AKRAMANUJAN.ORG25th JULY 2019 Function 4

CONTINUED FROM BLOG 118

NOTE:- MEP JUNE 2O19 Question on exhaust valve replacement has not been answered but declared as repeat question . The error was pointed out by Mr Roshan Mohan and this question is answered. Accordingly the JUNE 2018 MEP paper has been reposted with the correction.

ORAL Q AND A FROM EXAMINATION CENTRES

KOCHI

March 2015 Mr John

(1) Draw the B&W engine manoeuvring diagram or sketch.(a)Answer:- Starting :- The control air selection

cock is to be put in the position selected ie either Bridge control or Engine control room.. The air starting is accomplished by putting the engine control lever to air start position. The main air auto start valve opens and allows main starting air to flow to each air starting valve on cylinder head through a common air manifold. The starting air also flows through a branch to the air distributor which connects to each starting valve at top to actuate the valve. The opening of Air start valve is approximately 140degrees to provide for a good overlap so that starting is accomplished in any position of the crankshaft. When the engine picks up speed the engine control lever is moved to fuel position and the engine starts on fuel. When control moves to fuel position the air to air starting manifold is shut

by the main auto air start valve. This is a safety arrangement on the main air start valve which will not allow air to flow to the air start valves when the main engine is working on fuel.

Setting the direction of rotation is done by moving the reversing lever to the required direction. Air enters the Reversing servomotor cylinder, actuating the piston connected to the fuel pump roller and air distributor roller actuating links.. the respective rollers are moved to the correct position for the direction of rotation. The sketch shown as under explains the action.

The speed of the engine is controlled by the load or speed control lever actuating the fuel control lever.

The movement of fuel control lever is controlled by a ‘ Ward- Leonard’ positioning device. The Main Engine drives a tacho generator which is a DC motor fitted with permanent magnets so that its voltage generated is directly proportional to the RPM of the engine. The Electrical arrangement is a DC “ Wheatstone—Bridge” . The voltage drop selected at control speed is matched with the voltage produced by the tacho-generator and the fuel control motor will stop at a position when both voltage drops are same, which corresponds to the speed selected.. The arrangement is shown by a figure as given under.

(2) Explain the setting of the B&W engine fuel pump timing

Answer:-

When a fuel pump plunger sticks due to seizure it is necessary to lift the plunger and the barrel fit a spare one. The pump barrels and plungers are mated together and hence not interchangeable.. The barrel and the plunger are lifted together in the assembled position by an extractor. The extractor is screwed with the plunger by the central rod. The barrel is bolted with the flange of the extractor.

The same tool is used for mounting a new set of barrel and plunger. After the plunger is guided inside the regulator arm, the top cover with suction valve etc is fitted back.

Main adjustment is the adjustment of fuel pump lead i.e.to measure the lift of the plunger in relation to barrel with the particular unit piston at T.D.C. The measurement is taken by placing the legs of a measuring instrument on the adjusting nut and the central pin on plunger.

The pump barrel is attached to the underside of the top cover by four studs in the top edge of the barrel and the four cap nuts at top of the cover. Steel shims fill up the space between upper edge of the barrel and the underside of the cover. Slight adjustment of lead can be done by altering the position of pump barrel relative to the plunger by removing or inserting shims.

The cam is designed for high acceleration and short filling period. Advancing the cam in its direction of rotating increases the T.D.C. lift, the admission is early. If the barrel is guided down in relation to plunger by the adjusting device the admission occurs early.

The lead can be adjusted by an amount of ±2 mm in this way. Any adjustment beyond thus must be done by shifting the cam

(3) Draw and explain the liner wear profile of a slow speed main engine liner.

Answer:-

As seen from the sketch above , the wear is a maximum around the position of the top piston ring at TDC . This is where lubrication is the most difficult due to high temperature and pressure exerted on the back of the ring is the highest.Wear rate decreases down the liner as conditions becomes less arduous

and an oil film is built up. There is an increase in the wear rate as the rings pass over the scavenge port bars in a two stroke engine because of the decreased surface area and because the piston is slowing down. The point at which gauging takes place reflects these different wear rates. As shown on the diagram, there are several gauging points close together at the top of the liner and then further down the points are regularly spaced out further apart.

