Lagersmit – The effect of Environmentally Acceptable Lubricants (EAL) on stern tube bearings
In October 2019, DNV-GL published a newsletter with the results of a study regarding Environmentally Acceptable Lubricants (EALs) and stern tube bearing failures. Since the introduction of the Vessel General Permit (VGP) in 2013, there could be noticed a sudden and big rise in the use of EALs as stern tube lubricants for ships. Coinciding with the introduction of EALs, there was a global increase in the number of stern tube bearing failures.
To ensure the correct hydrodynamic lubrication in the bearing under all operating conditions, the lubricants viscosity is a key property. The DNV-GL study has discovered two viscosity related characteristics where EALs differ from mineral oils. These will affect the safety margin of the aft stern tube bearing during some critical operating conditions.
THREE MAIN FINDINGS
We summarized the study and found three main findings:
1. Most bearing failures occurred early in the lifetime of the vessels and during sharp turns at higher speeds. The sharp turns at higher speeds cause peak loads in the aft part of the bearing, due to bending of the shaft.
2. The viscosity of EALs while under pressure, is lower compared to mineral oils (when using the same viscosity grade). This means that when peak loads occur on the aft part of the bearing, the use of EALs will result in a lower safety margin.
3. At lower temperatures EALs have a lower viscosity when compared with a mineral oil of the same viscosity grade. This results in a lower safety margin during for example cold-starts and mooring trials of the vessel.
RECOMMENDATIONS BY DNV-GL
For existing vessels
When changing the stern tube lubricant from mineral oil to EAL oil; choose an EAL oil with a higher viscosity (one viscosity grade up).
For new-building vessels
Optimize the aft stern tube bearing design (for example double sloped bearings) to increase the contact area between shaft and bearing. Please note that DNV-GL revised the main class shaft alignment rules to account for the reduced safety factor of EALs.
If there is no other option than to use or switch to EAL, Lagersmit has EAL-seals available from stock. You can find more information on the list of compatible EALs and switch over procedure at www.lagersmit.com/eal. In line with DNV-GL’s recommendations, we do advise to closely look at the viscosity grade. In addition, please mind the effect that the originally advised mineral oil may have had, to compensate for shaft vibrations and ensure that the EAL oil is able to facilitate the same. Alternatively, air- or void-type seals may be used to avoid the above, remain in compliance with VGP/VIDA and stick to mineral oils.
Consider the use of an air- or void-type seal
In order to comply with the VGP/VIDA when it comes to oil-to-water interfaces, ship owners may choose either to use mineral oils with an air-type or void-space seal. With our Supreme Ventus® or Supreme Athmos® you avoid the use of expensive bio-oils and can keep sailing in American waters with mineral oils in the stern tube. The use of the Supreme Ventus or Athmos – which are available for retrofit and new building applications – will also help to avoid the effects as described by DNV-GL.
If you are interested in further exploring the possibilities or have any questions, please contact Lagersmit. Their team of sealing experts is happy to advise on new building, maintenance or retrofit applications.