Background – What is bitumen boil over?
A bitumen boil over occurs when hot bitumen comes into contact with ‘free’ water. The water immediately turns in steam expanding in volume by over 1600 times. The steam and hot bitumen form a hot foam which will rise up the tank and escape though any opening. If the foam cannot escape the tank will become pressurised. Both are extremely dangerous situations:
- Bitumen storage tanks are not designed to operate at pressure so rupture may occur.
- If the hot foam escapes it may be under pressure which is also extremely dangerous for anyone in the vicinity of the tank.
Brief description of the incident
- Prior to a holiday period the contents of the tank were run down and the tank heating was turned off. The heating to the tank was off for over 2 weeks.
- The heating was turned back on 18 hours prior to the planned bitumen delivery.
- Prior to the delivery the contents gauges and High-Level Alarms (HLAs) were checked. The temperature of the product in the tank was also checked to confirm it was at 120°C, the expected temperature.
- Towards the end of the bitumen delivery steam was emitted from the vent pipe followed by a hot bitumen foam. The hot foam was emitted for a period of 2 – 3 minutes. A video of the incident showed steam escaping from the tank shortly after commencing the delivery.
The driver could not see the steam being emitted from his location at the Homir valve on the delivery vehicle.
Root causes of the incident
The root causes of this incident were:
- Condensation accumulating in the tank during the period that heating was turned off. Also, it is a possibility that there may have been ingress of water into the tank. No checks were made to see if water had accumulated in the tank.
- Insufficient time was allowed for the bitumen in the tank to completely heat through. It is believed the surface of the bitumen in the tank was cold and therefore any water that had collected on the top of the cold bitumen had not had an opportunity to evaporate in a controlled manner.
- Although the HLAs on this tank are regularly checked there is no maintenance regime in place for the HLAs for this particular tank.
- The contents gauge for the tank was out of calibration and it is believed that the reading on the contents gauge was inaccurate
- The tank is over 20 years old and it is possible that carbonised bitumen on the side of the tank has reduced its safe working capacity (SWC).