(4)) Explain the construction of the Scotch boiler furnace and the cylindrical shell.

Answer:- The modern scotch boiler is now an all welded construction. The main water drum is a cylindrical shell with the end plates flat and flanged at the rim to join the cylindrical shell by welding.

The front end plate may have two or three furnace openings depending on the diameter of the boiler. The furnace openings are flanged inwards to make a good joint with the corrugated cylindrical furnace. The furnace is rolled to a cylindrical shape from corrugated plating and the seams are welded . At the inner end of the furnace , it joins the combustion chamber. The furnace and combustion chamber are held firmly by short bar stays as well as the smoke tube stays.

The end plates are held firmly by the long bar stays to prevent buckling of the end plates which are flat when they are subjected to pressure.

(5) Explain the principle of the double evaporation boiler and explain the need for the same.

Answer:- The main reason for the adoption of double evaporation boilers in tankers is that oil majors are concerned of damage caused by oil and sulphur which enters the feed systems through leaky steam heating coils used for heating the oil in cargo tanks. The double evaporation boiler is proof against this by virtue of the fact that the boiler proper operates on a closed circuit, its initial charge of high quality water being continuously evaporated in the boiler and condensed in a heat exchanger producing low pressure steam ,and requiring occasional make-up to replace incidental losses.

All the feed necessary to produce the low pressure steam is fed into the heat exchanger and ,as this is not fired. Dangers of damage to boilers by oil contamination and sulphur are eliminated in this non-fired heat exchanger.

Double Evaporation System

With the progress of time the transportation of bulk crude was passed on to tanker owners who exploited this system in a flexible manner whereby the primary boiler was used to produce steam in superheated condition to be fed to a reaction turbine driving an alternator for all the electrical load and for driving cargo oil pumps for discharging the cargo during port stay. The use of steam turbine driven cargo pumps the discharge of oil cargo is speeded up taking advantage of the high speed of the turbine . This is an added advantage of using turbine driven cargo oil pumps

(6) Where is the goose neck connection in the scotch boiler.

Answer: - The goose neck connection is in old riveted scotch boilers and it is the connection between the furnace end and the combustion chamber opening , which is in the shape of the goose neck.

(7) What is an expansion bellow and where is it normally fitted.. Why does it fail in service.Answer :- An expansion bellow is a device which allows free expansion of the exhaust manifold or piping when the pipe increases in temperature whilst the engine is on. It is made of thin heat resistant steel plating, but in course of time it fails by creep due to the frequent changes in the temperature. Creep causes loss of elasticity and the material fails .

(8) How does the labyrinth seal work and where is it fitted.

Answer:- The labyrinth seal is fitted in the T/C in the partition separating the air side from the exhaust side . The seal allows

the rotor shaft to pass through the partition without allowing the high pressure air to enter the exhaust side and this is achieved in the seal where mating fins alternately placed on the partition bush and the shaft bush retard the flow of the air causing it to lose its pressure energy, and thereby prvent flow.

Mr Kiran(9)Describe the procedure of the third survey ( 6th year) of the Aux boiler.

Answer:- As per classification regulations the auxiliary boiler and the Exhaust boiler or exhaust gas economiser are to be surveyed once every two years.

To carry out the survey the boiler has to be blown down completely according to the blowing down procedure. After blowing down and before commencing the cleaning operation the boiler must be safely isolated to prevent any accidents occurring and for this the steam outlet connection to the main steam line must be blanked using a strong spoon flange at the connection to the main steam manifold. Similarly the common feed line must be blanked to prevent accidental entry of feed water in the boiler under survey. These precautions are necessary to be observed in ships where there are two or three boilers connected to a common steam manifold.

Cleaning procedure:- The boiler’s fire side smoke side and water/steam side have to be thoroughly cleaned and made free of all deposits so that the bare metal surface is exposed. In water tube boilers rotary descaling heads mounted on a flexible shaft is very effectively used for descaling the internal surfaces of the water tubes where scaling occurs. In smoke tube boilers

the external surfaces are descaled manually. In small package boilers where entry into the boiler is not possible ,chemical cleaning or descaling is done by warming up the boiler with a very weak acid solution which loosens the scale and breaks it. The scale collected is flushed out.

