Understanding principles through the study of history – The serious accident in which a 200MW turbine was broken into 13 sections

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By studying history, we can understand principles, strengthen our beliefs, cultivate virtue, and take action. This accident is a very classic example; I’m sharing it here for everyone’s learning and discussion (I haven’t been to Kuang Forum in ages :lol). The source of the information is Sohu.com. However, a long time ago, an elder professor at Xi’an Jiaotong University gave a lecture on this case. Investigation and Analysis Report on the Accident of Unit 5 at Qinling Power Plant. The accident referred to in this article is the serious shaft failure that occurred at Unit 5 of Qinling Power Plant on February 12, 1988, at 16:06. **An accident expert investigation team was established to prepare this report. I. Introduction (1) Following the serious accident of shaft failure in Unit 5 of the Qinling Power Plant at 16:06 on February 12, 1988, the Northwest Electric Power Administration under the former Ministry of Water Resources and Electric Power set up an accident investigation team on February 14. Meanwhile, the former Ministry of Water Resources and Electric Power and the Machinery Commission jointly formed an “Equipment Appraisal Team” to conduct preliminary investigations into the accident, secure the scene, examine the scattered debris, and assess the extent of equipment damage. On March 6, they submitted a total of 8 documents to the experts, including an “Report on the Accident Investigation” and an “Investigation Report on Equipment Damage”. The expert group believes that the work on protecting the wreckage at the scene and investigating equipment damage was done well. (2) The expert group organized in the name of the National Work Safety Commission consists of 20 experts from various fields across the country: 6 from the former Ministry of Water Resources and Hydropower, 6 from the former Ministry of Machinery, and 8 from other ministries, commissions, and universities. Professor Zhong Qunpeng from Beihang University serves as the team leader, while Zhu Xin, former director of the Production Scheduling Bureau of the **Economic Commission, acts as the deputy leader. The expert group is divided into four subgroups: the first subgroup, dealing with materials and fracture analysis, is headed by Researcher Wang Renzhi from Institute 621 of the Ministry of Aeronautics and Astronautics; the second subgroup, focused on bearing vibration and strength, is led by Professor Zhu Jun from Xi’an Jiaotong University; the third subgroup, concerned with control and safety systems, is headed by Professor Wang Weijian from Tsinghua University; the fourth subgroup, an integrated group, is managed by Professor Meng Qingji from Xi’an Jiaotong University. (3) Before the expert team went to the accident site for investigation, on March 4, Comrade Ye Qing, former deputy head of the Economic Commission, met with the members of the expert team and conveyed the two directives from Li Peng, former Premier of the State Council, regarding the establishment of an **accident investigation team. He asked the expert team to \"adopt an objective and scientific approach, disregard all forms of interference, determine the causes of the accident based on facts, learn from the experience, and improve future work.\" ”For 6 and a half months, the expert group has been striving to work in accordance with this spirit. (4) Starting from its first investigation at the Qinling Power Plant on March 5, 1988, and up to September 29, the expert group held five research, analysis, and discussion sessions, as well as five meetings to seek opinions from the two relevant departments; each group also held one summary discussion session. In total, 42 days were devoted to centralized analysis and discussion. During this period, a total of 51 thematic analyses and experimental studies were conducted, two interim summaries were prepared, and six investigation briefs were issued. The editor filmed videos of the accident scene; a summary report on the investigation and analysis totaling around 300,000 words, along with a group research and analysis report, has now been prepared. To facilitate the work, an expert support team was established; they prepared a total of 7 equipment inspection reports, which were useful for the accident investigation. (5) During the expert group’s investigation, support was provided by the Ministry of Mechanical and Electronic Industry and the Ministry of Energy, as well as assistance from Dongfang Steam Turbine Factory, Dongfang Electric Machinery Factory, and Qinling Power Plant. The expert group received 4 analysis reports on this accident from two relevant agencies. Based on the various thematic analyses, the summaries for each stage, and the group summaries, as well as by referring to the accident analysis reports prepared by the relevant agencies and taking into account the opinions expressed during the discussions in the various