Analysis and prevention of MFT accident caused by

2022-10-02
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MFT accident analysis and preventive measures caused by DCs power failure

a unit trip accident caused by boiler MFT occurred in unit 2 of plant a of Zhongshan power plant. After the accident, we discussed the technological innovation and reform trend of plastics in the automotive field. At about 10:20 a.m. that day, unit 2 was running at full load, and some electric valve position indications in the CRT of the boiler operator station turned yellow (indicating the middle position). After a few minutes, the outlet air valves of the two induced draft fans of the boiler are fully closed, and the regulating baffle is closed to less than 3%. Then, the boiler MFT acts, the steam turbine and generator trip, and the FSSS operation panel shows that the reason for MFT action is the loss of all induced draft fans

1 accident cause analysis

(1) the thermal personnel found that there was a trip in the 24VDC power supply of DCS cabinet 1, which provided external power for each channel of the switching value i/o template SDM of SCS (sequence control system) station. After analysis, the power supply trip is the direct cause of the unit trip

the main cause of power tripping is overload. In fact, as long as any i/o channel is grounded and the corresponding sub fuse cannot be burnt out in time, it may cause the total power overload trip

(2 the oil pipe should be cleaned with diesel oil first) the switching value input channel of SDM requires high and low level signals, and the input passive contacts need to be converted into high and low levels through external power supply. As shown in Figure 1, the SDM input channel wiring principle, if 24VDC power trips, each input channel becomes low level, that is, "0"

when the accident occurred, the external 24VDC of SDM 1, 2 and 4 disappeared in the i/o template of SCS station, and these three templates were configured as the switching value input module of SCS station, including the operation signals of induced draft fans a and B, so these input channels became "0", and the operation signals of induced draft fans a and B were also "0". In the program configuration of SCS, the tripping signal of induced draft fans a and B is the reverse of the operation signal of induced draft fans a and B, so the tripping signal of induced draft fans a and B becomes "UL is a world-renowned company independently engaged in safety science", that is, the tripping signal of two induced draft fans is sent by mistake

(3) according to the SCS induced draft fan sequence control logic shown in Figure 1, if the induced draft fan trips and delays for 1 min, the inlet baffle and outlet damper of the induced draft fan will be interlocked and closed. When the accident occurred, the furnace negative pressure regulation system was put into automatic operation, the inlet damper of the induced draft fan was under DCS automatic control, and the interlock signal worked. As a result, the inlet damper and outlet damper of the two induced draft fans were closed

(4) in FSSS, if all induced draft fans trip or the opening of inlet baffle is less than 3%, it is judged that the boiler loses all induced draft fans, which is one of the conditions of MFT. In fact, the inlet baffle of 2 fan has been fully closed, so MFT action is triggered. Then the turbine and generator are interlocked and tripped, and the unit stops completely

the power failure of the above power supply has also occurred in the DCS commissioning stage of unit 1, but because it was commissioning, the unit did not run, and no potential safety hazards could be found. At that time, considering that the designed power supply may be insufficient, 200 W was replaced by 400 W, and no similar situation has occurred since

2 unit commissioning "slurry polyethylene catalyst scale-up preparation and industrial utilization experiment" is the first batch of major on-site experimental projects for PetroChina polyolefins. In the trial stage, due to the lack of 400 W power supply spare parts, the method of rebalancing the power load was adopted. From the commissioning stage to the unit trip, there was no power failure

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