Please help analyze the cause of the accident
Thread Content
During the normal operation of a boiler, at around 9:00 a.m., the boiler operator reported that one damper actuator was faulty and could not be operated. An on-site inspection confirmed the fault; the power indicator light on the servo board of the PS angular displacement actuator was flashing, which led to the assumption that there was a problem with the servo board. After replacing it, the power indicator worked properly (at that time, it wasn’t checked whether the feedback was normal). The boiler operator was asked to send an open signal, and once that signal was sent, the power indicator light started flashing again. The 220V voltage supplied to the card was normal, but the actuator still wouldn’t function. A fault was detected in the feedback system. Since soft servo control was being used, no output would occur without a valid feedback signal. It was found that the connection to the card was constantly interrupted; therefore, the feedback signal was forced to 0, and an operation signal was sent. As a result, the feedback signal began to fluctuate erratically. At this point, the operator noticed that the primary air pressure of the boiler was low, and multiple alarms appeared. About 5 seconds later, it was observed that the current of one fan was zero, and the low negative pressure in the boiler furnace triggered the boiler’s MFT mechanism. Approximately 20 seconds later, two exhaust fans and one supply fan stopped working. Once the boiler returned to normal operation, another actuator was installed; the operation was normal, but the feedback was incorrect. It was found that the feedback circuit was damaged – a module on the servo board of the original actuator had been destroyed. Upon checking the records, it was found that almost simultaneously with the operation of the dampers, the current in both exhaust fans suddenly increased to more than twice the level during normal operation; their rotational speeds rose from over 700 revolutions per minute to 947 and 890 respectively. At that time, all four fans were in speed-regulation mode, and this phenomenon indicates a transition from speed regulation to full speed operation. The speed-regulation system used an internal feedback device manufactured by Harbin Jiu Zhou. Two seconds before one of the fans tripped, an electrical alarm sent a signal to activate motor protection, and the fan tripped two seconds later. What remains unexplained at present is: 1. Whether the thermal control operation dampers will affect the two exhaust fans causing them to operate at full speed; the conditions for operating at full speed are: device failure, high-voltage switch tripping, manual reduction of speed, being in remote control mode, and the activation of the backup power supply. (The operation of the damper and that of either of the exhaust fans are not handled within the same control unit.) 2. What caused both exhaust fans to operate at full speed simultaneously? 3. Since there was no variation in current before the supply fan tripped, why was a protection activation alarm issued? What does this protection activation mean? 4. Why did both exhaust fans trip simultaneously after an MFT occurred? (The tripping of the other supply fan is understandable, as the interlock mechanism causes the supply fan to trip once the exhaust fans trip.) From a thermal control perspective, operating any damper has no relation to the supply and exhaust fans. The dampers are part of the secondary air system in the coal-fired furnace; they are controlled via passive digital switches to increase or decrease their operation. The commands for starting, stopping, adjusting the speed, or setting the fans to full speed, as well as the commands for operating the dampers, are not on the same circuit board and are located in different cabinets. The feedback signal regarding the position of the dampers is found in Cabinet No. 4, while the speed setting signal for the supply and exhaust fans is in Cabinet No. 2. The fan speeds are adjusted manually, as automatic combustion control is not active. So far, two thermal control faults have been identified: the feedback channel for the dampers is damaged, and one module on the actuator’s control board has failed. Could everyone please help analyze what caused the boiler to shut down?
1. Before the blower trips, an electrical alarm sends a signal to activate motor protection. The reason for this signal being sent, as well as the nature of that signal, are the factors that cause the blower to trip and lead to the shutdown of the boiler. This can be investigated in conjunction with electrical technicians； 2. The failure of the feedback circuit for the damper, as well as the breakdown of a module on the actuator’s control board, are likely caused by high voltage entering the circuit; it is possible that such interference led to both exhaust fans operating at full speed ； 3. Check the DCS logic to see if there are any interlock errors ； Feel free to point out any mistakes! Stay tuned!
1: The valve opening signal on the servo board may have reached its maximum (or minimum) value, and this signal is fed into the DCS’s analog input. The opening degrees of other valves are related to that of this valve. 2: The 220-volt power supply on the servo board enters the DCS via feedback lines, which can cause the analog circuit boards to be damaged and result in signals of maximum (minimum) opening degree, leading to incorrect operations. 3: The high current consumption by the fan is due to restarts; when switching from low speed to high speed, a restart is required. 4: The above reasons are provided for reference only.
1. First, check whether there is any issue with the DCS power supply section Was the module damaged by an instantaneous overcurrent? This caused the control circuit to malfunction, resulting in both motors starting at the same time. 2. I don’t know if your motors are controlled by frequency conversion? If it is frequency conversion control, you can check whether there have been any faults with the frequency converter, and then use the fault code to determine the cause. 3. You can also check what the rated current of the switch is. Because, as you said, the feedback channel is damaged, and you force it to 0 before sending out the control signal. This renders the motor’s operating condition unstable; the current either increases or decreases, which causes the motor protection device to trip. Because, as you said, a soft servo amplifier relies on feedback signals to determine the magnitude of the output signal it generates. 4. As for why the boiler stopped burning, I’m not sure; you’d better consult a professional in this area.