Lube oil is a one of the essential elements for operating any kind of machinery on board ship. Lube oil is responsible for lubrication and cooling of the parts which are operating relative to each other, giving rise to frictional and other types of stresses on the machinery. Without the use of lube oil, we cannot imagine any machinery operation on ship.
Different types and grades of lube oils are available for machinery, depending upon the working condition, operation, and requirements of the machinery itself. When it comes to marine engines, it is very critical to select the best grade of lube oil that can be used as crank case oil or cylinder oil. The lube oil is selected based on the properties which will improve the engine operation and reduce the wear down rate and hence the maintenance cost of the machine.
Important Properties of Lube Oil
The following are the most common and required properties of the lube oil used for marine machinery:
The lube oil alkalinity plays an important part in marine engines. When fuel burns, the fumes carry sulphuric acid which can cause acidic corrosion. For a trunk piston engine or four stroke engines, the main lube oil is responsible for piston and liner lubrication; hence it comes directly in contact with the combustible fuel. Therefore alkalinity of lube oil is important for controlling acidic corrosion.
For two stroke engines, separate grade of lube oil is used as cylinder oil and its alkalinity depends on the engine fuel grade (HFO or LSFO).
Lube oil is always in contact with air and thus oxygen presence in oil is inevitable. Moreover, at high temperature of the oil, the oxidation rate increases. After 85̊ degree C temperature, the increase in every 10̊ degree C of oil oxidation rates doubles itself leading to sludge formation, acid production and bearing corrosion. Hence additives are added to maintain keep these things in check. Lube oil temperature is controlled by passing it through lube oil cooler.
Load carrying capacity
It is also one of the important characteristics of lube oil which mainly depends upon the viscosity of the oil. The load subjected to different internal parts of the marine engine is very high; hence the load carrying capacity must be enough to withstand the pressure inside the engine. If this is not achieved then oil will be forced out and metal to metal contact will result in wiping out and wear down of the machine.
The internal parts of marine engine are always in movement producing heat energy. This heat energy has to be carried away or else it might lead to wear down due to thermal stresses. The lube oil must cool down the internal parts to avoid such a situation and must have a good thermal conductivity.
Detergency of the oil is obtained by adding some metallic based additives which will prevent the build up of small deposits in the metal surface. In two stroke engine, the cylinder oil detergency is very important as it removes the deposits from the ring pack area and keeps the combustion space as clean as possible
It is the property of the lube oil which prevents impurities to mix up with itself and keeps them suspended on the surface. This makes it easy for the separator or clarifier to remove it from the oil.
High Flash Point
The flash point is the minimum temperature at which the oil vaporizes to give an ignitable mixture of air. The flash point should always be on the higher side so that in case of increase in temperature of the oil, fire hazard can be avoided. Normally for marine engine lube oils, the flash point is always higher than 220̊ C.
Low Demulsification Number
It is not practically impossible to completely avoid contamination of oil with water. The low demulsification number of the oil helps in easy separation of water from the oil in the separator or when stored in the settling tank.
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