Gerta Zimmer

Modelling and simulation of steam turbine processes: individual models for individual tasks

Within power plants, several physical, chemical and mechanical processes are conducted to transfer the energy, stored in fossil fuel, into electrical energy. This energy conversion is divided into several stages. Hitherto, the largest conventional power plants employ steam turbines as prime movers to drive a generator. Hence, a steam turbine is one module to convert heat energy into mechanical energy. And thus it is one link in the chain of energy conversions with the aim of generating electrical energy. Today, steam turbine industry faces numerous challenges concerning efficiency, commissioning time, start-up times, operation, availability, safety, cost-effectiveness, etc. Many of these tasks can be supported by simulating the transient operational behaviour of the turbine in advance. For example, the commissioning time can be shortened if the turbine controllers are initialized with well-tuned pre-set parameters; cost-effectiveness can be increased by setting aside unnecessary devices and exactly determining material specifications; safety may be increased by predicting the impacts of failures and thus taking the necessary precautions. Different tasks require different details regarding the employed turbine simulation model. Thus, the turbine controller may be well tuned with less complex simulation models of turbine, generator and electrical grid, whereas detailed studies of failures, mainly the transient behaviour which may lead to serious damages, may require detailed modelling of the turbine-internal thermodynamic processes. Here, a brief overview of models which simulate the transient thermodynamic behaviour of a steam turbine is presented. Three different approaches will be introduced and compared with respect to different operating situations. Also, special attention is directed towards the time dependence of critical states, mainly turbine speed and pressure development in certain areas. The first model is based on a simple, linear approach and is suitable of giving a quick overview. The second one incorporates more details and is useful if the operating point is close to the design point. Finally, the last model incorporates mass and energy balances as well as the major non-linearities. Hence it depicts the turbine behaviour over a large range of operating points.

A New Emergency Stop and Control Valve Design: Part 2 — Validation of Numerical Model and Transient Flow Physics

In order to enhance steam mass flow through a turbine it becomes necessary to reduce the flow resistance of the turbine inlet valves. Consequently, a replacement of the high pressure turbine inlet valves is required. The valve combination described in this paper consists of a control valve and an emergency stop valve, opposite to the control valve. Both valves share a common valve seat. The control valve is a single-seat valve with integral pilot disc. A pre-stoke is introduced to allow for moderate opening forces. The emergency stop valve closes in countercurrent with the steam mass flow.The flow through the valve is analyzed by steady state and transient computational flow simulations. In addition to the steam mass flow, the forces acting upon the valve are determined. Transient behavior will be investigated by means of analyzing pressure fluctuations. Therefore frequencies caused by the steam flow are determined in the range up to 2000Hz.It will be shown that neither steady state nor transient simulations with a simple eddy viscosity turbulence model are capable to correctly predict the complex flow inside the valve. More sophisticated turbulence modeling like Large-Eddy simulation is thus inevitable. Furthermore, the physical phenomena causing the transient behavior are discussed. All findings are verified by comparison of the CFD with the measurements.Copyright

On Linearizing the Steam Mass Flow Relative to the Controller Output

In order to develop efficient control and to predict a steam turbine’s power output precisely, it is desirable to have a linear relationship between controller output, RA, and steam mass flow. Unfortunately, steam mass flow through a turbine is not only determined by the control valve stroke but also by the pressure in front of these valves. Even at constant pressure the relation between valve stroke and steam flow through the turbine is extremely non linear. The complexity is increased by the fact that a turbine is generally operated by two or four control valves which do not necessarily work parallel over the complete operational range. Additionally, the pressure in front of the control valve changes with the admitted steam mass flow. A counterbalancing method was developed that allows to include individual process data, such as pressure in front of the valves and at the turbine inlet in dependence of steam mass flow and valve throttle characteristics, as well as process engineering constraints, like valve staggering, for example. The developed method is applicable to new steam power plants as well as to retrofits. With the developed method it is also possible to predict individual valve strokes at valve testing or at deviating exterior conditions. The later feature is extremely useful for retrofit applications. Firstly, the correctness of the implementation with respect to the ‘old’ set-up can be verified, and then, secondly, the characteristics of the retrofitted components and thermodynamic conditions, respectively, can be substituted. The developed method was successfully applied for several power plants. A comparison of predicted data and commissioning data will be provided.Copyright

Controller benchmarking algorithms: some technical issues

(1) Strategy and Planning Coordinator (West Africa), BG Group, Reading, UK, (2) Professor of Automotive Engineering, Faculty of Engineering, Kingston University, London, UK, (3) Research Fellow, Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow, UK, (4) Siemens AG Power Generation, Muelheim an der Ruhr, Germany, (5) Siemens AG Power Generation, Muelheim an der Ruhr, Germany, (6) Manager, CESI SpA, B.U. Transmission & Distribution, Milan, Italy.

