FEBRUARY 2018

MARINE ENGINEERING PRACTICE

TIME ALLOWED - 3 HOURS

Instructions

1. Answer SIX questions only. THREE Questions from each section

2. All questions carry equal marks.3. Neatness in handwriting and clarity in expression

carries weightage.4. Illustration of an answer with clear sketches /

diagrams carries weightage.

Q1: Describe how the following conditions are prevented in auxiliary boilers: Feed contamination by oil from heating coil drains; Internal corrosion; Furnace blowback; Uptake fire.

Answer:-

The above line diagram shows the open feed system. Steam from the boiler is distributed to the various steam heating coils for heating the fuel oil and

lubricating oil. The heating steam gives its latent heat to heat the oil and in the process gets converted to water. The water coming out after giving its heat is drained into an observation tank. At the outlet of the drain at the tank a steam trap is provided to ensure that only water comes out and not steam. This ensures that the steam gives its latent heat to the oil. The trap also ensures that only water comes out and the water collected in the observation tank is in tranquil condition so that it can be observed. The level seen in the observation tank is maintained at the centre of the glass screen by a float controlled valve. The level in the tank is to be observed for any collection of oil.

The water from the observation tank drains into the cascade tank. The cascade tank has three chambers . the first chamber is provided with a filter to prevent solid dirt particles and debris from entering the boiler. The filtering medium is a combination of coke and coconut coir or loofa sponges. The water from the first chamber cascades into the second chamber fitted with six or eight filter cages covered with towel material to filter the oil collected in the water. The drained water from this chamber enters the third chamber which is similarly fitted with 6 or 8 filter cages. The water is double filtered for more purity. The water from the cascade tank is sucked by the feed pump and pumped through the feed heater and the feed controller through the main feed line and the main feed check valves into the boiler.

The boiler produces more steam than required especially in waste heat boilers and this excess steam is dumped into the dump condenser to recover the water. The condenser is maintained at a pressure of about 3kg/cm2 and this is done by a spring controlled back pressure valve or a screw down valve hand adjusted. If the back pressure valve is not provided all the steam will enter the condenser thereby starving the heaters of steam.

The condenser is provided with a change over cock at its steam inlet with a connection to the atmosphere so that the condenser can be isolated in case the boiler has to be operated in Dry -dock or if there is any defect in the condenser.

Prevention of oil contamination of the boiler water:- This is done by regular washing of the filter toweling fitted on the filter cages . A spare set of toweling is used so the filtration is continuous.

Regular observation of level in observation tank and detecting the leaky steam heating coil by operation of the heater drains after the traps, and taking suitable corrective action by repairing the leaky coils.

Internal corrosion :-The forms of corrosion encountered on ships due to the ocean environment are;

(1) Air corrosion caused by oxidation of steel exposed without a protective coating in a moist atmosphere leading to the formation of ferrous and ferric oxides. (2) Acidic corrosion caused by formation of chlorides which convert to hydrochloric acid causing corrosion of the exposed steel. This type of corrosion is predominant in boilers. (3) Galvanic corrosion which is caused by dissimilar metals immersed in an acidic medium like sea water, causing a galvanic current to circulate from the nobler non ferrous metal (bronze) to the baser metal steel through direct metallic contact and return via the sea medium to the nobler metal. This form of corrosion is predominant in coolers and condensers, where the tubes and tube plates are of nobler alloys compared to the steel waterboxes and casings.

Chemical treatment of boiler water is done to prevent internal corrosion. The water is continuously maintained moderately alkaline by maintaining sufficient alkalinity in the boiler water. The chemical used is caustic soda. Apart from this the hardness calcium and magnesium salts are suitably converted to harmless equivalent sodium salts by the addition of sodium phosphates. Air corrosion is prevented by mechanically removing air from the condensers in closed feed systems so that air entry into the boiler is thoroughly restricted. For oxygen dissolved in the water limited oxygen treatment is done by adding sodium sulphite in boilers operating at moderate pressures upto 42 bar and hydrazine for boilers working at pressures above 42bar.

The reasons for furnace blow back from auxiliary boiler are: (1) Purging of air insufficient.

(2) Water in the oil causing steam lock in the oil and making flow discontinuous and subsequent blow back. (3) Oil overheated causing gas lock and subsequent blow back. (4) Blow back can occur if the smoke side of the exhaust boiler or the forced circulation economiswer is sufficiently blocked by soot. This needs immediate cleaning. STANDING INSTRUCTIONS for avoidance of boiler furnace blow back: (1) When starting the boiler combustion system, start on manual mode

so that long air purge can be given (2) Ensure the fuel oil is sufficiently heated and this may require circulating the oil through the heater sufficiently till the oil temperature reaches the right temperature. (3) Ensure the fuel oil is free of water and this can be verified by operating the spring loaded drain valve on the oil service tank and ensuring the oil is free of water. (4) Change over to auto-mode only when steady state conditions are achieved.

