Combined Cycle Technology: A standard for flexibility

Markus Tacke, Robert Taud, Edwin Wolt, Siemens Power Generation Group (KWU), Erlangen, Germany

The Salta power plant, Argentina exemplifies, the use of the reference plant concept where flexibility and maximising use of standardized systems and components is required
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The challenges for today’s power generating equipment suppliers are tremendous. Customers now demand low installation costs or low life cycle costs, and high efficiency and power output. In addition, high reliability, availability and flexibility are required in the new power trading environments and must be combined in plants with very short delivery times. Furthermore, all this has to be achieved while adapting technology to local site conditions.

The Siemens pre-designed multi-shaft combined cycle power plant of the

780 MW class for 50 Hz applications, the GUD 2.V94.3A, is based on a plant concept that takes these specific demands into consideration.

The reference plant incorporates a number of pre-engineered options to allow site-specific variations and customer-required modifications to be easily accomplished at minimal cost. This approach assures high design flexibility while at the same time maximizing the use of standardized systems and components.

Project economies

A flexible approach to multi-shaft scope will maximize project opportunities
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Depending on the customer and the specific project, Siemens anticipates two main approaches for a successful project. First, the project development may be driven by the need to minimize initial investment cost for a required power output, for instance if capital costs are high. Second, a customer might set a goal of minimizing total costs during the lifetime of the project, and so needs to bring the life cycle costs to a minimum. There are some characteristics that support both general approaches.

For the GUD 2.V94.3A, a design concept was chosen that respects both possible approaches of a customer to his project. It is based on a split of the possible scope into a base and options (see Figure 2).

Starting from the base concept, the power plant may be modified into a low life cycle plant by adding pre-engineered options. The add-on process requires little time since a comprehensive set of options has been developed.

While dealing with low life cycle cost concepts, three contributors to these costs have to be considered:

  • Initial investment costs and capital costs
  • Fuel costs and efficiency
  • Operation and maintenance costs (O&M).

These three cost categories strongly determine the long term cost structure of the project. However, the turnkey contractor, together with the original equipment manufacture (OEM), has only partial influence on some of these contributors. They will strongly influence initial investment as well as O&M costs, but will have only a minor influence on the fuel costs. Hence low operation and maintenance costs through simplified O&M procedures play the crucial role in modifying the base concept to a low life cycle concept.

Six key factors lead the development and design of the reference concept and are derived from market requirements.

  • Efficient plant design minimizes initial investment costs
  • High output and efficiency reduce plant operating costs
  • High functionality meets individual customer requirements
  • Optimized O&M procedures increase customer benefit
  • Quality standards assure high availability and environmental performance
  • Low project risk and smooth project completion.

Efficient plant design

Reference plant basic scope and options: the synthesis of standard and individuality
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The plant layout of the GUD 2.V94.3A is characterized by a compact arrangement of the two gas turbine islands with the steam turbine island placed lateral to them. This grouping gives a low combined length of high pressure and cooling water piping and reduces the overall footprint of the plant.

The gas turbine centreline-to-centreline distance was established by reviewing space requirements for the gas turbine auxiliary systems, the power control centre, the erection procedure and maintenance access requirements.

The steam turbine island is placed lateral and perpendicular to the gas turbines. This arrangement reduces the amount of high pressure steam piping and simplifies the steam pipe routing adjacent to the steam turbine. It also allows the optional installation of a single stationary overhead crane to reach both gas turbines as well as the steam turbine for maintenance. A single house covering all turbines becomes possible.

The cooling method is strongly determined by site requirements. Consequently, the cooling tower, pre-engineered in the base configuration, may easily be replaced by once-through cooling or an air-cooled condenser without the need to rearrange major components of the plant.

The condensate extraction pumps are located as close to the condenser hotwell as possible on one side of the steam turbine. The feedwater pumps are placed next to the boiler on the steam turbine side. From here, both boilers are supplied with high pressure (HP) and intermediate pressure (IP) feedwater. This outside location keeps the space between the boilers free for additional systems such as the fuel gas preheater.

The electrical equipment is packed into power control centres (PCC) that are prefabricated and pretested modules. These PCCs are located close to the major components to be controlled to achieve a minimum combined length of power and communication cables.

The control room is a functional building that combines the major control equipment needed to operate the plant successfully. It may be extended to a single administration building, comprising the control room, several offices, social areas, the store and the adjunct workshop.

Standard modules in a GUD 2.V94.3A reference power plant
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The structured arrangement supports a smooth and efficient erection process for a reduced lead time and superior accessibility to all major plant components. There is generous space to reach the main machine sets with heavy lifting equipment for erection as well as maintenance. A clear separation of pipe and cable routing supports the transparent plant design.

All these features together can reach the goal of high project economics, irrespective of the approach preferred – low investment or low life cycle cost.

The redundancy concept of the water steam cycle has attracted considerable attention as it is often affected by customer preferences and operating philosophies. With reliability and availability becoming more and more crucial, utmost care in design and component selection has to be applied.