Optimized Steam Turbine Operation by Controlled Clutch Angle Engagement

Some single shaft CCPPs are known to display increased vibrations if engaged at certain angles and a very smooth operational behavior if engaged at other angles. Neither the reason nor a mechanical mitigation for this behavior is known.Roughly, the engagement angles can be clustered into four 90° sectors, one of them is deemed unfavorable and another one is classified as favorable with respect to vibrations of the overall power train shaft. Although the first priority is to have an assembly that runs smoothly at arbitrary engagement angles, an option to engage at a predefined ‘good’ angle provides an I&C back-up solution to avoid undue vibrations.To meet this requirement a control method was developed to run up the steam-turbine thus that it will engage at a pre specified angle. The basic idea of the developed control algorithm is to increase the acceleration of the steam-turbine during run up if a prediction reveals an engagement angle that would be too small to be within the desired area and, conversely, decrease the acceleration if the anticipated angle would be too large.A control algorithm to exploit this idea was developed and tested with the aid of a simulation model. The simulation model comprises a detailed thermodynamic model of the steam-turbine, the control valves and their actuators, the steam-turbine shaft and the shift clutch. I&C was modelled with all details relevant to turbine speed control. The gas-turbine/generator unit, however, is cut down to angular velocity and relative angle, respectively.The performance of the controller was validated to comply with parameter uncertainties, different sampling times and frequency fluctuations of the electrical grid during coupling. In parallel, a high precision angular measurement device was developed and integrated into the existing turbine I&C.Hardware-in-the-loop tests with simulated turbines and a hardware I&C system implementation also revealed a very satisfactory performance.On site implementation of a prototype was successfully accomplished and resulted in the predicted accuracy of the preset engagement angle.Copyright

A New Emergency Stop and Control Valves Design: Part 1 — Experimental Verification With Scaled Models

For the experimental verification of a new emergency stop and control valve design extended tests on a low pressure as well as a high pressure air test rig were performed. As the required thermodynamic parameters for a full scale test cannot be met, a scaled version of the valve design was tested. The scaling was done taking into account the laws of similitude. The inherent valve characteristics as well as pressure distributions and forces were gathered by means of steady and unsteady probes. While the tests in the low pressure test rig were performed at similar Mach numbers, the high pressure tests were performed also with Reynolds numbers sufficiently similar. A transformation of the pressure pulsations to the real steam valve was done by means of the Strouhal number. In the low pressure test rig ambient conditions were used for the inlet air. A vacuum pump was delivering the airflow through the tested valve model. The valve model was equipped with approximately 50 test points. For the high pressure test rig a six-stage radial compressor with interstage cooling was used. The valve model was equipped with approximately 40 test points. Due to the limitations of the compressor and other adjacent systems the tests in the high pressure test rig were conducted with a stepped operational concept using different mass flows and inlet pressures for the tested valve model. The inherent flow characteristics as well as the pressure pulsations of both measuring campaigns were compared with one another. Matches as well as mismatches are discussed. Additionally, the results were compared with steady state and transient CFD simulations, which is described in Part 2 (GT2014-25117).Copyright

Design Philosophy and Dynamic Calculation Method for Optimized Load Rejection Characteristics of Steam Turbines

A load rejection disconnects the generator from the electrical grid. The resulting power excess accelerates the turbo set. Reacting to the load rejection, the turbine governor rapidly closes the steam admission valves. The remaining entrapped steam expands, thereby continuing to power the turbine. Thus the turbine speed rises till a dynamic equilibrium of accelerating and braking forces is reached. Thereafter the turbine speed decreases. If the maximally attained turbine speed remains below the trip threshold, immediate re-synchronization to the electrical grid is possible. Consequently, a forced outage of the steam turbine can be avoided and operational reliability is increased. Furthermore, functional safety requirements demand that the maximum turbine speed remains below test speed under all failure conditions. Accordingly, steam turbine design has to account for the impact of overspeed for a reliable and safe operation of the turbo set.In order to manage load rejection requirements for steam turbine operation, the design engineer applies standard rules and overspeed calculation methods. These rules limit standardized overspeed estimation by defining maximum steam volumes, valve closing times, and I&C reaction times, as well as type and number of non-return valves.A more thorough turbine overspeed investigation is necessary for several reasons, such as to evaluate this behavior under undesired failure conditions e.g. failure of non-return valves or blocking of control valves. A second justification for this investigation would be to predict changes resulting from turbine modifications — e.g. turbine upgrade or change at I&C systems.In this paper, basic and advanced overspeed calculation tools are illustrated and compared, with respect to required effort as well as accuracy of prediction. It is shown how system parameters which are most sensitive with respect to overspeed can be identified and their influence assessed. Thus, firstly it is already possible to identify and improve critical overspeed behavior during design. Secondly, the impact of particular failures can be accurately predicted, thus allowing for due implementation of appropriate counter measures.The methods, presented in this paper, were developed by the authors and their predecessors at SIEMENS AG for large steam turbo sets with a power range between 100 MW and 1500 MW.Copyright