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Condition monitoring of turbine blades

Blade tip timing system leads to improved operational strategy

condition monitoring of turbine blade tip timingThe increased demand for operational flexibility causes low pressure turbines to be more often run under sub-optimal conditions. This leaves them more susceptible to vibrations, which can ultimately damage the blade. Through blade tip timing, operators can better estimate the risk of vibrations and adjust plant operation accordingly. Laborelec managed to bring this theoretical concept into practice by installing a tip timing system in the steam turbine of the French DK6 Power Plant in Dunkerque.

Europe is moving towards more renewable energy generation units. As a result, turbines are being pushed towards lower load and operational flexibility, which they were not designed for. This can lead to increased stress on the turbine blades, in the form of vibrations.

Measuring vibrations and blade sensitivity

The last row of low pressure turbine blades is especially sensitive to vibrations when the turbine operates at sub-optimal conditions, such as low load or incomplete vacuum. Tip timing consists of sensors mounted in the inner rotor casing that determine the passing time of each rotor blade. Using special software, we can use the data to calculate vibratory characteristics, such as the frequency and vibration amplitude of each blade. In this way, we obtain an indication for vibrations and can identify the blades that are most sensitive to them. As such, the technique could be the basis of a condition monitoring tool that assesses turbine blade health. Power plant operators can use this information to better define the risks of operating the turbine under certain circumstances and adjust the operational flexibility accordingly. Moreover, they can better tailor the maintenance plan and take early action for the blades that are more at risk for damage, thus minimizing unexpected downtime and repairs.

Careful positioning of sensors

During May and June ’13, Laborelec installed sensors at the low pressure steam turbine of the French DK6 Power Plant in Dunkerque. We carefully investigated the positioning of the sensors, since this requires the delicate and meticulous drilling of holes in the inner casing. Moreover, the installation needs to happen during the short outage period. Our experts reengineered the inner casing turbine walls and precisely measured the distance between the rotor and the casing. They also determined the relative expansion of the turbine. The biggest challenge was determining the perfect angle so that the sensor would overlap with the blade tip. We installed four sensors in order to obtain sufficient measurement accuracy. Analysis of data collected over the past four months is ongoing.

It should be noted that, while a blade tip timing system can be installed on almost any turbine, it remains an invasive measure. However, once the system is installed, the results—and subsequent rewards—are tangible.

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