FAULTY LEVEL INDICATIONS CAUSES FIRE

 A fire broke out on the vacuum distillation unit in a refinery during its shutdown. The unit had been restarted the day before following the acceptance of the work, while other job sites were still underway at the site. The reheating operation had begun during the night, and the unit was still in the power build-up phase. At around 9.15 am, thick black smoke was observed coming from the stack (fire in the furnace), with flames shooting from the open explosion vents. This situation was preceded by hammering in the pipes and rising pressure in the tower increase and the opening of valves: hydrocarbons began spilling outside. Following the inquiry, it appears that erroneous level indicators caused the tower to be overfilled then the backflow of liquid into the furnace via the vacuum system (backflow of incondensable materials). A brief summary of the findings: the local levels were not visible, the chain associated with the control levels in the bottom of the tower had not been completely checked (card), and the configuration of the system and notably the extraction levels were not correct.

Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION

TRIP BYPASS WITHOUT APPROVAL CAUSES INCIDENT

 Subsequent to a tube break inside a refinery, fire ignited on a furnace. The emergency shutoff system was tripped and the unit became depressurised via the tube that broke inside the furnace. During this incident, the unit was on a return path to its nominal flow rate. Roughly 24 hours prior to the break, following another incident, the reforming unit was operating at an extremely low flow rate over a 3-hour period. The low flow rate safety system had been bypassed without implementing any compensatory measures. The next day, this information was not even relayed to the daytime shift, with the abnormal situation leading to the quick coking of the tubes and accelerating their creep. The fire had originated from overheated tubes tied to an internal coking operation, caused by operating at an insufficient flow rate (in a breach of safety rules). In underestimating the incident occurring the previous day, the subsequent shift had not been properly informed. The environmental agency requested strengthening the refinery’s safety management rules and verifying their strict implementation, in addition to installing an alarm management system. The agency also requested: formalising both the resources to be notified in the event of a process-related incident outside of plant operating hours and the rules for overseeing unplanned shutdowns and corresponding start-ups; revising the periodic safety test acceptance protocol; and expanding training and recycling programmes thanks to the Company’s new tools, in emphasising furnaces and incident management.

Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION

HUMAN ERROR IN ENTERING DCS INPUT CAUSES AN INCIDENT

 At 9:03 pm, the control room operator responsible for the catalyst section of a refinery’s fluid catalytic cracking (FCC) unit entered an erroneous opening value for the atmospheric relief valve at the discharge of a compressor blowing the air needed to suspend a catalyst inside the regenerator. This valve deviated some air flow to the compressor discharge in order to protect the compressor from pumping phenomena. The operator on duty had input, then validated, an erroneous valve opening control value (less than 10%), when he actually wanted to lower the value from 20% to 19.5%. This instruction wound up increasing air flow to the regenerator and subsequently tripping the safety mechanism for the compressor and then for the entire unit. The 15-minute unit decompression caused flare emissions, followed by a gradual shutdown. The facility management brought the unit back online incrementally between 11 pm and 5 am, resulting in new flare emissions. The updated guideline requested the panel operator to no longer enter a value, but instead solely use the «up» or «down» arrow commands to increment the initial value by 0.5% or max 1%.

Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION

RUNAWAY REACTION DUE TO LOSS OF UTILITIES

 At 10:46 pm during a thunderstorm, an electrical outage interrupted polystyrene (PS) production at a Seveso-classified site. A safety disc broke and styrene was released. To minimise the effects of micro-outages (due to thunderstorms) on PS output quality, the site operator typically switched shop power supply onto the 4 electric generating sets of the facility’s Peak Day Withdrawal (PDW) unit. This manoeuvre was performed at 10:20 pm, with 3 sets still available. At 10:43, the thunderstorm knocked out the 1st set. Since the 2 remaining sets were no longer sufficient, the unit entered into safety mode at 10:46, closing all utilities. An employee tried to restart the PDW unit; the on-call electrical maintenance operator was called at 10:53 pm. By 11:05 pm, pressure on the 1st synthesis reactor had begun to rise. As per emergency procedures, gyro monitors started up at 11:15 to remove eventual vapours at the reactor line vent. The site was connected to the grid at 11:18 but the units were only allowed to resume operations a short time later. At 11:20, the disc on the 1st reactor burst at 5.8 bar, spraying a liquid mix containing 10 tonnes of PS and 3 tonnes of styrene.The runaway reactor was caused by the loss of utility service. The control room operator opened the vent too late, given all the actions required to put the 3 polystyrene lines into safe mode, in accordance with procedures.

Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION

AUTOMATION INCIDENTS IN PROCESS SAFETY

 "The factory of the future will have only two employees, a man and a dog. The man will be there to feed the dog. The dog will be there to keep the man from touching the equipment.» Warren G. Bennis, North American consultant,1996"

HCL LEAKS FROM FURNACE FOR 10 MINUTES In a chemical plant, 0.6 tons of hydrogen chloride (HCl) escaped during a 10-minute period from all furnaces and vents within the potassium sulphate workshop while cleaning the HCl circuits. An employee living adjacent to the site notified the guard house of the presence of a cloud originating from the plant. The emergency sprinkling system connected to the washer was turned on to stop these emissions. Poor calibration of one of the two devices used to measure gas pressure at the furnace outlet (not directly related to the ongoing works), causing the control valve on the gas evacuation circuit to close, was responsible for this incident: since gases were no longer being drawn, they escaped from the furnaces. The lack of an alarm on this control parameter slowed personnel response, and the absence of any means for comparing the 2 pressure measurements prevented the detection of sensor drift. To reduce the probability of repeat occurrence, an alarm was installed to detect deviations between the 2 pressure readings; also, a procedure laying out the most sensitive steps, in particular those requiring a supervisor's presence, was issued.

Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION