AKRAMANUJAN.ORG21ST DECEMBER 2019 BLOG229ramanujan1933@gmail.comFUNCTION 4 AND 6

CHENNAI ORALS.

QUESTIONS CONTRIBUTED BY MR VISHNU SHANKAR

41Refrigerant leak test using semiconductor method?

Answer:- Electronic leak detector is based on the principle that when halogen vapour is heated, positive ion concentration is increased and this increase is suitably magnified to an audible or visual signal. Thus the leak detector consists of a sampling tube through which air around the refrigeration system is drawn by means of a small fan. The air sample is passed over a heated platinum element. Under normal (clean air) conditions positive ion current flows under a voltage of 240 V between the ion emitter and collector (anode and cathode), However, when a sample containing halogens is drawn into the probe, an immediate increase in positive ions results and this is magnified by the electrical circuit so that an audible or visual signal is produced. Exact location of the leak can be determined as the strength of the signal increases as the probe is moved towards the leak and decreases when it is moved away from the leak. In some leak detectors a reference leak is built for comparison and adjustment of sensitivity of the detector. It is also possible to make compensation for background contamination. Electronic leak detector is highly sensitive and it can detect leaks as small as 0.25 to 0.50 oz. Per year. The operation is clean, quick and free from any fire hazard. However, it is important to maintain the input voltage for good sensitivity of the detector. They are designed to detect refrigerant leaks but should not be immersed are kept in refrigerant streams, for example from the refrigerant cylinder.

 

 

42Immediate checks and action taken when the vessel is floated in drydock?

(1) Answer:- During flooding of the dock, the flooding is stopped when the level of water in the dock reaches just above the high suction box. All the sea suction valves under the floor plates should be checked for any leaks. In this situation the ship is not floating and is still sitting firmly on the blocks. This verification is necessary to avoid any mistakes made by shipyard staff whereby some valves have not been fitted their cover joints and gland packings. A second docking can be avoided by this check.

(2) The Master and C/E should visit the dry dock before flooding and verify the following before giving orders for flooding :-

(a) The rudder fastening bolts are tight and cemented, the pintle cover plates are in place and welded.

(b) The propeller cone fitted and all bolts cemented.

(c) The rope guard in place and properly welded

(d) The sea suction box grids fitted and locked with locking wire.

(e) Bilge keel repairs are completed.

(f) Both anchors hoisted and housed in their respective hawse pipes.

(g) F/W can be supplied in the dry dock as required , but it should be ensured that the soundings of F/W tanks at the time of un-docking is the same as soundings when docking.

43Boiler water tests and their values

Answer:- The testing of boiler water is complementary to treatment and must be done on a regular basis so that suitable corrective action or changes in treatment are done to suit the correction required. The various tests and their purpose is given as under.

(1) TDS or density of total dissolved salts as the name suggests is the density of the water with dissolved salts. The water may contain un-dissolved salts such as phosphates of calcium and magnesium which does not matter. The density is therefore found by a hydrometer (salinometer) or a dipping type electronic density meter. The density is measured as soon as the water is collected in the sample jar and when the temperature reaches 940C. The density should be converted to ppm. The sample water is filtered and collected in a sample bottle for further tests.

The purpose of measuring TDS is to ensure that the density of boiler water is within the recommended level and if not to take suitable corrective action.

(2) Hardness Test :- Take 100 ml of boiler water sample in a bottle with a glass stopper. Add 0.2 ml of standard soap solution (reagent) at a time shake vigorously after each addition of soap until a lather persists for a least five minutes. The bottle is laid horizontally so that a large area of surface is obtained and the thickness of the persisting lather should be at least 3mm.

The calculation in ml of soap solution used x X 10 = P.P.M. of CaCO3 equivalent hardness.

Standard soap solution is a soap which is capable of reacting with one milligram ofequivalent CaCO3 to form a persistent lather for 5 minutes, 100 ml of boiler water sample used has x mgs of CaCo3.

But x mgs of CaCo3 is contained in 100 ml of the boiler water sample.

