TELEMETRY FOR STEAM TURBINE BLADE
1.) WHAT IS TELEMETRY ?
- Telemetering is defined as the complete measuring, transmitting, and receiving for indicating at a distance, by electrical translating means, the value of a quantity.
- The measurement is made by such transducers as the resistance strain gage, thermocouple, pressure transducer, accelerometer, etc.
- The transducer converts the measured quantity to an electrical signal, which is then transmitted without a conductor to a receiver.
2.) NEED FOR TELEMETRY IN BLADE VIBRATION MEASUREMENT
- Sensitive data such as blade surface stress, pressures and vibration from high speed rotating machine have been traditionally obtained using slip ring arrangements that typically involve the transfer of electrical signals from rotating conductive rings to stationary brushes in contact with these rings
- Slip ring assemblies have limitations on rotational speed, restricting their use in high-speed rotational systems like turbines.
- A viable alternative is the use of a telemetry system to transmit data from a high-speed rotating source to the laboratory data acquisition system using radio signals in the FM bandwidth.
- The advantage of this system is the in situ amplification of source signals and non-contact type transmission of these signals, allowing high-speed testing and minimizing noise. The quality of data and the versatility of use make telemetry an excellent option for acquiring much-needed blade data.
3.) TYPICAL TELEMETRY SYSTEM BLOCK DIAGRAM
4.) CHALLENGES FOR APPLICATION OF TELEMETRY IN STEAM TURBINE
- Telemetry systems for steam turbine applications face severe challenges not associated with the common, long-range telemetry systems used in commercial and personal communications.
- One of the most challenging requirements of rotor telemetry is that of providing power to the rotating transmitter modules
- The steam-turbine environment is characterized by high temperatures
- high accelerations on rotating elements
- surrounded by highly conductive metallic materials.
5.) TYPICAL POWER SUPPLY SCHEME FOR TELEMETRY
6.) SENSOR USED IN TELEMETRY SYSTEMS FOR BLADE VIBRATION MEASUREMENT
STRAIN GAGE SENSORS
PIEZO FILM SENSOR
The sensitivity of piezo film as a receiver of mechanical work input is awesome. In its simplest mode the film behaves like a dynamic strain gage except that it requires no external power source and generates signals greater than those from conventional foil strain gages after amplification.
Frequency response is thus free from any limitations imposed by the need for high gains and will extend up to the wavelength limit of the given transducer.
The extreme sensitivity is largely due to the format of the piezo film material. The low thickness of the film makes, in turn, a very small cross-sectional area and thus relatively small longitudinal forces create very large stresses within the material. It is easy to exploit this aspect to enhance the sensitivity
PIEZO FILM SENSOR & BASIC CIRCUITRY
SIGNAL CONDITIONING
- The primary principles of signal conditioning include:
- Filtering—Electrical filters designed to give the desired band pass and band-rejection characteristics.
- Averaging—If the desired signal exhibits periodicity, while the undesired signal is random, signal averaging can increase the signal-to-noise ratio.
- Common Mode Rejection—By wiring two equal areas of a piezo film electrode out-of-phase, unwanted common-mode signals can be made to cancel.
7.) FEATURES OF TELEMETRY
- No Slip Rings
- No Shaft Modifications Required
- Transmitters available for Strain, Torque, Pressure, Voltage, Temperature…. almost any type of signal
- Clamp-on Collar houses Transmitter(s) and Induction Power Converter. Contains embedded transmitting antenna
- Transmits Sensor Signals via Radio Transmitter to a Stationary Receiver Immune to electromagnetic interference, dust, oil, moisture, steam, etc.
- Excellent precision and linearity (0.1%)
- Small and light enough to install to narrow space to take data
- High ability of frequency response
- Multi-Channel Systems Available (36 channels)