thematic groups, the expert group prepared the investigation and analysis report on this accident (first draft). From September 20 to September 29, 1988, a full meeting of the expert group was held at Tsinghua University to conduct discussions and have thorough exchanges with Comrades from the two departments, during which the general report on the investigation and analysis was revised. II. Overview of the unit and the accident: At 16:06 on February 12, 1988, the No. 5 200MW steam turbine generator unit at Qinling Power Plant suffered a severe accident involving a shaft breakage during a test of the emergency speed-raising protection device. Six coupling bolts on the shafting and five parts of the shaft body broke, resulting in a total of 13 broken sections; the main unit was essentially destroyed. As for the basic details of this unit, the turbine was manufactured by Dongfang Steam Turbine Factory in 1983, with the factory serial number 14; it was a model that represented a transition from D05 to D09, and the control components were also modified. The generator was produced by Dongfang Electric Machinery Factory in 1984, with the factory serial number 84—12—6—20. The unit began trial operation on December 13, 1985, and was officially put into production in February 1986. Up to the accident on February 12, 1988, it had accumulated 12,517 hours of operation, 5,988 hours of maintenance (including 1 major overhaul, 11 minor overhauls, and 27 emergency repairs), 461 hours of downtime, and 59 start-ups and shut-downs (including 47 times of connecting to the grid). The emergency governor speed-raising test was conducted 6 times, for a total of 31 hammer strikes, with the unit reaching a maximum speed of 3373 r/min. During operation, there were a total of nine protection trips, including 5 cases of single-phase automatic reclosing (with three-phase tripping occurring after four reclosing attempts failed). The generator experienced one instance of non-synchronous paralleling under partial-phase conditions; there was also one case of bearing burnout due to oil depletion. Additionally, in one incident, the locking screw of the medium-pressure throttle valve came loose and entered the medium-pressure cylinder, resulting in damage to the flow passage. During the commissioning of the new unit on December 14, 1985, and upon restart after bearing replacement on July 18, 1986, a small amount of low-frequency components were detected in the vibrations. During the bearing bush replacement work in July 1986, the shaft bearings were scraped; after the major overhaul from June 6 to August 3, 1987, the alignment of bearing 6 changed slightly, and the top clearances of bearings 4, 5, 6, and 7 were greater than their side clearances. After October 1987, it was found that the vibration of tile No. 5 increased from 20 μm to 48 μm. The unit was originally equipped with 11 recording instruments. Among the real-time records of the 8 instruments provided to the expert group, 3 (main steam flow, vacuum, and deaerator pressure) were operating abnormally and could not provide records of the actual conditions at the time of the accident. The remaining 5 are functioning normally. Furthermore, the accident memory loses its signal within 30 seconds after the power is cut off due to a cable fire, failing to provide the necessary information. Although this unit is equipped with manual switches for monitoring main bearing vibration, there was no dedicated person to monitor it during the experiment. Digital tachometers are available both at the unit site and in the control room; their display update interval is 2 seconds, and the displayed speed lags behind the actual speed. Overview of the accident process: This test of the emergency safety device for increasing rotational speed was carried out after the unit was disconnected from the power grid at 5:52 on February 12, using an overspeed test valve, with the main steam parameters approaching their rated values and the primary bypass in operation. During the testing process, the following occurred: when conducting the test to increase the rotation speed of hammer No. 1, the operator of unit No. 6 misread the speed indicated on the tachometer of unit No. 5 as 3328 r/min when it was actually 3228 r/min; he then pressed the shutdown button in the control room to shut down the unit, without contacting the testers at the unit’s location, which led them to believe that hammer No. 1 had already started operating. According to those present, during the test to increase the rotational speed of flywheel No. 2, when the unit’s speed reached 3302 r/min, a sound similar to that of valves operating was heard. The testers assumed that flywheel No. 2 had started to operate and therefore released the handle used for the overspeed test; it was later confirmed that flywheel No. 2 had not actually started to operate. When the rotational speed dropped to 3,020 r/min, the test supervisor walked around the unit to check the vibration and temperature of each bearing. No abnormalities were found. After consulting