201226 Flocculation of high asphaltene fuel oil due to overheating

One of our vessels reported frequent operational problems on the main engine. The fuel oil had been in storage for some weeks and the line filters were getting choked with rubbery particles which required the engine to be stopped and the solid matter to be removed. Analysis showed that the fuel contained about 8.5% asphaltenes. Asphaltene content over 6% is considered high. Records showed that the storage tank had been heated excessively for some time. The heating was then temporarily turned off during a three week long stay at anchorage at an intermediate port where the ambient temperature was below 10°C, meaning the fuel temperature was reduced to below the normal range. Such a large variation in temperatures causes the asphaltenes to flocculate and solid particles to form.

Fuel oil in storage tanks should not be heated indiscriminately. In cold climates, it is advisable to control steam heating carefully to maintain a temperature of not more than 10 degrees above the pour point of the fuel. Temperature of the oil in the tank should be monitored continuously and recorded to prevent flocculation, which can render the engine unreliable and jeopardise navigational safety.