Exploring the Intricacies of a Vessel’s Stern Tube Lubricating Oil System

Date: August 29, 2023

By: Daniel G. Teleoaca – Marine Chief Engineer

When it comes to the complex machinery that powers maritime transport, every component plays a vital role. One such component that often goes unnoticed but is crucial for the smooth operation of a vessel is the stern tube lubricating oil system. This intricate system ensures the longevity and efficient functioning of a vessel’s propulsion system, making it an indispensable element of maritime engineering.

The stern tube, which is oil lubricated, serves as a bearing support for the propeller shaft and is sealed at both ends with lip seals, and the shaft is supported by two bearings situated between the seals. Seals are installed at the stern tube’s outer and inner ends. The seals at the stern tube’s aft end prevent water from entering and oil from escaping out to sea. Seals at the forward end of the stern tube keep oil from flowing into the machinery space.

Oil lubricated stern tube. Source and credit: Eagle Industry Kemel

Understanding the Stern Tube Lubricating Oil System

Lubrication for the stern tube (tail shaft) serves to discharge waste heat from the shaft, reduce friction in the shaft bearings and seals, and provide corrosion prevention. At the same time, the oil maintains pressure equilibrium at the seals and determined by the size and specified draught of the vessel, the lubrication oil system is supplied as a loss system with gravity tank or as a circulation system with circulation pump.

In case of smaller systems with only minor variations in the specified draught, the oil-filled chamber between the outer and inner seals is connected to a tank above the water line and as a result, there is a static pressure inside the chamber. The pressure is somewhat higher than that of the surrounding saltwater, preventing seawater infiltration into the vessel. Oil is delivered to both the radial seal ring and the bearings at the same time. The waste heat is removed by free convection via the oil and water in the tank through which the stern tube travels and the bearing temperature is determined solely by external stimuli with this sort of loss lubrication. Two gravity tanks are arranged at separate heights if necessary to prevent major pressure changes at the ship’s seals with two distinctly different draughts.

The forward seal has, in general, two sealing rings and oil pressure for the seal is supplied by two pumps. One of the pumps operates as the duty pump and the other pump is selected as the standby pump, which will be started automatically should the duty pump fail to maintain the LO pressure. The pumps are selected at the local pump selector switch for OFF/STANDBY/RUN. The pumps take suction from the stern tube LO sump tank via suction filters and deliver the oil under pressure to the space between the two sealing rings. The aftermost sealing ring seals the lubricating oil in the stern tube bearing and both sealing rings face outwards (aft). The oil outlet pipe is connected to the top of the seal housing and the oil flows back to the stern tube LO sump tank via the return line which is fitted with a sight glass which allows the oil flow to be monitored.

The aft seal consists, usually, of three parts:

  • Four rubber lip sealing rings.
  • The metal housing which carries the lip sealing rings.
  • A chrome steel liner which rotates with the propeller shaft.
Aft shaft seal arrangement

The aftermost sealing ring is No.1 seal ring and this faces outwards (aft), as does No.2 sealing ring. No.3 and No.3S sealing rings face inwards (forward). No.3S sealing ring is a backup sealing ring and can be brought into operation should No.3 sealing ring become damaged. In the event of No.3 sealing ring becoming damaged the valves on the oil lines to the space between No.3 and No.3S sealing rings are closed. The oil supply for the aft seal and the stern tube bearings comes from the stern tube aft seal and LO line pump, which takes suction from the stern tube LO sump tank.

Oil is supplied to the after seal and the stern tube bearings and the system gravity tank ensures that the correct oil pressure is applied to the aft seals. The gravity tank is fitted with a level alarm and the oil supply line to the space between No.3 and No.3S sealing rings is equipped with a sight glass, so that oil flowing to the space can be monitored. There is also a sight glass on the oil supply line for the space between No.2 and No.3 sealing rings and on the supply line to the stern tube bearing.

Challenges and Maintenance

Failure of the after seal rings can result in sea water entering the stern tube and lubricating oil can also leak into the sea if the after seal rings fail. The space between No.2 and No.3 sealing rings is connected to the lower header tank and this space contains oil but the oil is not circulating. In the event of the after seal rings failing sea water will enter the space between No.2 ring and No.3 ring and this water will find its way into the LO header tank.

If No.3 sealing ring fails, oil will leak from the bearing into the space between No.2 and No.3 rings. This oil will flow to the lower header tank and the level will rise. The header tank has a level alarm and if this is activated it indicates leakage of water past the No.1 and No.2 sealing rings or oil past No.3 sealing ring. If No.3 sealing ring fails the valves which connect with the space between No.3 and No.3S rings must be closed and No.3S ring will then act to seal the stern tube bearing.

