Turbovisory Instruments

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Differential expansion

Axial Shift

Shaft Eccentricity

Turbine Speed

Bearing Vibrations

Sealing

To Condenser

Eccentricity

• What is eccentricity?

The circle can be said is an ellipse with zero eccentricity.

Reason of Eccentricity• Sagging due to weight ( Mechanical related)• Uneven expansions ( Temperature related )• Bearing Problems ( Mechanical Related )

What is eccentricity and how it is measured?

• Deviation of mass center to geometrical center.

• Generates vibrations

• Increases with speed

• Measured in microns / millimeters

Type of measurements

• Inductive type ( 50Hz to 20KHz)

• Proximity Type ( 500 Khz to MHz)

DM LP Filter4rth order AMP

Peak Detector4-20mA0 – 500 microns

10Khz

A1 A2 B2 B1

OSC

Proximity Sensor• It is a flat coil of wire located at the end of a ceramic tip,

protected by epoxy fiber glass.

• Probe generates EMF at radio frequency.

• As the EMF cuts any conductor nearby its vicinity eddy currents are produced which absorbs the power.

• The loss of power is proportional to the gap between the conductivity meter and the probe.

• This sensor also generates voltage oscillations proportional to the air gap and hence the signal is demodulated to stable DC output as that of inductive sensor.

Vibrations

• The most vital parameter to be logged during startup, shutdowns and also during normal running conditions.

• Can be transmitted radially as well as in axially .

• Calls for immediate action, since they lead to catastrophic disaster.

Why vibrations are created?

• Due to unbalance conditions

• Load changes

• Mechanical looseness

Vibrations measurement• They are measured in microns ( amplitude

of vibrations) or mm/sec ( speed at which it vibrates)

• They are normally measured in mm/sec as it gives fatigue and the other gives stress.

• There are two types of transducers that can measure velocity– Magnetic type ( 10hz – 150hz, < 6g)– Piezo electric type ( Accelerometers )

Magnetic Type sensors

• Based on the fundamentals of fleming’s right hand rule.

• Here there is a permanent magnet which when vibrated causes relative motion wrt a coil, generating emf directly proportional to the rate of vibrations.

Accelerometers

• Main constituent is Piezo electric crystal

• Vibrations induces force on the crystal.

• The vibration produces force on the crystal which then generates proportional voltage.

Axial Shift of the Rotor

• The thrust bearing is the point at which the rotor is connected to the stator axially, at the front end of HP turbine.

• Result of thrust in HP, IP, and LP turbines

Causes for increment of axial shift

• Increment of axial thrust– Resistance to steam flow inside turbine– Bearing failure– Oil flow failure

Axial Shift Detector

• Reluctance type transducer supplied with 10V rms and 1953 Hz supply in bridge configuration

• Basic components are as follows– Detector coils (LVDT)

– Amplifier

– Rectifier

– Lineariser

– amplifier

Differential expansion of Rotor and Cylinder

• The difference in their rate of expansion is due to thermal inertia inbetween rotary and stationary elements

• It is basically rotor axial expansion – stator axial expansion

• Normally it is a positive value

Occurrences of high differential Expansion

• High Positive value– During Startup

– Low Load running

– High exhaust temperature

– Restrain to casing expansion

• High Negative value– During Shutdown

– High Load running

– Low exhaust temperature

Detector of Differential Expansion

• It is same as axial shift detector , expect here we have two axial shift detector that are arranged in bridge configuration.

Casing expansion

• Turbine is anchored nearly in the middle of LP Casing

• Expansion in the front is cumulative of the casing from the anchor point. It also shows the degree of thermal soaking done.

• Expansion is also measured in the middle pedestal

• Abnormal expansion indicates serious fault– Less jamming, lack of freedom of movement

– More permanent creep

Casing expansion Measurement

• LVDT – Linear Variable Differential Transformer ( More Used )– Does not have mechanical linkages, hence life is more

• Rectilinear Potentiometer

– Has mechanical linkages , hence less life

• LVDT – Linear Variable Differential Transformer ( More Used )

OSC10KHz

AMP RectifierSmoothening

ckt

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Speed Measurement

• This is the most important parameter of the power plant, and if not monitored properly may led to irreparable disaster

• There are three ways of measurement– Slotted wheel – voltage pick ups when subjected to magnet field– Permanent magnet is connected in the shaft which when rotated generates voltage

proportional to the speed– Eddy current pick ups.– Laser