Marine lube oil system or any lubricating oil system in particular works on four key types of lubrication; hydrodynamic, hydrostatic, boundary and elastohydrodynamic lubrication.
Adequate lubrication of parts is required to avoid heat buildup and wear; thus playing an essential role in the working of ship machinery and marine diesel engine in particular.
Many a time engine lube oil is used to determine or predict the actual health and condition of the engine. A lubricating oil, not just lust lubricate but also acts as a cooling medium, protects against corrosion, carries residue to filters, neutralizes acid and reduces wear.
Further in 48% of case lubrication failure or lubricating oil system is the cause. Making it the topmost cause for the main engine failure. Therefore it is important to ensure adequate lubrication at all times
This is often achieved by having an efficient lubricating oil system. The lubricating oil system is responsible for producing a thin film of oil between two moving parts via a constant supply of lubricating producing less wear and friction between moving parts.
Ship’s Main Engine Lubricating Oil System
The lube oil of a two-stroke marine engine consists of two individual systems; cylinder lube oil system and main lubrication system. But for four-stroke marine engines, there is a single lubrication system for both crankcase and cylinder lubrication.
The main or crankcase lube oil system consist of six main elements; sump/drain tank, strainer, lubricating pumps, filter, lube oil cooler, and distribution manifold.
Under normal circumstance, the lube oil pump takes suction from the drain sump through a strainer and pump it to the cooler via a filter. From lube oil filter it goes to different units via distribution manifold.
1 ) Lube Oil Sump
The main engine lubricating oil sump is a large space used to store lubricant surrounded by cofferdams. It is located below the crankcase under the main engine in double bottom.
It is the location in the engine room where all the lubricating oil needed for the main engine operation is stored in one place. Usually, the lube oil pump takes suction from these oil sump located under the engine or a drain tank beneath the engine block.
Further, it is the place where all the return lines for the lube oil meets and drain. Therefore it is included with major instruments such as level gauge, heating pipes, sounding pipes, suction port, etc.
2 ) Strainer
A strainer is a type of filter with a large mesh size; usually fitted at low pressure or suction side of a pump. Strainers are quite easy to clean and are installed to resist very large particle size.
It needs to be cleaned regularly otherwise the pressure difference across its sides may become un-operational. On ship’s main lube oil system a strainer is fitted on the suction of the oil pump inside the sump/drain tank.
There it restricts the metal particles from getting into the system.
3 ) Main Engine Lubricating Oil Pump
The lube oil system contains two screw pumps used to deliver lubricating oil to all the parts of the engine. Under normal conditions, one pump is operational while the other on standby.
The pump takes suction from the engine sump/drain tank and pumps it to the distribution manifold via filter and cooler.
From the distribution manifold, the lube oil reaches various working units of the engine i.e bearings, guides, under piston space, hydraulic power supply unit, etc.
These pumps are mostly engine driven for slow speed engines and motor-driven for medium and high-speed engines. Further, the outlet valves are of nonreturn type.
In event of a failure, the pumps are simply changed over; having no impact on the operation of the lubricating oil system.
4 ) Lube Oil Filter
Marine lube oil filters are fitted just after the pumps towards its discharge side. They are installed to prevent very small particles from getting into the system. At any point in time out of the two filters; only one is operational while the other is on standby.
The lube oil filter removes wear metals and foreign particles that may increase friction and cause wear. In old times this friction used to be duplex filter type; later replaced by auto backwash and centrifuge filters.
5 ) Lubrication Oil Coolers
The operating lube oil temperature at engine inlet is between 45oC. If it reaches or goes beyond 60oC; an alarm will come and the engine will slow or shut down based on the lubricant temperature.
Thus to maintain lubricant temperature at 45oC; lube oil coolers are installed in the system. In a typical lubricating oil cooler, the lube oil passes through the pipes with water surrounding it.
A bypass valve is provided to regulate and control the amount of lubricant oil flowing through the cooler. This plays a critical role in increasing, decreasing or maintaining the lube oil temperature.
In the lubricating oil system, the cooler is installed after the filter to reduce the pressure drop across its inlet and outlet valves. Plus if the cooler is installed before there is a higher chance of contaminants being settled in the lube oil cooler; affecting the rate of heat transfer.
