Source: Thermal Control Circle
TSI System Overview
The steam turbine safety monitoring and protection system mainly includes the monitoring and protection system (TSI), the critical blocking system (ETS) device, and the automatic crank operation device.
The TSI system can continuously monitor various important parameters of the steam turbine, such as speed, overspeed protection, eccentricity, shaft vibration, cover (watt) vibration, shaft displacement, expansion difference, thermal expansion and other parameters, so as to help the operator identify machine faults, so that these faults can be cut off in time before serious damage is caused to ensure the safety of the unit.
With the help of diagnostic data, maintenance personnel can analyze possible machine failures and help propose machine prediction maintenance plans, which can predict the maintenance needs of rotating machinery, make machine maintenance more planned, reduce maintenance time, and result in reduced maintenance costs and increased availability of steam turbine units.
The main principles and functions of TSI
The TSI system is mainly composed of sensors and intelligent boards
A sensor is an electromechanical conversion device that converts the amount of mechanical vibration, displacement, and rotational speed into electricity. According to the performance of the sensor and the requirements of the test object, the eddy current sensor is used to measure the rotational speed, eccentricity, shaft displacement, shaft vibration and expansion difference of the steam turbine unit (pure ESC).
Cover vibration is measured using a speed sensor
Thermal expansion is measured using a linear variable differential transformer (LVDT).
A differential magnetic sensor is used to measure the rotational speed of the unit
Eddy-current sensors
Working principle: The relative displacement and static displacement of the vibration of the object are measured by the gap change between the end coil of the sensor and the measured object (conductor), and there is no direct mechanical contact between it and the measured object, and it has a wide range of use frequency (from 0~10Hz)
There is a coil at the end of the sensor, the coil passes through the alternating voltage with a higher frequency (generally 1MHz~2MHz), when the coil plane is close to a certain conductor surface, because the coil flux chain passes through the conductor, the surface layer of the conductor induces an eddy current IE, and the magnetic flux chain formed by IE passes through the original coil again, so that the original coil and the eddy current "coil" form a certain coupling mutual inductance, and finally the original coil feedback an equivalent inductance. When the material is given, the coupling coefficient K1 is related to the distance d, K = K1 (d), when the distance d increases, the coupling weakens, the K value decreases, so that the equivalent inductance increases, therefore, the change of the equivalent inductance is measured, and the change of d is indirectly measured.
Schematic diagram of an eddy current sensor
Since the inductor voltage fed back by the sensor is an amplitude modulation signal with a certain frequency (carrier frequency), it is necessary to detect the voltage waveform that changes with time in order to obtain the gap with time. That is, according to the above principle, in order to realize the eddy current displacement measurement, there must be a special measurement route. This measurement route (called a preamplifier) should consist of a stable oscillator with a certain frequency, a detector circuit, etc. The eddy current sensor plus a measuring line (preamplifier) is shown in the block diagram: the voltage Vd output from the preprocessor is the voltage proportional to the gap d, which can be divided into two parts: one is the DC voltage Vde, which corresponds to the average gap (or initial clearance), and the other is the AC voltage Vac, which corresponds to the vibration gap.
Schematic diagram of the preprocessor
Speed sensor
How it works: Based on an inertial mass and moving housing, the sensor has a permanent magnet, which is fixed to the sensor housing, and around the magnet is an inertial mass coil, which is connected to the housing by a spring. When measuring, the sensor is rigidly fixed on the measured object, and as the measured object vibrates, the magnet moves to produce a magnetic field motion. The coil has a large inertial mass because it is fixed on the spring, that is, the object that vibrates at a relatively high frequency is relatively stationary. In this way, the coil moves in a straight line in a magnetic field, generating an induced electromotive force that is proportional to the linear velocity of the coil movement (i.e., the velocity of the casing). By detecting the induced electromotive force, the linear velocity of the measured object can be obtained.
Schematic diagram of a speed sensor
LVDT sensors
Working principle: using the mutual inductance phenomenon in electromagnetic induction, it is essentially a transformer, the primary coil W on the transformer is composed of two secondary coils W1 and W2 with the same parameters, the center of the coil is cut into the cylindrical core, the secondary coils W1 and W2 are connected in series with reverse polarity, when the primary coil W is added to the alternating voltage, the secondary W1 and W2 produce induced potentials e1 and e2 respectively, and their size is related to the position of the core.
