A reference for Success
Power market liberalization has caused equipment manufacturers to innovate new ways to meet the demands of time, efficiency and reliability. Siemens` reference plant, equipped with V94.2A gas turbine technology, is aiming to achieve this.
As a result of the energy market`s current trend toward privatization and liberalization, and the advent of independent power producers (IPP), power projects are increasingly evaluated in terms of profitability.
Power plant life cycle costs must therefore be minimized through low investment and operating costs, as well as short planning and power plant erection times. A high level of quality and mature, proven technology to ensure high availability is also needed. Today`s customers will also expect solutions tailored to their specific requirements.
Many original equipment manufacturers (OEMs) have therefore developed new designs and concepts to cater for these needs. German OEM Siemens has developed its `reference plant` concept.
Siemens` reference power plant is a pre-engineered design which has been optimized in terms of economics for a specific output range and market segment while meeting typical customer requirements. This avoids the problems associated with conventional planning such as expenses on time, manpower and money. The modularized design allows flexibility and can reduce project lead times.
The reference plant concept comprises function units, such as the boiler system, the gas turbine and steam turbine system, and so on. The function units are in turn made up of modules, such as the fuel supply system and the intake system. The power plants are designed such that the greatest possible number of identical modules are used. Adaptation to meet plant requirements is accomplished by swapping modules.
Standard configurations are the single-shaft plant with a triple pressure reheat cycle or the multi-shaft plant with either a dual pressure or triple pressure reheat cycle.
Single-shaft: Countries with a low total installed capacity or with limited, local needs for additional capacity require GTCC power plants in the 100 to 300 MW output range. The single shaft design, e.g. with a V94.2A gas turbine, is suitable here.
Multi-shaft: The multi shaft GTCC reference plant producing 500 to 600 MW with V94.2A gas turbines and a triple pressure reheat cycle is best suited for countries with a large installed capacity, high growth rates or unstable grids. Countries with low fuel costs or domestic gas reserves prefer plants with several multi-shaft units and a dual pressure cycle.
Siemens has used its reference plant concept at both King`s Lynn and Didcot B power plants in the UK. These feature V94.3 and V94.3A technology, respectively.
Life cycle costs
Siemens` objective with the reference plant is to minimize costs not only for the initial investment but over the entire life of the power plant. The effect of major parameters on cost is evaluated with the use of computer-based methods and tools.
Life cycle costs are a function of capital, fuel and operation and maintenance (O&M) costs, which are to some extent interdependent. Use of the V94.2A gas turbine, which is a high efficiency, high reliability unit with low NOx emissons, allows life cycle costs to be lowered.
However, some efficiency and output potential has been deliberately left untapped in favour of better service reliability and availability.
To ensure the robustness and reliability in the V94.2A design characteristics of the V94.2 gas turbine, which has more than three million hours of operation, were retained and enhanced with solutions from the 3A family.
Retained design characteristics from the V94.2 are:
à‚-Single casing, single shaft machine design with horizontal casing joint
à‚-Stacked rotor design with central tie-bolt and Hirth facial serations
à‚-Two journal bearings
à‚-Sixteen-stage compressor design
à‚-Ceramic tile lining in the combustion chamber
à‚-Four-stage turbine design
à‚-Free standing compressor and turbine moving blades
à‚-Axial exhaust gas diffuser
à‚-Generator drive flange at the compressor inlet
à‚-Pitch adjustment for first stage compressor inlet guide vanes.
The higher output and greater efficiency compared to the V94.2 was achieved through a number of measures:
à‚-A 100 degrees C increase in turbine inlet temperature
à‚-A 26 per cent increase in the compressor pressure ratio
à‚-A 2.4 per cent increase in compressor mass flow
à‚-Improved component efficiencies.
Efficiency was increased by 1.9 percentage points from the V94.2 to the V94.2A. This results in a 2.1 percentage points increase in combined cycle efficiency. The higher exhaust gas temperature now permits the economical use of a triple pressure reheat steam cycle and fuel preheating, which will permit a 3.3 percentage points increase in efficiency to 55.6 per cent.
Depending on the region, fuel price and service factor, power generation costs for a GTCC power plant with V94.2A gas turbines are from seven to ten per cent lower than those for plants using V94.2 machines.
The single-shaft design consolidates the main generation components into a single turbotrain, further reducing the number of peripheral plant systems This simplifies open-and closed-loop control, and enables optimal operating sequences during fully automatic startup, power operation and shutdown of the unit.
High flexibility is achieved via the self synchronous clutch between the steam turbine and the generator. The gas turbine can be started in a matter of minutes with the steam turbine uncoupled. The steam turbine is started as soon as the gas turbine exhaust begins raising appropriate steam in the downstream heat recovery steam generator (HRSG). The gas turbine can also be run by itself in simple cycle mode.
