Flange bolt break, EH oil leak, unit trip!
First, the equipment profile:
Unit 6 is a 1030MW ultra-supercritical secondary reheat coal-fired generating unit, the boiler is manufactured by Harbin Boiler Co., Ltd., boiler model: HG-2752/32.87/11.01/3.45-YM1, the turbine is designed and manufactured by Shanghai Steam Turbine Co., Ltd., using german Siemens technology, steam turbine model: N1030-31/600/620/620. The turbine control system adopts the Siemens T3000 DEH control system. The generator adopts QFSN-1000-2 type three-phase AC hidden pole synchronous steam turbine generator produced by Shanghai Generator Co., Ltd.
The EH oil system equipment is supplied by SAIC Motor Plant synchronously, and the EH oil pump is an axial piston pump with a rated flow rate of 2.4m3/h, an outlet pressure of 16MPa and a rated power of 30kW.
Second, the pre-accident working conditions:
At 6:46 on May 17, 2018, Unit 6 loaded 1011MW, AGC operation mode, A, B, C, D, E, F coal mill operation, unit operating parameters: main steam pressure 31.6MPa, main steam temperature 599 °C, primary reheat steam pressure 10.2MPa, primary reheat steam temperature 610 °C, secondary reheat steam pressure 3.2 MPa, secondary reheat steam temperature 617 °C, feed water flow 2732t/h, A, The opening degree of the ultra-high pressure gate on both sides of B is 38%, and the high-pressure gate and the medium-voltage gate are fully open. A EH oil pump current 23A, EH oil pressure 16.0MPa, EH tank oil level 531mm, B EH oil pump standby.
III. Events:
At 6:47:27 on May 17, 2018, the operator found that the oil level of the EH tank was one value lower (438mm) alarm, the oil level of the tank dropped rapidly, the A EH oil pump current rose from 23A to 49A, the EH oil pressure suddenly dropped from 16.0MPa to 15.1MPa, and the operator immediately went to the scene to inspect the EH oil system.
At 6:48:03, the A EH oil pump current rose from 49A to 60A, the EH tank level dropped to 391mm, and the EH oil pressure was 15.0MPa.
At 6:52 minutes and 04 seconds, the oil level of the EH tank was 208mm, the A EH oil pump current was reduced from 60A to 35A, and the oil pressure of the EH bus tube began to drop from 15.0MPa.
At 6:52 minutes and 23 seconds, the oil pressure of the EH bus tube dropped to 12.0MPa, the outlet oil pressure of the A EH oil pump < 11.5MPa, the B EH oil pump was started, and the EH oil pressure continued to decline without an upward trend.
At 6:52:34, the outlet pressure of the A and B EH oil pumps ≤ 1.0 MPa, and the A and B EH oil pumps tripped.
At 6:52 minutes and 48 seconds, the oil pressure of the EH mother tube dropped to 10.5MPa, the EH oil pressure low protection action (three to two) The steam turbine trip, the generator was de-trained, the boiler MFT action, and the plant power switching was normal.
After inspection, it was found that a large amount of oil leaked from the EH oil station of Unit 6 to the ultra-high pressure valve oil pipe flange on the A side.
At 19:59, Unit 6 was joined by system recovery.
Fourth, the cause analysis:
The reasons for the tripping of Unit 1 and Unit 6 are:
Unit 6 EH oil station to A side ultra-high pressure valve tubing flange bolt break, EH oil leakage, resulting in rapid decline in the oil level of EH tank, resulting in EH oil pump work abnormal tripping, EH oil pressure low protection action, Unit 6 tripping.
The fracture bolt specification is M14*50, the design material is Q235-A-O, and the spectral analysis of the fracture bolt is carried out on the spot, and the material meets the design requirements.
2. Analyze the causes of EH tubing flange bolt fracture as follows:
1) Bolts may have manufacturing defects during production and processing.
2) When the bolt is installed, there may be a situation where the installation tension is too large.
V. Exposure Problems:
1. The importance of the EH oil system is not understood. The accident of the EH oil system in the sister unit did not attract enough attention and did not deeply learn the lessons of the accident.
2. The investigation and management of hidden dangers is not in place. During the installation and operation of Unit 6, no major hidden dangers of EH tubing flange bolts were found to be broken.
3. Technical supervision and management are not in place. The review of the bolt material of important parts is not included in the scope of supervision.
VI. Preventive Measures:
1. Replace the flange bolt of the oil supply pipeline of the EH oil station of Unit 6 with a high-strength bolt (12.9 level).
2. Conduct a comprehensive inspection of the EH oil system pipeline of Unit 6 and Unit 7, find abnormal vibration, and carry out reinforcement treatment.
3. Comprehensive inspection of flange bolts of unit EH oil system of Unit 6 and No. 7, all replacement of high-strength bolts when the unit is out of operation, and strict process discipline and installation standards when replacing.
4. Take the tightening of the flange bolts of the EH oil system of Unit 6 and No. 7 as the scope of daily inspection, and deal with the problems in time.
5. The flange bolt of the EH oil system is included in the scope of metal supervision.
