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Marine Engineering Materials-Free Download
What is the term fatigue cracking and state with reasons two factors of crankshaft operation which have the greatest influence on the likelihood of fatigue cracking ?
Fatigue cracking occurs when the primary cause of crack propagation is due to the fluctuating nature of the stress applied to the component. The following factors could produce fatigue cracking:
• High combustion pressures, increasing the bending stress applied to each crankshaft throw
• Excessive crankshaft bending due to a main bearing failure, which increases the crankshaft bending stresses.
How a detuner or torsional vibration damper can reduce the effects of torsional vibration ?
The fitting of either a detuner or vibration damper will reduce the vibration levels of the crankshaft when operating in areas of high torsional vibration, such as close or within a critical speed range. The detuner will change the stiffness of the shaft and hence the natural frequency, thus separating the excitation frequency from the component’s natural frequency, whereas the damper will absorb the vibration within the shaft, reducing the effects of the torsional vibration. Explain how a fatigue failure is identified ?
Fatigue failure is identified as starting at a stress raiser or defect, then the crack generates through the material before causing sudden failure. The crack progress is shown as smooth, rippled formation known as striations or beach marks, whilst the sudden failure is a classic brittle fracture with rough appearance.
Describe how a fatigue crack may be initiated ?
The initiation site will be where the local stress is high enough to increase the minute cracks which occur on the metal surface. The stress can be increased locally by a surface defect, or even an extreme stress concentration caused by high applied stress.
The main causes of fatigue cracks are:
These can be reduced by ensuring a smooth surface finish to all area where high stress is applied especially in the web/pin radii area Oil holes
These should be minimised whenever possible, and the oil hole opening have wide and smooth radii Tensile stresses
Fatigue strength is reduced when tensile stress are present, so the radii areas are often cold rolled to ensure that the fatigue strength in these areas are increased. Stress applied on hardened materials
Fatigue cracks can grow faster when the material is harder, as the dislocations in the metal concentrate the stress on a smaller area of the material structure, hence any hardening of the crank pins must not be applied to the highly stressed radii areas.
Describe the events leading to a crankcase explosion on a main engine ?
The atmosphere inside a crankcase is stable and will not allow combustion or an explosion to occur as there is no ignition or fuel source. Hence the first event is the production of an explosive mixture. This will occur when the lube oil in the crankcase is heated by a “hot spot” and lube oil coming into contact with this will be evaporated. The evaporated oil then rises within the crankcase, and then condenses in a cooler part of the crankcase. The resultant white mist is within the explosive range, and is thus flammable. The second event is the ignition of this white mist by either the same or another hot spot within the crankcase. When the oil mist is ignited, a crankcase explosion will occur, which will raise the pressure within the crankcase.
State how overheating might be indicated other than by mist detector ?
One of the common areas of overheating is the various bearings within the crankcase. Hence bearing temperature monitors could be used to indicate that a bearing is overheating and could be oil mist generation site.
State how the severity of a crankcase explosion is limited ?
The rapid pressure rise within the crankcase can cause the engine structure to be blown apart, causing physical damage, and the resultant flame travelling across the engine room space causing personnel injury. This pressure rise is limited by the statutory use of relief doors fitted to the crankcase. These doors will open when the pressure rises above 0.02 – 0.1 bar, and prevent the over-pressure of the engine structure. The doors also perform the added function of preventing fresh air ingress into the crankcase where hot burning gases are present, by the quick closing action of the relief door. Emission of flame has in the past caused severe burns to personnel during a crankcase explosion, despite the addition of flame traps. Discuss the procedure in the event of overheating being indicated ?
As the explosion is an uncontrolled event, then great care must be taken to ensure the safety of the engineers within the engine room. MAN B&W recommend that:
1. Move away from the crankcase doors immediately
2. Reduce speed to slow, and ask the bridge to stop
3. When the engine has stopped, close the fuel supply
4. Stop the auxiliary blowers
5. Open the skylight and/or stores hatch
6. Leave the engine room
7. Lock engine room entry doors and keep away from them
8. Prepare the fire fighting equipment.
9. Do not open the crankcase for at least 20 minutes after stopping the engine, and ensure that the oil mist detector alarm (or bearing temperature monitor) has reset
10. Stop the LO circulating pump. Shut the starting air, and engage the turning gear.
11. Locate the “hot spot” (source of the oil overheating)
12. Make a permanent repair to the fault
What are the problems associated with effective lubrication of the liner and piston assembly of a large slow speed engine ?
