【Summary of recent turbine runaway accidents】 with pictures and analysis!

Upon inspection, the upper and lower casings of the turbine’s rear cylinder were found to be cracked; all of the blades in stages 8 and 9 had fallen off, the outer ring of the blade in stage 7 was worn out and three blades were deformed. The outer rings of the blades in stages 1 through 6 were also worn out. The diaphragms in stages 7 through 9 were broken, and all of the diaphragm seals, radial seals, and front/rear seals were worn out. The shaft of the main oil pump was broken, as well as the oil seal rings; the oil baffle ring of the main oil pump was broken as well. All of the thrust bearings were worn out, with bearings #1 through #4 being damaged. The shaft diameters of bearings #1 and #2 on the main shaft were worn out, and both the axial displacement sensors and the speed measurement probes were worn out. The coupling and gears on the turbine side were displaced, the housing of the turning motor was broken, the stator and rotor of the exciter were damaged due to friction, the connections to the rectifier disc were broken, and the fan blades were worn out. Approximately 20 copper tubes in the condenser were damaged, and the generator rotor needed to be sent back to the factory for repair and testing.

III. Analysis of the accident cause: Under normal conditions, after a power failure in the system, the generator set will automatically disconnect, and its speed will rise rapidly. The electrical and mechanical protection systems of the turbine will then activate, causing the main steam valve to close automatically. However, since the Uninterruptible Power Supply (UPS) was in bypass mode and the operator failed to press the emergency stop button in time, no signal was sent to close the main steam valve, resulting in the turbine running out of control. The specific reasons are as follows: 1. After the system lost power, the control room operator started the emergency oil pump in accordance with the operating procedures, but failed to press the emergency stop button in the control room (this action is not required in the emergency operation procedures for power generation outages, and the operator was unaware of this function), which prevented the main steam valve from being closed in time and was one of the main causes of this accident.

2. The on-site waste heat power generation uninterruptible power supply unit (UPS) was in bypass mode; as a result, it was unable to supply power to the unit’s control system in the event of a power outage, which prevented the main steam valve from closing automatically in a timely manner. The UPS being in bypass mode was one of the main causes of this accident. (After consulting the manufacturer, it is understood that an uninterruptible power supply will automatically switch to bypass mode for self-protection when the external power supply fails and its own battery level is low; additionally, the bypass mode can also be switched manually. Gui Ding Company failed to detect this issue in time during routine inspections and restore normal operation.)

IV. Preventive Measures 1. Take advantage of startup and shutdown opportunities to test the electrical overspeed protection of the unit; if the unit has been in operation for more than 6 months, a mechanical and electrical overspeed test should be conducted on it, with proper records kept. 2. During startup and shutdown, test the tightness of the main steam valve and high-pressure control valve, and keep records. 3. Strengthen the daily maintenance and management of uninterruptible power supplies (UPS), identify existing problems, and take timely corrective actions to ensure that the system remains in good condition. 4. Further improve the DCS control system by displaying the normal signals from the uninterruptible power supply (UPS) on the central control interface; an automatic alarm will be triggered in case of abnormal conditions. 5. Under normal operating conditions, the generator set should undergo weekly testing of the system’s auxiliary equipment, such as the oil pump system and vacuum system, and records of these tests should be kept. 6. Drawing lessons from this incident, the \"Central Control Operation Procedures for Power Generation Systems\" were revised and improved. Steps for operating the central control emergency stop button were added to the section on emergency procedures in the event of a complete power loss across the waste heat power generation system, thereby standardizing operational procedures. Training was also provided to key personnel such as those in central control roles to enhance their skills in power generation operations, with a focus on proper procedures in emergency situations.

