Large Steam Turbine Research Articles

The surge-stagnate model for complex design

Industry is facing ever-increasing demands for more performance at less cost. Current emphasis is on improving the design process to yield designs, at least technically equivalent to the present, more quickly. The surge-stagnate model (SSM) is introduced to provide an analysis framework, describing the way in which progress in design occasionally plateaus before striding forward. It has been found that there are two methods of accelerating the design, both may be achieved by the use of 'understanding'. The existence of the SSM has been verified through observation of case studies, and the results of one such study of a large steam turbine design are presented.

A metastable wet steam turbine stage model

A model for the prediction of the efficiency of axial flow steam turbine stage is described, where the flow through turbine cascade is considered non-homogeneous and metastable. At the exit an oblique shock brings it to equilibrium. The losses in the cascade are expressed according to Dunham and Came (Trans. ASME (1970)) and Kacker and Okapuu (J. Eng. Power (1982)) which is imprivemente of Ainley and Mathieson (1951) method. Two phase flow frictional multiplier is used as a correction factor for pressure coefficient. The model is compared with data of performance evaluation of large steam turbines of PWR power plants and results shows a good agreement.

The European Efforts in Material Development for 650.DEG.C. USC Power Plants. COST522.

Advanced Steam Power Plant is one of three working groups within the frame of European COST 522 with the aim of developing and evaluating ferritic steels for steam conditions up to 650°C. Today's state-of-the-art large fossil-fired steam turbines comprise live steam conditions of up to 610°C/ 300 bar and re-heat temperatures of up to 630°C. These ultra super critical steam parameters significantly increase plant efficiency and reduce fuel consumption and emissions of CO 2 . Ferritic materials should be used for thick-walled components to maintain high operational flexibility of such large plants. Rotors, casings, bolts, tubes/pipes and waterwalls are the critical components under current investigation. The class of the 9-12 % Cr steels offers the highest potential to meet the required property level for critical components. Therefore a significant effort to increase the application temperature of these steels was the focus of study within the European COST 501 programme and has led to improved materials for 600°C application of forged and cast components and for pipework. These 600°C materials are already being successfully utilised in a number of advanced European power plants. Further potential for improvement in creep strength seems possible after taking into account the oxidation resistance for T> 600°C. A large number of new ferritic-martensitic compositions, which have been designed on the basis of the positive outcomes attained in previous studies as well as on the results obtained with advanced thermodynamic calculation tools are under investigation in the new COST 522 programme. Full-size cast and forged components have been manufactured from the most promising compositions and now are being evaluated by intensive mechanical testing.

Two-valve control of a large steam turbine

Abstract The non-linear control system of a two-stage steam turbine is presented. The non-linearity in the plant model comes from the application of low pressure (LP)-valve to a control. The control system developed in this paper is based on feedback linearization and linear quadratic regulator. The performed experiments have shown that simultaneous operation of both high pressure (HP)- and LP-valves gives better results. The simulation results have been presented. The comparison of non-linear and linear control systems has been performed.

Neural Network-Based Modeling of a Large Steam Turbine-Generator Rotor Body Parameters from Online Disturbance Data

A novel technique to estimate and model rotor-body parameters of a large steam turbine generator from real time disturbance data is presented. For each set of disturbance data collected at different operating conditions, the rotor body parameters of the generator are estimated using an output error method (OEM). Artificial neural network (ANN)-based estimators are later used to model the nonlinearities in the estimated parameters based on the generator operating conditions. The developed ANN models are then validated with measurements not used in the training procedure. The performance of estimated parameters is also validated with extensive simulations and compared against the manufacturer values.

Neural network based modeling of a large steam turbine-generator rotor body parameters from on-line disturbance data

A novel technique to estimate and model rotor-body parameters of a large steam turbine generator from real time disturbance data is presented. For each set of disturbance data collected at different operating conditions, the rotor body parameters of the generator are estimated using an output error method (OEM). Artificial neural network (ANN)-based estimators are later used to model the nonlinearities in the estimated parameters based on the generator operating conditions. The developed ANN models are then validated with measurements not used in the training procedure. The performance of estimated parameters is also validated with extensive simulations and compared against the manufacturer values.

