Rotary lip seal operation with environmentally acceptable lubricants

Stern tube seals are a type of rotary lip seals used in the propulsion system of ships. These large-sized elastomeric components are placed at both ends of the stern tube of a ship preventing the lubricant spillage to the environment while, at the same time, avoiding the entrainment of seawater to the stern tube. The ideal leakless situation does not occur and, in reality, a continuous amount of lubricant is discharged to the ocean. The continuous spillage of lubricant is normalized in the marine industry and it is typically referred as the stern tube consumption. To limit the environmental impact of the oil, new legislations replaced the traditionally-used mineral oil-based lubricants for less environmentally harmful products, i.e. the Environmentally Acceptable Lubricants (EALs). However, these lubricants have brought all sorts of issues with the already existing stern tube system of a ship, especially the stern tube seals. The investigation conducted aimed to shed some light on the operation of the stern tube seals in combination with these greener lubricants. On the longer term, the work presented will give the seals and lubricants manufacturers the opportunity to redesign their products resulting in a less contaminant sailing while extending the service time of stern tube seals.