LVDT schematic
Differential magnetic induction sensor
How it works: Utilizes a differential sensing element. The element consists of two magneto-sensitive semiconductor resistors connected in series on a permanent magnet (both semiconductors are of the same material and geometry). In a sensor circuit, these two resistors form a differential inductive bridge (e.g., a Wheatstone bridge). When the triggers of a magnet or steel are close to or far from the sensor and are at right angles to each other (i.e., the magnetic field generated by the magnet on the surface of the sensor probe is at right angles to the edge of the trigger), it interferes with the magnetic field inside the sensor, causing the differential inductor bridge to lose balance and output a voltage. By measuring this voltage, the change in the gap between the DUT (i.e. the trigger) and the sensor probe can be obtained.
In the practical application of TSI measurement, we generally use the magnetic induction sensor to measure the speed of the unit, which is to obtain the actual speed value by measuring the number of pulse signals formed by the high and low voltage changes between the probe and the tachymeter tooth wheel.
Smart boards
After receiving the power signal of the corresponding sensor, various measuring boards are shaping, calculating, logic processing, etc., and display accurate and intuitive monitoring data and alarm indications. Outputs standard analog signals and relay contacts. The intelligent board can detect the sensor connection and its own operation, has a computer communication interface, can configure the measurement range and logic output, and has the functions of buffering the sensor signal output. Redundant configurations can be used for critical measurements, enhancing reliability
Rotational speed and zero rotational speed
The rotational speed value display is an important parameter for the start-up, shutdown and stable operation of the steam turbine unit, and the correlation between the vibration value and the machine speed is very important for the final analysis of the machine performance. For example, a sudden drop in speed during the shutdown of the machine will mean that there is a large area of metal friction inside the machine. The zero speed is the pre-set shaft rotation speed, when the running machine needs to stop, the machine speed reaches the zero speed set point, the relay contact action, so that the crank gear mesh, so that the shaft continues to rotate slowly, to prevent the shaft from bending, so as to avoid damage to the machine due to shaft bending in the ensuing start-up
The measuring chain is composed of two sensors and plates that are installed in the front box facing 60 (or 134) chainrings, when the machine rotates, the top and bottom of the tooth of the tooth chainring pass through the probe, and the probe will periodically change the output signal, that is, the pulse signal, the plate receives this pulse signal for counting, display, and after comparing with the set value, the relay contact output is driven. Measuring range of rotational speed: 0~5000rpm; Zero speed setpoint: less than 4rpm; Speed alarm value: 3240rpm.
Schematic diagram of rotational speed and zero-rotational speed measurement
Overspeed protection
Overspeed protection should have fast response and error redundant voting logic, so this measurement chain uses a "two-of-three" approach. It consists of three eddy current sensors mounted in the front box, facing the 60-chainring and three tachometers set at 3300 rpm. The same principle as the rotational speed measurement, the rotational speed value = (pulse frequency/number of teeth) ×60. The measurement range of overspeed of each unit: 0~5000rpm.
Schematic diagram of overspeed measurement
Shaft vibration
For rotating machinery, the radial vibration amplitude of the rotor is the most basic indicator to measure its comprehensive mechanical condition, and many mechanical faults, including rotor imbalance, misalignment, bearing wear, rotor cracks and friction, can be detected according to the vibration measurement. The rotor is the core component of rotating machinery, and whether the rotating machinery can work normally mainly depends on whether the rotor can operate normally
Monitoring and detecting vibration faults from rotor movement is more straightforward and effective than extracting information from the vibration of a bearing housing or casing. Therefore, the measurement of shaft vibration is becoming more and more important, and the measurement of shaft vibration is very useful for machine fault diagnosis
According to the principle of vibration, the axial trajectory can be obtained by synthesizing the vibration in the X and Y directions. When measuring shaft vibration, an eddy current probe is often mounted on the bearing housing, and the probe becomes one with the bearing housing, so the measured result is the vibration of the shaft relative to the bearing housing. Since the shaft is not necessarily intrinsically related to the vertical and horizontal directions, that is, the vibration in the vertical direction (Y direction) is already large, while the vibration in the horizontal direction (X direction) may be normal, so one probe is installed in the vertical and horizontal directions. Due to the effect of the horizontal facet on the installation, the two probes are actually guaranteed to be perpendicular to each other, as shown in the figure below. When the gap between the end of the sensor and the surface of the rotating shaft changes, the sensor outputs an AC signal to the board, and the board calculates the peak-to-peak (P-P) value of the gap change (i.e., vibration). Measurement range of unit shaft vibration: 0~400μm; Alarm value: 125μm; Standstill value: 250μm.
