"The erroneous 93% reading from the defective level transmitter still indicated an ongoing safe level condition in the tower but there was still no flow of heavy raffinate from the splitter tower to the storage tank as the level control valve remained closed; instead of the hydrocarbon liquid level being at 8.65 feet (93% level) as indicated, it had actually reached 67 feet."
Pretty much speaks for itself.
Four potential scenarios could have produced this excess pressure:
(a) Vapor pressure of hydrocarbons due to excessive thermal energy
(b) Steam generation from the presence of water at high temperature
(c) Non-condensables (nitrogen) remaining from the tightness testing
(d) Improper feed to the unit or introduction of “foreign material” in the feed
(e) A combination of the above.
Several steps in the startup procedure were omitted or not followed. The Board Operator overfilled the Splitter and overheated its contents without understanding that the very high liquid level and base temperature would contribute to a high pressure. The outside operators used local practices to control unit pressure instead of a purpose-built system, without understanding the possible implications.
Supervisory staff did not verify that the correct procedure was being used or followed, and were absent from the unit during shift relief, and key stages of the startup. There was a lack of clarity around who was supervising the startup. Although the startup procedure was not up-to-date, if the procedure had been followed, or if one of several possible interventions had been made earlier, this incident would not have happened.
Several trailers were located within 150 ft of F-20 and acted as a congregating point for non-operations personnel. Management of Change processes did not consider the possibility of significant release of hydrocarbons at the stack. This potential had not been considered in any previous site study. The injured were not notified in advance of the impending startup, or alerted when hydrocarbons were discharged from the stack, which led to them remaining in place and being exposed to the hazard. Both the trailer location and not alerting personnel increased the severity of the incident.
Much more at the link