In the steam turbine TSI (turbine monitoring instrumentation system), the core of axial displacement measurement is to monitor the axial position changes of the rotor in real time through non-contact sensors to prevent axial friction between the rotor and stator components (such as partitions and impellers) due to thrust bearing wear. Its specific measurement principle is as follows:
1. Measurement objects and purposes
Axial displacement refers to the axial movement distance of the turbine rotor relative to the stator. During normal operation, the rotor maintains a stable axial position under the constraints of the thrust bearing; when the thrust bearing wears, the lubricating oil film is destroyed, or the axial force increases abnormally, the rotor will deflect along the axis. If the deflection exceeds the safety limit, it may cause the rotor to collide with stationary parts (such as steam seals and partitions), resulting in equipment damage.
The purpose of TSI axial displacement measurement is to capture this offset in real time and trigger an alarm or emergency shutdown when it exceeds the limit.
2. Sensor selection and working principle
Axial displacement measurement generally uses eddy current sensors (non-contact), whose principle is based on the "eddy current effect" in electromagnetic induction:
1. Sensor structure: composed of probe (built-in high-frequency coil), extension cable and pre-sensor. The probe is installed on the stator (such as a bearing seat), facing the side of the thrust plate of the rotor (the thrust plate is an annular boss on the rotor, used to transmit axial force).
2.Eddy current effect:
The proximitor provides high-frequency alternating current (usually 1~2MHz) to the probe coil, and an alternating electromagnetic field is generated around the coil.
When there is a certain gap between the probe and the side of the thrust plate (metal conductor) (usually the initial gap 1~2mm), eddy current will be induced on the surface of the thrust plate. The reverse magnetic field generated by the eddy current will weaken the magnetic field of the probe coil, causing the coil impedance to change.
3. Signal conversion:
When the axial displacement changes, the gap between the probe and the thrust plate changes synchronously, and the coil impedance changes accordingly. The preprocessor converts this impedance change into a linear DC voltage signal (usually -2~+2V, corresponding to the 0~2mm gap change).
The magnitude of the voltage signal directly reflects the axial displacement: for example, the initial zero position corresponds to 0V. The voltage increases when the rotor moves toward the probe (forward displacement), and decreases when the rotor moves in the reverse direction.
3. Measuring point setting and zero calibration
1. Dual probe redundant design: In order to improve reliability, 1 sensors are usually installed on both sides of the thrust plate (or 2 sensors are installed on one side) to monitor the gap changes on both sides of the thrust plate and mutually verify the accuracy of the data.
2.Zero calibration:
When the unit is installed in a cold state, adjust the initial gap between the probe and the thrust plate (usually 1mm). At this time, it is set to the "mechanical zero position" (corresponding to the axial displacement 0 value), which is the normal position of the rotor on the thrust bearing working surface.
During operation, when the rotor moves in the direction of the non-working thrust pad (the thrust bearing is worn), the axial displacement is displayed as a positive value; when it moves in the direction of the working thrust pad, the axial displacement is displayed as a negative value (the specific definition of positive and negative is according to the manufacturer's specifications).
4. Protection logic
The system presets the alarm values (such as +0.3mm, -0.3mm) and danger values (such as +0.5mm, -0.5mm) for axial displacement:
When the alarm value is reached, an audible and visual alarm is triggered to remind the operator to check;
When the dangerous value is reached, TSI outputs an interlock signal to ETS (emergency shutdown system), immediately cuts off the steam source, and forces the turbine to shut down to avoid collision accidents.
Installation and debugging steps
As shown in the figure, the schematic diagram of the axial displacement installation of the steam turbine TSI, write down the installation and debugging steps of the axial displacement?
(1) When the steam turbine is completely cooled, install and adjust the zero position;
(2) Determine the probe model, sensitivity, measurement range, and zero-point voltage; confirm the TSI sensor probe, extension cable, and proximitor;
(3) Determine the thrust clearance;
(4) Determine the zero point of the shaft displacement and the position of the thrust plate---the thrust plate is close to the working surface, and the three parties on inspection, maintenance, and supervision of shaft pushing shall sign and confirm;
(5) Determine the installation direction of the probe and zero the sensor;
(6) Calculate the installation voltage: front-end device output voltage = zero-point positioning voltage ± theoretical display value × sensor sensitivity, (± represents the direction of axis displacement);
(7) TSI configuration and linearization;
(8) Measurement value comparison;
(9) Alarm and shutdown protection action test
(10) Check DCS display;
(11) Check the perpendicularity between the measuring element and the measuring plate;
(12) Fixed bracket.
Axial Displacement During the debugging process, how to simulate the axial movement of the steam turbine?
When the steam turbine is in a cold state and the thrust plate is close to the zero position of the working surface, the query voltage is -10VDC. Place the dial indicator on the sensor bracket and set the dial indicator indication to the zero position. Adjust the mobile sensor bracket to simulate the axial movement of the large shaft during operation. Select 4 points in the full range except the zero position, and two points in the negative and positive directions to simulate the axial movement of the steam turbine in the full range.
(Source: Instrument News Agency)