Moving parts of ship’s machinery require constant lubrication for reduction of friction between them. The selection of lubricating oil for a particular machine depends on various factors such as
- Type of machine
- Viscosity of oil
- Water content
- Flash point
- Total base number
- Purity of oil
Most of the machines run on a cyclic lubrication system. This means that there is an oil sump where the lubricating oil is stored. The moving parts use oil from this sump and after usage the oil comes back to sump. This continuous usage of the same lubricating oil leads to the reduction of the oil quality over a period of time, even though centrifuge purification technique is used to clean it.
It is for this reason that the lubricating oil of the whole lubricating system should be changed at regular intervals of time. However, replacing of the lubricating oil requires large quantity of oil and is also extremely expensive.
To reduce the number of intervals of changing oil, most of the ship owners either add chemicals or find out ways to preserve the properties of the lubricating oil. This way they can drag the oil replacement date as long as possible. However, at some point of time replacement is inevitable. But then how to know the exact time for oil replacement? Let’s find out.
Reading Between Lines (Signs)
There are few common signs that are important and which help in identifying the exact time of replacing the oil.
The easiest way of doing this is by checking the viscosity of the oil. Lubricating oil, when comes in contact with high temperature and oxygen oxidizes and thus reduces the viscosity of oil. Reduction in oil viscosity leads to decrease of oil film between the moving parts. This leads to metal contact, scuffing and other damages due to increase in friction. When the viscosity of the lubricating oil changes by more than ten percent, it is time to change the oil.
In diesel engines, the fuel pumps or injectors have a tendency to leak. Often, this leaked oil finds it way to the lubricating oil sump. The mixing of fuel oil with the lubricating oil leads to the reduction of the flash point of the later. The main drawback of this mixture is that in presence of hot spots it becomes a potent source of fire, one of the main causes for crankcase explosion. Thus the ideal time for changing the lubricating oil is when the flash point drops by 150 degree Celsius.
Water ingression into the lubricating oil sump is one of the biggest evils that lead to degraded oil. Water finds it way into the sump through leaking cooling water system or defective o-rings and gaskets. The presence of water not only degrades the lubricating oil but also gives rise to bacteria and fungus growth, which leads to corrosion and oxidation of oil. This drastically changes the quality and properties of lubricating oil, making it useless. Thus, if the water content in the oil reaches 2%, it’s time to change the oil.
One more aspect of the lubricating oil is the total base number. The main engine’s cylinder liners use low grade of lubricating oil with high sulphur content. For this reason, it should be monitored that the TBN doesn’t get below 20%. Sometimes, additives are also added to the lubricating oil to restrict the corrosive effect caused by the evaporation of sulphur.
Lastly, the level of impurities in the lubricating oil should be maintained. If the solid impurities in the oil increase, they interfere with the soft metal parts, damaging them completely in the long run. The oil should be changed if the impurities cross the 5% mark.
Gresham, R.M. & Totten, G.E. (2008) Lubrication and Maintenance of Industrial Machinery: Best Practices and Reliability. Florida: CRC Press