Method for determining the untwist of turbine blades

DETAILED DESCRIPTION The present invention is directed to a method for determining the untwist of turbine blades under dynamic conditions.
The method is comprised of the steps of producing a first pair of blade passing event signals in response to a blade tip's movement past a first pair of fixed sensors.
The signals comprising the first signal pair are compared to one another to establish a first differential value.
Another data point containing information relevant to the vibrational condition of the turbine blades is produced.
The first differential value and the other data point are evaluated to discriminate between blade untwist and synchronous vibration.
According to one embodiment of the present invention, the step of producing another data point includes the steps of producing a second pair of blade passing event signals when a blade tip passes a second pair of fixed sensors.
The second pair of sensors is displaced rotation-wise from the first pair of sensors an amount which is a nonintegral multiple of the wavelength of the resonant frequencies of the blade.
The signals of the second signal pair are compared to one another to establish a second differential value.
The first and second differential values are then evaluated to discriminate between blade untwist and synchronous vibration.
According to another embodiment of the present invention, the step of producing another data point includes the steps of changing an operating parameter such as turbine speed, steam flow rate, or temperature, from a first value at which the first differential value was established to a second value.
A second pair of blade passing event signals is produced when a blade tip passes the first pair of fixed sensors.
The signals of the second signal pair are compared to one another to establish a second differential value.
The first and second differential values are evaluated to discriminate between blade untwist and synchronous vibration.
According to another embodiment of the present invention, the degree of blade untwist can be used to infer the condition of the lashing wires