Axial Position Monitoring

Setting of Thrust Probe Voltage

• Ideally, the center of the transducer’s linear range should correspond to the center of the rotor’s float zone

• But it is difficult to position rotor in the middle of the float

• So we set the rotor to one end of the float and set the voltage correspondingly.

• In this case, center of linear range is 10.0 V. Float is 16 mils, half of which is 8 mils. If rotor moves to one side voltage will increase or decrease by 8 x 200 mv/mil =1.6 V

• So when rotor is in active side set voltage to 10 + 1.6= 11.6V

Adjusting the Meter Reading

• First check the calibration of the monitor independently using any DC voltage source

• With rotor in active side set probe voltage to 11.6 V.

• Adjust the meter zero pot to bring the reading to 8 mils ‘normal’.

• If the rotor is now pushed to the inactive side the meter should read 8 mils ‘counter’

Calibration of Axial Position (thrust) Monitor

Setting Alert and Danger Levels

• Hot float zone will be more than the float measured in cold

• With +/- 8 mils float the hot float will be around +/- 12mils

• Set alarm so that some babbitt will be removed before alarm actuates – about 6 mils beyond hot float or about +/- 18 mils

• Danger or Trip may be set 10 mils beyond Alert or at +/- 28 mils

Axial Probe ArrangementsOne at end of shaft, One at Thrust disc

• Advantage – If thrust collar becomes loose big difference would develop between the readings of two probes. Thrust collar probe would read same, shaft end probe reading would go up.

• Disadvantage – If Thrust Monitor is on ‘voting trip’, it would never cause a trip even if rotor contacts stator

One Probe at Thrust Collar One at End of Shaft

Axial Probe ArrangementsBoth Probes Looking at End of Shaft

Better

• Advantage – Will always detect changes in axial position of rotor. If connected to trip will always prevent rotor-stator rub.

• Disadvantage – Will not directly detect looseness of thrust collar. However, looseness of thrust collar can be inferred indirectly from the fact that bearing temperature will not go up even when thrust position reaches alert level

Both probes observing end of shaft

Axial Probe ArrangementsTwo Probes at End of Shaft, One at Thrust Collar

Best arrangement

• Best possible arrangement, will not miss anything• Probe looking at Thrust disc can be used for many purposes

• To detect loose thrust collar• To confirm bearing wear due to electrostatic shaft current• To measure and analyze axial vibration

Dual Axial Probes for Thrust Position Measurements

Dual Thrust Position Voting Logic

Voting Thrust Position with Temperature

• Voting dual thrust position monitors have very much reduced the occurrence of false trips due to probe / proximitor / cable faults

• However, some plants have tried to vote thrust position with bearing temperature through an external ‘AND’ gate thinking this would further improve reliability.

• Seems logical but can be dangerous under following circumstances

Voting Thrust Position with Temperature

• There are several situations when real high axial position may not be accompanied by high bearing temperature

1. Loose Thrust Collar – Axial load would not be transferred to thrust bearing. Bearing temperature would never increase

2. Electrostatic discharge damage – As babbitt is slowly eroded shaft would move axially but temperature would not increase much.

3. Lost temperature indication – this is much more common than thrust probe or monitor malfunction

Thrust Position V/S Thrust Bearing Temperature

• During thrust bearing failure the thrust monitor would always give the first alarm before the bearing temperature monitor. • If thrust position is voted with temperature, in rapid failures, this delay could mean more serious damage

Thrust Bearing temperature measurement locations

Location of thermocouples on sleeve bearing

Location of thermocouples on tilting-pad bearing