Schematic diagram of shaft vibration measurement
Bearing vibration (cap or pad)
In the measurement of shaft vibration, it has been shown that the vibration of the large shaft can be transmitted to the bearing housing, and the speed sensor is used to measure the movement speed of the housing relative to the free space, and the plate detects and integrates the velocity signal from the sensor into a displacement value, and calculates the corresponding peak-peak position signal as shown in the figure below. The measurement range of the unit watt vibration: 0~100μm.
Schematic diagram of the measurement of cover vibration
eccentricity
The eccentric position of the rotor, also known as the radial position of the shaft, refers to the radial average position of the rotor in the bearing, in the case of the normal operation of the shaft without internal and external loads, the shaft will float in the position determined by the design under the action of oil pressure damping, but once the machine bears a certain external or internal preload, the journal in the bearing will be eccentric, and its magnitude is represented by the peak-peak value of the eccentricity, that is, the difference between the extreme value of the positive and negative direction of the shaft bending
Eccentricity measurement
Eccentricity measurements can be used as an indication of bearing wear, as well as preload conditions (e.g. misalignment); Rotor eccentricity (bending at low speeds) measurement is an essential measurement during start-up or shutdown, allowing you to see the magnitude of shaft bending due to heat or gravity. The eccentricity monitoring board accepts two eddy current sensor signal inputs, as shown in the figure below. One is for eccentricity measurements, and the other is for keyphasor measurements, which are used in peak-to-peak signal conditioning circuits. The keyphasor probe looks at a keyway on the shaft, and with each revolution of the shaft, a pulse voltage is generated, which can be used to control the calculation of peak-to-peak. Of course, the key signal can also be used to indicate the phase of the vibration, as shown in the figure below. When the angle between the vibrometer probe and the keyphase probe is known, the location of the unbalanced mass, i.e., the location of the high point of the rotor, can be determined. This is important for the balance of the shaft. The measurement range of unit eccentricity: 0~100μm. Alarm value: 30μm greater than the original value.
Schematic diagram of vibration phase measurement
Shaft displacement
During the operation of the shaft, due to various factors, such as changes in load, temperature, etc., the shaft will move in the axial direction. In this way, there is a possibility of dynamic and static friction between the rotor and the stator, so it is necessary to measure the change of the rotor relative to the axial position of the stator, i.e., the clearance of the shaft in the axial direction relative to the thrust bearing
Because the monitor used may send an alarm signal for the failure of the sensor as an axial displacement fault, it may cause the unit to stop by mistake. According to the requirements of the API670 standard, two probes can be used to detect an object at the same time, which can avoid false alarms
It is required that the distance between the installation position of the two probes and the thrust flange on the shaft should be < 305mm, if it is too large, due to the influence of thermal expansion, the measured clearance cannot reflect the gap between the flange on the shaft and the thrust bearing
As shown in the figure, two eddy current probes measure the axial change of the rotor, and output the DC voltage value proportional to the gap between the probe and the flange to be measured. In order to avoid false alarms, the shutdown logic is output as "AND" logic. The measurement range of the axial displacement of the unit: -2~+2mm.
