Setting the Standard

In the world of the competitive kilowatt hour, equipment manufacturers have re-examined and re-thought their products and strategies. The result is units designed to meet the complex needs of the competitive power market, such as ABB`s STF steam turbines.

Franz Suter, Alexander Zbiorczyk,

ABB Power Generation Ltd.,

Switzerland

Dr. Alexander Tremmel,

ABB Kraftwerke AG,

Germany

The focus in the power generation business today has shifted from satisfying a power demand to delivering a competitive kilowatt hour. In fact, the market is increasingly shaped by deregulation, globalization, privatization, availability of finance, environmental pressure, changing energy demands, and the drive for economic growth in developing countries.

This has caused equipment suppliers to re-think their strategy and bring their product ranges in line with market needs. In order to support utilities and independent power producers in regions such as India and Southeast Asia, and to meet the many complex market demands, ABB has developed a range of standardized reheat steam turbine frames (STF). The sizes of these STF units has been determined by market factors.

ABB`s development of the STF range has differed from its other development strategies. The focus has not been on components alone but on the whole turbine island. With high system integration of the steam turbine, turbogenerator and related auxiliary systems, a balance has been found between process performance on one hand and the economical boundary conditions on the other.

The range includes the STF60 with a sub-critical and supercritical solution, the STF25 ranging from 210 to 280 MW and the STF15 – a simple and robust unit for a rated output range of between 120 and 150 MW.

Design principles

ABB designed the STF15 as a simple, compact turbogroup which combines high operational flexibility, short delivery time and proven design principles.

The STF15 consists of a two-casing steam turbine and an air-cooled turbo-generator arranged in a highly integrated design. The equipment is optimized for a conventional steam power plant with a single reheat water steam cycle and six-stage feedheater configuration. It can be adapted within the application range given in Table 1 and for lower evaluated processes with fewer extractions.

The STF15 is available as a table- or floor-mounted design. Both foundation concepts offer a compact layout for cost effective integration into the overall power plant with a total length of the rotor train of around 20 m. This layout requires less space and, consequently, lower civil work investment costs. This is beneficial to the customer.

The fixpoint of both designs is the steam turbine and the location of the oil supply unit. Common for both layouts is also a rigid foundation design for the turbo set and the condenser with a compensator between the systems. All dimensions of casings remain the same for every design point within the specified application range.

The outside dimensions and the interfaces of the turbogroup are fixed to simplify the power plant planning process. The steam path of the STF15 has been defined with a high degree of flexibility regarding the blading that ensures maximum efficiency under project-specific boundary conditions.

The HP turbine is of double shell design to maintain operational flexibility at low stress levels. The inner casing is split into two halves at the horizontal joint and held together by shrink rings. This design ensures a rotationally symmetrical inner casing, which results in homogeneous stress distribution and great thermal flexibility during load changes. It permits rapid start-up and shutdown and excellent behaviour during load changes. It also allows easy maintenance with minimal overhaul downtime.

The IP/LP turbine consists of an IP section and a flanged-on single-flow LP section. The IP turbine is of the reverse flow type and equipped with cast outer and inner casings.

The inlet scrolls in the IP section ensure low aerodynamic losses. The IP inner casing is split horizontally and bolted together by elongation bolts.

The IP turbine has two reheat stop valves and two reheat control valves, mounted directly on the IP outer casing. The diffuser of the reheat control valve serves also as inlet pipe into the inner casing. Each valve has an independent actuator.

The main components of the single-flow LP turbine section are the welded outer casing and the LP blade carrier which is supported at the turbine centerline and guided by adjustable elements in such a way that it can expand freely.

ABB`s high performance blading, first introduced in 1994, is used. The pre-twisted rotating blades with integral shrouds ensure excellent damping and consequently low stress levels.

In addition, all rotating blades except the last stage blades are equipped with integral shrouds to improve blade dynamics and minimize tip leakage.

The last two stages of the advanced LP module family, which are common for all ABB steam turbines, have an optimized transonic blading with a 3D-vane design that gives an uniform radial flow distribution where hub separation is avoided.

The free-standing last row design results in reliable operation due to the exactly calculated natural frequencies. Also aerodynamic losses from wires and interconnecting elements are avoided and individual blades can easily be replaced without casing disassembly.

The last stage rotating blades have induction-hardened leading edges for optimum erosion resistance.

The rotors, fully assembled and bladed, are balanced at nominal speed and subjected to a three minute overspeed test at 120 per cent of nominal speed. At least two balance planes are available, accessible from the outside, for field balancing without casing disassembly.

The HP turbine has an impulse wheel, two differently sized nozzles and two live steam control valves. This simple concept permits constant pressure operation with nozzle group control and three optimum load points or sliding pressure operation with fully open live steam control valves. The live steam control valves have actuators with integral electro-hydraulic converters which are connected with the electronic turbine controller, realized with the Advant Power instrumentation and control system.

The two control valves open simultaneously during start-up. Therefore the turbine is warmed up uniformly during start-up due to parallel operation of the valves. After synchronization with the grid, the partial-arc admission programme, pre-selected by the operator, is started and the turbine loaded to the adjusted target load. The change-over from one opening sequence to another is possible without load changes.

