On January 12, Unit 4 of a power plant experienced an unplanned shutdown triggered by a boiler MFT (Main Fuel Trip), and an investigation has revealed multiple contributing factors including inadequate operator skills, poor coordination among shift personnel, and ineffective leadership by the shift supervisor during abnormal condition handling.

Event Background and Equipment Overview

The Unit 4 boiler is a supercritical once-through boiler independently developed, designed, and manufactured by Harbin Boiler Company Limited. It features a single intermediate reheat, variable pressure operation, single furnace, balanced draft, solid slag removal, full steel frame, full suspension structure, ∏-type layout, and open-air arrangement, with the boiler model designated as HG-1100/25.4-YM1. The boiler is equipped with two 29.5-VI(T)-2500-QMR type three-sector Ljungstrom air preheaters manufactured by Harbin Boiler Company. The rotary air preheater consists of heat transfer elements, rotor seals, transmission devices, rotor thrust bearings, rotor guide bearings, as well as fire protection, fire monitoring, steam soot blowing systems, and cleaning systems. The unit was commissioned on December 26, 2012.

Pre-Event Operating Conditions

On January 12 at 04:15, Unit 4 was operating in AGC mode with a stable load of 207MW. The A, B, and E pulverizing systems were in operation, with a total coal flow of 87.8t/h, total air flow of 769.5t/h, and feedwater flow of 600.6t/h. The A/B air preheaters were operating normally, with outlet exhaust gas temperatures of 112°C and 115°C respectively.

Sequence of Events

At 04:19:30, the unit load was 210MW with furnace negative pressure at -55Pa. The 4A air preheater exhaust gas temperatures were 122°C/106.3°C/115.4°C. The lead operator monitoring the panel noticed an inconsistent air preheater rotor stoppage alarm on the 4A air preheater and immediately reported to the Phase II shift supervisor while dispatching an inspection worker to check the air preheater operation on site. At 04:20:26, the DCS issued a 4A air preheater measurement stoppage alarm. The inspection worker reported back after checking the 4A air preheater support bearing that the air preheater had stopped rotating on site, the 4A air preheater main motor coupling had disengaged, and the air preheater main motor was running but the reducer was not actually turning. At 04:21:30, the 4A air preheater exhaust gas temperature began rising rapidly to 134°C (peak), and furnace negative pressure reached +12Pa. The lead operator was responsible for monitoring the air-flue gas system while arranging deputy operator Shen to monitor the steam-water system and deputy operator Xu to monitor the pulverizing system. At 04:23:42, the lead operator switched A and B forced draft fans to manual control and induced draft fans A and B to manual control, manually adjusting the A forced draft fan moving blade from 26% to 39% and the B induced draft fan moving blade from 32% to 46%, with negative pressure at -392Pa at that time. At 04:24:36, the Phase II shift supervisor ordered switching to local to stop the 4A air preheater main motor, and the auxiliary motor started automatically in low-speed operation with a motor current of 44A. At 04:24:41, it automatically switched to high-speed operation with a motor current of 22A, and the inspection worker visually confirmed at the air preheater support bearing that the air preheater was still not rotating. The A-side air preheater exhaust gas temperature was 191°C, and the B-side air preheater exhaust gas temperature was 123°C (peak). At 04:24:55, the lead operator continued opening the 4B induced draft fan moving blade to 53% and closing the 4A induced draft fan moving blade damper to 25%, with furnace negative pressure at -550Pa. At 04:25:36, the DCS interlock protection stopped the 4A air preheater auxiliary motor, both the main and auxiliary motors of the 4A air preheater were fully stopped, and the single-side air preheater trip triggered RB action, interlock tripping the 4B coal mill and A primary air fan, and automatically putting the A-layer plasma and E-layer oil guns into service. The lead operator immediately arranged for the deputy operator to check turbine-related parameters, adjust auxiliary steam pressure and condensate pressure, and arranged for another deputy operator to monitor boiler-related screens, check plasma and oil gun activation, and reduce coal feed to 52t/h. At this time, the 4A air preheater exhaust gas temperature was 205°C (peak), the unit load automatically reduced to 156MW, and furnace negative pressure was -580Pa. At 04:27:27, the lead operator engaged automatic adjustment of the induced draft fans, with A induced draft fan moving blade opening at 26% and current at 151A, and B induced draft fan moving blade opening at 56% and current at 259A, with furnace negative pressure at -202Pa. At 04:28:27, the A induced draft fan moving blade opening was 29% with current at 155A, and the B induced draft fan moving blade damper opening was 61% with current at 284A. At 04:28:30, the A induced draft fan issued a stall alarm. At 04:30:12, the lead operator switched the induced draft fans to manual adjustment, adjusting the A induced draft fan moving blade opening to 37% and the B induced draft fan moving blade opening to 47%. At 04:30:45, the A induced draft fan stall alarm disappeared. At 04:32:28, the unit load was 164MW, the RB ended, and furnace negative pressure was -1441Pa. The A induced draft fan stall alarm was issued again, and the A induced draft fan moving blade was manually closed to 38% with current at 177A, and the B induced draft fan moving blade damper was closed to 40% with current at 244A. At 04:33:25, the 4A induced draft fan stall alarm disappeared, and negative pressure dropped to -524Pa. The A and B induced draft fan moving blades were at 38.8% and 41.3% respectively, with currents at 149.5A and 207.7A; the A and B forced draft fan moving blades were at 26.7% and 23.4% respectively, with currents at 26A and 23.7A. At 04:33:48, furnace negative pressure was -1558Pa, and the lead operator manually closed the 4A forced draft fan moving blade to 9% with current at 25.3A, and closed the 4B forced draft fan moving blade to 7% with current at 23.9A. At 04:35:08, furnace negative pressure dropped to -2344Pa, with the left furnace negative pressure measurement point 1 at -2558Pa and the right furnace negative pressure measurement point 2 at -2580Pa, below the protection action value of -2500Pa. The left furnace pressure 1 low-low, left furnace pressure 2 low-low, and right furnace pressure 2 low-low actions were triggered, activating the furnace pressure low-low protection, boiler MFT, turbine trip, and generator interlock trip actions operated normally. At 04:36:00, the Phase II shift supervisor ordered shutdown processing to ensure safe unit shutdown while reporting to the Phase I shift supervisor to notify relevant personnel to arrive on site.