For this reason, the impact of possible redundancy concepts was investigated in a study undertaken by an independent organization, the T

This redundancy concept was found to be optimal in terms of cost-benefit considerations. However, pre-engineered options allow modifications in the redundancy concept to respect customer requirements and local regulations. For example, 2 x 50 per cent and 3 x 50 per cent configurations have been pre-designed for the feedwater and condensate pumps given, in case of a single pump failure, reduced or full plant output respectively.

The design criteria of the water steam cycle therefore grant low initial investment costs while providing the required flexibility in design and chosen availability of the power plant through options.

Functional concepts

Once the decision for the construction of a new power plant has been taken, the time period elapsing until the first power is delivered to the grid becomes important to the profitability of the project. Short time intervals, including the design phase as well as the erection and commissioning period, will reduce capital cost and lead to early power sales. Standardized concepts like the

GUD 2.V94.3A reference power plant can meet this target in several ways.

Pre-designed power plant concepts may seem to be a contradiction to flexible designs and high functionality, but this has been overcome by a high level of built-in modularity. This allows specific customer and site requirements to be accomplished without having to undertake costly and time-consuming redesigns. Modularity is achieved by grouping the combined cycle plant into functional units starting from the main components such as the gas turbine and steam turbine island. These functional units are then further subdivided into modules that can be exchanged as requested by customer or service requirements, fuel, local conditions, NOx restrictions and so on.

Today’s market requirements for flexible scope packages – such as econopac, power island, or turnkey scope – has also been considered in the design of the 2.V94.3A (see Figure 2). The econopac is the smallest functional unit that should be considered, giving the customer the basic building block for the power plant. It can be expanded to a power island so that guarantees for combined cycle gross power output and gross efficiency become possible. The EPC scope and contract allows net power output and net efficiency to be guaranteed as well as total construction time. The benefits associated with these three basic scope definitions vary with customer and market requirements, hence the GUD 2.V94.3A supports all three of them.

Pre-engineered options

The base version of the GUD 2.V94.3A was designed as a plant with low initial investment costs and optimal performance at baseload conditions. However, actual power plant projects may require adaptation to local conditions and operational specifications. These requirements are taken into account by several pre-engineered options which supplement the base version.

The options selected depend on customer requirements in conjunction with local conditions. The following examples demonstrate some typical applications of the various options.

The turbine hall may be preferred if harsh weather conditions predominate. It may be equipped with a main crane if mobile hoisting systems are not available at short term and at reasonable costs. On the other hand, if a crane is not available but weather conditions are favourable, an optional crane without a house is also available.

The superior plant flexibility even allows for the possibility to adapt lead times by choosing the phased construction option with a bypass stack. The bypass stack not only allows adaptation of lead times through phased construction, but also an operation independent of the water steam cycle to increase operating flexibility.

If the operating mode of the plant requires frequent start-up and shut-down procedures, the option of a bypass deaerator is advisable. For even faster start-up and shut-down an auxiliary steam generator can be incorporated into the system. The steam is used to pressurize the labyrinth seals of the steam turbine during the time plant is not in operation to reduce air intake into the water steam cycle and hence shortening the waiting time for steam purity during start up.

Fuel flexibility is introduced with the option to use fuel oil No.2 as a secondary fuel, where water injection for NOx reduction is another option.

Optimized O&M

The importance of O&M costs is increasing and in the near future their influence on life cycle costs will grow further. Siemens has taken this into consideration with the GUD 2.V94.3A reference plant. Various goal oriented activities have been identified to accomplish a beneficial integration of O&M into the plant.

The maintenance aspect of a power plant is quite often thought to be a component issue only. However, the plant design has significant influence on the maintainability of the installation.

For the GUD 2.V94.3A, maintainability aspects have been considered in the plant layout, with special emphasis on easy access to all components. For the base version, locations for mobile lifting equipment during the maintenance period have been predetermined and optimized by a full 3D erection and maintenance study. The optional crane for maintenance is capable of lifting the heaviest part to be moved.

Another important aspect in reducing the maintenance effort is the water-steam cycle. Here the pipe routing of all steam pipes is close to ground level at the shortest distance possible behind the boiler to facilitate easy access to all major valves. In addition, the amount of isolating and control valves in the water-steam cycle has been optimized to simplify maintenance and lessen the stock of spare parts needed.

Similarly, the optimized redundancy concept for the main pumps such as feed water pump, condensate extraction pump and cooling water pump leads to less components needing maintenance and an optimized spare parts stock.

Consequently, not only the maintenance effort but also the capital needed for spare parts is reduced.

If harsh weather conditions prevail, a turbine house might become necessary to cover both gas and steam turbines during operation and maintenance and streamline maintenance procedures. The installation of a turbine house or crane is a typical example of where the low investment cost concept is transformed into a low life cycle cost approach, reflecting changing priorities in customer requirements.

The recommended maintenance schedule of the entire plant is based on the maintenance requirements of the gas turbine. The gas turbine itself is designed for easy maintenance to shorten down-time during inspections. For the base version, the concept proposes the transferal of the larger parts to a neighbouring or temporary building for storage. Sufficient laydown space for most of the disassembled gas turbine parts is available inside the turbine building, if the optional, pre-engineered house has been chosen.

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