In addition, having a cooler after the filter unit allows for better filtration of less viscous fluid/oil. This increases the efficiency of the filtration unit and further reduces the viscosity drag.
6 ) Distribution Manifold
After lube oil cooler the lubricating oil goes to the distribution manifold from which it is distributed to all the parts of the engine. The major part of the lube oil is then sent to the crosshead via a telescopic pipe.
From the crosshead, the oil is further diverted to the under piston space, guides and crosshead bearings. Now from the drillings in the connecting rod, it goes to the bottom end bearing.
The remaining oil goes to the thrust bearing, chain drive, hydraulic power supply unit to actuate exhaust valves, main bearings, and vibration dampeners. From there the return oil drain back into the oil sump.
Cylinder Lubrication System
Unlike in high speed or trunk type engines where there is a single lubrication system for both cylinder and crankcase lubrication. Modern two-stroke engines have a sperate lubrication system for its cylinders, liners and piston rings.
The cylinder lubricating oil system consists of four main parts; a storage tank, a daily tank, a distribution manifold, and a cylinder oil lubricator. The role of cylinder lubrication is to lubricate the liner and the piston ring. This reduces the friction between the liner and the piston ring.
Further, it helps creates combustion seal and neutralizes acidic combustion by-products reducing wear due to acidic corrosion. In the trunk type engine, this is done by splash lubrication.
But in slow-speed large marine engine, it is done by quills controlled by cylinder lubrication control system.
Having a correct grade of cylinder oil is important to efficient engine operation. In a marine engine, the grade of lubricant used depends on the sulfur content in the fuel oil.
For example, a sperate tank is kept to store separate lubricating oil for LSFO ( low sulfur fuel oil ). At present modern marine engine runs on BN 30/40/50 for LSFO and BN 70/100 for HSFO.
1 ) Cylinder Oil Storage Tank
There are two separate tanks kept on board to store cylinder lubricating oil. In one tank lube oil is with TBN 70/100 is kept while in other TBN 30/40/50 is kept. The TBN stands for total base number; indicating the alkalinity of the lubricating oil.
A lube oil with high alkalinity/TBN is used against fuel oil with a high concentration of sulfur to neutralize the acidic effects of sulphuric oxides formed as the by-product of the combustion process.
In emission-controlled areas, TBN 70 is to be changed over to TBN 50/30. Further to ensure quick cutoff of tank supply in event of a fire; a quick closing valve is fitted at the outlet of the tank.
2 ) Cylinder Oil Daily Tank
From a cylinder oil storage tank, the lube oil flows into the cylinder oil daily tank based on daily lube oil consumption. The lube oil is filled with a manual valve so there is no accidental or automatic filling of lube oil.
Having a daily tank helps keep a check on a sudden increase or decrease in lube oil consumption on a daily basis. Further, it is the same tank responsible for lube oil delivery, purification and storage.
3 ) Distribution Manifold
After cylinder oil daily tank the lube oil goes to the distribution manifold via a duplex filter. The oil is then sent to the lubricating drive to send it to different units based on the crankshaft position.
From the distribution manifold, the lube oil goes straight to the lubricator via a backpack valve.
4 ) Cylinder Oil Lubricator
The job of the cylinder oil lubricator is to pump lube oil into the groves/quills fitted on the liner body. These groves in the form of quills allows for even distribution of lube oil by forming a “W” shape.
In a complete revolution of the crankshaft; the lube oil is delivered twice in the cylinder. Once while the piston is moving down and another when it is moving up. The amount of lube oil to be inserted depends upon the engine speed or load.
If the amount of lubricant is too low wear happens; but if it is too much it leads to carbonization forming deposits. Thus to maintain efficient lubrication at all times modern engines utilize an electronically controlled lubrication system and its lubricator.
For manual operation, the feed rate of the lubricator pump unit is controlled/adjusted by increasing or decreasing its mean radius. In a typical cylinder, there are at least four quills in a zigzag manner.
The oil is released just before the piston ring pass through those holes and carry the lube oil with them lubricating the liner forward. The majority of oil is used to create a seal; lubricate liner and piston ring while a small percent is finally burned off in the combustion space.
Note: This article is produced on request from Aman Garg. Further, I will look forward to your helpful comments and recommendations to improve this Article.
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