Ultrasonic stress measurement of power station bolts

1 bolt is a very important joint fastening part of thermal power plant. It bears the important role of connection of flange, valve door cover, cylinder joint surface and ensuring steam tightness. Bolt working

1 bolt is a very important joint fastening part of thermal power plant. It bears the important role of connection of flange, valve door cover, cylinder joint surface and ensuring steam tightness. The safety of the bolt work is directly related to the safe operation of the unit.

In the operation of the thermal power plant unit, the phenomenon of bolt damage sometimes occurs, which brings hidden dangers to safety production. The causes of bolt damage are various, some are due to bolt design, manufacturing is unreasonable, improper installation and maintenance, etc. External reasons are due to the long-term operation of bolts under high temperature and high stress conditions, the bolt material itself changes, resulting in bolts Failure or damage to the station When the high-temperature bolt is tightened, a certain initial tightening force, that is, the pre-tightening force must be applied. The bolt produces a certain elastic deformation under the action of this preload. After the equipment is put into operation, as the running time increases, part of the elastic deformation changes into plastic deformation, that is, relaxation occurs, and the stress generated in the remaining part is called residual stress, and the value is not less than the seal required to maintain the tightness of the flange. stress. If the pre-tightening force is too small, it will cause the leakage pre-tightening force to reduce the service life of the bolt and even cause the bolt to crack or break. During the overhaul of a power plant unit, it was found that there were 7 cracks in the wheel bolts of the 12 high- and medium-pressure rotors of the 12 steam turbines. The analysis showed that it was caused by improper disassembly and assembly and excessive pre-tightening force during maintenance. Pre-tightening force measurement was carried out on two of the 12 bolts of the machine. The result showed that the pre-tightening force of the bolt was 305-398 M Pa, which was about s, exceeding the allowable stress of the material (0.5-0.6 σ). Therefore, the manufacturer has a clear regulation on the pre-tightening force of the bolt: generally 300 MPa for the pearlitic alloy steel is 200 M Pa.

At present, during the actual installation and maintenance of power station bolts, the torque method, the angle of rotation (arc length) method, the tension method, etc. are often used to control the bolt pre-tightening force. Most of these methods are controlled by the bolt elongation ΔL according to Hooke's law σ= E? ΔL / L to control the bolt pre-tightening force σ Ultrasonic stress measurement is to directly measure the ultrasonic sound velocity through the bolt to be tested, according to the relationship between the speed of sound and the stress, the axial stress of the bolt is obtained.

Usually, the shear wave coupling is difficult in precision measurement. Therefore, the stress value of the bolt is often calculated by inversion (1) by measuring the longitudinal wave sound and its variation and supplementing the temperature measurement.

For different bolt materials, the ultrasonic properties are different, and there are not many materials commonly used for high-temperature fastening bolts in power stations. Therefore, a representative material is selected for testing, and the method of inputting the stress meter to measure the bolt stress after programming the test results is feasible. Figure 1 is a graph showing the relationship between stress, temperature and sound time of 20Cr1Mo1VNbTiB steel.

The measurement results of stress, temperature and acoustic time relationship of different bolt materials are fitted into mathematical formulas by regression analysis method, and these relations are programmed and written into the EPROM of the intelligent bolt stress meter to form the bolt stress measurement test. basis. The principle of ultrasonic bolt stress meter is shown in Figure 2.

The working contents of the ultrasonic stress meter mainly include measuring the sound velocity of the ultrasonic wave propagating on the workpiece, using the material data stored in the instrument, and determining the stress of the workpiece according to the sound velocity and temperature in the workpiece.

3Using the ultrasonic stress meter When measuring the bolt stress with ultrasonic waves, the average sound velocity of the sound wave propagating inside the bolt is measured. When the bolt is working, the axial stress distribution along the bolt is different, and the sound velocity of each segment is also different. The average sound velocity of the sound wave propagating in the bolt is measured. Figure 3 is the axial stress distribution diagram of the stud bolt. The distribution of stress along the axial direction of the bolt can be expressed as follows: According to the definition of the average value, the relationship between the average stress σ and the stress of the straight portion of the bolt can be written: the difference in the bolt structure is different, and the influence on the speed of sound is also different.