Schematic diagram of shaft displacement measurement
Expansion difference
Differential expansion is the relative thermal growth between the rotor and the cylinder, and friction can occur when the difference in thermal growth exceeds the allowable clearance. In the process of starting and stopping, due to the difference between the mass of the rotor and the cylinder, the coefficient of thermal expansion, and the coefficient of heat dissipation, the thermal expansion of the rotor and the expansion of the cylinder are not the same, in fact, the temperature of the rotor rises faster than the temperature of the cylinder, and the difference of its thermal growth if it exceeds the allowable dynamic and static clearance tolerance, friction will occur, which may cause an accident. Therefore, the purpose of monitoring the expansion difference value is to take necessary measures to ensure the safety of the unit before friction occurs
It is generally stipulated that the rotor expansion is greater than the cylinder expansion in the positive direction, and the reverse direction is negative. In addition, if the range of expansion difference measurement is large and exceeds the linear range of the probe, bevel measurement and compensation measurement can be used. Since it was not possible to install eddy current sensors in the cylinder, the sensor was fixed on the platform of the bearing housing using a sliding pin system
Schematic diagram of the difference in expansion measurement
Thermal expansion
The steam turbine expands its cylinder due to heating during the start-up process, and if the expansion is uneven, the cylinder will become oblique or warped, and this deformation will cause huge stress between the cylinder and the foundation, resulting in misalignment, and this phenomenon is usually caused by the "jamming" of the sliding pin system. Knowing the cylinder expansion and expansion difference, it is possible to determine the expansion rate of the rotor and cylinder
Schematic diagram of the measurement
Connect the core of the LVDT sensor with the cylinder, when expanding, the core moves, generates a proportional electrical signal, inputs the measuring board for linear processing, and displays and outputs a 4~20mA signal, as shown in the figure:
ETS Introduction
ETS is the steam turbine critical blocking system, which accepts the alarm or shutdown signal from the TSI system or other systems of the steam turbine generator set, performs logic processing, and outputs the indicator light alarm signal or the steam turbine blocking signal. In order to be easy to use and reliable to operate, we have selected a dual-machine PLC (programmable controller) for logic processing. The dual-machine PLC works at the same time, and the alarm signal can be output in any action. When any one fails, the PLC sends out a fault alarm signal of the local machine and automatically cuts off its shutdown logic output, while the other one can still work normally. The device can communicate with other systems to meet the automation needs of power plants.
The internal logic of the ETS device is realized by a programmable controller (PLC), which replaces the traditional relay logic, and a dual PLC structure is adopted in order to improve the reliability and safety of the ETS device.
The hardware configuration is as follows:
CPU Module, Input Module, Output Module, PLC Power Supply, External Power Supply, Other Accessories
How ETS works
The two PLCs of the ETS unit work at the same time. After the input signal from the field enters the device, it goes to machine A and machine B at the same time, and the corresponding output signal is given after automatic processing by internal logic. Taking "steam turbine overspeed" as an example, when the electric overspeed shutdown signal is input into this device, it enters machine A and machine B at the same time for processing, and outputs the shutdown signal at the same time
Schematic diagram of the working principle of the dual machine
ETS unit power supply
Control power supply: AC 220V, single-phase 50Hz
Input response time: about 10ms
Output response time: about 10ms
系统MTBF >10000h
The availability rate ≥ 99.9%.
The ETS device is equipped with a two-way power supply switching circuit, when a power supply fails, it will automatically switch to another power supply circuit to continue working, and if the main and auxiliary power supplies fail at the same time, an AC power supply power loss alarm signal will be output
function
a) Overspeeding of the steam engine
b) Generator tripping
c) The main fuel of the boiler trips
d) Large axial displacement
b) Ehāhāhhā(32)
f) 凝汽器真空大(3取2)
g) Lubricating oil pressure is too low (2 out of 3)
h) Manual shutdown
i) DEH failure
j) The vibration of the steam turbine bearing is large
k) High bearing temperature
l) The expansion difference exceeds the limit
m) Standby (2-way)
Turning device
The crank device used in the 600MW steam turbine of Harbin is a low-speed crank, which is installed between the low-pressure cylinder B and the generator, driven by an electric motor, and equipped with a pneumatic operating mechanism, with the main features:
When the speed of the steam turbine drops to zero speed, it can be operated remotely or manually cranked on the spot.
After the B disc device lifts off the motor, the turning speed is 1.5 rpm. The cranking device is automatically withdrawn after the turbine is punched.
There is an interlock between the C-crank device and the top axle oil pump system to prevent the crank from being put into the crank before the oil pressure is established.
