Main Engine start Air Systems

With reference to a main engine air start system:

state, with reasons, the purpose of the slow turning system and how it functions; state with reasons, Three safety features incorporated in an air start system;

State with reasons, why an engine may fail to turn over on air

Explain how the cause of an engine failing to turn over on air may be determined.

Ans)

The slow turning system consists of a small bore pipe complete with an automatic valve, bypassing the main air start automatic valve. When opened the slow turning automatic valve provides enough air for the engine to turn over slowly on air, but not fast enough to do any damage should water or fuel be sitting on the piston due to a leak, which would cause the engine to ‘hydraulic’, damaging head and running gear had full air throughput been used.

Two situations in which the slow turning valves is used:

On a slow speed engine, during maneuvering. If the engine has been on ‘stop’ awaiting a movement for, more than a set period (usually 20-30 minutes) then when an order is given and the engine control moved to start, instead of the main air start opening, the slow turn valve will open. If the engine satisfactorily completes a revolution then the main air start opens allowing the engine to start.

On a medium speed diesel electric installation where engines are on standby for immediate start, a slow turning valve may be timed to turn the engines every hour, an alarm sounding if they fail to complete 2 revolutions, However air used for this slow turning must be dry to prevent corrosion of liners over a period of time when the engines are not needed.

Non return valve: In case of a pressure rise in the air start manifold above that in the air bottle( for instance because of a detonation in the air start manifold) the non return valve will close.

Flame Trap: if a cylinder air start valve is jammed open, the flame from combustion could travel back down the air start manifold igniting any oil vapor in the line. To prevent this, a flame trap consisting of a brass plate with drilled holes is placed between cylinder air start valve and manifold.

Relief valve or bursting disc: placed in the air start manifold to relive any increase in pressure

Main air start valve closed: operator error, Turning gear engaged, or if disengaged, the air operated interlock switch may not be venting. If a CPP is part of the installation the pitch may not be set to zero( an interlock) .On a reversing engine, the camshaft may not be in the correct position for the direction required. The air start pilot valve for the particular unit receiving air may be sticking shut, the engine will therefore have a “dead spot”

The engine is not receiving the start signal from the bridge or control room.

Always check the simplest cause first. When preparing an engine for sea, it is usual to blow the engine over using compressed air before shutting the cocks. It is here that the problem may manifests itself.

Check that the isolators on the correct air receivers are open and that the automatic valve is in the automatic position .

Check that the turning gear is out and that no interlock warning lights are showing. on a unidirectional engine fitted with a CPP check that the pitch is at zero.

On a reversing engine check that the camshaft is in the correct position for the direction required. Now the engine should turn when the automatic valve is operated by hand, if not then the fault will probably lie with the pilot valve for a particular unit. It is not unknown, because of moisture in the air for these to corrode and stick. Note the position of the flywheel (mark with chalk)on a reversing engine try an astern start, on a unidirectional engine turn the engine so another piston has just come over TDC, disengage turning gear and try again. If the engine now turns when operating the automatic valve manually, try again from the control room.

If the engine will start locally, but not from the control room or the bridge, then the fault must lie with the air signal to he automatic valve. The investigation from here must follow a logical series of steps with the help of the manual and the air start drawings.

Engine system vary, but basically a pneumatic or solenoid valve allows an air signal to operate the main automatic valve. At the same time an air signal allows air to the air start distributor which will allow the cylinder valves to open in the correct sequence. Before the air signal can reach either of these devices, switches either electrical or pneumatic relays, must confirm that the previously mentioned interlocks are clear and that the camshaft is in the correct position. some engine builders include a fault finding system which involves checking the air start relays which are all together in a box by the engine side local controls. An indicator will show which relay valves are open and these are checked off against the fault finding list.

Crosshead Bearing Unscheduled Failure

As a recently appointed chief engineer you are requested to survey the crosshead of a main engine following an unscheduled repair due to bearing failure.

Out line the information you would request prior to the survey

Describe the survey procedure you would adopt stating, with reasons, the areas which should receive particularly close attention.

State with reasons, what information you would request and the operation s you would require observe after reassembly of the crosshead

Ans)

Information requested prior to survey:

Previous survey reports: Engine hours when survey was carried out, details of recorded clearances, details of any of any possible marking on pin, bearing condition

Manufacturers service letters appertaining to crossheads, which may give a clue as to reason for failure.

Recent LO analysis; Any deterioration in the lubrication quality due to contamination by cylinder oil or fuel may give a guide to why the bearing failed.

Sample of any detritus found in Lo filters, plus any records of anything found in the days leading up to the failure. If white metal is or has been found, it may indicate that the bearing material was failing over a period of time.

Engine room log for the weeks leading up to the failure. And since the repair, giving details of oil pressures and temperatures, fuel rack readings, exhaust temperatures etc. plus power card readings before the failure. This will give an indication whether overloading has taken place. Reports of engineers made at time of repair. As to action taken, conditions found etc together with any photographs they may have taken.

The old pin(if replaced ) and bearing shells.

(b)

Assuming that the crosshead pin and piston rod assembly is resting on the support brackets bolted to the guides and that the conrod is swung down to crankcase door level inspect bearing half in con rod(the loaded half) for evidence of scoring, squeezing, wiping, cracking of bearing metal, scoring will indicate hard particles in LO or damage to crosshead pin. Squeezing and cracking points toward overloading, whilst wiping will indicate poor lubrication and/or overloading. Some manufacturers have an overlay of a lead/tin alloy on the tin aluminum bearing metal, and the state that the bearing should be replaced if this overlay has worn away. Other manufacturers have wedges machined in the bearing material and recommend replacement when the wedges are reduced to half original width.

If it is considered necessary to inspect the top half of the bearing, then the con rod must be reconnected the engine turned to BDC and the top bearing cover lifted.

Inspect closely the underside of the pin. this is the side which is continuously loaded. The finish should be mirror like with no evidence of scoring or roughness. If any is found then the pin must be replaced or polished. Because the crosshead bearing is not hydrodynamic ally lubricated, and relies on boundary lubrication at the beginning and end of the swing then any damage to the pin will damage the beading material.

Examine the guide shoes and guide strips for evidence of squeezed out metal, also check the sump pan for white metal. If there is any doubt as to the condition of the guide shoes, then the con rod must be removed and the pin must be lowered to the crankshaft so that the guide shoes can be examined.

On reassembly the crosshead pin and the guide shoe clearances should be taken and recorded. Incorrect clearances will not allow a LO film to be built up when the rubbing speed is at a maximum. The engine should be turned through several revolutions and the turning ,motor ammeter observed for any change in load. This will ensure that there is no binding between moveing parts The oil pumps should be switched on and the flow through the bearing and guide shoes observed. This will ensure that there is an adequate flow for cooling and lubrication purposes. The oil pressure should be recorded.

After boxing up and preparing the engine, the engine should be run for 30 seconds,2 minutes, 10minutes and 30 minutes, on low load, opening up after each run and checking the bearing for signs of overheating or squeezing out of white metal. This should help prevent damage to the pin if the bearing does, wipe. Load on the engine should be slowly built up over a period of time and then a set of power cards taken to ensure that the engine is not overloading or unbalanced.

Main Bearing Failures

The vessel which you are about to join as Chief Engineer has recently suffered the main bearing failures. State, with reasons for your answer, each of the following.

The information and documents you would require in order to enable you to assess the cause of such failures.

Ans)

The most recent bearing clearance readings and when they were taken with details of shims fitted or removed. Bearing clearances on thin wall bearings are non adjustable: thick wall bearing clearances are adjusted with shims . The wrong clearance will lead to breakdown of hydrodynamic film as will uneven shim thickness either side of bearing

Crankshaft deflection readings for ballast and loaded condition: This will give an indication of the alignment of the crankshaft in the bedplate. If outside of manufactures recommendations, it may cause overloading of bearing s which are high.