Cause Analysis

Regarding the boiler MFT cause: During the handling of the 4A air preheater stoppage and trip abnormality, operators switched to manual adjustment of induced draft fan output to control the rapid rise in exhaust gas temperature. The large output deviation between the two induced draft fans caused the A induced draft fan to stall. While handling the A induced draft fan stall, furnace negative pressure was low, and the induced draft fan moving blade opening was not reduced in time. Simultaneously, the forced draft fan output was significantly reduced, which exacerbated the furnace negative pressure decrease, ultimately reaching the furnace pressure low-low protection action value of -2500Pa, causing boiler MFT. Regarding the air preheater trip trigger cause: The 4A air preheater main motor input shaft end cover lower gasket had inadequate process quality standards and improper installation, causing the motor fixed bracket lower part to open, the coupling elastic block to wear, metal parts to make hard contact and wear, and the claw teeth to fracture and disengage, causing the air preheater to stop rotating. After the auxiliary motor started automatically, the rotation signal was not detected within 60 seconds, and the "air preheater inlet flue gas temperature and outlet secondary air temperature difference higher than 100°C signal" triggered the air preheater motor shaft breakage protection to stop the auxiliary motor. The main motor had already been switched to local and the automatic start failed, and both motors stopping triggered the 4A air preheater trip signal.

Exposed Problems

First, operators had insufficient skills and did not thoroughly understand the boiler balanced draft principle, leading to improper operation. When furnace negative pressure was low, they failed to promptly reduce induced draft fan output to control furnace negative pressure, instead reducing forced draft fan output to control furnace negative pressure, further worsening the furnace negative pressure condition. Second, operators lacked coordination and mutual monitoring during accident handling. When the deputy operator noticed increasing negative pressure and fan stall, only verbal reminders were given without actually reviewing the air-flue gas system screens to stop the lead operator's misoperation. Third, the shift supervisor's organization of abnormal accident handling was ineffective, failing to control key risk points during the abnormal handling process and failing to properly assume monitoring and command responsibilities. Fourth, the air preheater reducer external repair quality was poor, with low repair process standards and coupling misalignment, causing air preheater coupling elastic block wear and motor disengagement. Fifth, the air preheater shaft breakage protection logic parameters were unreasonably set, with the 60-second delay to trip the running motor not fully considering the motor stall time during hot-state startup of the air preheater. Sixth, defect management was inadequate. The analysis of the air preheater coupling elastic block wear cause was insufficient, risk analysis and pre-control measures were inadequate, operations had not prepared for accident scenarios and drills, and maintenance had not promptly eliminated equipment defects.

Preventive Measures

First, strengthen operator skills training. Through professional knowledge training and simulator accident drills, effectively improve operators' abnormal accident handling capabilities and teamwork abilities. Second, improve equipment external repair quality control capabilities. Notify the air preheater reducer manufacturer to immediately come to the plant for comprehensive analysis, evaluation, and inspection of repair quality, thoroughly address remaining issues, and completely eliminate safety hazards. Third, optimize and improve the air preheater shaft breakage protection logic. During air preheater startup, collect the hot-state startup stoppage alarm disappearance time, and modify the delay time based on equipment operating conditions and motor stall. Fourth, continuously strengthen equipment defect management. Actively carry out defect elimination work, conduct risk analysis and pre-control measures for defects that cannot be eliminated in time, and implement unit anti-unplanned-outage measures.

Preventive Measures Implementation Table

1. Strengthen operator skills training. Through professional knowledge training and simulator accident drills, effectively improve operators' abnormal accident handling capabilities and teamwork abilities. 2. Improve equipment external repair quality control capabilities. Notify the air preheater reducer manufacturer to immediately come to the plant for comprehensive analysis, evaluation, and inspection of repair quality, thoroughly address remaining issues, and completely eliminate safety hazards. 3. Optimize and improve the air preheater shaft breakage protection logic. During air preheater startup, collect the hot-state startup stoppage alarm disappearance time, and modify the delay time based on equipment operating conditions and motor stall. 4. Continuously strengthen equipment defect management. Actively carry out defect elimination work, conduct risk analysis and pre-control measures for defects that cannot be eliminated in time, and implement unit anti-unplanned-outage measures.

Appendix 1: SOE Records Before and After Boiler Trip. Appendix 2: Furnace Negative Pressure Adjustment Curve. Appendix 3: 4A Air Preheater Coupling Back Wheel Misalignment.

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