I. Accident details: At 3:59 on April 26, 20##, an overspeed accident occurred in the 18MW turbine of Generator Set No. 2 at a certain company. At 9:30 p.m. on the same day, relevant experts and technical personnel arrived at the scene to understand the course of the accident and conduct an inspection of the accident site. Upon on-site inspection, it was determined that some of the damaged equipment needed to be sent back to the factory for repair. The equipment was installed by June 9, oil circulation was confirmed to be satisfactory on June 10, and startup testing was carried out. The system was connected to the grid again at 3:52 on June 11 (after being offline for approximately 47 days). II. Accident details: The company’s No. 2 generator set was connected to the grid for power generation on March 31, 20##, with a designed capacity of 18 MW. At around 11:20 on April 24, 20##, a fault occurred at the 220kV substation supplied by the higher-level power supply department, and Generator Unit #2 was taken out of service at 11:40. During the operation of Unit 2 in the early stage, there were issues with no feedback display for the speed measurement from the local panel, one of the speed measurement feeds from the TSI protection cabinet, and the speed measurement on the control screen of the DEH (a digital electro-hydraulic control system based on digital computers). The company arranged for inspections and replacements to be carried out. During the inspection, it was found that two of the 7 speed measurement devices on the turbine were severely damaged (namely, the on-site local panel speed measurement device and one of the three speed measurement devices in the TSI turbine safety monitoring system protection cabinet), and the fixing point between the front speed measurement disk and the rotor shaft was loose. After resolving the above faults, corresponding inspections were carried out on the remaining 5 speed measurement devices, and feedback was received from each measurement point. At 23:47 on April 25, the turbine began to spin up, during which it experienced three trips due to complete failures in the speed monitoring system; DEH alarms were triggered and the main steam valve closed automatically. During this period, relevant professionals inspected the fault, but without conducting a thorough examination, a fourth start attempt was made at 3:37 the following day. At 3:59, out of the three DEH control speed measurement devices, two suddenly stopped showing any speed readings. Due to the system’s interlocking mechanisms, the turbine valves automatically opened to 68%, causing the speed to rise instantly to 3850 r/min. The turbine’s safety valve activated, and abnormal noises and vibrations were heard on site; as a result, the main steam valve was closed and the machine was shut down. III. Inspection Results Upon reviewing the DCS trends and records, during the fourth warm-up cycle at 1200 r/min, the ETS overall protection was disabled at 3:52:29. At 3:59:38, a full failure signal for the speed channel triggered a shutdown; at 3:59:57, the TSI issued an overspeed shutdown signal at 110%; at 4:00:00, the OPC overspeed protections at 103% and 110% were activated (the protections did not function as the overall ETS protection had been disabled). At 3:59:48, the DEH speed detection system detected that both the speeds of unit 1 and unit 2 were 0 r/min. The DEH program selected 0 r/min as the preferred speed (the program uses a two-out-of-three voting mechanism) for speed control. Since this value was significantly different from the target speed of 1200 r/min, an instruction to increase the valve opening was issued at 3:59:41. By 3:59:55, the valve opening had reached 70% (the maximum allowable value for the valve is 70%). By 4:00:00, the turbine reached its maximum speed of 3850 r/min, and at the same time the main steam valve began to close, causing the machine to shut down.

This post was last edited by Desert Fish on 2021-7-4 21:43. V. Cause Analysis: 1. During the maintenance on the 24th, it was found that two speed sensors were worn out and the gear discs were loose; these were replaced and tightened respectively. It was assumed that the looseness of the gear discs was the cause of damage to the speed measurement device, without conducting a deeper analysis, which was the direct cause of this incident. \2. During the fault inspection, the on-site inspectors failed to establish appropriate preventive measures in accordance with the system’s interlocking mechanisms; they did not adequately consider the possible consequences, and blindly disabled the ETS main protection. This violated the company’s regulations regarding the protection of main equipment, resulting in the ETS protection failing to function when a fault occurred in the unit – which was the main cause of this accident. 3. Another cause of this accident was that the company’s and department’s supervisors were not present when the main equipment was started up to organize and arrange for the staff to carry out operations in a proper manner.