Burners in the furnace were turned on to pre-heat raffinate going into the tower and to heat the raffinate in the tower bottom, two more were lit at 11:16am. The required temperature for the tower reboiler return flow was 135 °C (275 °F) at 10 °C (18 °F) per hour but the procedure was not followed. During this start-up, this return flow temperature reached 153 °C (307 °F) with a rate increase of 23 °C (41 °F) per hour.[14] 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 (2.64 m), i.e. 93% level, as indicated, it had actually reached 67 feet (20 m). Just before midday, with heat increasing in the tower, the actual fluid level had risen to 98 feet (30 m). Pressure started to build up in the system as the remainingnitrogen in the tower and associated pipework became compressed with the increasing volume of raffinate. The operations crew thought that the pressure rise was a result of overheating in the tower bottoms as this was a known start-up issue, so the pressure was released.