Uptake fires :- Uptake fires in forced circulation economisers are caused by accumulation of oily soot in the spaces between the tube fins which being saturated with oil will catch fire by a burning piece of soot from the engine exhaust. When the main engine is running at full power the excess air supply is enough to aid the fire started and will continue till all the accumulated soot is burnt.

Such fires are avoided by regular use of sootblowers where provided. Apart from this periodical water washing of the economiser must be carried out since just soot blowing is not sufficient.

Q2. Write a report to your Engineer Superintendent describing a breakdown of the main refrigerating plant for cargo or ship provision giving reasons for the breakdown, the method of repair and the action taken to prevent a recurrence.

Answer:-

To the superintendent Engineer,

Global ship management company,

Sub:- main cargo fridge break down

Dear sir,

On the 30th of march 2018 the main cargo fridge compressor no 1 was found to be non functioning although it was running. It was in cut-in mode for over an hour and on investigation it was found that the suction and discharge pressures were observed to be the same

The number 2 compressor was started and put on operation. The cargo hold temperatures were checked and were found to be 10 degrees higher but normalised within half an hour of changing over to the stand by compressor no 2.

On dismantling the valve chamber head it was found that the bursting disc separating the high pressure and the low pressure side had ruptured so causing this condition.

The bursting disc was replaced by the only one spare disc available, and the compressor tried out. It was observed that the disc which ruptured was made of brass whereas the manual states it is of nickel and .05 thickness

I have kept this compressor on stand by duty since the replacement is also of brass.

I request that at least two original bursting discs from the makers be supplied at the disport on arrival. A suitable indent is attached to this report for your necessary action.

Yours faithfully

Q3. A. During the weighment of CO2 bottles required for total flooding of Engine room, it was observed that few bottles are less than the original capacity. State the reasons for the same and checks / tests to be made prior refilling; B. State how often the CO2 bottles are required to be weighed and pressure tested.

Answer;- The section of the CO2 bottle of the portion with the attached head is shown in sketch as under

The leaks occur froim the thread connections indicated as 1 and 2 caused by wear down of the threads.

The checks required prior to filling is defective pressure heads must be renewed with the corresponding bottle threads rethreaded by remetalling and rethreading.

The statutory interval for weighment is every two years and the hydrostatic pressure testing of the bottles is every 10 years.

Q4. During the past 4 months since you joined the ship as 2nd engineer a number of main engine exhaust valve have suffered from cracking and corrosion at the seating faces; Write a report to the superintendent engineer covering the following points; An

explanation detailing how the problem becomes evident; Your action upon recognizing the extent and seriousness of the problem; Your reasoned view regarding the possible cause of the problem; Your recommendation to avoid future incident.

Answer:- Dear sir,

Please find attached a report on the four numbers of exhaust valves of the main engine which are having their valve seats and lids cracked during the last 4 months after I have joined the ship.

Report

(a)The problem became evident when all the four valves had their seats and lids burnt within a total of 240 hours of operation, when i got concerned and started investigating the problem . I carried out a detailed inspection of these valves after dismantling and cleaning the valves . i have observed that the cooling water spaces around the valve seats are heavily scaled .

I measured the external length of the valve cage and the depth of the water space from the same datum and observed that the residual thickness was high which allows for increasing the depth and thereby enhance the cooling of the valve seat. I have shown these facts in the sketches given under.

(b)If the heads and seats of exhaust valves become overheated they soon begin to distort and burn. To prevent this, conduction of the heat away to the surrounding media is relied upon; consequently, with large valves there are greater

thickness of metal, the rate of cooling by conduction is slower and the process must be accelerated.

The cooling of exhaust valve seats becomes very necessary in engines burning high-viscosity fuel. This is due to the combination of the sodium and vanadium content in the fuel. Under some circumstances at high temperatures the sodium and vanadium content in the exhaust gases is very corrosive.

(1)Cooling space length as existing

(2) cooling space extended .

(c)It was observed that the cooling water passage up to the bottom of the valve cage can be extended by about 7mm which will enhance the cooling of the valve seat and lid. Accordingly I suggest and recommend that the valve cages cooling water spaces must be soaked by filling the spaces by a weak acid solution for a period of 24 hours so as to de-scale and de-sludge the spaces especially at the bottom. I further request you to advise the engine makers / valve makers to increase the length of the cooling water space by 7 mm.

Q5. A. Describe the events leading to a crankcase explosion; B. State how overheating might be indicated other than by a mist detector; C. State how severity of a crankcase explosion is limited. D. Emission of flame has in the past caused severe burns to personnel present during a crankcase explosion. Despite the addition of flame traps. Discuss the procedure in the advent of overheating being indicated.