Sets of recent power and draw cards: This will give an indication as to the cylinder power balance of the engine. Excessive loading in a particular unit, or overload of the engine in general will lead to higher bearing loads.

Details of recent Lo tests on the crankcase oil shipboard or from spectrograph analysis ashore. On board tests will give a guide to water content which can cause corrosion especially sea water corrosion, viscosity, too high indicates contamination by cylinder oil/products of combusting causing poor heat transfer and damage to bearings by calcium deposits; too low indicates contamination by fuel oil, leading to breakdown of hydrodynamic film. Spectrograph tests will give an indication to particles of metals present which may indicate if the filtration/purification system is efficient as well as a clue to which engine parts are wearing.

The last survey reports for the main bearings prior to failure together with dates, clearances, journal condition and diameter, (ovality can lead to failure) and any other information, which may be available e.g., photos. If the failed bearings were surveyed at the same time, there could have been mistakes made when reassembling (tightening sequence, tightening pressures)

Reports as to the events leading up to bearing failure; indications of failure, damage reports.

Ovality of journals is due to the varying load on the crankshaft due to the varying cylinder pressure if the joviality approaches 25% of the bearing clearance then the hydrodynamic lubrication will be affected, This leads to failure of the bearing through wiping of the white metal, regular measurement will allow the problem to be rectified before damage occurs. Avoid overloading.

Increased surface roughness of journal: This will cause metal contact with the bearing material, and therefore wiping, because the oil film is no longer thick enough to separate the two surfaces. This increased roughness can be caused by abrasive damage to contamination, corrosive damage due to sea water contamination (tin oxide encrustation) or acidic oil. Avoided by correct filtration, purification and replenishment of lub oil when required.

Spark erosion of the bearing material due to potential difference between hull and prop shaft due to the bronze propeller being immersed in sea water (an electrolyte ) This earths down through the main bearing shells.

Cracking of bearing material: Can be caused by incorrect bonding between white metal and backing, but also caused by fretted bearing housings (inadequate for steel shell backing), misalignment between bearing cap and housing ( incorrect tightening of nuts )and overloading. Avoided by following manufacturers instructions when refitting bearings; use only engine builders spares, tighten bearing cap nuts equally and opposite, not diagonally, if 4 are fitted.

Regular checking and recording of main bearing clearances when carrying out a crankcase inspection. At the same time check for squeezed out white metal and white metal in bottom of crankcase. Any misalignment of the crankshaft investigated and rectified. As well as leading to bearing failure, this can lead to crankshaft failure due to excessive bending(cyclic stressing). Misalignment may be due to uneven bearing wear down indicating the likelihood of another imminent failure, fretting of chocks or hull distortion .

If the engine is a two stroke crosshead engine, and there is evidence of contamination of the crankcase lub oil with cylinder oil then it may be prudent to discontinue the practice of purifying the drains from the piston rod stuffing boxes to the main engine sump.

Regular L C analysis can help prevent damage to bearings by water/acid attack. Early warning of presence of water will allow investigation into cause, Efficient purification will remove water and solid particles. Efficient filtration will prevent scoring of shaft and bearing

Ensure the shaft is earthed via slip rings to the hull to prevent spark erosion. Correct fuel injection can be verified by regular power/draw cards, which will prevent overloading or uneven loading of the engine. On a medium speed engine a power balance can be obtained from compression/peak pressures.

If the crankshaft main journals are damaged by scoring or are oval, then this can be corrected by grinding and fitting undersize bearings.

Turbocharger and overhaul

Q) Following a recent turbocharger overhaul it is observed the scavenge air pressure a lower was previously and the engine power output is also reduced.

State, with reasons, the possible causes of the problem.

State, with reasons, what engine operational information should be gathered in order to enable the possible cause of the problem to be detected.

Explain how the defect may be rectified.