Hence the concentration is (x/105) i.e. [(x / 1000) / (1/100)]

or [(x X 10) / (1000 X 100X 10)]

or 10 x PPM

In all calculations, the result is given in P.P.M. of CaCO3 equivalent because the molecular weight of CaCo3 is 100 and it simplifies the calculations.

The purpose of hardness test is to monitor the conversion of hardness salts to equivalent salts of sodium, and ensures that hardness does not exceed a predetermined maximum value.

(3) Alkalinity Test:-

Test for alkalinity to phenolphthalein: Take 100 ml of sample of boiler water. Add 10 drops of phenolphthalein – result a pink coloration. Titrate with (N/50) H2SO4 so as to clear the sample.

Calculation ml. Of N/50 acid used X 10 = P.P.M. of CaCO3 equivalent

Phenolphthalein is less alkaline than hydroxides or carbonates and when it is added in a sample containing hydroxides and or carbonates it will turn pink in colour. The acid used after this coloration will first neutralise the hydroxides, forming salts. It will then react with the carbonate molecules present forming bicarbonate molecules. Bicarbonate molecular are less alkaline than phenolphthalein. Hence, the pink coloration will disappear once all the hydroxides and carbonates have been dealt with by the acid one bicarbonate molecule is formed form two carbonate molecules. Hence, in this test the quantity of acid used is a measure of the alkalinity by the hydroxides present and half the carbonates.

Total alkalinity:

Continue with the same sample used initially for alkalinity to phenolphthalein add 10 drops of Methyl-orange -- result. Yellow coloration. Titrate with N/50 Sulphuric acid until the colour becomes pink.

Calculation: Ml. Of N/50 acid used for both tests x 10 = P.P.M. OF CaCO3

equivalent for total alkalinity.

Methyl orange indicator is less alkaline than phenolphthalein and Bicarbonates. It can be used initially in place of phenolphthalein or as in more normal, as a continuation after the alkalinity to phenolphthalein test. If no yellow coloration results when the Methyl orange is added to the sample no Bicarbonates are present. Hence, no carbonates must have been present. Therefore the Alkalinity as determined in the Alkalinity to phenolphthalein test has been due to Hydroxides alone. Hydroxides and Carbonates can co-exist together in a soluation but Hydroxides and Bicarbonates cannot.

For all practical purpose the Alkalinity to phenolphthalein can be considered as caustic Alkalinity (The difference is not much) and the residual Alkalinity, i.e. total Alkalinity minus. Alkalinity to phenolphthalein as Alkalinity due to carbonates.

General Explanations for the Calculation:

For complete chemical reaction we have:

CaCo3 +H2SO4 = CaSo4 + CO2

i.e. 100 ml. Of CaCo3 will combine with 98 mol. Of H2SO4 for complete chemical reaction. But both these substances are dissolved in water and hence we must relate them to their concentrations to obtain the correct mass.

One normal solution is defined as the Gram equivalent weight of a chemical dissolved in water to form a litre of its solution.

Equivalent Wt. = Molecular wt. / valency.

The valency of H2SO4 is 2, hence equivalent wt. Of H2SO4 = 98/2 =49.

Therefore one normal solution of H2SO4 is made by dissolving 49 gms of H2SO4 in 1000 ml. of solution. If “a” ccs of H2SO4 is used in the reaction, then the mass of H2SO4 used will be:

a x (1/50) x (49/1000) gms of H2SO4

Let the mass of CaCO3 in the sample be denoted by x.

Then x = a x [49/ (50 X1000) x (100/ 98)] = ( a /1000) gms but x is

dissolved in 100 ml. of boiler water sample and hence in terms of “a” it will be

(a/1000) X (1/100) which is [(a X 10) / 106] in P.P.M.

Hence the quantity of (N/50) H2SO4 used X 10 = P.P.M. OF CaCo3 equivalent.

CHLORIDE TEST:

Continue with the same sample add 2ml. of Sulphuric Acid, add 20 drops of potassium chromate indicator. Titrate with N/35 Silver Nitrate solution until a brown coloration results.

