Ever decreasing cycles

    Ever decreasing cycles

    Today`s competitive power market demands high efficiency, reliability and short commissioning cycles – traditionally a tough combination where steam turbines are concerned. But in a new approach to this technology, GE has developed a family of standardized steam turbines which can be delivered within 12 months of order.

    Henry Stueber, Bill Gorman,

    GE Power Systems,

    USA

    Steam turbines have traditionally been designed and optimized to meet specific thermal cycle conditions. While the typical steam turbine would use mostly standardized component hardware, the steam path would be customized for each new application.

    With today`s customers demanding shorter delivery and commissioning cycles, however, a new approach was required. In response to this, General Electric developed a new, standardized family of steam turbines designed to perform with high thermal efficiency while offering a significantly shorter delivery cycle. Increased design standardization, or structuring, will enable these turbines to be shipped in 12 months from the time they are ordered compared to 18 months for customized units.

    The first application of this structured approach is being used with D11 steam turbines which have been configured specifically for installation in 207FA (60 Hz) and 209FA (50 Hz) combined cycle power plants.

    Since the 1986 introduction of F gas turbine technology, a total of 230 7FA and 9FA gas turbines have been ordered, including 181 for combined cycle applications. The steam turbines for these power plants form a relatively narrow, but increasingly important, sub-group of the overall steam market, and therefore they were prime candidates for standardization.

    The demand for F technology combined-cycle systems peaked dramatically in 1998, driven largely by an upturn in the U.S. market. Two key factors spurred this burst of activity: high temperatures in many parts of the country during the summer, driving demand for electricity to record or near-record levels; and declining reserve margins. Due to uncertainty over deregulation, many utilities over the past few years delayed or stopped buying equipment and as a result, reserve margins overall in the U.S. were down in the 12 per cent range in 1998.

    Faced with the competitive pressures of a deregulated market, power producers are demanding higher performance and greater efficiency from their equipment. These market requirements drove GE several years ago to raise steam cycle conditions for 7FA and 9FA class combined-cycle power plants to nominal levels of 1800 psig (124.1 barg) inlet pressure, and reheat temperatures of 1050 degrees F (566 degrees C). The structured D11 steam turbines are designed to accommodate these nominal inlet steam conditions, including supplementary fired applications.

    These key market forces – the immediate need for additional capacity, coupled with the demands for higher performance and efficiency – provided GE with a strong impetus for pushing forward with the development of the structured steam turbine concept.

    The structured D11 steam turbine family includes five basic models. Three versions of the 60 Hz unit are available, with low-pressure sections using 76 cm, 85 cm or 102 cm last stage buckets, depending on site requirements. Two versions of the 50 Hz machines are available, using 85 cm or 107 cm last stage buckets. The 60 Hz models are for applications with two 7FA gas turbines, while the 50 Hz machines are for applications with two 9FA gas turbines, forming 207FA and 209FA combined cycle systems, respectively.

    The 60 Hz structured D11 steam turbine is rated at 175-200 MW unfired and up to approximately 250 MW at maximum firing, while the 50 Hz machine is rated at 260-300 MW unfired and approximately 370 MW at maximum firing, depending on ambient conditions and condensing pressure. Those values represent a performance improvement of about 2 MW over previous conventional units.

    Cycle optimization

    The first, fundamental step in developing the structured D11 concept was to ensure optimization of the thermal cycle. Since the basic bottoming cycle parameters were already determined, the search for potential improvements focused largely on the secondary parameters. These were (1) reheat pressure, and (2) the intermediate pressure (IP) to low-pressure (LP) crossover pipe pressure where LP steam is commonly admitted.

    Variation in reheat pressure has a relatively flat influence on bottoming cycle steam turbine generator output. Although not critical to theoretical cycle performance, the hot reheat pressure does have a major influence on HRSG and steam turbine design. The hot reheat pressure for the cycle is set by the flow passing area of the first IP turbine nozzle area. For the D11 design, the hot reheat pressure for the unfired base case was established very close to theoretical optimum and results in a cost-effective and mechanically conservative HRSG and steam turbine design.

    The second parameter investigated for cycle improvement was the LP turbine crossover pipe pressure. The steam from the IP turbine exhaust passes through the crossover to the LP turbine. The IP turbine exhaust also serves as a convenient location to admit steam from the HRSG low pressure superheater. The crossover pressure is determined by the nozzle area of the first double-flow LP stage. The ideal pressure is much lower than the pressure that typically would be seen in a traditional, conventionally fossil fired boiler steam turbine cycle with regenerative feed water heating.

    The crossover pressure effectively sets the LP admission pressure for the combined cycle. The saturation conditions in the LP drum, combined with judicious selection of HRSG surface area, determine the HRSG stack exit temperature and therefore directly impact the efficiency of the bottoming cycle. Once the optimal crossover pressure and the IP turbine inlet pressure were established and fixed, a standard set of IP stages could be designed.