Diagram of the crank control cabinet
1-Ammeter .2-Voltmeter .3-Crank Indication .4-Throw in Place Indication 5-Meshing in Place Indication 6-Top Shaft Oil Pressure Normal Indication.7-Lubricating Oil Pressure Normal Indication.8-Crank Allowed Indication .9-Motor Stop Indication. 10-Power Indication.11-Tilting Allowing Selection Switch.12-Hand/Auto Selection Switch.13-Test Light Button.14-Solenoid Valve Action Button.15-Jog and Put Button.16-Stop Turning Button.17-Inspection and Bypass Button.18-Throw Off Button
Control schematic diagram of the turning device
Turning device function
The three-phase power supply is connected to the crank device, and the AC contactor of the control loop is connected to the soft starter of the motor, and the soft starter of the motor is output to the crank motor to drive the crank motor. According to the control requirements, the PLC controls the solenoid valve of the hydraulic mechanism for oil intake and discharge and the forward and reverse rotation of the crank motor.
This device can meet the requirements of zero-speed automatic cranking, manual automatic cranking, emergency cranking, and has the functions of low oil pressure protection and remote control.
How it works
According to the logic requirements of the crank device system: under normal circumstances, the control cabinet of the automatic crank operating device must receive the effective signal of the pressure contact with normal oil pressure of the top shaft and normal lubricating oil pressure, that is, to meet the external allowable cranking conditions. Otherwise, the control cabinet will refuse to start the crank motor.
Zero-speed automatic loading of the turning car
After satisfying the external allowable cranking conditions, allowing cranking and selecting the automatic cranking mode, as long as TSI sends a zero-speed signal to the cranking control cabinet, the PLC of the cranking control cabinet will automatically turn on the solenoid valve after confirming that it has received the zero-speed signal (usually, it will automatically delay for about 30 seconds after receiving a stable zero-speed signal before being confirmed by the system), so that the hydraulic actuator and the steam engine cranking gear are meshed. The crank control cabinet PLC begins to detect whether the meshing signal is received 30 seconds after the solenoid valve is energized, if the meshing signal is not received, the crank motor will periodically start the motor forward rotation through the soft starter for a short time, so that the crank motor is micro-moving, so as to facilitate the meshing between the gears
If the meshing signal is received, the motor will automatically start and maintain the operation of the crank motor, and at the same time, after the motor runs for 10 seconds, the solenoid valve will automatically power off, so as to complete the automatic crank car input.
Pressing the "Stop Turning" button at any time will stop the turning motor. The stop-stop button is an alternate switch when the meshing in place is effective, so as to prevent the stop-stop button from automatically starting the crank motor when the stop-stop button is released because it is not thrown off or the meshing in place signal still exists.
Manual automatic turning mode
Under the allowable condition of cranking, select the manual crank mode, press the "solenoid valve action" button, make the hydraulic mechanism complete the crank gear meshing, if the crank control device receives the meshing signal in place, it will automatically start the crank motor, and disconnect the power supply of the solenoid valve after 10 seconds. If the meshing signal is not received, it means that the coil gear has not been meshed. The following are two ways to facilitate the meshing of the crank gear, one of which is the same as the automatic crank mode, that is, the device automatically periodically timing micro crank motor until the smooth meshing; The second is to select the "hand/automatic selection switch" in the middle position, press the "jog and input" button to complete the manual micro turning motor, if the meshing is in place, and then the "hand/automatic selection switch" to the manual position, it will automatically start the turning motor to complete the turning input.
Pressing the "Stop Turning" button at any time will stop the turning motor. The stop-stop button is an alternate switch when the meshing in place is effective, so as to prevent the stop-stop button from automatically starting the crank motor when the stop-stop button is released because it is not thrown off or the meshing in place signal still exists.
Emergency loading of the car
Emergency input crank is in the case that external conditions do not allow cranking, for example: low lubricating oil pressure, abnormal top shaft oil pressure, or other electrical faults, in order to ensure that the turbine rotor can rotate, it is necessary to use the emergency cranking method, which will cause additional wear or damage to the bearing pads. Please use with caution!
In the case of manual cranking, at any time, as long as the "inspection and bypass" button is pressed, all the safety protection of the motor fails, such as pressing the "jog and input" button, directly start the cranking motor to run. Of course, you can also press the "solenoid valve action" button first to try to complete the mesh. It must be noted that "Inspection & Bypass" is an alternate button, and under normal circumstances you must press this button to turn off the indicator light on it to disable this function.
Pressing the "Stop" button at any time will stop the turning motor and the "Inspection & Bypass" function will be automatically cleared and the light will turn off.