Two-Valve Control of a Large Steam Turbine

The non-linear control system of a two-stage steam turbine is presented. The non-linearity in the plant model comes from the application of LP-valve to control. The control system developed in this paper is based on feedback linearization and linear quadratic regulator. The performed experiments have shown that simultaneous operation of both HP and LP valves gives better results. The simulation results have been presented. The comparison of non-linear artd linear control systems has been performed.

APPLICATION OF ENERGY PROCESSOR MODEL FOR DIAGNOSTIC SYMPTOM LIMIT VALUE DETERMINATION IN STEAM TURBINES

With growing importance of quantitative technical condition assessment in critical machinery, the need for adequate determination of diagnostic symptom limit values is becoming vital. Such determination may be based on the energy processor model of a machine [1]. The general model should, for each specific case, be developed in order to account for unique features of machine design and operation. The paper describes such an approach for large steam turbines, operated by utility power stations. The energy processor model, adopted for these machines, is described and its mathematical description is presented, based on resonable simplifying assumptions. Possibilities of the determination of model parameters from data obtained during normal operation are outlined and discussed.

Calculation and Measurement of Torsional Vibrations in Large Steam Turbosets – New Technique

ABB has been carrying out torsional vibration test on steam turbine sets for many years, with sizes ranging from 350MW coal fired steam turbines to 1000MW nuclear steam turbines. Rated speeds have been 1,800 rpm and 3600 rpm. Torsional shaft vibrations are excited in large steam turbine generators sets by electrical disturbance torques occurring in the active part of the generators. Due to the low torsional damping of the shaft trains, resonance can cause high torsional vibration amplitudes. Some thermal power plant operators require experimental verification of the natural torsional shaft frequencies. ABB has developed a new measurement technique which allows torsional vibration to be measured in a selected measurement plane along the shaft. With today's highly sensitive sensors and modern measurement and data analysis equipment, the excitation due to normal random disturbances in the electrical grid provides sufficiently good results. All natural torsional frequencies in the range of interest are obtained with an accuracy of ± 0.2 Hz and identified using the calculated natural frequencies. The new measurement technique, places no restrictions on normal power plant operation.

Analysis of shaft torsional phenomena in governing large steam turbine generators with non-linear valve stroking

The paper focuses on how shaft torsional problems can be initiated in some situations by the fast action of steam control valves. It demonstrates that shaft flexibility can exert a significant influence on steam turbine-generator response following severe supply system disturbances, particularly for large machines where the turbine has nonlinear valve stroking and fast valving. The effect can be minimised by careful location of the speed sensor on the turbine shaft and by filtering to reduce speed input or acceleration signals due to troublesome low-frequency torsional vibrations to an insignificant level. Digital implementation of steam-turbine control systems is described. Response for a range of control philosophies where digital control procedures are used are given. Two large machines where digital speed sensing and filtering techniques in steam valve control are implemented are examined.

Some material toughness properties of a series of turbine casing bolts removed from service after over 100 000 hours

The present study was aimed at examining the effects of temperature on both the Charpy and slow strain rate fracture toughness properties of a series of large steam turbine casing bolts which had been subjected to temperatures of around 500°C for some 122 000 h. Also, the various toughness properties were assessed in terms of the existing fracture toughness–Charpy correlations. The present CrMoV steel bolts had suffered varying degrees of Reverse Temper Embrittlement (RTE) during service and exhibited strong fracture toughness–temperature trends. Although no single fracture toughness–Charpy correlation effectively described the full toughness–temperature transition curves, it was evident that, for embrittled and partially embrittled bolts, the Wallin correlation best decribed the lower temperature portion of the curves. At the upper temperature region all embrittlement conditions exhibited reasonable agreement with the upper bound correlation of Barsom and Wolfe. Finally, it was shown that the fracture toughness transition temperatures, T K, and the Charpy Fracture Appearance Transition Temperatures (FATT) exhibited good agreement with the fracture toughness transition temperature being equal to the temperature at which the fracture toughness value was 3000 N/MM 3/2.

On efficiency measurements for large steam turbine low-pressure stages

Current trends in the development of power engineering are turning attention towards modernization of the machinery and equipment. Modernization of the LP (low-pressure) part of a turbine is one of the most promising solutions. A natural question which arises concerns the effectiveness of such modernization, i.e. a comparison of the characteristics (efficiency) of this turbine part before and after modernization. Such a comparison may be based on measurements of the efficiency of the LP part and the last stage before and after the modernization, provided that the efficiency gain achieved exceeds the measuring error. This paper deals with the estimation of the accuracy of such a measurement, the principle and methodology of which have been developed at the Institute of Fluid-Flow Machinery (IFFM) for a real turbine.