The electro-hydraulic actuators used for STF15 turbine control valves have excellent dynamic properties. Quick load changes as a demand of the grid as well as rapid load reduction and load shedding to house-load can be maintained. The STF15 steam turbine can operate well within a frequency fluctuating or unstable grid.

STF15 turbogenerator

The STF15 turbogenerator incorporates a combination of knowledge and experience together with proven components to ensure reliability and availability.

For the rotor and stator, air is used as the cooling medium. The air is recooled in a closed circuit by means of air/water heat exchangers.

The coolers are arranged in a separate cooler box, attached to the generator casing, directly mounted to the foundation plate. This design allows a simple layout of the foundation, as no cooling air channels are needed in the foundation. The cooling air flow is generated by axial fans on the shaft. the stator core is cooled through radial slots; the stator winding is indirectly cooled.

The STF15 generator casing design follows a multi-functional concept. The housing encloses and protects stator, rotor and heat exchangers, and at the same time serves as a sound dampening enclosure during operation. Generator casing and cooler box are transported to the site as two separate modules and are directly mounted to the foundation plate.

The implemented system integration of the turbogenerator and the steam turbine – i.e. common lube oil, jacking oil and cooling water supply – leads to a compact layout of the whole steam turbine frame. As a consequence, the STF15 turbogenerator can easily be transported by railway or ship and allows a streamlined installation and commissioning work on site, reducing the cycle time of the project.

Fast track schedules

By establishing pre-engineered documents, well-defined interfaces as well as pre-assembled modules, a significant reduction of the cycle time for the steam turbine generator down to 14 months from project start to commercial operation can be realized.

With this cycle time the steam turbine generator is far away from the critical path of the boiler or other components. For example, Figure 3 shows a typical boiler construction period of 21 months for a 150 MW plant which also defines the overall project time schedule.

Highly standardized footprints of the steam turbine generator and the availability of detailed pre-engineered documents and drawings makes it possible to deliver all project specific documentation necessary for a precise plant planning procedure within only one month after contract comes into force.

Hence the steam turbine island can be used as a fixpoint within the planning process. For example, with the pipe routing and the interfaces available it is obvious that pipe collisions below the foundation can be avoided at an early design stage.

The hardware delivery of the steam turbine generator can be shifted within a range of seven months depending on the project criteria. After that, procurement and manufacturing activities start in parallel so that all transportation modules are ready for shipping at the latest after nine months. In this special case, a two month shipment phase is assumed.

For shipment and erection purposes another development issue of STF15 is of importance. With the design optimization of the steam turbine generator components, the number of transportation modules is reduced significantly. These are highly pre-assembled and ready for immediate use. The whole generator consists of only five transportation modules.

Moreover, turbine systems such as the turbine controller are pretested in the workshop, so that a high quality level can be maintained on site, a factor which is absolutely necessary for the realization of a six week erection phase of the steam turbine generator.

The commissioning is also scheduled for a six week period, starting with system checks overlapping the erection activities. This includes the optimization of the steam turbine generator under site condition and a 72 hour trial run.

As a first consequence of this time schedule, the amount of interest to be paid during construction is reduced significantly. Also capital allocated to the project is freed up in such a way, that initial down payment comes seven whole months later than would have previously been necessary.

Servicing units

Additional advantages of the STF15 arise from standardization. Due to the growing fleet experience with this type of machine, the basis for an extension of the operation periods between the overhauls is provided. For the first time there is the opportunity in the steam turbine business to implement a spare part inventory concept different from the historical approach of storing huge spares packages at customer side.

In the framework of a long term co-operation agreement, ABB keeps an inventory of parts for the STF fleet on behalf of its customers. Emergency spares delivery is possible within 24 hours to an international airport.

In a long term view, the centralized stock enables the implementation of continuous development improvements on spare parts. This concept, in combination with a 24-hour contact hotline and the implementation of shared tools for exchange of operational data, results not only in minimum downtime, increased availability and short response time but has benefits also in lower investment and reduced operating cost.

Meeting demands

The steam turbine frame STF15 is designed for conventional reheat steam power plant applications with the focus on the IPP and developing country markets and is expected to meet complex market demands.

The STF15 steam turbine is a unit which is fixed in outer dimensions and footprints. Thus easier and more precise planning processes can be established as a result of the availability of pre-engineered documentation and drawings, highly pre-assembled modules, which are delivered ready-made.

The STF15 has high flexibility of integration within project specific boundary conditions, namely live steam parameters and the conditions of the cold end of the plant. Therefore, this configuration is standardized and fixed where no flexibility is asked for and it leaves room for adaptation.

The technology and design therefore reduces project risk, so the insurance issue becomes easier and the financing potential becomes higher. The standardization gives a completely new opportunity of the spare part inventory at the supplier, resulting in less investment and lower operating cost.

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Figure 1. ABB`s family of steam turbine frames is designed for competitive markets

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Figure 2. STF15 steam turbine cross-section

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Figure 3. The steam turbine generator time schedule is reduced through standardization

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Figure 4. Advanced LP blading