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TURBOVISORY SYSTEM

Introduction:

Mass of the Thermal turbine is very huge and when operating a machine of this size with the small

clearences required to achieve high plant efficiency. The correct interpretation of the informationprovided allows the plant to be run up and loaded in the minimum time consistent with safe operation.

It is the purpose of the turbine supervisory equipment to provide this information.

The measurements covered by the Turbovisory system include:

1.) Eccentricity2.) Turbine Speed.3.) Speeder gear Position.

Eccentricity:

It is defined as the out of centre excursion of the axis of rotation of shaft. It is normally measured as

the diameter of the locus, traced out by the shaft centre. For practical reasons, the measurement is

made on the shaft, within the bearing pedestal. The change in radial air gap within the cylinder is

inferred from this measurement.

Eddy Current Measurement System:

The system for measuring the eccentricity, consists of a transducer that operates on Eddy current

principle. The tip of the transducer contains a coil of wire, which is connected to the signal sensor.

The signal sensor contains an oscillator circuit, a detector or a demodulator circuit.

In this system, the transducer coil and connecting cable form a part of the oscillator circuit whichgenerates a high frequency current in the measurement coil, at a frequency of 1 or 2MHz. This current

produces a magnetic field around the pickup coil and the turbine shaft that brought into the proximity

of the coil links with the magnetic flux, producing eddy currents in the conducting material. These

eddy currents alter the operating point of the oscillator, modifying the amplitude of the signal,

proportional to the gap between pickup coil and shaft.

The average voltage is used to indicate gap from the transducer tip. The varying voltage is the AC

vibration signal. The slope of the curve, showing the change in the transducer output voltage as the

gap changes is the sensitivity of the system (2V/mm).

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Speed measuring through Toothed wheel and probe:

Of the variety of the speed detection methods, the toothed wheel, and magnetic probe is probably the

most useful choice. It is difficult to design toothed wheel systems which will inherently produce an

accurate sinusoidal output. In practice they normally produce output pulses of which, only the leading

edge is significant. No information is transmitted except at the instant that the pulses occur whichneed a high pulse repetition rate, if good stability is required. In general, the angular machining

accuracy of the teeth and mounting eccentricity of the wheel on the shaft will be independent of the

number of teeth. A probe system producing substantially a square wave output is usually desirable. It

must be suitable for use where a relatively large air gap exists between probe tip and toothed wheel,

which will accommodate the varying hydrodynamic conditions in the bearings.

Speeder gear Position:

Wire wound potentiometers are used for the measurement of speeder gear position and this istransmited to the control room by electronic system.