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The monitoring system of large power plant steam turbines (TSI) is the “nerve center” that guarantees the safe operation of the unit, and the specifications and reliability of sensor installation directly relate to the safety of the entire power plant. Below, we will elaborate on the installation specifications and common issues of three key sensors: vibration, axial displacement, and speed.
1.Vibration Sensors
There are mainly two types of vibration sensors:eddy current sensors (used to measure relative vibration of the shaft) and velocity/acceleration sensors (used to measure absolute vibration of the bearing housing). Modern large steam turbines typically install both types simultaneously to synthesize the absolute vibration of the shaft.
Vibration Sensor Installation Specifications
1.Selection and Verification of Sensor Type:
•Shaft vibration: Use an eddy current sensor. Before installation, its linear range and sensitivity must be verified to ensure that the probe operates near the midpoint of the linear region under the preset gap voltage (usually -10VDC or -12VDC corresponding to the gap).
•Bearing vibration: Use a velocity sensor or an acceleration sensor (with an integrator). Before installation, verify that its frequency response characteristics meet the vibration frequency range of the steam turbine (usually 10Hz~500Hz).
2.Installation Position and Orientation:
•Shaft vibration probe: Typically installed at each bearing location at 90°±5° along the X (vertical) and Y (horizontal) directions to capture the trajectory of the shaft. The probe should be perpendicular to the shaft surface and avoid discontinuities such as keyways.
•Bearing vibration sensor: Installed at the most rigid position of the bearing housing that can best transmit vibrations, usually on the machined mounting surface of the bearing housing. The direction should be consistent with the measured vibration direction.
3.Installation Gap (for Eddy Current Sensors):
• This is the most critical step. The initial gap must be set strictly according to the probe specification. For example, for a probe with a linear range of 2mm, the initial gap is usually set to 1.0mm (corresponding to a gap voltage of -10VDC).
• Usemicrometers or dedicatedgap gauges for precise adjustment, and monitor the gap voltage with a multimeter to ensure accuracy.
4.Fixing and Protection:
• The probe must befirmly locked to prevent loosening due to vibration during operation. The bracket should have sufficient rigidity to avoid self-resonance.
•Cables: Must use coaxial cables specified by the manufacturer. Connections should be clean, dry, and tight, with protection against oil, water, and high temperatures. Cables should be secured to avoid friction damage.
5.Grounding and Shielding:
• The entire measurement system (probe extension cables, preamplifier) must besingle-point grounded, usually at the preamplifier end. The shielding layer should also be grounded at one end to avoid forming ground loops that introduce interference.
Common Issues in Vibration Sensor Installation
1.Inaccurate gap voltage: Not calibrated accurately during installation, causing the probe to operate in a nonlinear region, resulting in distorted measurements.
2.Loose probe: Locking nuts not tightened, causing the probe to gradually withdraw or penetrate during operation, leading to measurement drift or sudden changes.
3.Mechanical damage: The probe head may be damaged during installation or maintenance; cables may be corroded or worn due to high temperatures or oil contamination.
4.Electromagnetic interference: Poor cable shielding or improper grounding introduces 50Hz power frequency or other high-frequency interference, causing vibration signal readings to be too high or unstable.
5.Bracket resonance: Insufficient rigidity of the installation bracket may cause resonance at certain frequencies, amplifying the vibration signal and causing false alarms.
6.Probe facing mechanical defect points: If the probe is directly facing scratches, rust spots, or keyways on the shaft, it may generate false pulse signals.
2.Axial Displacement Sensors
Axial displacement is usually measured usingpaired eddy current sensors to eliminate the influence of radial shaft wobble.
Axial Displacement Sensor Installation Specifications
1.Paired Installation and Thrust Disk:
• At least two probes must be installed, located on both sides of the thrust disk (working face and non-working face), and aligned on the same axis.
• The probes should be aimed at the flat end face of the thrust disk, not at sloped or toothed areas.
2.Zero Position Setting:
• This is the core of the axial displacement sensor installation. In the unit’s cold state, with the rotor pushed towards the working face, adjust the gap of the two probes so that the sum of their gap voltages is zero (or a preset zero position value).
• This setting ensures that when the rotor is in the theoretical middle position, the instrument displays zero.
3.Calibration and Linearity:
• It must be ensured that the characteristics (sensitivity, linearity) of the two probes are consistent; otherwise, measurement errors will be introduced.
