A turbocharger is a high speed rotating machinery, which provides additional air to the combustion chamber and increases the efficiency of the marine engine. It is a form of waste energy recovery system on ships which uses the engine exhaust to rotate the turbine, providing fresh air to the combustion chamber through compressor fitted in the same shaft.
With such high speed rotating machinery, bearings are installed on the shaft, which moves at extremely high speeds and carries remarkable loads with ease and efficiency. But for a turbocharger bearings to perform without any fault, it needs to be adequately lubricated.
Property of Turbocharger Bearings:
Must handle high loads: The turbocharger bearings need to carry heavy rotating parts, and the load will increase when the engine is in operation. The bearings should have high load carrying capacity.
Withstand challenging and extreme conditions: The turbocharger fitted on the ship has to operate in conditions different than that on land. The bearing should run smoothly even in rolling/ pitching/ yawing condition or in extreme weather.
Enhanced operational reliability: The bearing should be able to operate even during long-running hours of the oil or when the oil is contaminated.
Reduced temperature peaks: The turbocharger speed can be relatively high during the starting up of the engine or when the engine is suddenly stopped during maneuvering, leading to an increase in bearing temperature. The bearing should be able to cope up with this temperature peak occurring for a short period of time.
Does not react: The bearing material should not react with lubricating oil provided in the turbocharger or with other parts of the turbocharger made from different materials.
Compensate high thrust loading: High boost pressure acting on the compressor wheel can create significant thrust loads. The thrust is even higher in VGT turbochargers with variable geometry turbines, as they are designed to provide higher boost pressure at low load. The bearing should be able to withstand and run smoothly in high thrust load situations.
Oil supply delay tolerant: Some turbochargers do not have their oil sump and oil supply pump for the bearing. The lubrication oil is provided from the main engine system through long-running lube oil pipes which will lead to delay in oil reaching to the bearing after startup. The bearing should work correctly even in oil starved condition.
Properties of Lube oil for turbocharger system:
– Should have high temperature resistant and good thermal stability
– Synthetic in nature with good demulsibility
– Should not react with material of the turbocharger and pumps
– Should have high load carrying capacity
– Should have resistance to foaming
– Should provide rust and corrosion protection
– Should have air release property
– Must have high flash and low pour point
– Should have a good viscosity index
There are mainly two types of turbocharger bearings popularly used in marine turbochargers. They can be defined by the locations of their fitting in the turbocharger shaft:
I) Outboard bearing type
II) Inboard bearing type
Outboard bearing: In the outboard bearing type, the location of the bearings are outside the rotating turbine and compressor blade on the shaft. Usually, Ball/Roller bearings are used in these positions. Arrangement and properties of outboard bearing type are:
- Ball and roller type combination bearing is used
- Bearings are mounted in resilient type housing having laminar springs, which provide axial and radial damping and they do not allow the bearing surface to chatter or flutter when stopped
- The primary advantage of ball & roller bearings is that they entail minimum frictional power loss and heat produced. However, these bearings are extremely sensitive to even transient overloading and may fail in service rapidly under such conditions
- Dirt inclusion either while assembling or contaminated lubricating oil, damage to the running ‘race’ due to ‘brinelling,’ any misalignment while running due to thermal shocks, etc. are all such causes. These bearings are resiliently mounted with circular leaf spring pack support.
For medium and large turbocharger, outboard bearings offer multiple benefits such as
- They are readily accessible for servicing and maintenance
- Easy to fit and remove compared to another bearing types
- The bearings do not require removal of the compressor wheel, eliminating the strenuous physical effort and times, especially with large shaft weights
- New technology allows using sleeve type bearing in place of roller bearings (ABB VTR turbocharger)
Lubrication System for outboard bearing:-
- Ball and Roller bearing uses integral lube oil supply system for lubrication, i.e., it is fitted with a self-contained gear type pump (centrifugal pump in some models) operated by the rotating shaft of the turbocharger provided on both blower & turbine side
- An independent oil sump is provided on both sides of the casing of the turbocharger, and the pump draws oil from the sump. A gear pump is provided on both blower & turbine side
- A sight glass is provided in the casing with maximum/ minimum oil level mark (oil should be renewed at stated intervals)
- The outboard bearing may be lubricated by an external system (M/E LO System) through a fine filter or by the independent system (Pump, filter, cooler, oil, sump, alarm, etc.)