The effective lubrication of the cylinder liner and piston assembly requires a constant lubricant feed over the whole liner surface, and that piston movement will generate the oil pressures required to separate the surfaces.
However in the real situation the following problems occur:
• The oil is injected at defined points which can lead to an oversupply at the feed points, and an under supply away from these points
• The residual fuel normally used contains acids and abrasives that will reduce the lubrication properties of the oil
• The normal operation of the piston will cause the piston movement to stop at top dead centre, causing any oil pressure developed between the ring and liner to collapse
• The high temperatures present at top dead centre reduce the effectiveness of the lubricant
• The feed rate of the lubricant is regulated usually by the speed of the engine, which will cause a mismatch between actual lube oil requirements over a wide range of engine operation, with usually too little an amount of oil injected at low loads and during engine load changes.
Describe the appearance and state the causes of cloverleafing and microseizure on cylinder liners ?
Cloverleafing will be caused when the supply of lube oil is not uniform around the radial bore of the liner. The normal effect is for the oil to reduce in alkalinity away from the injection point, thus if the oil becomes acidic then high corrosive wear rates will result. This will cause uneven bore wear rates, with heavy wear in the areas furthest away from the oil injection points. Microseizure is caused when the liner and piston ring material is pressed together causing localised welding of the material in the absence of sufficient lube oil. The causes are insufficient oil and/or excessive cylinder pressures causes heavy ring/liner contact forces. The appearance is heavy scratching/tearing in the vertical direction, together with a localised hardening of the ring and liner material.
Describe the composition of a cylinder oil suitable for a main engine operating on residual fuel ?
The cylinder oil will require:
• Ample viscosity to separate the surfaces under high loading conditions
• Sufficient alkaline reserve to neutralise the acids formed by combustion of residual fuel oil
• High levels of detergent to maintain piston ring cleanliness and free ring movement
• A level of anti wear properties to minimise micro seizure
• The ability to burn without residue
With reference to bridge control of a large MAN B&W slow speed propulsion engine explain how starting and reversal are achieved ?
For large propulsion engines the Bridge Control is achieved by using the telegraph to select the desired speed and direction. When the bridge telegraph is placed in Stop, the fuel is prevented from injecting as the fuel pump puncture valves are energised. When the bridge telegraph is moved to an ahead command, the air start and fuel cams are placed in the required direction, and the start command is given. This will admit starting air to the engine. Once the engine is turning above the starting speed, the air is closed and fuel is admitted. The quantity of fuel admitted will depend on the position of the telegraph handle, i.e. slow ahead, full ahead, etc. When the bridge telegraph is moved to an astern direction, the air start and fuel cams are reversed. Once the air start is reversed, the engine will start as detailed for the ahead start.
Describe the investigation and remedial action required if the engine fails to turn on air ?
If the engine fails to start on air the following points would be investigated, and remedial action taken.
• No air pressure at engine manifold. The valves from the air receiver would be checked, and opened if found shut.
• Low air pressure at engine manifold. This could indicate that the air compressors are either not working or excess air is being used. All air compressors would be started, and air usage restricted to engine manoeuvring only.
• Turning gear engaged. The turning gear position and the interlock switch would be visually checked. The gear would be removed if found engaged.
Describe the investigation and remedial action required if the engine turns on air but fails to fire on fuel ?
If the engine fails to fire on fuel the following points would be investigated, and remedial action taken.
• Check for any shut-downs still active. The shut down panel would be examined, and if a trip is active the cause would be investigated and system made operational. Following this the trip reset button would be pressed.
• Low fuel pressure at the engine manifold. The fuel system would be investigated, and all valves checked to be opened, and all booster and supply pumps checked for running.
• Sticking fuel linkage. The action of the fuel linkage would be checked during the start sequence. The fuel linkage should admit a “starting level” of fuel after the engine start sequence has been completed. The physical movement of the fuel linkage would be checked, and any mechanical friction reduced by lubrication.
Describe the investigation and remedial action required if the engine fails to reverse ?
If the engine fails to reverse the following points would be investigated, and remedial action taken.
• Air start distributor not reversing. The engine controls would be moved ahead and astern whilst observing the movement of the air start distributor. If the distributor is not moving, then the air servo cylinder would be checked to ensure that it is free to move, and that air is being admitted and vented as required to achieve the movement required.
• Air start valve not opening. It is possible that the air distributor does reverse as expected, but that the required air start valve would not open to rotate the engine in the new direction. This would be investigated by turning the engine in the ahead direction so that it stops in a new position. The engine would again be tested in the astern direction to test if a starting air valve has failed to open. Any valve found faulty would be removed, freed and refitted.
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