Questions and Answers for Marine Engineers – Part 1
Questions and Answers for Marine Engineers – Part 1
Explain the term water hammer indicating possible effects on the system ?
Water hammer occurs when steam is admitted into a cold pipeline. The steam condenses producing both water, and a vacuum (when the water seals the pipeline from the steam supply). This vacuum causes the water plug to be drawn into the closed end of the pipe with increasing velocity producing high impact forces on the pipework. This impact force can be high enough to rupture the pipeline.
State how the risk of water hammer may be avoided ?
Water hammer is avoided by slowly admitting the steam into the cold line, and draining/venting the cold line to minimise the vacuum forming, and assist in draining the condensate.
State with reasons why the testing and treatment of boiler water is essential ?
Testing of the boiler water is important to determine that:
- The level of chlorides is within acceptable limits (below 200 ppm) to prevent hard scale and pitting
- The reserve of boiler chemicals within the boiler water is accepted (P Alk above 100 ppm, and Phosphate above 20ppm)
Treatment of the boiler water with chemicals will ensure:
- Alkalinity reserve against boiler water space corrosion
- Scale build-up is minimised
- Enough coagulant levels exist to minimise sludge build-up within the boiler
- The strength of the boiler drum is maintained
State with reasons how boiler problems may be minimised if no water treatment is immediately available and a steam supply must be maintained ?
Boiler problems would be minimised by:
- Reducing the steam load and demand on the auxiliary boiler to reduce firing temperatures
- Ensuring hotwell feed temperatures are at the recommended 95°C to minimise oxygen levels in the feed
- Minimise blowdown of boiler water, whilst ensuring chloride level is not excessive, to avoid loss of the boiler chemicals
Why wear down in main bearings is critical to the condition of the crankshaft and propeller shaft system on ships ?
The operational stresses present within the crankshaft is influenced by the bending stresses present. If the crankshaft alignment is completely straight then the bending stresses are minimal, but should uneven main bearing wear down occur, then these bending stresses will greatly increase.
When the main bearings wear down, the alignment of the crankshaft to the propeller shaft will change. This wear down will increase the bending stress on the propeller shaft, and the external moment at the engine / shaft interface, increasing the crankshaft stresses.
When the crankshaft is subjected to these high stresses, the small defects present within the shaft could develop and propagate into cracks that could lead to shaft failure.
Why total reliance is placed on frictional grip in conventional built up crankshaft on main engine ?
Frictional grip is the usual method of crankshaft construction for built-up crankshaft. This grip is subjected to the full torque of the engine output, and hence subjected to high torsional stresses. If there are any defects at this joint, then the resulting stress concentration would trigger a crack and possible shaft failure.
To minimise the possibility of such defects, then only a frictional grip is permitted under Classification Rules for crankshaft construction, hence pins, keys, etc are not allowed.
Why oil holes are given large fillets in crankpins and journals ?
Defect growth from a small surface defect into a crack that propagates through the shaft material require high levels of stress. Such levels of stress are possible when the stress is concentrated at section changes. The oil hole will inherently increase the local stress levels, and thus to minimise these increases, the oil hole will have a significant radius at the surface. The size of this radius will significantly influence the local stress levels, and should be closely monitored during crankshaft construction, and possible crankpin surface repairs by grinding.
What are the causes and effects of torsional vibrations in crankshaft ?
Torsional vibrations are inherent within diesel engines, due to the varying torque produced by the piston and crank arrangement from each cylinder. This torque variation is further compounded by the arrangement of the firing order of the crankshaft.
The effects of such vibrations is to increase the shear stress and hence total stress levels carried by the crankshaft in service, when other stresses such as bending and combustion loads are present.
What is meant by the term critical speed and why it can be a problem on ships ?
The critical speed of a shaft occurs when the shaft rotational speed is at or close to resonant conditions. In this condition the torsional vibration of the shaft increases greatly, and will impose very high shear stress on the crankshaft. These levels of stress could even cause crankshaft failure.
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:
Stress raisers
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:
- Move away from the crankcase doors immediately
- Reduce speed to slow, and ask the bridge to stop
- When the engine has stopped, close the fuel supply
- Stop the auxiliary blowers
- Open the skylight and/or stores hatch
- Leave the engine room
- Lock engine room entry doors and keep away from them
- Prepare the fire fighting equipment.
- 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
- Stop the LO circulating pump. Shut the starting air, and engage the turning gear.
- Locate the “hot spot” (source of the oil overheating)
- 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.
Questions and Answers for Marine Engineers – Part 1