Off-design flow analysis of low-pressure steam turbines

In this paper, a method for flow calculation in large steam turbines is presented. The method is based on the through-flow theory and the finite element procedure. It includes some extensions and improvements in the form of a new combination of loss correlation, radial distribution of losses and spanwise mixing, a new procedure for density calculation and an extension to encompass reverse meridional flow. The method is applied to the LP (low-pressure) part of a 165 MW steam turbine for which the flow field and performance over a wide range of mass flow and input pressure are calculated. The flow at far off-design loads, at which flow reversal behind the last stage occurs, and the last stage changes to ventilation are especially analysed. The comparison of numerical results with experimental data shows good correspondence.

Analysis of torques in large steam turbine driven induction generator shafts following disturbances on the system supply

The paper first summarises the advantages of steam turbine-driven induction generators over conventional generators such as low cost, less maintenance, rugged and brushless rotors (squirrel cage type, no need for synchronisation, etc.), together with problems concerning excitation (VAr compensation at loads etc). A mathematical model of the induction generator simulated in direct-phase quantities where saturation of the magnetising reactances is simulated and saturation of stator and rotor leakage reactances is ignored is developed and employed for detailed simulation of the machine. Discrete-mass models of the machine shaft where both steam and electrical viscous damping is simulated are employed in comparing transient shaft torsional response evaluated by time domain simulation and frequency domain analysis following incidence and clearance of severe system faults. The paper then investigates torsional response following incidence and clearance of severe supply system disturbances, when the rotor is stationary and when running at close to synchronous speed unexcited, and following malsynchronisation when excited by a controlled VAr source, together with torsional response following bolted stator-terminal short-circuits at full-load and no-load following switching in of the induction generator onto the system supply. It examines precision of predicting torque in turbine-generator shafts by frequency domain analysis not analyzed for induction generators in the literature heretofore following incidence and clearance of worst-case disturbances on the supply. Effect of steam and electrical damping on maximum shaft torques predicted by frequency domain analysis is also illustrated. The results illustrate there is no tendency for shaft torques to become more onerous as the fault clearing time is increased as is the case for shaft torques in large synchronous machines. Three large two-pole machines of rating of up to a few hundred MWs are analysed.

Service embrittlement trends in large CrMoV steel steam turbine bolts

The present paper examined the reverse temper embrittlement, RTE, characteristics of nearly two hundred large intermediate pressure (IP) and high pressure (HP), CrMoV steel turbine bolts which have been removed from two identical 120 MW units which had been in service for over 120 000 hours at elevated temperatures of around 500°C. It was immediately clear that the extent of toughness loss or RTE response was primarily controlled by three parameters, viz, grain size, d, accumulated service strain, %ϵ, and bulk phosphorus level, wt.% P. In an effort to establish the various average RTE responses from a significant bolt population they were separated into groups in terms of bolt size and unit from which they were removed. It was shown that, essentially, the various IP bolt groups exhibited strong influences of accumulated strain on the degrees of embrittlement recorded while the different HP bolt groups revealed significant effects of grain size. Finally in order to predict the average RTE response of the present CrMoV steel bolts the average embrittlement trends were portrayed in terms of an embrittlement factor, Q, which was a function of grain size, accumulated strain and bulk phosphorus level. It was established that the degree of embrittlement, % EMB, could be adequately predicted from an expression containing √ Q. Hence, the individual influences of the various parameters affecting embrittlement could be easily portrayed by use of Q and % EMB.

Turbine Oil Reservoir Study

Flows in a typical large steam turbine oil tank were modeled in a one-fourth-scale Plexiglas mockup. The flow patterns in the lank were observed using dye injection into water. Measurements of particle concentrations established the mechanisms of particle removal with and without filtration. A simple lumped-parameter mathematical model describing the oil clean-up process has been developed and verified experimentally.