All the 10 listed mountings are dismantled , overhauled and surveyed. The furnace lining is suitably repaired by renewing broken bricks ,the fire cement wherever cracked is suitably excavated and fire cement mortar mix made and filled up.

Classification of defects:- The types of defects encountered during a survey are classified as follows:

(a)Wastage (external)(b)Wastage(internal) (c)Overheating.(d)Grooving.(e)Caustic embrittlement.(f)Wastage caused by exhaust gas.

(a) External wastage: (1) Wastage of the shell plating around the mountings especially on the top of the boiler caused by water leakage falling and collecting around the mounting. The space around the mounting is made bare by the formation of a well by the thick boiler insulation and the steel cladding to protect the insulation. The leakage is due to loose glands or a leaking joint and the leaked water collects in the well causing air corrosion.

Repair :- The wasted shell plating around the mounting is suitably built up by welding using a compatible electrode, as recommended by the class surveyor. The welding has to be done by a class approved welder with the recommended preheat, and ground smooth. The corrective action suggested should be done when the residual thickness is not less than 75% of original thickness.

(2)Leakage from manhole door:- Leakage from manhole door on a continuous basis causes wastage of the plate flanging, which results in increased clearance between flange and sphigot resulting in the joint getting squeezed out and leakage increasing

Recommended repair:- The wasted portion of the flange is to be built up by welding and done by a class approved

welder using compatible electrodes and under recommendation of class surveyor. (3)Wastage of support stools.:- Water collecting in the coaming around the boiler and stagnating causes air corrosion of the boiler stool. This collection occurs when the funnel scupper is choked and the scupper overflows into the coaming space around the boiler, stagnating and causing corrosion.

Recommended repair:- The choked drain should be cleared and wasted portion of the stool should be suitably cropped and renewed. Since this location is beyond the pressure vessel there is no need for a class approved welder to carry out this repair.

(b)Internal wastage :- (4) Wasted and necked stays:- Necked and wasted stays which have wasted more than 25% of its cross section area are to be renewed completely and attaché in the same manner as they were originally attached, which means if they were screwed in and fitted with check nuts , the same method of fixture should be adopted for the

renewed stays. If they are welded to the plating, the renewed stays should be similarly welded.

(5)Holed and plugged tubes:- These should be renewed. In low pressure boilers the tubes are placed in position and expanded using a roller set. The material of the tube is mild steel seam welded with a carbon content of not more than 1% , and a ductility corresponding to 23 % to 26 % elongation during a tensile test. Each tube before fitting and expanding in position is to be subjected to a collapsing test. This test is done on the work spot before fitting the tube as follows:

The tube is placed in position , its length adjusted and the extra length about 20 to 30mm long is cut out. This piece is then collapsed in a vice till its diameter is reduced to half and becomes elliptical in shape .The elongated end surfaces are examined for crack formations.. If cracked , the tube should be rejected and another tube tried out.

(10)How would you carry out crank case inspection of the main engine and what clearances you will check.Answer:-The following precautions should be taken prior to carrying out a crank case examination of the main ngine.

At least 20 minutes should elapse after the engine is stopped before any attempt to enter the crank case.

The safety check list for entry should be carried out and the check list duly filled up.

the crank case must be thoroughly wiped and made free of smeared oil so that no one slips inside and injures himself..

Obtain propeller clearance from deck department till completion of check.The following checks and clearances are to be measured.

The clearances , using the appropriate or long feelers, of the top end bearings, the bottom enfd bearings and the main bearings must be taken.

When checking top end bearings the clearance is taken from the top when the crank is on TDC. When checking bottom end bearings, the clearance is taken on the bottom side with crank on BDC. Guide clearances are to be taken at top, mid stroke and

bottom. All holding bolts must be hammer tested for tightness and

the locking devices checked for their intact condition. The bottom end bearing must be checked by nipping the

bearing using a long crow bar to check for free movement of the bearing on the crank pin.

The Main bearing clearances are checked from the top side of the bearing with the webs turned to the starboard side.

The tie rod pinching screws must be checked for their tightness. Missing pinching screws must be fitted and suitably tightened.

The sweepings from the crank case are checked for metal particles and other impurities . This is necessary to investigate any undue wear from bearings and other components.

The cam shaft chain sag on the slack side is to be measured for record.