The TSI (Turbine Supervisory Instrumentation) system serves as the most critical safety barrier for steam turbine units; key protection functions—such as monitoring axial displacement, shaft vibration, differential expansion, rotational speed, and shaft eccentricity—rely entirely on signals captured by TSI probes. Issues such as incorrect probe gap settings, loose mounting brackets, or substandard wiring practices can lead to measurement drift and frequent nuisance alarms, or, in severe cases, result in unplanned unit outages. Professionals in thermal control instrumentation know well that there are no trivial matters regarding unit protection: the stability of the main unit hinges on the stability of TSI measurement points. All critical monitoring, interlocking, and trip protection functions for the steam turbine rely on TSI probes. Ultimately, many on-site issues—including false alarms, signal fluctuations, and protection system anomalies—stem from inaccurate installation gaps, uncalibrated zero points, insecure mounting, or non-standard installation practices.
I. Axial Displacement Probe Installation (Core Protection for the Main Unit)
Axial displacement monitoring tracks the relative axial position between the rotor and the casing to prevent rubbing between rotating and stationary parts; it is one of the highest-level protection systems for steam turbines.
Comprehensive probe calibration must be completed prior to installation: checking for physical damage and verifying electrical insulation integrity. Linearity must be verified on a test bench using a dial indicator, and the 4–20 mA and 0–10 V outputs, as well as alarm and trip relay actuation states, must be checked to ensure good linearity.
On-site installation standards: Dual probes are installed on the same side of the main shaft, facing the low-pressure casing. Probe specifications: Φ14 mm; sensitivity: 3.937 V/mm; proximitor power supply: -24 V. The distance between the probe and the thrust collar must be less than 305 mm to avoid measurement distortion caused by thermal expansion.
- Measurement range: -2 mm to +2 mm (cold state).
- Installation voltage: -9.75 ± 0.2 V.
The dual-probe trip logic utilizes an "AND" configuration to effectively prevent nuisance trips caused by single-point failures.
II. Absolute Expansion Probe Installation
During steam turbine startup, shutdown, and load changes, temperature fluctuations in the casing cause thermal expansion. If sliding keys bind or expansion is uneven, risks such as casing tilting, abnormal foundation stress, and shaft misalignment can arise.
Two LVDT absolute expansion sensors are installed while the unit is in a cold state; they are mounted on dedicated brackets with their cores securely connected to the casing.
Casing expansion drives the displacement of the sensor core, generating a linear electrical signal. After processing by the circuit board, a standard 4–20 mA signal is output to monitor the overall casing expansion status in real time. The installation zero point must be aligned with the scale's zero mark.
III. High- and Low-Pressure Casing Differential Expansion Probe Installation
Differential expansion refers to the difference in expansion between the rotor and the casing.
The rotor has low mass, heats up rapidly, and undergoes significant expansion, whereas the casing has high mass and heats up slowly; consequently, their expansion rates are not synchronized. If differential expansion limits are exceeded during startup or shutdown, rubbing between rotating and stationary parts (or rotor-to-stator contact) is highly likely to occur. Installation parameters – Probe specification: Φ25mm eddy current probe
- Sensitivity: 0.8V/mm
- Measurement range: -2mm to 10mm
- Standard zero-position voltage (installed facing the generator):
- High-pressure (HP) differential expansion: -11.10V
- Low-pressure (LP) differential expansion: -3.80V
Industry convention: Rotor expansion toward the generator side is defined as the positive direction for differential expansion, facilitating unified monitoring and trend analysis.
IV. Installation of rotational speed and zero-speed probes
Rotational speed is a core parameter for steam turbine control; zero-speed detection is primarily used for the turning gear interlock during shutdown to prevent rotor bowing due to static resting.
Technical parameters
- Measurement range: 0–5000 rpm
- Zero-speed actuation value: <4 rpm
- High-speed alarm: 3240 rpm
- Probe gap: 0.8–1.0 mm (calibrated using a feeler gauge)
Two magnetic induction probes are installed facing the toothed wheel; rotational speed is calculated via pulse frequency:
Rotational speed = Pulse frequency ÷ Number of teeth * 60
When the unit speed falls below the zero-speed threshold, the turning gear automatically engages to maintain continuous slow rotation of the rotor.
