ultimate target of each upgrade is to increase its value for the customer by making it more competitive than the original product. Upgrade targets include increasing the performance, output, life, reliability and availability. Incorporating lessons learned from the previous phases of the improvement cycle and using proven technologies from other products ensures that development risks and time-to-market are minimized.