State the instructions, which should be issued with respect to future turbocharger .. order to prevent similar incidents.

Ans)

(a) A turbocharger overhaul consists of removing the rotor and nozzle ring for inspection and cleaning, renewing the bearings where necessary, inspecting of the labyrinth glands, cooling water spaces and gas inlet protection grid,

Causes of low scavenge pressure on reassembly could be due to:

Damage to turbine blades. This could have occurred when removing refitting rotor. The efficiency of the turbine will be reduced if the blade profile is damaged. May also cause vibration.

Compressor axial clearances incorrect due to badly fitted bearing: Allowing air to leak back from discharge to suction, or excess air to leak down back of wheel to exhaust. May also cause compressor wheel to rub against casing. If the turbocharger system is fitted with a waste gate or a charge air by pass, then these could be jammed open or not operating correctly,

(b) Turbo-charger speed: The faster the turbocharger revolutions, the greater mass flow of air produced. And the higher the scavenge pressure slow turbocharger rpm could be an indication of all of the above faults. Vibration (and noise): some turbo-chargers have vibration meters fitted. If not, monitor visually and by placing the end of a screwdriver or special “listening rod” on the casing, and listening at the other end. Vibration can indicate damaged unbalanced rotor: noise indicates rotor in contact with casing.

Exhaust gas inlet temperature to turbo-charger: .A higher than normal temperature (along with higher exhaust valve temperatures) are an indication of a high exhaust gas back pressure could be caused by damaged balding not allowing smooth passage of gas.

Exhaust gas temperature difference across turbine: A smaller than normal differential will indicate that the exchange in energy is not taking place across the turbine.

Pressure drop across air inlet filter: If the turbocharger is operating at the normal speed, but the scavenger pressure is low. Then the compressor is not operating efficiently. This could be because the axial clearances are incorrect or because of damage or choked compressor wheel. The pressure drop across the air filter will be lower than normal. Could also be an indication of dirty air filter or choked air cooler(pressure drop across air cooler will be high) is the turbocharger prone to surging, esp at load changes? If so then it indicates disruption in either the gas or air flow through the charger.

(c) If the charger is vibrating or unusually noisy then it must be stopped and the cause investigated. This will entail removal of the rotor. Before removal check run out of bearings with a clock gauge. Check compressor wheel, shaft, labyrinth, turbine wheel for signs of damage or rubbing. Check casing and nozzle ring for damage. If damage has occurred then it must be rectified before the rotor is rebalanced and refitted with the correct bearings. If there is no vibration or noise, check simplest possible faults first. Check dump valves, bypass and waste gate if fitted. Check that the air filter is clean. It may be necessary to clean the air cooler on the air side. If none of these are successful, then it may be necessary to open up the turbo- charger and check that it has been assembled correctly with correct axial clearances and that there are no blockages in the nozzle ring or protection grid.

(d) The overhaul of the turbocharger should be overseen by the 2^nd engineer or another senior member of the engineering staff as appointed by the chief engineer. When the rotor is removed for overhaul/cleaning, only chemicals recommended by the manufacturer are to be used at the recommended strengths. After cleaning, the turbine and compressor wheels are to be examined closely for any sign of damage before re-assembly. New ball and roller borings to be fitted if the running hours are within 20% of the recommended maximum hours for the bearings. Normally this will be the case. Only manufacturers spares, sealed in tins are to be used. When fitting run out must be recorded and should be within 0.01 mm .Axial clearances must be checked and recorded after comparison with the original clearances (stamped on makers plate) As soon as the casings are oil tight the compressor and turbine end are to be filled to the correct level with the correct grade of oil. The overseeing engineer is to ensure that no gloves, rags, tools etc are left in the casing or exhaust pipes before boxing up. And that the protection grids are clean and in position. The air filters and air cooler is to be cleaned at the same time that the turbocharger is overhauled. Bypass valve and wastes gates are to be checked for correct operation on completion of overhaul.

