With reference to large slow speed diesel engines:
(a) Explain about side chock and end chock.
(b) Describe two methods used for chocking.
(c) State the advantages and disadvantages of this system of chocking compared with on employing steel or cast iron.
(a) Side Chock
- Fitted to prevent engine from moving sideways due to movement of vessel or thrust from reciprocating and rotating parts.
- Prevents chafing of supporting chocks and tank top. Chock is welded to the foundation plate.
- When vessel rolling, helps the holding down bolts to resist lateral forces
End Chock (aft end of Engine only)
- Provided at each end of long girder to position engine to match with crankshaft alignment.
- Absorbs collision or axial loads.
- In case of integral thrust block, it absorbs propeller thrust and propeller exited vibration.
(b) Two methods used for chocking Cast Iron chocks
- Underside of bedplate is machined, foundation plate with 100:1 taper to facilitate fitting of chocks.
- The engine is aligned with output shaft using wedges or jacks.
- Cast iron chocks are hand-fitted between bedplate and foundation plate by hand scraping until about 80 % of contact is achieved.
- Holding down bolts fitted through bedplate, chock and foundation plate are tightened after removing wedges or jacks.
- Alignment to be rechecked.
Resin Chocks
- No machining of bedplate or foundation plate is required. (mating surface)
- After aligning engine with output shaft, foam rubber dams fitted to form moulds for the resin to be poured.
- Bolt holes if already drilled plugged or special thrust sleeves fitted in predrilled holes.
- Two-part epoxy resin mixed according to manufacturer’s instructions and poured into moulds, a slight head ensuring resin flows to back of mould.
- Setting time varies according to temperature, usually 48 hours during which no vibration is allowed and use space heaters
- Holding down bolts fitted and tightened after removing supporting arrangement.
(c) Advantages
(1) Used on all sizes and types of main and auxiliary machinery.
(2) Instillation time is very fast, measured with hours, not a day.
(3) Withstand service temperature up to 80˚C. Reduce noise level
(4) Modulus of elasticity helps to maintain crankshaft deflection.
(5) System gives reliable and permanent alignment without machining bedplate.
(6) Resists degradation by fuels, lubricants and eliminates chock area corrosion.
(7) By filling of all cavities of surfaces, more contact surface area can be obtained than C.I chock, which can obtain only 70 %.
(8) Gives uniform and precise mounting, non-fretting condition permanently.
Disadvantages
(1) Maximum Operating temperature (60˚~80˚) is limited depending on classification societies.
With Reference to main engine holding down arrangements:
(a) Describe, with the aid or sketch, a resin chocking arrangement currently used for Main Engines;
(b) State, with reasons, the advantages of the arrangement described in Q (a) compared with an alternative arrangement;
(c) Describe the effects which may occur if the arrangement described in Q (b) becomes slack.
(a) Resin chocking arrangement
- The sketch shown is a resin chocking arrangement for a large slow speed crosshead diesel engine.
- The engine is direct drive and the thrust block incorporates within the engine, the holding down bolts, adjacent to the thrust block, transmit the thrust to the hull.
- Unlike the older arrangements using a thrust brackets, modern arrangement uses a thrust sleeve.
- This allows for a reduction in the number of holding down bolts by about 40% and the numbers of side chocks by about 50%.
- Holes are drilled in the tank top, and after aligning the engine, the thrust sleeves are fitted and the resin poured.
- Holding down bolts are fitted and tightened after removing supporting arrangement.
- As the load bearing qualities of resin chock is less than the cast iron chock, the area of the chocking is greater and almost covers the whole area between longitudinal girder and bedplate.
(b) Advantages of Resin chock over Cast iron chock
1. It can be used on all sizes and types of main and auxiliary machinery.
2. Less installation time (measured with hours).
3. No machining of bedplate or foundation plate is required. (mating surface).
4. The system gives reliable and permanent alignment without machining bedplate.
5. It resists degradation by fuels, lubricants and eliminates chock area corrosion.
6. Almost 100% contact surface area can be obtained than C.I chock which can obtain about 70 ~ 80 %.
(c)
1. If the resin chocking arrangement becomes slack, wear will occur between the chock, bedplate and foundation plate.
2. Wear will increase over time and will lead to vertical misalignment which could lead to excessive bending of crankshaft.
3. The bolts will be subject to bending stresses as they try and transmit the thrust as well as possible vibration which could lead to fatigue failure.
4. This could also lead to longitudinal alignment and possible thrust bearing damage.
Regarding Crankshaft alignment
(a) How to check the crankshaft alignment by means of dial gauge?
(b) How to record measurement and How to know the figure is good condition or not?
(c) How would you verify the accuracy of reading?
(a) Checking crankshaft alignment by means of dial gauge
Before entering crankcase, carried out risk assessment, follow enclosed space entry procedure and fill up safety check list
- The crank measured turned toward BDC and placed clock gauge as close as possible to connecting rod and set “zero” with slight pretension. (most engines have punch marks for clock gauge position).
