PREVENTING THRUST BEARING FAILURES:

Rotor Axial Position and Thrust Bearing Temperature Monitors:

Machines such as turbines and compressors can be severely damaged if the thrust bearing fails. Such failures can be prevented if careful consideration is given to the machinery with regards to protective instrumentation.

Axial thrust position is the measurement of the relative position of the thrust collar to the thrust bearing. The primary purpose of an axial thrust position monitor is to ensure against an axial rub between the rotor and the stator.

Once a thrust bearing is loaded, the only additional change in position in the loaded direction short of failure is the combined effect of pressing the thrust bearing components more tightly together, thinning the oil and in some fabricated designs, elastic deformation of the thrust bearing support.
	Damaged Thrust Bearing Pads and Thrust Collar

In practice, this means that a loaded thrust bearing subjected to an increasing load will not allow much shaft movement right up to the point of failure. When failure occurs, it is usually catastrophic, and movements of 6 mm (0.236 ins) or more can occur in a matter of seconds causing severe internal damage. Thus, even though absolute limits for axial position can be established, the small movement which occurs up to the point of failure, combined with the rapid movement of failure, will generally allow thrust bearing damage even on machines protected with continuous position monitors on automatic shutdown. What the monitor can do however is to prevent contact between rotating and stationary components such as impellers and diaphragms if alarms and trips are set up correctly.

	To anticipate a thrust bearing failure due to overload soon enough to prevent damage to the thrust bearing itself, temperature sensors embedded in the bearing are required. With this method, an increasing load is seen as an increasing metal temperature and it is a highly responsive indicator of an overload condition.
A change in thrust temperature is probably one of the best indicators of a locked gear coupling. It can also warn of unusual operating conditions that might alter the thrust balance of a machine and overload the thrust bearing.

Axial position and thrust bearing temperature measurements are perhaps two of the most important parameters to monitor on turbo-machinery, as measuring a single variable at a single location may not ensure sufficient protection. Hence, both systems should be installed but if cost is a constraint, fit a thrust temperature measuring system in preference.

Note: Do not rely on temperature sensors in the oil discharge line. Most thrust bearings intermix the oil from the active and inactive sides of the thrust bearing before it reaches the drain outlet line temperature indicator and this temperature does not indicate the actual temperature of the active face of the thrust bearing, i.e., the metal temperature. This system is too slow to react to abnormalities and hence will not prevent thrust bearing failures.

 

Calibration of Axial Position Thrust Monitor

Calibration Curve for Axial Position

Contact Machinery Vibration Consultants Limited for advice on the following:

1.0 RECOMMENDATIONS TO HELP PREVENT THRUST FAILURES
1.1 Rotor Axial Position and Thrust Bearing Monitoring Systems
1.2 Temperature Measurement
1.3 Axial Position Measurement
1.4 Oil Temperature Sensors
1.5 Bearing Metal Temperature Limits
1.6 Thrust Bearing Metal Temperature Alarm and Trip Levels
1.7 Thrust Bearing Axial Position Alarm and Trip Levels
1.8 The Balance Drum
1.9 Increasing Thrust Bearing Temperatures
1.10 Start-up Conditions, Partial Load Operation
1.11 Shutdown Justification
1.12 Wear on Balance Piston Labyrinth
1.13 Compressor Surge
1.14 Lubrication

2.0 POINTS TO NOTE DURING OVERHAULLS 2.1 Machine Alignment / Thermal Growth 2.2 Relative Axial Positions of Machines in a Train 2.3 Electric Motors’ Magnetic Centre 2.4 Magnetic Centering Forces 2.5 Axial Forces With Gear couplings 2.6 Shimming Thrust Bearings 2.7 Thrust Collars 2.8 Thrust Bearing Pivots 2.9 Welding on Machine Casings 2.10 Replace Worn Bearings 2.11 Location of Temperature Sensors