Calculation Ml. of M/35 solution used x 10 = P.P.M. of Cl. Or ml. of N/50 Silver Nitrate solution used x 10 = P.P.M. of CaCO3 equivalent for chlorides.

Purpose:- Chlorides may be present in the boiler water sample. It is essential that they be measured as they would be an indication of salt water leakage from a leaky condenser or result of priming of fresh water generator. The Alkalinity to phenolphthalein sample taken has had the hydroxides and Carbonates dealt with and they will play no further part therefore in the test conducted for Chlorides. The sample is made definitely acidic by the addition of a further small quantity of acid. This is to speed up the chemical reactions which next take place. Silver Nitrate has affinity for potassium chromate and chlorides. Its principal preference however is for the chlorides. When it has reacted with the chlorides present in the sample, it is then free to react with the potassium chromate. In doing so, it produces a reddish brown coloration. It is therefore apparent that the amount of silver nitrate solution used is direct measure of the chloride content of the boiler water sample.

PHOSPHATE TEST (RESERVES)

This test is conducted for boiler to determine the phosphate reserve in the boiler which is necessary to deal with the extra feed being taken in.

Take 50m of hot boiler water sample, add 4 grains of potassium nitrate crystals, put 25ml. of the above into a test tube. Stand test tube in a water bath at 38 degree to 45 degrees C. Add 5ml, of ammonium molybdate reagent.

A sample of boiler water which contains sodium phosphates and dissolved potassium nitrate will turn cloudy when ammonium molybdate is added. This reaction to produce a cloudiness in the sample is quicker the larger the quantity of phosphates present. If the cloudiness does not result until after five minutes have elapsed, the phosphate reserve in the boiler water is too low (below 20 P.P.M.). If the cloudiness results before two minutes have elapsed, the phosphate reserve is too high (above 70 P.P.M.). It will be appreciated that the sample of boiler water must be hot in order to dissolve effectively the crystals.

44What is hot corrosion and cold corrosion?

Answer:- High-temperature corrosion

Vanadium is the major fuel constituent influencing high-temperature corrosion. It cannot be removed in the pre-treatment process and it combines with sodium and sulphur during the combustion process to form eutectic compounds with melting points as low as 5300C. such molten compounds are very corrosive and attack the protective oxide layers on steel, exposing it to corrosion.

Exhaust valves and piston crowns are very susceptible to high-temperature corrosion. One severe form is where mineral ash deposits form on valve seats, which, with constant pounding, cause dents leading to a small channel through which the hot gases can pass. The compounds become heated and then attack the metal of the valve seat.

Low-temperature corrosion

Sulphur is generally the cause of low-temperature corrosion. In the combustion process the sulphur in the fuel combines with oxygen to form sulphur dioxide (SO2). Some of the sulphur dioxide further combines to form sulphur trioxide (SO3). The sulphur trioxide formed during combustion reacts with moisture to form sulphuric acid vapours, and where the metal temperatures are below the acid dew point (1600C) the vapours condense as sulphuric acid, resulting in corrosion.

The obvious method of reducing this problem is to maintain temperature in the engine above the acid dew point through good distribution and control of the cooling water. There is always the danger that an increase in temperatures to avoid low temperature corrosion may lead to increased high temperature corrosion. Attack on cylinder liners and piston rings as a result of high sulphur content fuels has been effectively reduced by controlled temperature of the cylinder liner walls and alkaline cylinder lubricating oil45 Why TBN is important? TBN values of ME and AE lub- oil

Answer:- TBN is the total base number and it represents the alkalinity of the oil which is necessary to neutralize the acidic nature of the products of combustion caused by sulphur di oxide.

For aux medium speed engines the TBN IS 12.

For slow speed main engines it applies to the cylinder oil which has TBN 18

46 There is a crack on ME air line, how will you manage to reach port?

Answer:- It depends where the crack is. If it is on a branch line connection from the manifold to the cylinder start valve, the branch can be suitably blanked and the pilot air valve connection from air distributor also blanked. The air start valve has been isolated and made inoperative. The engine will start with some more revolutions on air. By this process the leak is prevented and waste of air also prevented

Welding on a cracked high pressure air pipe is difficult since it needs professional expertise and the use of special electrodes compatible with the pipe material.