- Turn crankshaft ahead by means of turning gear and record values in both horizontal crank pin positions and at TDC.
- Turn crankshaft until dial gauge is as close as possible to connecting rod on other side.
- Record value at that position. This would correspond with first value (zero) if the gauge has not been moved.
- The difference between values at BDC and TDC indicates the amount of crankshaft deflection during one revolution.
- Crankshaft deflections gives an indication of high and low bearings.
- If a bearing between two cranks is higher than other side of it, both sets of crank webs will tend to open out when cranks are on BDC and close in when they are on TDC.
- Vice versa, if low bearing between two cranks
(b) Record of measurement
- Vertical misalignment compared with figures mentioned instruction book.
- In MAN B&W – MC engines, mentioned 3 figures.
- Normal
- Realignment recommended
- Absolute maximum permissible.
- If the figures met (1) & (2), crankshaft alignment is within limit.
- If the figures reach (3), we need to check the causes of faults.
(c)
- (T + B) and (P+S) nearly the same.
- No. (6) reading required to be repeated.
(a) Explain why crankshaft deflections are taken.
(b) Describe the precautions which must be taken to ensure that crankshaft deflection readings are as accurate as possible.
(c) Explain the action to be taken if some crankshaft deflection readings are outside acceptable limits.
(a) Crankshaft deflections are taken
1. To check horizontal and vertical alignment of crankshaft in bedplate.
2. To check bedplate alignment with the tail shaft on a direct drive engine.
3. When ship’s hull and engine bedplate move and flex, crankshaft alignment vary within given limits.
4. If alignment is out of limit, crankshaft will excessive bending stresses, lead to failure.
(b)
1. Crankshaft deflection take when vessel is in light ship condition, not be taken when vessel is loading/discharging cargo as hull will hogging or sagging
2. Ensure that dial indicator is correctly positioned between crank webs, gauge is free to move, does not stick and has calibration certificate.
3. Open indicator cocks, air pressure on top of pistons cause the crank webs to open and close, increasing reading
4. Ensure that journals rest on lower half of main bearings; checked by the feeler gauges. If the crankshaft is bridging a main bearing, decrease reading
5. Turn engine to reading point and then kicked back on turning gear. This will unload the turning gear, to lift crankshaft.
(c) If the readings are outside the acceptable limits, the following actions should be taken;
1. Faulty reading obtained due to Human error: re-check deflection.
2. If readings are correct and still out of limit, check for ship loading condition
3. Journals resting on lower half of main bearings; checked by feeler gauges. Floating journals cause faulty readings.
4. If not, check for journal eccentricity (not exceed 25% of bearing clearance) and bearing wear down. If the cause is worn bearing, renewing worn bearing, deflections back within limits.
5. Cause is due to worn or fretted chocks, collision or grounding, necessary realignment
6. If deflections are still outside limits, cause must be investigated and corrective action taken.
7. Inform DPA and superintendent, head office. Follow maker’s instructions.
8. If engine operated with excessive deflections, run engine at reduced load as per engine maker’s instruction.
(a) Identify with reasons, the cause & effects of misalignment in the slow speed engines crankshaft
(b) What are the purposes of crankpin fillet?
(a) Cause and effects of crankshaft misalignment
(1) Wear of main bearing lower halves
(2) Wear and oval shape of journal pin
(3) Crack or deformed main bearing pocket
(4) Distortion of engine bed plate, engine support structure
(5) Foundation chocks wear, crack or fretted
(6) Uneven tightness or Slack or broken tie rod bolts
(7) Holding down stud bolts loose or fracture
(8) Hull deformation due to improper loading, vessel grounding and E.R fire
Effect of Misalignment
- If crankshaft is misaligned, load distributed on the bearings unequal and high level of cyclic bending stress imposed on the pins and webs, while the engine is running.
- This stress cause fatigue crack on the crankshaft and eventual fracture.
- Engine running with undue vibration and damage to main bearings.
(b) Purpose of crank pin fillets
- At change of section between journal and web, web and crankpin, fillet radii are machined, there is not a sharp corner to act as a stress raiser.
- Crankshaft subjected both to bending and torsional load reversals, designed to resist failure by fatigue.
- To improve fatigue resistance under bending loads, crankshaft is provided with generous fillet radii, at least equal to 3% of diameter of pin or journal.
(a) Explain how scavenge fire occurs in large two stroke engine.
(b) How to prevent scavenge fire?
(c) How to deal with scavenge fire and what will be done after extinguished?
(a) Scavenge fire occurs due to presence of
(1) Oxygen
(2) Combustible material(oil)
(3) Heat source
1. Oxygen (air): available plenty from charge fresh air.
2. Combustible materials
(a) Unburned fuel from the combustion space
(b) Oil-leakage from piston cooling system into scavenge space due to damaged O-rings between piston rod and crown and/or skirt.