Here, the concept of the effective length of the bolt is introduced, so that when measuring the bolt stress, the corresponding effective length is selected for the bolts of different structures to ensure the accuracy of the stress measurement. The effective length of the stud is shown in Figure 4. The effective length of the other structural bolts can be found in the corresponding manual.

The current ultrasonic bolt stress meter has strong storage capacity due to the application of computer technology. The raw data of each bolt can be numbered and stored in the stress meter, saving a lot of data input work for the next measurement.

Electric Control System of Steam Turbine in Weinan Thermal Power Plant Zeng Fengru1, Ma Lijun2, Zhao Yongkai2, Su Tongchun (1.Hebei Electric Power Survey and Design Institute, Shijiazhuang 050031, China 2. Harbin Power Station Engineering Company, Harbin 150046, Heilongjiang) The composition and control functions of the ESC provide experience for improving the automation control level of the medium and small capacity double pumping adjustable turbines.

1 Overview The 2×50 MW steam turbine of Weinan Thermal Power Plant is a double pumping, adjustable and condensing steam turbine of Harbin steam turbine cylinder. Its rated parameters are: three-stage extraction of power turbine, which is located in front of the main steam cylinder. The pressure is 0.294 M Pa and the flow rate is 28 t/h. It is taken from the five-stage extraction of the steam turbine and is located in front of the rotating partition in the medium-pressure cylinder.

The double-pumping adjustable steam turbine requires control not only to adjust the electrical load, but also to adjust the industrial extraction and heating heat load. Since these three loads interact and interact with each other, decoupling control and static auto-tuning are required. Due to the use of liquid control, there are disadvantages such as complicated structure of the hydraulic part, too low precision of the integrated slide valve, large hysteresis, inconvenient adjustment of the operating personnel, and the shortcoming of load jump between the domestic electric and liquid adjustment, and the same domestic capacity. The unit control level is not high, and in order to meet the control level requirements of the unit, after repeated argumentation and comparison, it is decided to adopt a pure electric control system. The control device uses the Woodward 501 controller. The module includes dual redundant power supply modules, CPU modules, DI/DO modules, AI modules, actuator power amplifier drive modules, and speed measurement modules. The controller drives the high, medium and low pressure three oil motives through the power amplifier module to adjust the high pressure cylinder adjustment door, the medium and low pressure cylinder adjustment door and the low pressure cylinder rotating diaphragm to control the electric power, the industrial heat load and the heating heat load respectively. The basic control strategy is to adjust or change any one load, and should not cause changes in the other two loads. This requires that when any one of the adjustment doors is in the adjustment process, the other two adjustment doors should be adjusted accordingly to offset the change of the other two loads caused by the door change, that is, the static self-tuning function. The basic control structure of the unit is shown in Figure 1.

2 ESC control mode 2.1 Pure electric control mode cancels the relevant parts of the liquid adjustment, such as synchronizer slide valve, governor slide valve, integrated slide valve, etc., adopts the start slide valve and only controls the main valve. Switch, keep the emergency security department set. The electric/liquid converter is controlled by the Woodward 501 controller. The electric/liquid converter output is a straight stroke lever and is connected to the intermediate slide valve to control the opening of the intermediate spool valve misalignment to adjust the pulsating oil pressure and pass through the oil. The engine slide valve and the oil motor feedback slide valve control each of the adjustment doors. For the key hydraulic parts of the electric / hydraulic converter, according to its high precision requirements, the internal structure is fine and complex, easy to jam, the use of independent oil source for its oil. The independent oil source and the electric/liquid converter form a closed oil system, which is completely separated from the lubricating oil system of the steam turbine body, which greatly ensures the operational reliability and control precision of the electro-hydraulic converter.

Conclusion There are still some problems in the bolt pre-tightening control work. The method of controlling the bolt pre-tightening force of the power plant is still primitive. However, with the continuous improvement of the ultrasonic stress meter and the universal application of the ultrasonic measurement bolt axial stress method, it is scientific, simple, and The time to accurately control the bolt preload is not too far.

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