    Steam turbine condensing pressure is highly critical to cycle efficiency, and is highly variable depending on plant geographic location and available condensing medium. With a known optimum required crossover pressure, it was possible to design a series of standardized LP turbine sections with different last-stage buckets and annulus areas, for different condensing pressures to match the fixed IP turbine. These standardized LP turbines sections also include provisions for dearation extraction or other thermal cycle or process purpose if needed.

    Design features

    The optimization of the 207FA and 209FA thermal cycles made possible the development of the structured D11 family of steam turbines. The D11 steam turbine evolved from the opposed flow HP/IP turbine with double flow LP that has been used in fossil and combined cycle applications for many years.

    Main steam enters the turbine at the underside of the high pressure shell via two off-shell stop and control valves. The combined HP/IP section utilizes single-shell construction consistent with earlier opposed flow HP/IP shell designs for 1800 psig/566 degrees C (1050 degrees F) steam conditions. There are two HP/IP shell designs, one for 207FA and one for 209FA applications. Variability in the steam path design is limited to the HP section. Each shell design is fixed, with the diaphragm grooving and supports designed into the shell.

    The HP staging is customized for each application. The HP nozzles and buckets are designed to achieve the specified inlet pressure and to match the cold reheat pressure that will be seen because of the fixed IP steam path design and the selected reheater section and piping pressure drop.

    Since two 7FA or 9FA gas turbines provide a predictable exhaust energy and the HRSG surface areas are somewhat standardized by the constraints already discussed, it is possible to limit the number of HP stages to 11, and closely limit the range of stage diameters. With the fixed staging of the IP section, it is possible to closely control the HP/IP rotor design in terms of forging size and bearing span. Since rotor dynamic criteria have been thoroughly analyzed, the relatively small steam path variations allowed in the HP section result in reliable operating characteristics.

    Previous combined cycle LP turbine designs evolved from original conventional fossil applications and were not initially designed as an integrated, interchangeable set of turbine sections. Provisions for feedwater heating extractions from the LP turbine in these early combined cycle designs were included only if required by the specific application. In establishing the structured D11 product offering, consistency of low pressure turbine application was given high priority. In addition, each low pressure turbine design includes extraction capability if desired.

    GE`s Six Sigma quality programme played a major role in the D11 development. In 1997, GE Power Systems launched a number of Six Sigma projects based on customer expectations for product delivery, then aimed at matching the focus of new product introductions with the projected sales volume and, with that data, create a structured product. The D11 is the first of those products.

    Reduced delivery time and increased performance were identified as critical areas in the D11 design. Using Six Sigma methods in the analyse phase, the team identified alternative design concepts and then developed one high level design. This was developed into a detailed design of the structured D11 steam turbine, as well as the detailed design of the process from customer order to customer delivery. In the verify phase, the product was launched for full production.

    Six Sigma allowed the team to design a steam turbine for the combined cycle system using the system`s requirements and then flowing the requirements down to the steam turbine level. By applying the discipline and the tools of DFSS to the design of the structured D11 steam turbine, GE was able to develop a family of high quality, standardized products with the flexibility to meet specific site requirements. Manufacturing and delivery cycles have been dramatically reduced, without sacrificing thermal performance.

    Delivery cycles

    Design standardization allows the structured D11 steam turbine to be shipped only 12 months from the time the order is received. The items requiring long lead times can be completed in advance. By forecasting volume with experienced suppliers and reserve capacity, GE will be able to shorten delivery cycles for rotor forgings, castings and exhaust fabrication.

    Critical customer drawings will be available almost immediately when the notice to proceed is given. The D11 is structured so that minor adjustments in the HP steam path to configure the turbine for the thermal conditions of a particular site do not change the outline dimensions, component weights, sole plate layout or foundation loadings. This consistency allows architect engineers to get a head start on the turbine foundation design, overhead crane specification, auxiliary equipment placement, and piping and electrical system design.

    Since structured D11 turbines have many common components, spare parts inventory can be reduced. Such items as valve stems, valve discs, journal bearings, thrust bearing, shaft end and interstage packing, spill strips, horizontal joint shell bolting, auxiliary system components and various gaskets are standard for all models in the structured D11 family.

    Another key benefit is reduced installation time. The HP/IP section of the D11 turbine will be shipped from the factory already assembled with diaphragms and rotor installed and properly aligned, and with the horizontal joint shell bolts fully tightened. This saves about four weeks of field erection time.

    In the future, the structured D11 steam turbine will serve as a template to design other structured families in the present steam turbine line.

    Since the introduction of the structured D11 concept, 11 units have been scheduled for production in 1999, including several units targeted for applications in the surging U.S. market, with 20 more scheduled for production the following year. The first structured D11 is expected to go into service in mid-2000.

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