V. Installation of bearing cap vibration (bearing shell vibration) probes
The bearing shell vibration sensor captures vibration velocity signals from the bearing housing; these are filtered and integrated to convert them into peak-to-peak vibration displacement values, monitoring the overall vibration level of the bearing.
- Measurement range: 0–100 μm.
Channel calibration is required prior to installation; a standard signal is input to verify that the loop is functioning correctly, ensuring accurate and reliable measurement.
VI. Installation of shaft vibration probes (Critical)
Shaft vibration measurement points provide the most sensitive and direct data for diagnosing shaft system faults.
Two Φ8mm eddy current probes (sensitivity: 7.87V/mm) are installed at each of the unit's bearings (No. 1 through No. 6).
The two probes are positioned at a 90° angle to each other and at 45° to the horizontal plane, monitoring vibration in the X and Y directions, respectively. Installation Standards
- Standard gap voltage: -9.75 ± 0.2 V
- Corresponding physical gap: Approx. 1.2 mm
Ensure operation across all conditions remains within the probe's linear range.
Field experience: While minor fluctuations in the shaft vibration zero-point are permissible, precise zero-point calibration significantly improves the accuracy of subsequent fault analysis.
VII. Installation of Shaft Eccentricity Probes
Eccentricity monitors rotor bowing and is a critical parameter to check before startup; rolling the turbine is strictly prohibited if eccentricity exceeds limits. Standard installation gap voltage: -10 V.
Key-phasor probes must not be installed directly facing the keyway.
Key-phasor signals also provide vibration phase data for spectrum analysis and fault diagnosis.
VIII. General Installation Precautions for TSI Probes (Essential for Field Personnel)
- Do not twist the lead wire to tighten the probe
Use a wrench to hold the probe body/platform steady while tightening; twisting the signal cable is prohibited. The fine internal wiring of eddy-current probes is susceptible to hidden damage, which can lead to erratic signal spikes or intermittent connection faults during operation—issues that are extremely difficult to troubleshoot.
- Cables must be neatly bundled and securely fastened
Ensure connectors are firmly plugged in and internal wiring is organized to prevent abrasion, tension, or grounding hazards.
- Probe face must be perpendicular to the target surface
Maximum tilt angle ≤ 1°; tilting directly causes linearity errors and measurement inaccuracies.
- Accurate alignment for displacement and differential expansion probes
Align the probe center with the center of the target flange (or slightly offset outward) to ensure effective measurement across all operating conditions.
IX. Common Commissioning Faults and Standard Remedial Actions
- High/Low-Pressure Differential Expansion Zero-Point Shift
Causes: Loose probe, bracket displacement due to vibration, shaft zero-position shift, or uneven cylinder temperature.
Remedial Action: Check low-pressure cylinder temperature, tighten the probe bracket, recalibrate the standard zero-point voltage, and secure the measurement point position.
- Continuous increase in axial displacement (rising to ±0.89 mm)
Immediately conduct a comprehensive inspection: check unit load, main and reheat steam temperatures, condenser vacuum, thrust bearing pad temperatures, differential expansion trends, local audible checks, bearing vibration, and exhaust temperature; coordinate with I&C personnel to verify instrumentation and promptly adjust operating conditions to prevent protection system actuation.
- Abnormal vibration exceeding limits (unit "red-line" criteria)
Before medium-speed warm-up: If bearing vibration > 0.03 mm, immediately trip the unit.
During critical speed passage: If bearing vibration > 0.1 mm or shaft vibration > 0.254 mm, force a trip; strictly prohibit forcing passage through the critical speed—instead, reduce speed and perform warm-up.
Prioritize checking lube oil pressure, oil temperature, and hydrogen temperature; assess the risk of shaft system rubbing or rotor bowing.
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