with the chief engineer present, the test on the operation of Flyweight No. 2 was continued. Based on the recollections of those present (including the test personnel and other people who were there at the time), first a sound indicating an increase in speed was heard, followed by dust being emitted from the auxiliary exciter; then there was a muffled noise, and the end of the generator caught fire. At that moment, a worker was hit in the waist by debris. The personnel at the front of the vehicle heard a muffled noise and then saw that cover No. 1 flipped up, steam was ejected from the high-pressure seal, and the test personnel fell to the ground. The time from hearing the speed-up sound to the ignition at the end of the generator was about 6–8 seconds; during this period, those present said they did not feel any intense vibrations, with a few people reporting another sound after the ignition at the generator’s end. Relevant leaders at the power plant directed the staff and firefighters in their efforts to extinguish the fire. The fire was completely extinguished at 16:28, without causing any other accidents; aside from one person who was scratched by debris, there were no other injuries or casualties. During this critical test of increasing the speed of the safety device, the shift supervisor was called away for a meeting, which reduced the number of people involved in the test; after the emergency stop was triggered accidentally during the testing of Hammer No. 1 in the control room, the person in charge of the test was not informed at all; after the accident, it was not possible to provide data on the vibration levels of the bearings while the unit was speeding up. There were shortcomings in the organization of this test. The repeatedly changing witness values provided by relevant personnel regarding the observed maximum turbine speed and the speed at the time of tripping have increased the difficulty of the investigation. III. Basic conclusions of the analysis: The analysis indicates that this accident was initiated by the instability of the oil film, resulting in severe damage to the shaft system caused by sudden and intense composite vibrations. The low stability margin of the unit’s shaft system and the excessive speed rise of the unit to 3500–3600 r/min were the main causes of this accident. It was a technical accident caused by a combination of factors. (1) This accident started with the instability of the oil film. The evidence for oil film instability is as follows: (1) Based on the analysis of the fracture surfaces at both ends of the motor rotor and the circumferential cracks nearby, it is concluded that the damaging stress is alternating stress, which is consistent with the nature of stress experienced during oil film instability; (2) Measurements and reconstruction of the motor rotor debris indicate that its bending deformation resembles the first-order vibration mode; (3) The reversal of the orientation of Bearing No. 2 indicates signs of oil film instability in the shaft system; (4) Around 2500–3361 r/min, low-frequency components at 1140 r/min and 650 r/min were detected in the vibrations of this machine; (5) According to incomplete statistics, approximately 15% of machines of the same type as Machine No. 5 have experienced oil film instability. 2. Sudden and intense composite vibrations caused severe damage to the shaft system. Due to the high rotational speed and rapid acceleration rate, the stability margin of the shaft system was low, leading to sudden oil film oscillations. These oscillations intensified during the rapid increase in rotational speed, causing extremely strong vibrations in the unit. As a result, certain fasteners (such as couplings and screws securing the bearing shells) became loose, resulting in shaft system imbalance, failure of some oil wedges, or self-excited dry friction. Friction and collisions between stationary and rotating components (e.g., damage to slip rings) as well as loosening of certain rotating parts further damaged some critical components, generating significant unbalanced forces. Meanwhile, the first-order critical speed of the intermediate- and low-pressure rotor extension shaft and the second-order critical speed of the generator fell within or close to the range of this rapidly increasing rotational speed. The sudden onset of oil film oscillation and the combined effects of various factors exacerbated the severe damage to the shafting. 3. Based on the fact that the stress at the fracture site of bearing 7 during bending vibration is much higher than that at the fracture site of bearing 6, that the outer end of bearing 7 was subjected to a severe impact from the upper bearing block prior to shaft breakage, and the differences in the fracture morphologies at bearings 6 and 7, it is believed that the severe damage to the entire shaft system originated from the fracture of the motor shaft at bearing 7. (II) The low stability margin of the shaft system and excessive speed rise of the turbine unit were the main causes of this accident.