Lubricating oil analysis and its test results, conclusions

The analysis of oil may be used as a method of monitoring the condition of the equipment that it lubricates.
Explain briefly how shore analysts might test the oil.
State the type of information that would be expected.
Give possible reasons for an excess of
Iron
Copper
Antimony
Tin
Silica


Ans)

A) The shore analyst test the oil by the following procedures;

1. FLASH POINT: By Pensky martin apparatus. This method determines the flash point of the LO in a closed cup. This gives the lowest possible ignition point of the oil . The test portion is slowly heated at a constant rate, whilst continuous stirring is provided . A small flame is directed through an open shutter at regular temperature interval with simultaneous interruption of stirring. The flash point is the lowest temp at which the application of the test flame causes the vapour above the test portion to ignite.

2. BASE NUMBER: This method determines the alkaline properties of the oil by titration with perchloric acid. The sample is dissolved in an anhydrous mixture of chlorobenzene and glacial acid and titrated with a solution of perchioric acid in glacial acetic acid. The rate of neutralization of the mixture is indicated using an electrical potential bridge arrangement. Measurements of current are plotted against quantity of the titrating acid mixture. The graph indicates the alkaline reserve at the point where the graph plot inflect

3. KINEMATIC VISCOSITY: The measurement of viscosity is carried out by measuring the time taken for a controlled volume of flow under gravity through a fixed capillary. In order to produce an accurate result, the driving head and the temperature must be controlled. The time taken is directly related to the kinematics viscosity

4.DENSITY: A glass hydrometer is used to measure this. While the test is carried out the temperature should be maintained constant. It is also important that the hydrometer is allowed to reach the same temperature as the measured liquid.

5. INSOLUBLES: This method uses both toluene and pentane to find the level of insolubles. A sample of oil is mixed with pentane and centrifuged. The oil solution is decanted and the precipitate washed twice with pentane, dried, and weighed to give pentane insolubles and then for toluene insolubles , a separate sample of the oil is mixed with pentane and then centrifuged, the precipitate is washed twice with pentane , once with toluence alcohol solution and once with toluene. The insoluble material is then dried and weighed to give the toluene insolubles.

6. WATER CONTENT: In shore testing, water content is found by the distillation method. The oil mixture to be tested is heated under reflux with a water immiscible solvent which co-distils with the water in the sample .Condensed solvent and water are continuously separated by a trap, the water settling in the graduated section of the trap and the solvent returning to the still.
b) type of information expected from the above tests would be the following

1. the flash point of a standard LO is set ( 220 deg c for M E ). Any reduction would surely indicate fuel oil contamination.
2. The base number indicate the level of alkaline reserve. A sharp increase of this indicates contamination of crankcase oil by cylinder oil. Or may be due to leaky stuffing box and excess cylinder lubrication. For auxiliary engines, reduction in BN indicate levels of acid formation due to combustion .
3. If the used oil has lower viscosity it may be due to light fuel dilution like D.o. (leakage from nozzle cooling line etc in auxiliary engine)
4. The pentane insoluble gives the measure of all insolubles due to dirt, wear particles and carbonaceous products from combustion. Whereas toluene insolubles give dirt and inorganic particles and hence a flow of the quantity of combustion desired insolubles in the oil assessed. If the level of insolubles is rising, it indicates the efficiency of the oil cleaning plant are lower ( fine filters and centrifuges). More carbonaceous products indicate an occurrence of poor combustion characteristics.
5.
Any water in the lub oil affects the primary property of lubrication and hence load bearing capacity.
High water content may be indicative of leakages from the water cooled components.

Excess of iron: Excess wear and tear of liners, crank shafts timing gears, camshafts, piston heads, piston rings etc. Iron ore carriers may suffer from these due to the suction of iron ore particles through intake air fitters.

Copper,antimony, tin: due to excess wear and tear from top end bushes, crank pin and main bearings., these are main constituent of white metal overlay

Silica: Due to air- borne or dirt ingress. Also, if the fuel oil contained higher degree of catalytic cracking fines, (mainly alumina and silica), it may contaminate crankcase oil through leaking stuffing box.

