Rotary lip seal operation with environmentally acceptable lubricants
Due to the COVID-19 crisis measures the PhD defence of Xavier Borras Subirana will take place (partly) online in the presence of an invited audience.
The PhD defence can be followed by a live stream.
Xavier Borras Subirana is a PhD student in the research group Surface Technology and Tribology (STT). His supervisors are prof.dr.ir. M.B. de Rooij and prof.dr.ir. D.J. Schipper from the Faculty of Engineering Technology.
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. This project was divided in three parts: the data collection, the modelling and the validation.
The first part consisted in obtaining the necessary information for developing the computational models. The characteristics of the tribo-system and the window of operation were investigated. The knowledge on Environmentally Acceptable Lubricants is limited and hence especial attention was paid to comparing the common mineral oil-based lubricants to the EALs. Additionally, the seal and shaft materials, the garter spring and the surface roughness were analysed.
The modelling part began by building a robust axisymmetric static model of the stern tube seal, i.e. when the shaft is not rotating. This thermomechanical model served as base on which to build more complex models of the seal. Next, the dynamic operation of the seal was modelled. Due to the complex alignment between the propulsion shaft and the stern tube seals, it is likely that the seals operate under non-concentric conditions. The focus of the research is placed on the lubrication mechanisms that develop as a consequence of such a misalignment. Two misalignment-induced hydrodynamic pressure build-up mechanisms are presented complementing the primary lubrication mechanism theory governing the operation of rotary lip seals.
Validation has been done on many aspects of the models developed. The unavailability of data on stern tube seals required from specialized setups for the validation of the models. Three specialized setups has been developed and used in this research: a split-shaft setup, a static glass shaft setup and a dynamic setup. The radial force between the shaft and the seal was measured via a large-sized split-shaft setup. The width of the contact and the percolation threshold of the seal was measured using the glass shaft test rig. Ultimately, the third setup made it possible to study the behaviour of stern tube seals under real operating conditions.
With the knowledge gained on the EALs, the models developed can be used to significantly shorten the effort required to develop a EAL-suitable seal design. 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.
