“Smooth seas do not make skillful sailors.”
It is a wise old saying, but a smooth engine doth make a healthy ship, and we from the marine community can take it in literally. The said smoothness in the working of a ship can be ensured by The Lubrication System onboard; So seafarers, let’s talk about it.
The job of the lubrication system is to distribute oil to the moving parts to reduce friction between surfaces which rub against each other. In IC Engines an oil pump circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine. This lubricates the bearings, allows the use of higher-capacity fluid bearings and also assists in cooling the engine.
Table of Contents
What Does It Do?
The lubrication system has many tasks to undertake within its working;
- Heat dissipation; the lube oil (LO) applied acts as a coolant in the machinery and cools down the components.
- Reduce wear and tear; the components are in continuous contact, leaving a chance for wear and tear due to friction. LO reduces the friction.
- Avoid corrosion; forms a protective layer around them. Further, the alkalinity of the cylinder oil neutralizes the acidic corrosion arising from the combustion region.
- Removes debris; the impurities and debris present gets washed out.
For executing these duties, the LO incoming must have specific features tied to them.
What Are Those “Specific Features”?
- Viscosity: Viscosity is a crucial factor on par with the clearance between the two components. It is preferred in a fair value since one with higher values may result in slugging, and one weaker may not work correctly.
- Carbon Residue: The higher the carbon residue, the greater will be the maintenance cost and work. Also, the system may get charred and corrupted.
- Flash Point: The temperature at which LO has sufficient vapor so that when it is exposed to a spark, fire will be induced.
- Pour point: The temperature at which LO ceases to flow.
- Cloud point: The temperature at which LO develops wax crystals on them, which hinders a proper flow.
- Total Base Number (TBN): Inside the engine, there would be a mixture of air, fuel, and LO comprising of ;
- 97% – Hydrocarbons
- 2.5% – Calcium/Potassium
- 0.5% – Sulphur
The reactions inside the cylinder are;
|S + O2 –> SO2|
|2S + 3O2 –> 2SO3|
|H2O + SO3 –> H2SO4|
Sulfur oxidizes to form sulfur dioxide and sulfur trioxide, which further react with the moisture content in the cylinder outlines to form sulphuric acid. This highly corrosive acid needs neutralization.
That’s why the cylinder incorporates an alkaline LO with a high TBN value.
H2 SO4 + Ca(OH)2 —> CaSO4 + H2O
(Neutralization reaction) CaSO4 is the byproduct, a non-corrosive naturally occurring salt.
Main Engine Lube Oil Sump stores LO and provides steam heating using Steam Coils to attain a temperature(T) of 400 – 450 C. It has the following components concatenated;
- Level gauge
- Sounding pipe
- Heating steam coil
- Maintenance holes
- Suction pipe
- Airvent pipe
- Valves for LO pumps and LO purifiers
The working of the LO Purifier is similar to that of the fuel oil purifier, and it works continuously. It intakes oil from the bottom where sludge and other impurities are present and deliver the purified oil to the top. Since LO has a lower density than sludge, it will remain on the top, and hence the suction for the utilization of LO from the sump takes place at some predominant height above the bottom.
The Main Engine LO Pumps are 3 phase centrifugal pumps that handle a large LO and run on electricity, unlike positive displacement pumps that maintain high pressure.
There are two pumps in the Main Engine Lube Oil (ME LO) system, one of which is Active/Master/Running, and the other is on Stand-By, set for an automatic cut-in if there occurs a chance of oil pressure reduction or primary power failure.
At the end of the pump line, a Pressure Sensor (P) is present, further connected to an auxiliary line with a Pressure controller (PC). If P senses a deviation in the pressure value than required, LO can flow forward or backward accordingly through the auxiliary line to correct the P-value. The supply pressure might depend upon the designs and requirements, mostly appropriated to 4.5 kg/cm2.
These employ autotransformer start, which leads to recirculation of LO through it, generating a gradual increase in the T of LO. The advised value of T at the inlet to the engine is around 450 C.
At the start of the pump, there is a chance for backpressure, which causes overload in the pump. In such situations also LO recirculation through the auxiliary line is implemented to prevent any catastrophe.
