Turbine Axial Displacement: Definition, Causes of Abnormalities, and Countermeasures
Time: 2025-12-12
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Core Definition of Axial Displacement
The axial displacement of a turbine refers to the relative axial movement of the rotor along its axis with respect to the stator. Using the designed clearance between the thrust collar and the working and non-working surfaces of the thrust bearing as the zero point, a positive value indicates the rotor shifting toward the generator side, while a negative value indicates the rotor shifting toward the high-pressure side of the turbine. This parameter primarily reflects the load condition of the thrust bearing and the axial balance stability of the rotor, and is a key monitoring parameter to ensure that the flowing components of the unit do not collide or wear.
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Causes of Abnormal Axial Displacement Changes
(1) Increase in Forward Displacement (Rotor Shifting Toward the Generator Side)
1. Thrust Bearing Failure: Wear or burning of thrust pads, excessive thrust clearance, insufficient lubrication oil pressure, or excessive oil temperature leading to oil film breakdown, resulting in loss of axial positioning support.
2. Steam System Abnormalities: Sudden drop in main/reheat steam temperature (rotor thermal contraction lagging behind the stator), water carried by steam causing water hammer, creating instantaneous forward axial thrust.
3. Load and Steam Extraction Disturbances: Sudden increase in unit load (rapid rise in steam flow, axial thrust increases proportionally with flow), non-operation of high-pressure heaters without timely load reduction, abnormal increase in high-pressure cylinder steam intake.
4. Flow Passage Component Failures: Low-pressure cylinder blade breakage or shedding, high-pressure cylinder diaphragm deformation or seal wear, stage leakage and changes in flow area leading to imbalance in axial thrust distribution. (2) Increase in Negative Displacement (Rotor Shifting Toward the High-Pressure Side)
1. Sudden Rise in Steam Temperature: A sudden increase in main/reheat steam temperature causes the rotor to expand thermally faster than the stator, generating negative axial thrust.
2. Load and Extraction Failures: A sudden drop in unit load (sharp reduction in steam flow, rapid decay of axial thrust) or sticking/closure of the low-pressure extraction valve can lead to an increase in reverse axial thrust on the low-pressure side.
3. Abnormal Reverse Force on the Thrust Bearing: Wear on the non-working surface of the thrust pad or excessive reverse thrust clearance causes the rotor axial position to shift toward the high-pressure side.
4. Uneven Steam Admission to the High-Pressure Cylinder: Sticking or deviation in the opening of the high-pressure main steam valve or governor leads to unbalanced steam distribution on the high-pressure side, increasing the proportion of reverse thrust.
- Axial Displacement Abnormality Handling Measures (On-site Operable)
1. Preliminary Monitoring and Assessment
- Immediately verify the axial displacement gauges (compare main/backup gauges), and simultaneously monitor thrust bearing temperature (normal ≤85℃, alarm ≤90℃), lubricating oil pressure/temperature, thermal expansion difference, steam parameters, as well as unit vibration and unusual noises, to rule out instrument faults.
- Determine the trend of displacement changes (sudden change/gradual increase) and, in combination with operating conditions (load adjustments, parameter fluctuations, equipment operations), identify the root cause of the fault.
2. Graded Response Operations
(1) Below Alarm Value (Gradual Increase)
- Parameter Adjustment: Stabilize main/reheat steam temperature and pressure, open relevant steam trap valves (to prevent water carryover), adjust the lubrication system to maintain oil pressure ≥0.15MPa and oil temperature between 40–55℃.
- Operating Condition Optimization: Gradually adjust load (avoid sudden increases or decreases). When the high-pressure heater is out of service, reduce load by 20%–30% according to procedures to balance axial thrust.
- Status Tracking: Record displacement and bearing temperature data every 5 minutes. If the trend does not improve, gradually reduce the load to a safe range.
(2) Exceeding Alarm Value (Below Trip Value)
- Forward Displacement Exceeds Limit: Immediately reduce load to below 50% of rated load, stop high-pressure heater. If thrust bearing temperature continues to rise, start the backup lubricating oil pump. If still ineffective, trigger load reduction shutdown.
- Reverse Displacement Exceeds Limit: Stabilize steam temperature (avoid further increase), check the low-pressure extraction valve status. If the fault cannot be handled immediately, reduce load to 30% of rated load and isolate the faulty extraction section.
(3) Exceeding Trip Value / Emergency Condition
- When axial displacement reaches the trip setting value (normal forward 1.2–1.5mm, reverse -0.8– -1.0mm, executed according to unit procedures), or in cases of water hammer or thrust bearing temperature exceeding 95℃, immediately perform a manual emergency shutdown. Cut off main/reheat steam admission and fully open all levels of steam trap valves.
- Maintain the lubrication system operation for more than 30 minutes after shutdown to cool the thrust bearing. Subsequently, disassemble and inspect the thrust bearings, thrust clearance, flow components, and steam valve status. Restart only after the fault has been resolved and verified.
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