Approach to residual life assessment of large steam turbine components

Ansaldo has manufactured and installed in Italy and all over the world, since 1909, medium and large steam turbines under its own design and various licence agreements, such as General Electric, Westinghouse, and, more recently, ABB. Since 1986 residual life assessment research programs have been implemented with a big effort, and a special approach to residual life assessment has been defined with our main customer, the ENEL Company, for the activity to be carried out in order to get the optimum balance between safety and cost consideration. The paper will illustrate this approach as well as the techniques developed for residual life investigations. In the paper are presented examples of the documentation for data acquisition, examples of the application of investigation techniques such as computer echotomography, boresonic and material sampling on rotors, and finite element analysis on critical main steam valves.

A Comprehensive Classification of Combined Cycle and Cogeneration Plant: Part 1: Presenting the Map of Turbomachinery Options for Heat and Power

An overall picture of combined gas and steam cycle options, without dependence on numerical analysis, is long overdue. A single equation is derived for plant thermal efficiency. This is based upon component efficiencies and the ratio of steam plant size to gas turbine size. Therefore, a unique map can be graphically presented covering the whole steam range from power generation through to cogeneration (that is combined heat and power) in association with gas turbines. Understanding of the map leads to recognition of an underutilized area. The gas turbine is normally integrated with the main steam boiler. Conversely, the unfixed combined cycle plant may be integrated with a conventional boiler and back-pressure turbine. The correct steam turbine-gas turbine size ratio must be chosen. A practical example is given of how a gas turbine and waste heat boiler may be introduced part-way along a large power station's main steam turbine. The integration is achieved in the steam cycle and not at the main boiler. This approach allows low-risk, low capital cost, simple repowering of large power stations. It is not a new thermodynamic cycle, but has the following advantages: 1. The tie between the extent of after-firing and waste heat boiler efficiency is avoided. The conventional boiler is fired and the waste heat boiler is unfired. Integration of the plant occurs at the intermediate pressure level of the steam turbine. 2. The above integration gives high thermal efficiency. It also retains the conventional boiler, or equivalent, which is free to burn a range of fuels. 3. The gas turbine waste heat boilers and the main boiler together fill a common steam turbine of an economic size. Two types of plant are considered to demonstrate the practical usefulness of the integrated steam cycle. Examples are given based on coal-fired, or heavy-fuel-oil-fired combined cycle plant, integrating conventional boiler, gas turbine plus unfired boiler and steam turbine. The data are based on real equipment, which is either at existing UK installations or plant of known performance. An explicit analysis is also presented to compare fuel running costs for the integrated steam cycle with costs for both unfired combined cycle plant and conventional steam plant.

The Aerodynamic Characteristics of Steam Turbine Reaction Stages. (Aerodynamic Characteristics of Multistages).

The aerodynamic characteristics of reaction stages of a large steam turbine have been studied for the purpose of performance improvement. For the second step, the characteristics of the multistage with typical reaction bladings were studied by two-dimensional low-speed cascade tests and three-stage air turbine tests. The principal results are as follows. (1) In stationary rows, profile losses are almost the same as the two-dimensional cascade test results in low-turbulence flow. (2) In rotating rows, profile losses are about double those of the stationary rows, which is the same phenomenon as observed in single-stage turbine tests. Secondary flow losses near base sections are fairly larger than the two-dimensional cascade data based on the estimated blade exit flow angles, sin-1λ. (3) Stage efficiency distributions along the blade height indicate the capability of performance improvement near the base diameters. (4) The internal efficiency of the tested multistage reaction turbine can be estimated by total-to-static efficiency for the last stage and total-to-total efficiency for the other stages.

Aerodynamic Characteristics of Steam Turbine Reaction Stages. (Aerodynamic Characteristics of a Single Stage).

The aerodynamic characteristics of reaction stages of a large steam turbine have been studied for the basis of performance improvement. For the first step, the characteristics of a single reaction stage with typical reaction blading were studied by two-dimensional low-speed cascade tests and single-stage air turbine tests. The principal results are as follows. (1) Loss distribution in the stationary row is similar to the two-dimensional cascade test results. (2) In the rotating row, profile losses are more than double those of the stationary blades, and secondary losses near base section are fairly larger than the prediction based on the cascade test results. (3) Turning angles of flow through the rotating row near the base diameter are fairly larger than the prediction. (4) Stage efficiency distribution along the blade height shows performance improvement near the base diameters.