1. Low stability margin of the shaft system
The shaft system of Unit 5 and similar units has a low stability margin. Calculations indicate that the critical speed at which shaft instability occurs is approximately 3,400 r/min, with a logarithmic decrement δ ranging from 0.06 to 0.08. This critical speed is sensitive to changes in the relative elevations of bearings No. 5 and No. 6. In 1986, after bearing bush damage occurred, the bearing bushes were reworked. After a major overhaul, measurements showed that the elevation of bearing No. 5 was 0.04 mm higher than that of bearing No. 6. Furthermore, following the overhaul, it was found that the radial clearance of bearing bushes No. 4, 5, 6, and 7 exceeded their lateral clearances. All these factors contributed to a decrease in the critical speed at which shaft instability occurs. 2. The main reasons for the sudden speed rise and overspeed of this unit are as follows: (1) Unit 5 is of type D09; under rated parameters and when the synchronizer is set at the 3000 r/min position, the upper limit of the operating range of its speed control system is approximately 3400 r/min (calculated value). This value is significantly lower than the upper limit of the speed control systems of other domestic 200 MW steam turbines (which is generally 3600 r/min). Additionally, during the overspeed test, the controllable area of the spool valve is large, while the maximum oil discharge area of the governor spool valve is small. These conditions create a scenario where the speed control system loses its ability to automatically regulate the rotational speed. When the parameter is below the rated value, the maximum speed will correspondingly increase. (2) According to the current analysis and calculations, starting from 3000 r/min and conducting speed-increase tests using the overspeed test slide valve with the primary bypass open and near the rated main parameters, the upper limit of the operating range of the speed control system is approximately 3400 r/min. Once the speed reaches 3360 r/min, rotating the control handle by another 20–30 degrees brings the system to its upper operating limit; the control handle being too short makes it difficult to operate, and combined with possible operational factors, this results in the speed control system losing its ability to automatically regulate the speed. (3) The steady-state oil consumption of the oil system in the Type D09 unit is greater than that in the Type D05 unit; the installation elevation of its oil tank is lower than that of the Type D05 unit. The oil ejector operates in a critical state, resulting in an increase in the activation value of the hydraulic overspeed protection device from the original 3401 r/min to approximately 3450 r/min. (4) The digital tachometer has a display cycle of up to 2 seconds, causing the displayed speed value to lag behind the actual speed. (5) The flyweight of emergency safety device No. 2 did not move at the set value of 3310 r/min. 3. Through analytical estimations in the following aspects, it is highly probable that the maximum overspeed of the shaft system during this accident was 3500 r/min–3600 r/min. (1) In the automatically recorded curve of the main steam pressure at the time of the accident, there was a steep downward trend during the speed-increase test; the pressure decreased by 1.47 MPa, or 15 kgf/cm². Although there are various explanations for the reasons behind this downward trend, analyses suggest that it can still serve as a reference for the maximum upper limit of the surge speed. Based on this analysis and calculation, the rotational speed of the shafting during this accident was no more than 3550 r/min. (2) Based on the elastic-plastic test calculations regarding the deformation (0.5–0.53 mm) and fracture of a certain pin in the last blade at stage 26, the results indicate that when only centrifugal force and additional forces are considered as loads acting on the blade, the rotational speed of the shaft system during this accident was approximately 3,600 r/min. (3) Based on the information provided at the test site and the thermal operating conditions of the turbine and boiler at the time of the test, an analysis was conducted regarding possible overspeed scenarios under various conditions. The results showed that the maximum possible speed ranged from 3456 r/min to 3600 r/min. Furthermore, referring to the data provided by the test operators, it was observed that the turbine tripped after reaching a maximum speed of 3,456 r/min. Considering the display lag of the digital tachometer and the response delay of the steam valves, the maximum speed of the unit should be above 3,500 r/min. (III) This accident was a technical incident caused by a combination of factors. Analysis indicates that it was characterized by suddenness in terms of timing, severe damage, and technical complexity; it was a major equipment failure resulting from multiple technical factors. 