Stern tube oil lubricating system (with air seals)

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I would like to share with you a short description of the operation logic of the stern tube lubricating system. I think it is helpful.

The stern tube assembly, which includes the forward and aft seals, serves two main purposes. Firstly, it provides support for the propeller shaft through two oil lubricated bearings. Secondly, it prevents the oil from leaking out of the stern tube into the engine room or the sea. Additionally, the aft stern seals are necessary to prevent seawater from entering the stern tube, as this could lead to contamination of the system, damage to the bearings, and corrosion.

To lubricate the stern tube bearings, oil circulates in a closed loop circuit. One of the two LO pumps pumps the oil through a tube cooler, which then directs it to the underside of the stern tube via two lines. The oil passes through the stern tube, surrounds the bearings, and returns to the stern tube LO tank.

The stern tube LO tank, positioned about 1m above the center line of the propeller shaft, has a capacity of 180 liters. The outlet pipe from the tank connects to the suction side of the two stern tube LO pumps, creating a positive head on the pumps, which are located at the engine room floor plate level. One pump operates while the other remains on standby. If the operating pump’s discharge pressure drops to 1.5 bar, the standby pump activates. The LO cooler is supplied with water from the central low-temperature cooling fresh water system. The stern tube assembly is also cooled by the surrounding tank, which is filled with water from the domestic fresh water system.

Maintaining an adequate oil supply in the stern tube is crucial for reducing friction and carrying away heat. Regular oil sampling and testing should be conducted to ensure that the oil still meets the manufacturer’s specifications. The presence of water in the oil indicates leakage at the aft stern seal, which can be detected by the presence of seawater in the drain collecting tank. The presence of metal particles in the oil suggests a failure in either the forward or aft propeller shaft bearings.

The LO pressure in the stern tube is regulated by an air control unit (ACU). The ACU supplies air, at a reduced pressure, to the space between the No.1 and No.2 seals of the aft seal and to the top of the stern tube LO tank. The air in the seal chambers is vented to the sea, and any change in the vessel’s draught is detected by the ACU. It automatically adjusts the air pressure in the stern tube LO tank to increase or decrease the oil pressure in the stern tube to the optimal level. A relief valve in the stern tube LO tank is set at 2.5 bar and relieves excess pressure to the stern tube LO sump tank.

The lubrication system for the stern tube bearings operates independently of other systems. There are three systems specifically dedicated to the stern tube: one for the bearing unit, one for the aft seal assembly, and one for the forward seal assembly. All these systems utilize the same lubricating oil.

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