Evaporation tower overpressure event

Thread Content

Event Name: Overpressure Incident in the Evaporation Tower
Occurrence Time: October 10, 1999, at 19:00
Event Overview: On October 9, 1999, the reforming unit entered the stage of chlorination renewal of the reforming catalyst, with the evaporation tower operating in single-tower circulation mode. At that time, a typhoon made landfall, bringing heavy rainfall; this resulted in extreme instability in the operation of the heating furnaces, thereby affecting the chlorination renewal process. From ~19:00, significant oil spraying occurred from the vent line at the high level of the compressor building. At that time, the flare line in the plant was not connected to the pipeline network; only material was being fed into the evaporation tower. It was therefore concluded that the safety valve of the evaporation tower had activated. Upon inspection, it was found that the two valves from the top of the evaporation tower to cooler E-3104/1.2 were closed (valves added during maintenance), which caused the DCS to show an inaccurate pressure reading for reflux tank D-3103; in reality, the evaporation tower was already under overpressure. After the inlet valve of E-3104/1.2 was opened, the pressure-controlled PIC-3102 was activated for pressure relief. Due to maintenance work, there was no valve in the gas pipeline connected to PIC-3102, allowing nitrogen-containing gas to enter the heater and causing it to shut down completely. At 21:00, re-ignition and heating are carried out to proceed with chlorination renewal. The safety valve of the evaporation tower was sent for inspection as it failed to return to its normal position after tripping. At around 18:30 on the 10th, sulfidation of the reforming catalyst was completed, and preparations were made to feed material in; the temperature in the evaporation tower was increased accordingly. At around 19:00, the workshop supervisor noticed on the DCS console that the pressure in D-3103 was rising rapidly; he notified the on-site control room, but all the operators were already present there. The shift supervisor immediately rushed to the reprocessing site to inform deputy director Lin Xu. Lin Xu found that the pressure at the P-3103 backup pump was above 2.5 Mpa, so he rushed to D-3103 to relieve the pressure. Fu Zhonglin activated the pressure-controlled PIC-3102, while director Wang Shengjun noticed from the DCS control panel that the temperature in the furnace of reboiler H-3102 was approaching 800°C, and he promptly took action to lower the temperature. The pressure in the evaporator returned to normal after adjustment. Cause analysis: 1. The excessively high temperature in the furnace of the reboiler H-3102 in the evaporation tower, rising from around 500°C to 800°C without being detected in time, was the direct cause of this incident. 2. The safety valve at the top of the evaporation tower was not properly set to maintain a constant pressure during reformation feed, and it was not installed; this led to excessive pressure in the evaporation tower, with no timely pressure relief, which was also one of the main causes of this incident. 3. There was a lot of complex work during the construction period, and the on-site communication equipment (walkie-talkies) did not work properly; moreover, there was no one in the on-site control room, which delayed the processing time. 4. The team operators failed to conduct a thorough inspection during the process modification for oil padding in the evaporation tower; they did not open the two valves from the top of the evaporation tower to cooler E-3104/1.2, which was the main reason for the safety valve to activate and cause oil leakage on the 9th. 5. A pressure test for connection was carried out before the device was put into operation. The crew responsible for this task had a vague understanding of the importance of such tests and of how to carry them out, as a result of which the section from the top of the evaporation tower to D-3103 was not connected, thereby missing the opportunity to identify and prevent problems at an early stage. 6. During the inspection, modification, and renovation work, two valves located between the top of the evaporation tower and cooler E-3104/1.2 were added, but the staff were not informed about this in a timely manner; as a result, the operators were not clear about the modified process. This is a management issue. 7. In the PIC-3102 pressure-controlled backside modification, there was a missing valve in the gas pipeline; the modification work was not thoroughly inspected. Nitrogen-containing gases from D-3103 could not be discharged into the flare line, which was the main cause of the heating furnace shutting down on the 9th. 8. Commencing work under adverse weather conditions was also a major cause of the overpressure in the evaporation tower on the 9th. Precautionary measures: 1. During operation, the workshop process supervisor or reforming process engineer should be in the control room to give instructions and monitor the operations, ensuring proper control of all stages. 2. The startup procedure is cumbersome; the main operator should concentrate fully on monitoring the system, with clear responsibilities assigned to them. 3. Replace the walkie-talkies with better communication capabilities; someone must always be present in the on-site control room to ensure continuous communication between the control center and the site. 4. Before starting work, the safety valve should be installed and tested, and lead seals should be applied to the inlet and outlet stop valves. 5. Major process introductions or changes should be carefully checked, with confirmation at three levels: the operator, the shift supervisor, and the workshop technician. 6. Clarify how to conduct steam penetration training sessions, emphasize the importance of penetration testing, and resolve any ambiguous understanding. 7. The workshop managers in charge of the renovation project should conduct pre-work briefings and check for any problems or potential risks associated with the renovation project. 8. In cases where severe weather may affect safety, consult the relevant departments of the company to postpone the start time of material feeding.