- All lubrication systems must contain adequate lubrication when tilted to an angle of 15° in any direction or a temporary tilt of 22½
- If Separate System is used, following criteria must be fulfilled –
a 6 m head of the tank; pressure >1.6 bar; supply time of 10 minutes
- Low friction losses at low loads
- High efficiency
- No need to remove rotor for bearing access
- Use in axial flow type turbocharger
- Can work at high temp
- No header tank or additional piping system is required
- No additional fittings such as cooler, filters are required
- Used mainly in a large size of turbocharger system as compared to an inboard bearing type
- Require high-grade lubricating oil
- Bearing subjected to brinelling
- Requires resilient mountings
- Requires greater alignment
- High operational and maintenance cost
- Shorter Time Between Overhaul (TBO)
Inboard Bearing: In the inboard bearing type, the location of the bearings are inside the rotating turbine and compressor blade on the shaft. Usually, sleeve type white metal bearings are used in these positions. Arrangement and properties of inboard bearing type are:
On the sleeve type white metal bearing, hydrodynamic fluid film lubrication prevails. Still, the friction losses and heat generated are better compensated than an outboard bearing.
The hydrodynamic film is used for lubrication of the bearing; and it is dependent on the maintenance of the correct oil viscosity, high purity, etc.
This type of bearings are not as sensitive to some misalignment as an antifriction bearing, and most importantly, failure in service is unlikely to be sudden, but usually preceded by warning symptoms.
To restrict the leakage of air from compressor side to gas side (turbine side of the turbocharger), labyrinth seal arrangement is fitted at the back of the compressor impeller
A thrust bearing arrangement is fitted into the main bearing assembly of the turbocharger to compensate for the axial thrust which is taking place during the startup, shut down & low loads operation of the turbocharger.
The main thrust is absorbed by sealing air acting on turbine disc. This air flows past leak off labyrinth at a rate dependent upon clearance.
The axial thrust is directly dependent on the turbocharger load, i.e., with an increase of load in the turbocharger, thrust will also increase resulting in rotating element to move towards compressor end and clearance at leak off labyrinth decreases, air flow reduces, thrust chamber air pressure acting on turbine disc increases.
Lubrication System for inboard bearing:-
A sleeve-type white metal bearing is fitted inboard to a radial type turbocharger. In this case, the shaft is centrally supported, and the thrust bearing is located adjacent to sleeve bearing.
There is no separate sump for inboard bearings. The lube oil is supplied from main lube oil system through a gravity tank. Lubrication oil is supplied through the pipeline connected to the bearing housing via a non-return valve.
Lube oil is supplied to the gravity tank by a main engine lube oil system through an orifice and one line from the tank goes back to the sump tank, thus maintaining the oil level in the gravity tank at all times.
The oil tank should have the capacity to supply oil for 10- 15 min after a low-level alarm for the oil level in the tank is sounded.
A long service life of the bearing is claimed because of a stable rotor on a larger supporting surface with a low rate of bearings wear.
Advantages of inboard bearings:-
- They are compact and takes very less space
- Low grade of lube oil can be used as compare to another type
- High operating temperature is possible as they have a better cooling arrangement
- They are not subjected to brinelling
- They have a longer Time Between Overhaul (TBO), hence more bearing life
- They have a very less chance of misalignment
- In case of blackout or pump failure, header tank will provide the lube oil to the bearings
- No additional moving parts (gear pump) is required in the casing
- High friction losses at low loads, thus low efficiency
- To access the bearings, the rotor needs to be removed
- The working hours to overhaul the bearing are higher than that of outbound bearings
- They have an inadequate response to a change of load
- Chance of oil contamination is higher as it used system oil of the main engine
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