• After installation, manually rotate the rotor to allow it to move slightly within the thrust gap, checking whether the instrument display values match the actual mechanical displacement.
4.Rigid Fixation:
• The measurement of axial displacement is related to emergency shutdown; its bracket must have extremely high rigidity, as any slight deformation can lead to serious measurement errors.
Common Issues in Axial Displacement Sensor Installation
1.Incorrect zero position setting: Incorrect positioning in the cold state, or setting while pushing from the non-working face to the working face, leading to an erroneous measurement reference during operation.
2.Inconsistent probe characteristics: Significant differences in sensitivity or linearity between the two probes lead to asymmetric and inaccurate forward and reverse displacement readings.
3.Probe misalignment: The two probes are not on the same axis, measuring displacement on a slope or different concentric circles rather than purely axial displacement.
4.Uneven target surface: The measurement surface of the thrust disk is worn, warped, or dirty, causing fluctuations in measurement values.
5.Bracket deformation: Deformation of the bracket due to temperature or mechanical force changes the relative position of the probe to the thrust disk.
3.Speed/Key Phase Sensors
Speed measurement typically uses eddy current sensors or magnetoresistive sensors, while key phase sensors must be eddy current sensors.
Speed/Key Phase Sensor Installation Specifications
1.Key Phase Sensor Installation:
• The probe should be aimed at the key phase mark on the shaft (a boss or groove). The mark should be sufficiently prominent with clear edges.
• The installation gap should meet the same requirements as the vibration probe to ensure signal strength. The key phase signal is used to provide a phase reference for vibration and accurate speed pulses.
2.Speed Sensor Installation:
•Eddy current: Align with a gear wheel (speed gear), ensuring the gear teeth are flat and evenly spaced. The gap setting specifications are the same as above.
•Magnetoresistive: The gap with the gear teeth should be adjusted according to the manufacturer’s specifications, usually smaller. Its directionality must be noted.
3.Signal Quality:
• After installation, the waveform must be observed with an oscilloscope. The key phase signal should be a clean, steep pulse wave; the speed signal should be a uniform sine wave or square wave.
• The amplitude and shape of the pulse must meet the triggering requirements of the TSI system.
Common Issues in Speed/Key Phase Sensor Installation
1.Key phase signal loss or weakness:
• The probe is too far from the key phase mark.
• There is oil contamination or rust at the mark, leading to a deterioration in the reflected signal.
• The mark itself is too smooth, with unclear edges.
2.Unstable speed signal:
• The teeth of the speed gear are damaged, uneven, or have magnetic materials attached.
• The polarity of the magnetoresistive sensor is reversed or the gap is improper.
3.Electromagnetic interference: Similar to vibration sensors, interference signals can superimpose on the speed signal, leading to counting errors, especially at low speeds.
4.Summary and General Recommendations
1.Follow Specifications: Strictly adhere to the installation manuals and maintenance procedures of manufacturers (such as Bently Nevada, GE/EPRO, Siemens, etc.).
2.Meticulous Operation: Sensor installation is a delicate task; rough handling is strictly prohibited. Use the correct tools (torque wrenches, micrometers, oscilloscopes, etc.).
3.Documentation: Keep detailed records of each sensor’s installation position, initial gap voltage, cable numbers, instrument ranges, etc., for future maintenance and fault analysis.
4.System Coordination: After installation, it must be coordinated with the TSI system for channel verification, simulating signal checks to ensure the PLC/DCS display, alarm, and shutdown logic are correct.
5.Regular Maintenance: During major and minor overhauls, a comprehensive inspection, testing, and calibration of sensors and their circuits should be conducted to prevent issues.
In summary, the installation of TSI sensors is a task that closely integrates theory and practice, where any minor oversight can pose significant safety risks. A rigorous attitude, standardized operations, and rich experience are key to ensuring their reliable operation.
You may also refer to:
1. Knowledge | Detailed Explanation of Each TSI Monitoring Probe of the Steam Turbine
2. Innovation | The First Fully Domestic Steam Turbine TSI Commissioned at Huaneng Laiwu Power Plant!
3. Original Quality | Installation Precautions for Small Steam Turbine TSI (Illustrated)
4. Original | An Incident of Unit Shutdown Caused by Aging of TSI Equipment
5. Analysis of Vibration Protection Action Trip of a Gas Turbine TSI System
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Source: Steam Turbine Brother (ID: qijibaoge)