| Lapsed time | Phenomenon and action taken | Deficiency, problem |
| 0 (base) | The urgent discharge valve for disposal facilities of oxidation line A was broken. The valve suddenly opened during line A operation. | |
| 13 min after | Oxidation line A was stopped by the interlock function (oxidation line B was operational). | Was the partial stop appropriate? Was full examination of the partial stop made? |
| 14 min after | Stopped lines A and B of the EDC decomposition process to regulate the production of EDC, which was only produced by line B of the oxidation process. So, the operating line for the EDC decomposition process changed to line A only. As a result, the operating load changed from 100% to 45%. | Was manual operation of line B oxidation process + line C EDC decomposition performed? Was the operator trained? |
| 31 min after | Because the stoppage was partial, the temperature balance of the HCl tower changed and the middle tier temperature fell from 80˚C to 57˚C. In order to recover the temperature condition, the amount of heating steam was increased and the reflux volume was decreased. At that time, the operators did not realize that they should control the top and bottom temperatures of the HCl tower. | The operating standard of the chloride tower was insufficient. The operator training was inadequate. |
| 1 h 1 min after | Although the temperature at the top of the HCl tower is normally −24˚C, it rose to 38˚C. Then VCM was mixed in the upper part of the HCl tower and the back-flow tank of the HCl tower added HCl. The operators decided that the HCl tower condition had become stable because the temperature of the middle tower stage recovered to 80˚C. | The alarm for detecting abnormal temperature was not set at the top tier of the HCl tower. It was not the point being monitored. The standard value of the temperature of each part of the HCl tower was not decided. |
| 2 h 18 min after | After VCM mixed in the back-flow tank of HCl, VCM mixed with the oxidation line B. The entire production facility was then stopped. The general manager, production manager, and section manager realized that the temperature at the top of the HCl tower stage was not normal and they assumed that VCM had mixed in the back-flow tank of the HCl tower. However, they did not know that 1,1-EDC was being generated, so they did perform any special operation. | There was no knowledge of the abnormal reaction and side reaction that are assumed to have occurred. Important information was not available. Important information was not included in the training manual. |
| 2 h 41 min after | The back-flow pump of the HCl tower was stopped. The level of the HCl back-flow tank was nearly 100%, and the tank was sealed. | There was no operating standard that ensured safety. |
| 5 h 1 min after | The HCl tower refrigerator was stopped. The back-flow tank of the HCl tower was disconnected from the HCl tower, so the back-flow tank of the HCl tower was sealed. | There was no operating standard that ensured safety. |
| 8 h 0 min after | The transfer of liquid from the back-flow tank of the HCl tower to the liquid HCl temporary receiving tank started. Although the pressure of liquid HCl in the temporary receiving tank was increasing, the operators were unaware of this phenomenon. | There was no operating standard that ensured that the safety pressure of the HCl receiving tank was monitored. |
| 11 h 20 min after | Operators realized that the pressure of the liquid HCl temporary receiving tank was rising, and they started pressure relief action. | The drop-in-pressure measure was insufficient. |
| 11 h 36 min after | The allophone sounded and white smoke was discharged from the top of the liquid HCl temporary tank during the pressure relief action. |
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| 11 h 44 min | The pressure of the back-flow tank of the HCl tower rose to 2.0 MPaG (the designed pressure of this tank is 1.9 MPaG). | |
| 11 h 45 min | The back-flow tank of the HCl tower ruptured, exploded, and burst into flames. |