If the crack is on the main manifold, a steel circumferential band must be fabricated of plate thickness about 2.5 mm with at least two bolts for tightening. The pipe surface should be scraped of all paint and covered with a copper strip at least 1 mm thick annealed so that it will form into the curvature and fill the surface completely like a joint when tightened by the clamp.

47 How to do air bottle survey? What all areas to check?

Answer:-The complete survey of the air bottle as per class requirements consists of draining , opening the air bottle man hole door , overhaul and inspection of all the mountings on the bottle in dismantled condition, internal examination inafter thorough cleaning and scrapping of rust collected at wasted areas.

After survey the wasted areas around the drain pockets which have been cleaned are coated with special polymer primary and finished coat paint. The mountings are reassembled with new packings and joints. The relief valve is tested and set separately using a test rig and a hydraulic pump , which will be witnessed by the surveyor.

48 Checks on AE after overhaul.

Answer:- After the A/E is overhauled, the sump is filled with fresh lub oil after the sump is thoroughly cleaned . The engine is started and made to run on idle speed for about 19 minutes when some primary checks for leaks is carried out after which the engine speed is raised to normal running speed and run for about 3o minutes. The engine is stopped , the crank case doors opened and the bottom end bearings and main bearings felt for any signs of over heating. If no over heating is observed the engine may be started and put on half load by paralleling with the running generator. The machine is kept running in this condition for about an hour during which time pressure card is taken for all the units with fuel on and off to obtain the compression and peak pressures . The heights of these bars should be checked for uniformity. During the running, the cooling water outlet and inlet temperatures of individual units, the exhaust temperature of all units, the luboil pressure, the T/C RPM, air manifold pressure are recorded and verified. If all is well the machine can now be put on normal load.

49 How to check Main bearing, Crosshead bearing, bottom end bearing clearances and their values

Answer:-The procedure is as follows:

This operation is done during crank case inspection.

· Obtain propeller clearance and engagwe turning gear .

· For top end bearings;

· Put crank on TDC.

· check the oil clearance between top half and crosshead pin on both sides. Use long feeler gauge since the leaves have to penetrate the full width of the bearing.

· For bottom end bearing:

· Put crank on BDC.

· check oil clearance between bottom half and crank pin. use long feeler gauge.

· For main bearing:

· Turn the crank to stbd side at half stroke . the clearance is measured between journal and top half bearing.

· The standard values are:

· TOP END 0.2 -0.3mm

· Bottom end 0.4-0.5 mm

· Main bearing 0.45-0.5 mm

50Defects in connecting rod bolts

Answer:- Connecting rod bolts of medium speed engines are prone for permanent set (elongation). They shoul be checked for elongation using the standard gauge length supplied by makers. Checked for surface serrations and cuts. If these defects are found they should be discarded.

51How to do over speed trip test of AE

Answer:- The engine is run with the full load on. It is best tested during shutting down the engine. The procedure of releaving the electrical load from switch board is followed and the reverse power trip is made to operate. A person at the

engine panel should observe the fluctuation of rppm when the load is pulled out. It is normally between 15 and 20 percent of full load RPM.

FUNCTION 3

QUESTIONS ON DECK MACHINERY COMPOSED BY AKR

DECK MACHINERY.

23 For large ships which type of windlass is preferred and state the reasons for the preference

Answer:- The hydraulically operated windlass has larger torque capacities because the hydraulic motor can operate at very low speeds even at 5 to 10 rpm, there by building a very high torque. The torque –speed characteristics of hydraulic system is much better and flexible than electrical systems. Their comparison is shown in the sketch given under(Fig10)

((Fig 10))

The Hydraulic machine is robust and can withstand a lot of shock. The components are less since the double reduction gear box and the slip clutch are eliminated thereby reducing the cost of the machinery. For safety a spring loaded shock valve is provided in the system which will connect the high pressure to the low pressure side when overload occurs.

24 sketch a section of the windlass or mooring winch motor and explain the major maintenance required.

