A novel technique for steam turbine exhaust-pressure limit control using dynamic pressure transducers.

摘要

A novel approach is presented to increase the operational flexibility of steam turbines. Pressure at the exhaust of the last stage of a condensing steam turbine is one of the most important parameters that limit the operation of a steam turbine - especially on days with hot ambient temperature. On these days when the power demand is at its peak, the power plant may be forced to reduce power production due to insufficient condenser cooling capability in order to maintain exhaust-pressure within specified limits.; The main concern in operating at these off-design high exhaust-pressures is that it can result in flow separation which can lead to aerodynamics instabilities and thus to blade vibration and failure due to high cycle fatigue. Current exhaust-pressure limits are generally established based on experience and are conservative in order to protect the blade from catastrophic failure. This research proposes a new method to place dynamic pressure transducers around the perimeter of the last stage blades to measure pressure variations caused by vibrating blades. This approach will enable real-time monitoring of the pressure signal, enabling real time detection of blade vibration, thereby allowing operation at increased exhaust-pressures without risk to the last stage blade, ultimately enabling the power plants to produce more power during times of peak demand.; A deep insight into the phenomenon of aerodynamics instabilities in steam turbines was obtained and parameters that influence stability were identified. Finite element analysis was used to predict the modal and structural response of a full row of blades. CFD analysis was performed to predict the impact of higher exhaust-pressures on the steam flow at the exit of the last stage blade. Two distinct experiments were conducted on a subscale low-pressure steam turbine. In both tests, different sets of blades, nozzles and steam path hardware were used. The first test was conducted to identify the appropriate pressure transducers and understand the proposed technique. In the second test, the transducers were applied at wide range of turbine operating conditions by changing steam mass flow, exhaust-pressure and turbine speed. The dynamic pressure transducers responded accurately in all operating conditions. FE analysis predictions were validated using test results. Dynamic pressure transducer response was compared with strain gauge response and an excellent agreement was observed between two sets of data. The transducers were clearly able to identify the lower as well as higher order modes of vibrations. Details are provided to define allowable dynamic pressure amplitudes based on the material capabilities of the blades, thereby allowing the blades to operate to their maximum capabilities.; Successful application of this technique will allow a wider range of operating conditions and hence a more reliable and profitable steam turbine. This unique technology will have a direct impact on the power production in high demand seasons and can result in increased profitability for the turbine operators.