Through the Null Return Valve, it reaches the Auto Backwash Filter and Manual Filter, which filters out the LO incoming and employs self-purification. Depending upon the design, some may have the inlet filter and backwash filter as a single unit or have the backwash filter after the cooler. The filter also has a magnetic core to filter out the metallic impurities present.
The filtered LO needs to is cooled before entering the engine section. LO cooler is either plate type or shell and tube type and employs a Low-Temperature Liquid Cooling System, a parallel set-up of a long tubular flow of freshwater.
At the end of the cooler, a Temperature Sensor and a Temperature controller are present, exploiting a 3-Way Valve to bypass LO directly from the filter into the engine inlet if there is a deviation from the T value sensed from the preferred.
After this whole journey, LO branches out to various specific zones, of which three are prominent;
- Main Lubricating Oil System
- Cylinder Lubricating Oil System
- Turbocharger Lubricating Oil System
This system can be a separate unit or connected to the ME, depending upon the design.
A turbocharger has running components, which will run for 15 – 20 minutes even after the complete stoppage of the engine. So for that phase, the LO flows in through a separate LO Gravity Tank present on the top of it, working by the effect of gravity.
Cylinder Lo System
This system differs from the ME system mainly due to the contrast between the properties of the LO employed in each, precisely the TBN values;
- ME LO = 6 – 7 mgKOH/g
- CLO = 60 – 70 mgKOH/g
And, viscosity grade of them are;
- ME LO = 75 – 85
- CLO = 85
High basic nature is preferred in Cylinder LO to enhance its Neutralizing Property, as stated before.
The system applies the Boundary Lubrication method to provide a seal between the cylinder liner and piston to reduce friction and heat. As per IMO 2020, low sulfur fuel oil should be used to run an engine, 0.50% m/m (mass by mass), so advice must be sought from corresponding engineers and system manufacturers to recommend the perfect LO.
The system is located under the engine, in the double bottom surrounded by a cofferdam, generally. Two pumps are also recruited, a master and a stand-by, to transfer LO of 45bar from its storage tank to The Accumulator/ Measuring Tank/Daily Oil Tank for daily exploitation. Further, oil levels and leakages are studied using Sounding Pipes.
From the Lubricators, these are sprayed circumferentially onto the cylinder liners through interconnected quills. The implementation is made possible by the motion of the piston, precisely applying LO between the liners and piston rings.
This system is Load Dependent such that the working depends upon the load, rpm, and such data related to the engine. The data are analyzed and processed and sent to the Solenoid Sensor, which ensures the actuation of LO and accordingly dispatches it to the cylinder liners through a Feedback Sensor, which further safeguards the functionality of the system.
Main LO System
The main LO system supplies to the camshafts, bearings, camshaft drives, and a branch also goes to the crosshead.
At the engine inlet, three prominent tasks are to be undertaken;
- Goes up and down the piston, cooling it in the process.
- To the crosshead bearings and its complimentary shoe guides.
- Drips down the bore in connecting rod to the crankshaft region.
A branch also leads to the Hydraulic Power Supply Unit in modern engines, activates valves, thrust bearings, moment compensator, torsional vibration damper, etc.
There are subsystems under the ME LO system;
- Cam-less engines;
Works on Hydraulic Power System, which actuates exhaust valves and fuel valves hydraulically. So the branch from the main engine must reach here.
- Camshaft driven engines;
In these engines, camshaft is connected to a sequence of Fuel Cam > Roller > Guide > Follower > Fuel Pump > Fuel Pump. It has a Barrel And Plunger Element. There is a chance for leakage from this element, which may further contaminate the whole LO system via the ME sump upon reaching the shaft region.
To prevent this, Older Engines generally had a separate Cam LO Tank. A cam – LO line, filter, and a cooling system came complimentary with it. Here the sump will be the Cam – LO tank and not the ME sump and are replenished periodically.
Modern Engines uses an Umbrella Seal, which eliminates all the aforesaid additional set-ups and is present at the bottom of the pump body. It collects leakage and is later drained off to the Drain Tank. Thus, no total contamination occurs.