Answer:- Maintenance.:- During lay ups at special surveys, the end cover is opened up and vanes as well as springs which are damaged are renewed. The oil seal and roller bearing also renewed if they are worn out.

The sketch of the winch motor with detail of vane assembly is shown under.(Fig 13b)

((Fig 13 b))

25 Sketch the stockless Bower anchor and explain how the anchor assumes a position of the plough when dropped in the sea.

Answer:- The anchor as sketched is in position to be dropped.

When it hits the sea floor , because of the curved bottom of the anchor head the anchor will fall on any side . The anchor head is hinged with a freedom of movement of about 15 to 20 degrees on either side permitted by the wedge shape cavity in the head int which the shank is connected by the pin. This freedom allows the head to assume the position as required for digging into the soil.

26 Why are the mooring rollers on the mooring winch conical in shape.

Answer:- When a ship arrives at the berth or pier it is placed parallel to the pier or jetty and engines stopped. The ship has to be bodily hauled in by the mooring ropes forward and aft. The mooring ropes ends formed into spliced nooses are slipped on the pier bollards and the mooring ropes on the ship are wound by two or three turns on the mooring winch cones both at forward and aft. When the cones are rotated by the mooring winch the tension created on both the forward and aft ropes pulls the ship bodily towards the jetty . The ship moves inwards and comes to rest when the ship makes contact with the jetty fenders.

The question usually asked is why is the mooring rope drum conical in shape and how is the large tension created in the rope capable of pulling the ship towards the pier.

The answer is as follows:

The Able seaman or boat swain holding the slack end of the rope puts his body weight on the rope and this body weight produces a tension on the rope at the other end of a high magnitude caused by friction between rope and cone surface. This resultant high force is based on the rope or belt Drive formula derived in Applied mechanics and is given by the formula

T1 /T2 = eμϴ

Where T1 is the tension on the tight side and

T2 is the tension on the slack side.

e is the natural number approximately 2.3

µ is the coefficient of friction between rope and steel and is approximately 0.3

Ɵ is the angle of overlap which in this case is 3 turns or approximately 19 radians. The body weight of a normal person is about 70 Kgs. The tension on the tight side therefore increases to 70 * 2.35.7 = 10080 kgs weight = 98.88 KNs which is the high tension.

The reason for having a conical shape is for ease of slipping the rope out by one or two turns . This high tension on the tight side is needed to create a momentum to advance , if maintained it will break the rope with fatal consequences . The broken rope can spring back with a high rebound hitting the able seaman or bosun holding the rope. There have been many instances of persons loosing their lives on such occasions.

A competent seaman will instantly reduce the turns when the ship starts moving so that the rope does not break by the high tension created and the conical shape provides for this convenience.

27 How is the power capacity of the windlass decided.

Answer:- 7.1.5 The windlass is to be capable of exerting, for a period of 30 minutes, a continuous duty pull corresponding to the grade of chain cable, as follows :-

36.8 dc N

for grade CC1

41.7 dc N

for grade CC2

46.6 dc N

for grade CC3

Where dc is the chain diameter [mm]. The mean hoisting speed is not to be less than 9 [m]/min.

The power is given by 36.8 dc* (9/60)/1000 KW FOR CC1 grade chain utilised windlass.

28 How is the capacity of the windlass brake and chain stoppers decided.

Answer:- The capacity of the windlass brake is to be sufficient for safe stopping of anchor and chain cable when paying out.The windlass with brakes engaged and release coupling isengaged is to be able to withstand static pull of 45 per cent of the tabular breaking strength of the chain without any permanent deformation of the stressed parts and without brake slip. If a chain stopper is not fitted, the windlass is to be able to withstand a static pull of 80 per cent of the tabular breaking strength of the chain without any permanent deformation of the stressed parts and without brake slip.

The chain stoppers and their attachments are to withstand a pull of 80 per cent of the tabular breaking strength of the chain without any permanent deformation of the stressed parts. The chain stoppers are to be so designed that additional bending of the individual link does not occur and the links are evenly supported.