Maintaining a stern tube lubricating oil system presents challenges due to the harsh maritime environment. Factors such as water contamination, temperature variations, and extreme pressures can impact the system’s efficiency. Regular maintenance, including routine oil analysis, filter replacement, and seal inspections, is essential to ensure the system operates optimally.

Nowadays, the repair and replacement of the stern tube aft seals can be done, in emergency situations, while the vessel is in service as the system is designed and in such way. Below there is a video example of Wartsila stern tube aft seal repair during vessel in service.

Warstila stern tube aft seal underwater replacement. Source and credit: Wartsilacorp and mySealTV

During engine operation the following checks must be performed:

  • Check the pressure gauge readings daily
  • Check the stern tube LO temperature daily
  • Check the forward seal LO temperature or casing temperature daily
  • Check for any discoloration of the LO and for the presence of water daily
  • Check the operation of LO filters and clean as required, or at least every month.
  • Check that the air control unit is functional and is working correctly.

It is important to note that the oil in stern tube system must be sampled and analyzed at intervals suggested by the oil supplier. Also, when the vessel is in dry dock the oil supply to the bearings and seals must be switched off and the stern tube drained. When the dry dock is being flooded the stern tube lube oil system must be restored.

Environmental Considerations

With growing environmental awareness, the maritime industry is under pressure to reduce its ecological footprint. Stern tube lubricating oil systems can potentially lead to oil leaks and spills if not properly maintained. As a result, ship designers and operators are exploring environmentally friendly lubricants and innovative seal technologies to mitigate these risks.

Nowadays, on the new modern vessels, the aft main seal is an air type seal operation, clean filtered compressed air is used as the means of controlling and maintaining the seal differential pressure.

Simplex airspace sterntube seal. Source and credit: BAHR visuelle Kommunikation

Air guard seal. Source and credit: Wartsilacorp

As can be seen air is supplied to the space formed by no.2 and no.3 sealing rings which then flows into the space formed by no.1 and no.2 sealing rings before flowing out to the sea. The air is supplied from a control unit and the flowrate is adjusted so that there is always the same differential pressure no matter what the draught of the vessel. The space formed by no.2 and no.3 sealing rings is open to the stern tube sump tank, so if the oil sealing rings no.3 and no.3S are damaged, any oil entering the space formed by no.2 and no.3 sealing rings is drained to the stern tube sump tank.

Stern tube bearing lubricating oil is supplied by one of the stern tube LO circulating pumps which take suction from the stern tube sump tank, which on modern vessel is maintained under pressure by air from the air control unit. The pressure in the stern tube LO tank provides the same effect as gravity tank fitted in the LO system, which is for emergency use if the air supply to the stern tube tank will fail.

The air control unit is equipped with flow regulators which are adjusted to provide constant air flow rate to the stern tube seal. Any alteration in the vessel draught is automatically compensated for by the changes in pressure of the air supplied.

The leakage past no.1 and no.2 sealing rings will be dependent on the general condition of the seals and the condition of the shaft liner. Any oil and sea water that may be present in this space is drained into the drain tank which should be checked regularly as it warns the duty engineer of any seal leakage problems. If the tank contains sea water, no.1 and no.2 seals are leaking, but if contains oil, then it is the seal no.3 that is leaking.

If the air supply fail or there is a disruption in the air supply from the air control unit to the stern lubricating oil tank the aft seal can be converted to the emergency seal condition. This is done by bypassing the lubricating oil tank and using the return pipe system to maintain a positive head on the oil in the stern tube. In the event of this kind of failure the system will trigger an alarm and system valves should be changed in order to ensure that an oil supply is maintained at the aft seal. In this case the stern tube bearing system must be converted to the gravity tank system in order to maintain the desired lube oil pressure, as this is necessary because there will no air pressure acting on the system.

The aft seal space between rings no.1 and no.2 may be flushed through with fresh water when necessary as the water is supplied at the connection to the air control unit.

In conclusion, the stern tube lubricating oil system might be concealed beneath the surface, but its role in maritime operations is undeniable. From enabling smooth propulsion to safeguarding against wear and tear, this intricate system is a testament to the marvels of maritime engineering. As technology advances and environmental concerns grow, the industry continues to refine and innovate this system to ensure safer, more efficient, and more sustainable maritime transportation.

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