Why is it two separate systems for the Main and Cylinder units?
- Cylinder oil is costly, and nobody wants to contaminate it.
- If anything happens to the cylinder side and the sumps are not different, the whole system of LO gets wasted.
- The rpm received from an engine is nearly 100 rpm. Since explosion and combustion time can be longer, CLO can utilize low-grade fuels also.
- The CLO must quickly remove carbon and combustion residue to prevent abnormal wear and enhance cleanliness.
- Some gases like NO2 need to be neutralized, which is facilitated by CLO.
- Due to such high demands of CLO, it is used only once and then replaced, unlike ME LO, which is reusable within limits.
The crosshead is an integral part of the engine that needs to be inevitably lubricated since it works on high load and is always in non-continuous motion with low oscillation.
Depending on whether the engine is MAN B&W, Wärtsilä, or Sulzer, the connection to the crosshead can vary, which invokes a change in the manner LO is to be applied;
- MAN BMW = Telescopic line
A telescopic line is flexible to move along with the crosshead, and LO is supplied from the top. The preferred pressure value is about 3 – 4 bar.
- Sulzer or Wärtsilä engine = Swinging arm line
The swinging arm connects pipelines free to move off their coupled point where LO is supplied from the bottom. Here the recommended pressure value is about 16 – 18 bar and employs booster pumps for the same.
BONUS> The answer to why a Sulzer engine uses booster pumps and has a different value of input pressure lies in its design.
Cooling is a significant task to be undertaken by the LO. A MAN engine has a set of channels machined to the lower crosshead bearing to admit a smooth flow of cooling oil, whereas a Sulzer engine has no such channels, and thus the LO has to be pumped with such force between the pin and the bearing to inject the LO.
- LO is supplied to the bottom of the bearing where it is in contact with the rod.
- LO stays within the oil grooves and hydrodynamically moves along with the crosshead for lubrication.
- As mentioned before, a part goes up the piston, and another goes down it.
The piston is the primary moving member of the engine, constantly in friction with the cylinder walls.
- They are lubricated internally through bores inside them.
- Lubrication occurs by the Cocktail Shaker Effect, which involves splashing the LO over the inside of the piston with the assistance of the repeated rocking motion.
- In some designs, nozzle plates and nozzles are provided, through which oil is sprayed to the bottom of its crown.
Main Bearing Lubrication
A separate branch leads directly to the main bearings, which are also in contact with the crankshaft, which is ever-rotating.
- Here, hydrodynamic lubrication is more viable.
- The applied LO forms an oil wedge.
- As the shaft rotates, this oil wedge takes up the load between the bearing and journal.
- As the rotating speed increases, the load spreads equally over the wedge, and the LO covers the whole surface.
Camshaft LO System
A camshaft has a fuel cam, inlet cam, camshaft gear, camshaft bearing, and exhaust cam, which all need lubrication for efficient working.
- The bearings involved are under-slung wide bearing.
- These have a groove in them through which LO will be supplied.
- The rotational motion of the adjoining shaft embarks hydrodynamic lubrication over the bearings due to weight formations.
- After working, LO returns to the ME LO sump.
Maneuvering Main Engine Lubricating Oil System Operation at a glance;
Consider restarting the engine; the operator must operate the system in the following manner;
- Check the sump level tank and replenish it if necessary.
- Ensure the working of the LT Central cooling System and proper circulation of freshwater.
- Ensure all Pressure Guages and instrumentations are open and correct.
- Ensure steam heating is provided to ME LO sump if T is low.
- Set up the whole flow line.
- Select one pump as master and the other as stand-by. An autotransformer is employed for starting and must be allowed to cool down for 10 – 20 minutes before another starting attempt.
- Keep the system running and allow T to rise to the preferred value.
- Check readings at T and P gauges.
- Start the engine only after ensuring appropriate stable T and P values.
Back To Home!
LO will be sucked back from the engine by the ME LO purifier after the cruise through work at about 900C for the most efficient separation by maximum density difference. The dirty oil will be sent to the Dirty Oil Tank, which upon purification is transferred back to the Clean Oil Tank and reused upon reaching the sump.