Answer:- (A) (a) The development of conditions favourable for a crank case explosion are to be noticed in the time the engine is warmed up and during the time the engine is continuously running. The Main engine jacket cooling water is initially warmed up by circulating the Aux engine cooling water through the main engine jacket for at least six hours before controls are tested. The aux engine cooler is by-passed during this operation so that the main engine is warmed up quickly. The heat from the jacket travels downwards heating the crankcase. The air within the crankcase warms up. About one hour before control testing the

main lub oil pump is started. The oil squeezing out of the bearings forms a fine spray and the droplets of oil mix thoroughly with the warm air making the mixture fit for an explosive condition. All it needs is a heat source to ignite the mixture of warm air and oil vapour. This condition is maintained constant throughout the main engine running period.

(a)(b)When the engine is running continuously, the initiation of an explosion can occur caused by an overheated bearing , overheating of the piston rod rubbing against the packing element or spark generated from chain guides in the chain casing. In trunk piston engines it can be caused by Blow- Back from a cylinder with worn piston rings.. Hence it is the hot spot or ignition source which is the third element of the FIRE triangle to start the explosion.

(B) Over heating is indicated by temperature sensors fitted to the main,top end and bottom end bearings as well as the chain guides. The alarms fitted to the temperature sensors get activated.

(C) The severity of the secondary larger explosion is avoided by the instant activation of the explosion relief valve.

(D) Personnel get injured if they remain close or in way of the flame movement. The flame trap gauze are required to be cleaned periodically if they have to remain effective. The gauze openings get blockrd by the oily dust prevailing in the machinery space.

On receiving a n alert from the temperature sensors or the mist detector, the bridge is to be informed and engine slowed and stopped as soon as possible.

Wait for about 20 minutes before opening the suspected crank case door.

Wait for chief engineers instructions after entry into crank case made and fault ascertained.

Q6. Describe the overhaul of a boiler safety valve and explain using sketches where necessary those parts which require particularly close attention; Describe the setting of boiler safety valve to comply with classification society requirement.

Answer:-

(b) With the safety valves completely dismantled, the following parts need close inspection. (1) Valve spindle:- generally inspect the spindle for any damage particularly cracks on the surface. Particularly check for bends if any and this can be done by verification of trueness by centering on the lathe. (2) Springs:- the permanent set or deformity can be checked by

comparing their heights with the spare spring on board. Springiness : boiunce them on the floor. Ring the springs to detect for any cracks. (3) Valve and valve seats :- Check the valve and valve seats for any indentation or grooving marks. If so the valves and valve seats will need machining. Valve and valve seat need lapping with very fine emery paste or jewellers rouge since they very rarely come into operation. (4) Valve chest, spring casings and covers:- Check for cracks by tapping

them and listening to the metallic sound. (5) Compression nut:- Try the compression nut on the cover and check

for radial and axial play. (6) Check the drain from bottom of chest is clear, as well as the connecting pipe up to scupper is clear. (7) Other parts require close inspection for deformity and cracks.

BOILER SAFETY VALVE SETTING PROCEDURE: 1) A Gagging tool is used alternatively on the valve to prevent simultaneous lifting of the valves at the set pressure. 2) Use the standard boiler gauge brought by the surveyor if the boiler pressure gauge is not calibrated. 3) Remove the caps keep the valve spindle of one valve (port) gagged. The other valve will be se first (stbd). 4) Keep the compression nut screwed down, on the valve to be set, till the previous marks. 5) Raise steam pressure slowly and bring pressure to set pressure. 6) A slight hissing sound will be heard. This is the floating condition of the valve. In this condition the valve will sit on its seat by slight pressure on the spindle with the palm of the hand and will not come back unless tapped. 7) Reduce pressure, remove gagging tool and fit out the other valve. Repeat

above operation on this valve for setting. 8) When both valves are set remove gagging tool. Fire the boiler at maximum capacity. Note full flow of waste team from waste steam pipe. Note pressure on gauge. The pressure should not exceed 10% of the set pressure. On satisfactory completion of setting, refit

caps, cotters, locks and easing gear. Test easing gear and following safety alarms and cut outs: - (a) H.P cut off and alarm (b) Low water alarm and cut off (c) High water alarm and cut off

Q7. What are the routine maintenance carried out to an a.c. motor; and- A. Explain the meaning of single phasing in a.c. machinery; B. State the dangers associated with single phasing and enumerate the protective normally fitted to counteract such dangers.

Answer:- The routine maintenance carried out aon an AC motor are (1) quarterly insulation readings taken by meggar(2) the routine overhaul of the motor after a fixed number of running hours.