1. Through the analysis of experimental research, the following possibilities were ruled out in this accident: (1) The possibility of the unit being disconnected from the power grid and then reconnected was ruled out, as was any direct connection between fatigue-induced wear over time and this accident; (2) The possibility of a hydrogen explosion in the generator was ruled out; (3) The possibility that the generator’s slip rings were the first components to fail was ruled out; (4) The possibility that the severe damage to the shafting was caused by the loosening of 9-watt screws first was ruled out; (5) The presence of old cracks on the fracture surfaces of the generator’s slip rings, generator shaft, and low-voltage rotor shaft was ruled out. 2. To date, the following questions remain unresolved: (1) What kind of sound was heard by the testers during the test with hammer No. 2? (2) Reasons for the relatively low local yield strength σ0.2 and elastic limit σ0.02 in the areas (or points) where the motor shaft or turbine shaft broke. (3) Details and time sequence of the entire shaft system failure process. Preliminary analysis suggests that these questions do not affect the basic conclusions of the aforementioned analysis. IV. Urgent technical measures and several suggestions to be taken at present To prevent a recurrence of the incident involving Unit 5 in the Qinling plant and to enhance the reliability of similar units, the following urgent measures and several suggestions are put forward. It is recommended that both parties jointly study and address these issues. (1) Regarding the operation test of the emergency safety device: 1. Improve the control system to raise its upper operating range appropriately, and refine the design of the overspeed test spool valve to facilitate accurate operation. 2. Conduct a thorough inspection of the governor spool structures for similar units as soon as possible. Before any improvements are made to the governing system of the D09 type units, it is recommended to use a synchronizer to raise the speed to a certain level (for example, 3150–3180 r/min) first, and then use the overspeed test spool; or alternatively, use a synchronizer directly for the speed-raising test. 3. Solve the testing problems of emergency safety devices and provide reliable testing equipment. Develop practical testing methods and standards. 4. Improve the operational reliability of the overspeed protection device. 5. Pay attention to the installation of thermal testing systems, strengthen the management and maintenance of instruments, and ensure a high operational efficiency of these instruments. 6. Accelerate the introduction and development of key components for turbine generator monitoring and protection devices, in order to enhance the monitoring of critical operating parameters of the units, as well as the recording and storage of protection-related data. (II) Regarding the improvement of the stability and reliability of shafting systems: 1. Develop bearings with good stability, and improve existing three-oil- wedge bearings to enhance their stability margin. 2. Determine the appropriate relative elevation for each bearing. 3. Improve the manufacturing quality of bearing shells and the installation processes, and establish repair and grinding procedures and standards for three-oil- wedge bearing shells. 4. Study the necessary preload for critical bolts in the shafting system and the anti-loosening measures. 5. Improve the manufacturing and installation quality of the extended shafts for medium- and low-pressure rotors. 6. Survey the low-frequency vibration components of the currently operating units to establish reasonable allowable values. V. The “85” public relations project: Given that 76 domestic 200MW units with three exhaust rows are already in operation, and the number is expected to rise to around 100 by 1990, these units will remain one of the main types of thermal power generators in China for some time to come. In order to take effective measures to improve their operational reliability and safety, the following long-term tasks are proposed for inclusion in the two key research projects under the “85” plan. (1) Research on the stability of the shafting system in large steam turbine generators (2) Research on improving the reliability of the control and safety systems in domestic 200MW units (3) Research on unit automatic monitoring systems and online fault diagnosis technologies (4) Research on the coordination between large power grids and large units under disturbances in power systems – dynamics of electromechanically coupled systems (5) Research on quality control and reliability throughout the life cycle of large units (6) Research report by a task force on expert systems for failure diagnosis and preventive decision-making in large units 1. Summary of work and attachments by the materials and fracture analysis team of the expert group investigating the accident involving Unit 5 at Qinling Power Plant 2. Summary of work and attachments by the bearing vibration and strength team of the expert group investigating the accident involving Unit 5 at Qinling Power Plant 3. Summary of work and attachments by the control and safety systems team of the expert group investigating the accident involving Unit 5 at Qinling Power Plant 4. Summary of work and attachments by the comprehensive team of the expert group investigating the accident involving Unit 5 at Qinling Power Plant http://5b0988e595225.cdn.sohucs.com/images/20170920/4c5dfc864ab94a4ba918d0b6ac6fb49c.png http://5b0988e595225.cdn.sohucs.com/images/20170920/3b3e77f109da42c7b041867b18385af1.png List of materials provided by the “Equipment Evaluation Team” jointly formed by the former Ministry of Water Resources and Electric Power and the former Ministry of Machinery (submitted on March 6, 1988): 1. Investigation report on the damage to equipment following the accident involving Unit 5 of Qinling Power Plant’s steam turbine generator set 2. Brief overview of the damage to the turbine and bearing shells of Unit 5 at Qinling Power Plant 3. Investigation into the damage to Unit 5 of Qinling Power Plant’s generator set 4. Inspection of the condition of the speed control system of Unit 5 after the accident 5. Results of checks on the speed measurement and electrical over-protection systems of Unit 5 at Qinling Power