The multi-shaft plant concept with the steam turbine located between the gas turbosets offers the advantages of a small footprint and shorter main steam lines. The plant does not include a feedwater tank. An availability analysis and experience with the single-shaft King`s Lynn plant in England showed that a feedwater tank is not necessary. Additionally, analysis shows that a design including two 50 per cent feedwater/condensate pumps also fulfils the market requirements for reliability, availability and maintainability (RAM), as well as common condensate recirculating pumps for both HRSGs.
Variants including a 3 x 50 per cent pump configuration, a parallel deaerator or an auxiliary steam generator are also possible. There are also variants for high availability plants as well as for frequent startups of the plant for peaking or daily cycling dispatching. By using an in-boiler bypass circuit during fuel oil operation, the external condensate preheater can be eliminated.
Multi-shaft plants are also suitable for phased construction of the plant. At first, only the gas turbines will be operated, with the HRSGs and steam turbines being added later. This allows the operator to use the income from open cycle gas turbine operation to help finance the relatively expensive bottoming steam cycle. Most of these operators have no need to maximize the efficiency of their plants because their fuel costs are low.
In this case, the reheat can be eliminated in favour of a dual pressure cycle, allowing use of less expensive HRSGs and steam turbines.
Bypass stacks can be used to decouple the gas turbines from the HRSGs so that they can be quickly brought to full load via the turbines` maximum permissible load gradients, providing extreme operational flexibility.
The same gas turbine-HRSG modules are used for both the single-shaft and multi-shaft plants. However, a bigger steam turbine with a greater intake capacity and a larger double flow low-pressure exhaust cross-section is used to accommodate the higher steam mass flow generated from two gas turbines in the case of the multi-shaft design.
O&M costs naturally have a major influence on life cycle costs, and Siemens has implemented a number of measures to reduce these. All the stationary and moving blades in both the compressor and the turbine can be replaced without removing the rotor. This makes repair and refurbishment particularly quick. The annular combustion chamber of the V94.2A can be entered via a manhole, simplifying inspection of the first rows of turbine blading and the hot gas path of the combustion chamber.
The inspection and major inspection intervals of the GTCC plant are essentially determined by the gas turbine, which must be inspected every 8000 equivalent operating hours for a base load operated unit, i.e. once per year. The HRSG, the water/steam cycle, the cooling water system, etc. are also inspected during the annual inspection of the gas turbine. The barrel-type HP and IP/LP steam turbines need to be opened every 100 000 equivalent operating hours.
The generator requires a routine inspection every 25 000 equivalent operating hours, in which a borescope is used to examine the inside of the generator. As a result of this measure, the rotor only needs to be removed every 100 000 equivalent operating hours (ten to 12 years for a base load operation regime).
With the increasing use of diagnostic equipment such as `Digest` for turbine diagnostics or `Diwa` for monitoring water chemistry, the trend is moving away from fixed preventive maintenance intervals and toward condition-based maintenance.
To take advantage of the benefits of standardization, as many components and systems as possible are integrated into modules. A module comprises a technical unit which provides the same function in different plants of the same type. A variety of engineering solutions can be applied within a module to provide its function. Each module is designed for easy transportation and service, usually tested at the factory.
Modularization has created a building-block system, so the advantages of standardization can be enjoyed without the disadvantages such as inflexible planning due to rigid output and configuration schemes.
The result of this modularization is cost-effective power plants with short delivery times since the modules are designed to take full advantage of parallel engineering. The mature, proven modules ensure the high service reliability of the plants.
The increasing deregulation of the global energy markets has led to the emergence of a new type of power plant operator – the independent power producer. The IPPs prefer the turnkey supply of power plants with firm contractual assurances of technical and operational aspects such as output, efficiency and availability since they primarily view power plant projects in terms of profitability. Low life cycle costs, short project lead times and high availability are the most pressing objectives. Technical details often take a back seat to financing aspects.
Traditional utilities are also facing increasing cost pressure as a result of deregulation and liberalization of the energy markets. Thus they, too, are adopting a similar behaviour to IPPs concentrating especially on shorter planning times, faster erection and lower power generation costs, and less in technical details.
Figure 1. The Didcot B multi-shaft GTCC power plant is equipped with two GTCC blocks each with two V94.3/V94.3A gas turbines with their corresponding HRSGs in a multi-shaft arrangement. Total plant net output amounts to 1332 MW
Figure 2. CAD drawing of a single-shaft GTCC plant with the steam turbine connected to the generator via a self-synchronous clutch
Figure 3. CAD drawing of a multi-shaft GTCC plant with a common steam turbine-generator between both gas turbines
Figure 4. Section through the V94.2A gas turbine with the annular combustion chamber