(B) Single Phasing

The loss of current through one phase in a three-phase supply is described as single-phasing . The open circuit in a phase often from a blown fuse, faulty contact or broken wire will prevent a motor from starting but a running motor may continue to operate with the fault.

A motor running with a single-phase fault will carry excess current in the remaining supply cables. Motor windings will have unequal distribution of current . The single winding A could have more than normal full load with the motor on half load . Maximum motor loading could bring current in A to double or treble this figure. Current in the remaining two supply cableS

tends to rise in almost the same proportion as that in A . Flow through winding B and C in series becomes higher than usual at full load . Single phasing in a running motor can be detected by overload device in the supply lines or through the overheating .

Motor protection :-.

Protection of motor is required mainly to prevent overheating which can causes deterioration of winding insulation and burn -out

, if severe . Overheating as the result of overload, stalling , single phasing or prolonged starting period can be detected by a rise in line current and by temperature change. Overheating as the result of high ambient temperature or poor cooling due blocked air passages can only be detected by temperature rise within the windings. Each of the three supply phases of the motor is fitted with an overload relay. Such an arrangement should detect single phasing . Where the symptoms are of high current in two supply line as well as straight overload . operation of any of the relays will close the circuit to energise the time delayed trip.

Q8. With reference to the main gearing and gearbox, state with reasons: (a) Why the first examination after commissioning is of special importance. (b) What part would receive the closets scrutiny and what defects might possible be found. (c) The gearing faults that are likely to develop early in the

life of the vesselAnswer:- The first inspection after commissioning is

important due to the scoring the teeth faces undergo caused by a combination of rolling and sliding . Theoretically the tooth profile is designed only for rolling contact with single point instantaneous contact , but practically sliding occurs causing scoring

.Scoring . The action of two involute tooth surfaces when rotating in unison is such that the contacting surfaces both roll and slide over each other At the pitch line point of contact, the sliding component is Zero and the contacting surfaces are in pure rolling contact. But the sliding component increases with the distance from the pitch line and is a maximum at the tooth extremities, can cause scoring of the tooth sufficiently severe can cause scoring of the tooth surfaces. This scoring or galling is an actual fusing or welding together of particles of the contacting surfaces. Under the continued motion, particles are torn from one surface and either deposited on the other surface or released.

(b) The part or parts which require close scrutiny are the gear teeth individually Scoring which results in a serious deterioration of the tooth surfaces is not tobe confused with the minor scratching of the surfaces that result from the passage of minute particles between the teeth. Another factor is the deflection of the pinion and gear shafts caused by tooth reaction which has a detrimental effect on the alignment of the shafts which are to be perfectly parallel. The third factor is the torsional deflection experienced by the shafts because they transmit the power torque.

(C) The faults that are going to be developed are: * Scoring with reasons stated as above. * excessive teeth play leading to gear chatter. * heavy wear of gear face profile. * In extreme cases gear teeth fracture.

Q9. Briefly explain the term fatigue and explain how fatigue failure occur; State the difference between high stress / low cycle and low stress / high cycle; State how defect in metal can influence the expected safe life of a component; State how fuel timing and cylinder power balance can influence the possibility of fatigue cracks developing in the bedplate.

Answer:- -Fatigue in a material is a measure of its behaviour when submitted to cyclic variable stressing. It may occur as variation of magnitude of stress or a complete reversal .

Materials under fatigue may fail at a lower stress value than would be required on the same material under laboratory static stress.

(b). The factors which affect the endurance of a material to fatigue are:

(1) Stress concentration caused by abrupt changes in section, keyway ,hole ,threads, corrosion on the surface, ridges and cuts on the surfaces.

(2)Magnitude and nature of stress applications. The cyclic stresses can be represented by sinusoidal curves for the two types of stress variations as under;

(1) Variation in magnitude only without reversal of direction.

(2) Variation of magnitude and direction with reversal of the stress.

Of these two, the second variety is more likely to fail earlier and at a lower stress value.

(3) Material defects such as surface cracks appear caused by fatigue and these are termed as fatigue cracks. They appear on shafts as surface cracks when the torque transmitted is varying in a cyclic manner. When a shaft is subjected to torque the shear stress at the surface caused by twisting is maximum at the surface and reduces as the radius decreases correspondingly. Similarly if the material is under cyclic bending moments surface cracks appear on the surface of the component caused by fatigue since the alternating stresses at the surface is maximum in magnitude and direction as well.

(c) The factors influencing cracking of main engine transverse girder is mainly attributed to faulty engine operation caused by un-even power produced in each unit with corresponding variations of peak pressures in each unit. This results in the variation of compressive stress levels in the transverse girders causing fatigue cracks to appear.

Another factor which can contribute to the first cause is the deterioration of the crank shaft alignment leading to high variations of reactions at the main bearings A