Plant 6. Collection of images describing the damage to equipment resulting from the accident involving Unit 5 at Qinling Power Plant 7. Labels and names of the debris that flew off during the accident involving Unit 5 at Qinling Power Plant 8. Report on the investigation into the shafting fracture accident involving Unit 5 at Qinling Power Plant Attachments: List of members of the expert group investigating the accident involving Unit 5 at Qinling Power Plant Serial Number Unit Name Technical Title Specialization Team Leader 1 Professor Zhong Qunpeng, Deputy Director of Beihang University Materials, Failure Analysis Deputy Team Leader 2 Engineer Zhu Xin from the former ** Economic Commission, Director Third Members 3 Researcher Wang Renzhi from Institute 621 under the Ministry of Aeronautics and Astronautics Materials 4 Researcher Wang Xiwen (female) from the Beijing Iron and Steel Research Institute Materials 5 Professor Zhu Jun from Xi’an Jiaotong University, Deputy Director Bearings 6 Professor Meng Qingji from Xi’an Jiaotong University Vibration 7 Professor Wang Weijian from Tsinghua University Control and Protection 8 Associate Professor Wang Zheng from Tsinghua University, Head of Laboratory Rotor Dynamics 9 Engineer Shu Junzhan, Chief Engineer from Xi’an Thermal Engineering Research Institute Steam Turbines 10 Engineer Fang Deming from Xi’an Thermal Engineering Research Institute Control and Protection 11 Engineer Chen Jigang from Xi’an Thermal Engineering Research Institute Materials 12 Engineer Wang Lishan from North China Electric Power Administration Steam Turbines 13 Engineer Chen Rongde from North China Electric Power Administration, Deputy Chief Engineer Electric Machines 14 Engineer Fu Rujie from the Energy Ministry’s Electric Power Construction Research Institute, Deputy Chief Engineer Vibration 15 Engineer Yang Guanghai from Shanghai Power Generation Equipment Design and Research Institute Dynamics 16 Engineer Sun Huilian from Shanghai Power Generation Equipment Design and Research Institute Materials 17 Researcher Wu Qingke (female), Head of the Reliability Center at the Mechanical Research Institute Fracture Mechanics 18 Engineer Shi Jianzhong from the Reliability Center at the Mechanical Research Institute, Director of the Center Bearings 19 Engineer Zhang Weijie from Harbin Electric Machinery Factory, Deputy Chief Engineer Electric Machines 20 Deputy Chief Engineer Wang Xixiang from Beijing Heavy Electric Machinery Factory Control Attachments: Detailed schedule of the work carried out by the expert group investigating the accident involving Unit 5 at Qinling Power Plant 1. From March 5, 1988, to March 18, 1988, a total of 12 days were spent conducting on-site investigations, analyses, and discussions at Qinling Power Plant. 2. On March 23, 1988, a report was submitted to the two committees (the former **Economic Commission and **Mechanical Industry Commission) and the two ministries (the former Water and Power Ministry and the Supervision Ministry). 3. On April 9 and 13, 1988, the opinions of the former Ministry of Water Resources and Electric Power and the former Machinery Commission were heard respectively. From April 21 to April 29, 1988, over a period of 8 days, a second on-site investigation, analysis, and discussion were conducted at the Qinling Power Plant. On June 9, 1985, discussions were held with colleagues from the former Machinery Commission and the former Water and Power Ministry on \"Urgent Measures and Some Suggestions for Improving the Reliability of 200MW Units with Three Exhaust Streams.\" From July 2 to July 4, 1986, a three-day expert group analysis and discussion meeting was held in Beijing. From August 10 to August 14, 1988, over a period of four days, some members of the expert group went to the Qinling Power Plant to further hear from witnesses and address questions related to the investigation. On August 30, 1988, an expert group meeting was held in Beijing to discuss the issues summarized by the team. From August 23 to September 15, 1988, each expert group conducted its own group summary and prepared relevant documents, with each group taking 3–4 days for this task. On September 5 and September 7 of 10.88, the members of the expert group in Beijing listened to the opinions of the two departments on the accident analysis. From September 20 to September 29, 1988, a summary, analysis, and discussion meeting was held in Beijing over a period of 19 days. Summary of Topics for Analysis and Experimental Research
I. Phase 1 (March 19–April 20): 20 topics related to analysis and experimental research
1. Experimental analysis of the material and fracture at the low-voltage end of the motor shaft (at bearing No. 6)
2. Experimental analysis of the material and fracture of the rotor shaft seal in the low-pressure cylinder
3. Fracture analysis at the bolts at the root of the motor fan blades
4. Deformation and hardness tests on the pin-connected blades in the middle and last stages of the medium-pressure cylinder, as well as their pins
5. Actual measurement report on the second-order critical speed of the motor rotor in a 200MW steam turbine generator set (fifth-order critical speed of the shafting)
6. Influence of the shafting elevation on shafting stability
7. Impact of loose bolts securing the bearing shells or failure of the oil wedges on bearing performance
8. Unbalance responses caused by the loosening of bearing No. 9, as well as unbalance resulting from imbalances in the slip rings, retaining rings, and fans
9. Estimation of the energy required for the motor rotor shaft segment to fly out
10. Further investigations and research
