On-LineTurbine Internal Efficiency Monitoring for Steam Turbine Condition Diagnostics

Abstract

The integral diagnostics factor used for the evaluation of turbine island condition is the turbine heat rate corrected to constant steam parameters and condenser pressure which is called “corrected turbine heat rate”. In case of excessive deviation of corrected heat rate from its normal value, it is necessary to implement a diagnostic subsystem for determining the source of the deviation. The following subsystems of the turbine island have to be taken into consideration: turbine steam path, turbine end seals, control valves, etc. For each subsystem it is necessary to select the calculated or measured parameter(s), which will be used as diagnostic factors for turbine condition evaluation purposes. For the turbine steam path subsystemthe internal efficiency of the high pressure turbine (HP) and intermediate turbine (IP) can be used. Selection of the internal efficiency as diagnostics factor is based on its high sensitivity to variations in steam path conditions (erosion, steam path fouling and so on), as well as to steam path diaphragm and radial sealing conditions. However, the basic internal efficiency calculation results of HP and IP turbines at different loads, using measured parameters, show very high sensitivity of the calculation results to random measurement errors, random components of the regulation process and variations in spatial and time distribution of the temperature and pressure fields. This high sensitivity causes the uncertainty of the calculation results to approach ± 1.5 % and means that the results can not be used for diagnostic purposes. To achieve acceptable results for uncertainty (± 0.3 %) special methods of mathematical analysis need to be applied, including robust estimation, precise multivariate approximation, filtration and others. Furthermore, the HP turbine internal efficiency depends on control valve position and thus can not be directly used as a diagnostic factor in an on-line turbine diagnostic system. To solve this problem, special methods were developed for correcting the real efficiency value to the valve wide open position. The developed method was verified by a set of specially designed tests and results have shown that this technique can be applied for on-line calculations. The described system for on-line turbine internal efficiency monitoring was developed in cooperation between IEC and Berman Engineering Ltd. The system was implemented on a 550 MW unit and has been in operation for two years. The results of operation show that the system yields the required expanded uncertainty in the range of ± 0.3 %. The system is characterized by high reliability and a friendly interface.