(c) Cylinder oil in scavenge space due to choked scavenge drains or excessive cylinder lubrication.
(d) Crankcase oil comes into scavenge space due to defective stuffing box.
(e) Oily rags left inside scavenge space after cleaning.
3. Heat source
(a) Overheated piston due to faulty fuel timing or cooling supply failure.
(b) Overheated piston rod due to stuffing box malfunction.
(c) Fire blow-pass from combustion due to worn-out/broken/sticking piston rings or excessive liner wear
(b) Prevention of Scavenge fire
1. Regular overhauled, fuel valves
2. Correct fuel injection system and timing
3. Keep piston, piston rings and liner in good order
4. Keep correct cylinder lubrication.
5. Avoid overloading on cylinder units.
6. Checked and maintained regularly stuffing boxes
7. Cleaned regular, scavenge trunk and exhaust ports
8. Checked scavenge drain valve at every watch.
(c) How to deal with Scavenge fire
If small fire
1. Inform bridge and slow down M/E
2.Stop auxiliary blower.
3.Cut out fuel to cylinder concerned.
4. Increase cylinder lubrication, & coolant supply to that cylinder.
5. Shut scavenge drain valve to prevent sparks blown out in the ER.
6. Stop the engine.
7. Do not enter at least 20~30 min after engine stops.
8. Find the cause of fire and correct it.
If big fire:
1. Inform Bridge and stop M/E gradually.
2. Stop auxiliary blower.
3. Maintain normal cooling system and lubrication system.
4. Turn engine by turning gear and supply cylinder lubrication manually to prevent seizure.
5. Apply CO2 or steams into scavenge spaces as required. (if fitted)
6. Do not enter at least 20~30 min after engine stops. (To reduce hot spot temperature and to avoid next explosion or fire.)
7. Follow enclosed space procedure and risk assessment.
8. Find causes of fire and correct it.
After fire has been extinguished:
1. Clean all scavenge spaces
2. Cleaned, inspected and tested, scavenge relief door
3. Inspect cylinder liner, piston, piston rod, diaphragm, stuffing box and tie rods near that cylinder, for cracks and deformation.
4. Turn engine by turning gear and observe ampere consumption to know whether seizure or not.
5. Piston rings renewed and engine run at reduced speed for at least 24 hours with maximum recommended cylinder oil feed rate.
6. Cylinder oil feed rate reduced gradually, while engine speed increased gradually.
(a) Explain how crank case explosion occur?
(b) How to prevent crank case explosion?
(c) How to act when you face crankcase explosion?
(a) Crankcase explosion occur
Fire triangle consists of:
(1) Fuel: crankcase oil
(2) Heat: hot spot
(3) O2: air in the crankcase.
- Main causes of hot spot are lack of lubrication and incorrect clearances in moving machinery parts
- For four stroke engines blow-by cause of ignition.
- When oil particles contact with hot spot, evaporate and oil vapour moving around in crankcase.
- When reach remote and cooler region of crankcase, condense and change to oil mist.
- Hot spot generates more oil mist and ratio of oil and air gradually reaches to the flammable range.
- contact with hot spot, primary explosion occurs.
- Condition depend on amount of mixtures.
- Due to primary explosion, pressure wave or flame front exists in crankcase Flame front increase amount of oil mist rapidly.
- Cause to open crankcase relief valve and relieve the pressure. Crankcase is in partially vacuum.
- Relief valve is not closed back properly or, engine room air is induced into crankcase.
- Cause secondary explosion again, very serve
(b) Prevention of Crankcase Explosion
1. When overhauling and inspection time, all bearings and moving parts’ clearances, adjustments and tightening torque maintained as per makers’ instruction
2. Checked lubricating pipes to main bearings, locking arrangements, chain lubrication points, chain system, and stuffing box condition
3. Adequate lubrication
4. lubricating oil quality on board test and lab regularly.
5. Due to the running hours. carried out crankcase inspection
6. Abnormal wear, bright marks, colour changed components
7. Checked and maintained regularly crankcase relief doors, breather pipes, main bearing high temperature alarms and oil mist detectors
Safety Devices
(a) Crankcase relief valve
(b) Breather pipe
(c) Oil mist detector (OMD)
(d) Main bearing high temperature alarm
(c) following immediate action should be done
1. Check the alarm is fault or not.
2. If confirmed, Reduce ME speed and inform to bridge and CE
3. Prepare to stop ME, shut fuel oil supply
4. Switch off the auxiliary blowers and engine room ventilation
5. Open skylight doors
6. Close engine room doors and leave the engine room.
7. Prepare the firefighting equipment.
8. After explosion happened, follow procedure according to contingency plan and enter engine room from highest entry.
9. Check engine as per maker instructions and restart.