a. Possibility and causes of severe overspeed during the 2.12 accident
11. Further analysis of the possibilities related to the overspeed protection system
12. Simulation tests on the failure of bolts holding the motor fan blades
13. Shear tests and simulations on the 25-stage pin-connected blades
14. Verification of digital tachometers and vibration tests
15. Reliability investigation of existing 200MW units in China and operational conditions of such equipment
16. Investigation and study of shafting fracture accidents in foreign countries
17. Exploratory tests on “combined vibrations” of shaftings
18. Comprehensive analysis of the causes of lateral shafting fractures in domestic and foreign generators
19. Mechanical model analysis of the impact of impact loads on the deformation of pin-connected blades and shear of their pins
20. Comparative analysis of the structure and performance of three domestically produced 200MW units

II. Phase 2 (April 30–July 1): 23 topics related to analysis and experimental research
(A) Topics for the Material Fracture Group
1. Comprehensive analysis of the material, fracture, and damage characteristics of the slip rings, along with their potential as sources of unbalanced forces in this accident
2. Fracture analysis of instantaneous high-torque torsional braking in the middle section of the shafting, and possibility of shafting breakage due to braking in the low-pressure cylinder
3. Further quantitative analysis of the macroscopic fractures at both ends of the motor shaft, as well as confirmation of its first-order vibration mode
4. Analysis of the material, chemical, and mechanical properties of the motor shaft and low-pressure rotor shaft, and assessment of the possibility of fracture due to material issues
5. Analysis of the fracture characteristics of the low-pressure cylinder blades, and assessment of the possibility of premature blade fracture
6. Fracture analysis of the emergency stop device (short shaft)
7. Literature review on the dynamic response of material mechanical properties

(B) Topics for the Shafting Vibration Group
1. Mechanical model analysis and calculation methods for the deformation and failure of pins on the sealing blades, as well as their use in determining the possibility and accuracy of overspeed
2. Further analysis of the sources of unbalanced forces in this accident
a. Analysis of the damage to bearing No. 2
b. Further comprehensive analysis of the factors that led to this accident
c. Further comprehensive analysis of the shafting fracture pattern in this accident

(C) Topics for the Control and Safety Group
1. Analysis of the dynamic characteristics of the control and safety system, as well as the conditions leading to severe overspeed
2. Analysis of steam energy under overspeed test conditions, along with estimation of the maximum speed
3. Conditions under which the speed control system of Unit 5 lost control, and estimation of the possibility of severe overspeed in this case
4. Estimation of the rate of speed increase during overspeed tests based on experimental parameters
5. Relationship between the rotation speed of the control valve during overspeed tests and the rate of speed increase

(D) Topics for the Comprehensive Analysis Group
1. Analysis of the stress conditions on components that have not yet deformed or fractured, along with estimation of the maximum speed
2. Analysis of radial clearance in the medium-pressure cylinder blades and deformation/failure of other components, along with estimation of the minimum speed
3. Comparative analysis of the structure and stability of the three domestically produced units, and their relationship to this accident
4. Comprehensive suggestions regarding preventive measures (for both old and new units)
5. Further ideas for future research topics
6. Further exploration of the possibility of shafting harmonic vibrations

III. Phase 3 (July 3–September 10): 8 topics related to analysis and experimental research
1. Summary of inquiries regarding domestic 200MW units under moderate operating conditions
2. Information on the operating conditions of Unit 5 at Qinling Power Plant before accidents 2 and 12
3. Information on the operation records of Unit 5 at Qinling Power Plant
4. Comparative analysis of the Dongfang and Harbin units
5. Analysis of the shaft breakage accident involving Unit 5 at Qinling Power Plant
6. Impact of the low yield strength of the motor shaft material, as well as loose screws No. 3, 6, and 7, on shaft breakage
7. Analysis of the conditions and reliability related to sudden oil film oscillations
8. Report on the investigation conducted at Qinling Power Plant from August 11–13, 1988

Appendices
Brief report on the investigation of the accident at Qinling Power Plant
1. In Phase 1, an expert investigation team was formed at Qinling Power Plant.
2. In Phase 2, the expert team began working at Qinling.
3. In Phase 3, cleanup work started at the accident site of Unit 5.
4. In Phase 4, the casing of Unit 5 at Qinling was removed.
5. In Phase 5, the expert team further discussed issues related to speed and vibration sources.
6. In Phase 6, relevant government departments listened to the expert team’s reports.

Appendix: Video footage of the 2.12 accident at Unit 5 of Qinling Power Plant (attached separately)
Some opinions regarding the work of the expert assistance team, along with the reports submitted
I. Some opinions regarding the work of the expert assistance team
1. The task of the expert assistance team is to assist in completing the tasks assigned by the expert team. Please assign responsibilities to comrades such as Qian Zhongpeng, Guo Jianlie, Shu Junzhan, and Zhou Xisheng.