Emergency Generators for a passenger ship of carrying 300 Passengers

The emergency generators for a passenger ship having capacity of carrying 300 passengers has to be completely renewed. The company contracted for the work has no previous experience of marine installation. You as the Second Engineer have been requested to write the specification for the type and installation to the generators.

A)Outline your proposal for the type of engine.

B)State the necessary requirements for approval and certification needed.

Ans)

a) A self- contained, self excited emergency source of electrical power is to be installed, having independent fuel, lubrication, cooling and starting arrangement,

2.The emergency source of elec. Power, transitional source of emergency power, emergency switchboard and emergency lighting switchboard are to be located above the uppermost continuous deck and be readily accessible from the open deck, they are not to be located forward of the collision bulkhead.

3. A fire or other casualty in spaces containing the main source of elec. Power or in any machinery space of catagory a will not interfere with the supply, control and distribution of emergency electrical power. The space containing the emergency source of electrical powers in not to be contiguous to the boundaries of machinery spaces of category a and those space containing the main source of electrical power.

4. The capacity of the electrical power is to be sufficient to supply all those services that are essential for safety in the emergency for a period of 36 hours.

5. The emergency generator is to be driven by a suitable prime mover with an independent supply of fuel having a flashpoint (closed cup) of not less than 43 deg c

6. To be started automatically upon failure to the emergency switchboard and to permit the emergency generator to carry its full land as quickly as is safe and practicable, subject to a maximum of 45 seconds.

7. To be able to operate at full rated power when the ship is listed by 22.5 deg and when the firm of the ship is 10 deg from an even keel or any combination of or upto these limits.

8. The emergency switchboard is to be installed in the same space as that of the emergency generator, No accumulator battery except for engine starting, to be installed in the same space as the emergency switchboard.

9. The emergency switchboard is to be supplied during normal operation from the main switchboard against overload and short circuit and which is to be disconnected automatically at the emergency switchboard upon failure of the main source of electrical power.

10. Provision is to be made for the periodic testing of the complete emergency system and is to include the testing of automatic starting arrangements,

11 Emergency generator engine is to be fitted with a speed governor, which with fixed setting, is to control the speed within 10 percent momentary variation and 5% permanent. variation when the total connected emergency statutory load is suddenly applied or taken off.

12. Emergency generator engine is to be fitted with are over speed protective device independent of the governor and to be so adjusted that the speed does not exceed 15% of rated speed

b) For a period of 36 hours, emergency lighting:

1. At every lifeboat preparation station, muster and embarkation station and over sides ,
2. in all alley ways, stair ways and exits, giving access to the muster and embarkation station,
3. in all service and accommodation alleyways, stairways and exits, personnel lift cars,
4. In the machinery spaces and main generating stations including their control positions, at each main and emergency switch board.
5. In all control station machines control rooms and at each main and emergency switchboard.
6. At all stowage positions for firemen’s outfits
7. at the steering gear; and
8. At the fire pump, the sprinkler pump and the emergency bilge pump and at the starting position of their motors;

b) For a period of 36 hours , the navigation lights and other lights required by the international regulations for prevention collisions at sea in fore

c) For a period of 36 hours,

1. All internal communication equipment required in are emergency;
2. The navigational aids as required by SOLAS
3. The fire detection, fire alarm, and sample extraction smoke detection system and the fire door holding and release system,
4. and for intermittent operation of daylight signaling lamp, the ships whistle, the manually operated call point and all interval signals that are required in an emergency.

d) For a period of 36 hours….

1. Emergency fire pump;
2. The automatic sprinkles p/p , and
3. The emergency bilge pump

e) The steering gear for a period of 30 mins.

f) For a period of half an hour

1. Any watch tight door of elec. Operated together with their control, indication and alarm circuits.

The arrangement to bring the lift cars in deck level for the escape of persons