[Weekly Accident Case Sharing] Accident Case 7: Interlock failure issue.

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I. Fault symptoms: The boiler in a company’s own power plant is of the natural circulation type, and its coal grinding system is of the intermediate storage type; eight ball mill coal grinders are used to carry out the coal grinding task for the unit. On June 24, 2016, the unit was operating at full load, with all eight coal mills functioning properly. The inspector on site suddenly noticed that the bearing of Mill C was red-hot, and notified the operators in the control room to shut down Mill C promptly. The operator promptly checked the bearing temperature of Coal Mill C and found that it had reached 210°C. Moreover, the temperature rose gradually, and eventually maintenance personnel confirmed that the bearing had burned out. II. Cause analysis: (1) As an important auxiliary machine of the unit, the coal grinder is equipped with a protection mechanism that shuts down the grinder when the bearing temperature exceeds 75°C. However, this protection did not activate when the temperature reached 210°C (it failed to function). Upon inspection by the thermal control personnel, it was found that the protection sensor used in the logic circuit was not the actual temperature sensor. (2) The sound alarm set on the control station was muted by the operator. (3) The operators failed to monitor the situation properly; it took as long as 2 hours for the temperature to rise, and during those 2 hours they did not detect any abnormality in the temperature of bearing C of the coal grinder. III. Identified issues: (1) The management of sound alarm settings for process parameters is inadequate, with operators making changes arbitrarily. (2) During the testing of the unit protection logic, the testers carried out simulations behind the panels in the cabinet room, rather than at the actual measurement points as required, which resulted in the interlock system failing to function. (3) The operators do not monitor the system carefully. IV. Preventive measures: (1) Strictly comply with standards and specifications; when the shutdown time of the unit reaches the value specified by the standards, static tests of the protection logic for all auxiliary equipment and the main unit shall be carried out to ensure that the interlocks function promptly and accurately. (2) For static tests, a method of applying a simulated signal at the source point is used. (3) Strengthen the management of personnel and equipment.