201554 Steam cleaning and flammable atmosphere don’t mix

Edited from IMO sub-committee III 1/WP.3

A chemical tanker in ballast was en route and the crew were preparing the tanks for loading. The crew had just completed washing of one of the tanks, which had previously carried benzene. The next steps were to strip the tank, ventilate it for a few hours, and then carry out tests to determine the cleanliness of the tank.
A crew member decided to carry out steam cleaning before ventilating the tank. A steam hose was inserted into the tank, steam pressure was increased and a cargo pump was started to remove any water collecting in the tank. A few minutes later there was an explosion and fire. Unable to contain the fire, the crew abandoned ship and were later rescued. However, one crew member went missing and was presumed deceased.
The investigation found that the explosion was the result of the ignition of the tank atmosphere, which contained benzene gas that was within the flammable limit. The source of the ignition was most likely an electrostatic discharge from the end of the steam hose coming into contact with the tank side or other structure. The steaming of the tank, which was performed immediately after washing and before ventilation, also likely gave rise to an electrostatically charged mist.

Lessons learned

• Prior to tank cleaning, a pre-cleaning meeting should be held to ensure that crew members understand their duties and the proper procedures to be followed. Any deviation from the procedures must be reported immediately.
• After carrying a flammable cargo, always assume that the atmosphere within a tank is flammable.
• Be aware of the extreme danger of using steam injection to clean flammable cargo tanks due to the risk of static electricity.
• Benzene is a significant fire and explosion hazard based on its physical properties, including its flash point, vapour pressure, and boiling point. It can quite readily form explosive mixtures in air as a result of its high vapour pressure. Preventive measures against the accumulation of static electricity should always be employed.