2. During periods when the expert team is temporarily away from the site, the main task of the expert assistance team is to identify important component debris. The specific identification items are listed in the attached table. 3. When conducting the identification of each significant wreckage item, the expert assistance team shall form a team composed of representatives from various parties. An appraisal report must be prepared for the appraisal results, and it shall be signed and approved by representatives from various parties. 4. Unless approved by the expert panel, the expert assistance team shall not conduct destructive tests on important debris pieces, nor take any of such debris fragments away without permission. 5. The expert assistance team shall establish a unified photography group, assign a specific person in charge, and store the film uniformly. Relevant units may develop the photos, but they must not be made public. As evidence-based original footage, it must obtain the approval of the expert advisory group. 6. When the expert team visits the site for further investigation for the second time (around mid-April), please provide the expert assistance team with the certification issued and the relevant original evidence materials. 7. Based on the work of identifying the wreckage, each unit is requested to conduct an analysis of the causes of the accident, and submit the analysis report to the expert group by mid-April. 8. Members of the assistance team have the right to access information related to the accident, including design calculations, drawings, manufacturing processes, commissioning data, as well as records of installation, maintenance, and operation. **Expert Investigation Team on the Accident of Unit 5 at Qinling Power Plant, March 21, 1988
II. Report Materials Submitted by the Expert Assistance Team
1. Summary of the Inspection Results Regarding the Damage to Unit 5’s Generator in Qinling Power Plant
2. Inspection Results of the Disassembly of Unit 5’s Turbine in Qinling Power Plant
3. Inspection Results of the Disassembly and Damage Assessment of Unit 5’s Turbine’s Speed Control and Safety Systems
4. Measurement Results of the Radial Gaps Between the High- and Medium-Pressure Rotor Blades and Their Rims in Unit 5 of Qinling Power Plant
5. Damage Status of the Coupling Bolts in Unit 5 of Qinling Power Plant
6. Inspection and Measurement Results of the Blades and Impellers in the Medium-Pressure Section of Unit 5 in Qinling Power Plant
7. Further Investigation into the Parts That Failed First in the Low-Pressure Cylinder

Section Number, Location of the Main Section, Location Where the Shaft Broke Off
1. The short shaft of the main oil pump and the emergency safety device; the coupling came loose. The shaft of the main oil pump remained in place.
2. Shaft breakage between the two emergency safety devices; the shaft of the first emergency safety device fell inside the front casing.
3. Breakage of the bolts connecting the short shaft of the emergency safety device to the main turbine shaft. The shaft of the second emergency safety device fell inside the front casing.
4. Breakage of the bolts connecting the medium-pressure rotor to the medium-pressure extension shaft. The medium-pressure rotor remained in place.
5. Breakage of the bolts connecting the medium-pressure extension shaft to the low-pressure extension shaft. The medium-pressure extension shaft flew out from the right side and landed on the 5-meter heater platform.
6. Breakage of the bolts connecting the low-pressure extension shaft to the low-pressure rotor. The low-pressure extension shaft flew out from the right side and landed on the 10-meter platform adjacent to Row B wall.
7. Breakage of the low-pressure rotor at the withdrawal groove with a diameter of Φ405 mm at the root of the 37th-stage impeller. The broken shaft of the low-pressure rotor lay sideways inside the bridge key.
8. Breakage of the low-pressure rotor at the withdrawal groove with a diameter of Φ405 mm at the root of the 32nd-stage impeller. The low-pressure rotor remained in place.
9. Breakage of the bolts connecting the low-pressure rotor to the generator. The broken shaft of the low-pressure rotor rotated 180 degrees and fell back to its original position.
10. Breakage of the generator rotor at a point with a diameter of Φ420 mm. The broken shaft of the generator rotor stood upright on the left side of the generator’s end face.
11. Breakage of the generator rotor at a point with a diameter of Φ420 mm. The main shaft of the generator remained in place.
12. Breakage of the bolts connecting the generator rotor to the main exciter. The broken shaft of the generator rotor passed through Row B wall and fell onto the 10-meter platform in the boiler room; the main shaft of the exciter remained in place. http://5b0988e595225.cdn.sohucs.com/images/20170920/feb3f6b2dc0643ce9d21557c2b2aaecf.jpeghttp://5b0988e595225.cdn.sohucs.com/images/20170920/a60a688d6a5349f89808bdc2df0509f0.png