Q3) Is there any regulation about air compressors - time ;required to fill the Air Bottles?
Ans) Two starting compressors must be fitted, of sufficient total capacity to meet the engine requirements.
Each compressor must be able to press up Air receiver from 15 bars to 25 bars in 30 minutes.
Two air receivers must to be provided.
Total air receiver capacity is to be sufficient for Twelve (12) starts of Reversible engines and six (6) starts for non-reversible engines.
Q4) TYPES OF EVAPORATORS IN REFRIGERATION SYSTEM?
ANS) In the large refrigeration and air conditioning plants the evaporator is used for chilling the water. In such cases shell and tube type of heat exchangers are used as the evaporators. In such plants the evaporators or the chillers are classified as: 1) Dry expansion type of evaporators
2) Flooded type of the evaporators
In case of the dry expansion type of chillers or evaporators the expansion valve controls the flow of the refrigerant to the evaporators. The expansion valve allows the flow of the refrigerant depending on the refrigeration load. In case of the shell and tube type of evaporators the refrigerant flows along the tube side, while the substance to be chilled (usually water or brine) flows long the shell side. In case of the flooded the
evaporator is filled with the refrigerant and constant level of the refrigerant is maintained inside it. In these evaporators or the chillers the refrigerant is along shell side while the substance to be chilled or freezer flows along the tube side of the heat exchanger.
Though this classification is also applicable to the domestic refrigerators and the air conditioners, the evaporators used in these systems are classified based on their construction. The evaporators are classified based on the construction as:
1) Bare tube evaporators 2) Plate surface evaporators 3) Finned evaporators
The bare tube evaporators are the simple copper coil evaporators over which the substance to be cooled flows.
The plate surface evaporators are commonly used in the household refrigerators. These evaporators are also in the form of coil, which is attached to the plate.
The finned evaporators are also made of copper coil with fins on the external surface as well on the internal surface.
BARE TUBE
FIN
Q5) WHAT IS ERMATO JOINT?
ANS) It is a kind of coupling to absorb vibration, fitted on pipes like scavenge drain pipe, in tank’s steam heating coils.
Q6) EXPLAIN PROPELLER SHAFT WITH DIAGRAM?
ANS) The propeller shaft is bolted to the main engine flywheel, passing through the thrust block then along the shaft tunnel. Here it is supported by the shaft bearings before passing through the stern tube to drive the ship's propeller.
The shaft is manufactured from forged steel, complete with coupling flanges. It is machined leaving a larger diameter at the location of the shaft bearings; this section has to have a fine finish to run within the white metal bearing.
The shaft coupling flange faces are accurately machined and the bolt holes reamed to accept fitted bolts. They are bolted together using high tension bolting, which is tightened using hydraulic tensioning gear.
The supporting bearings are cast in two halves and are usually white metal lined. These have oil scrolls cut into them to distribute the splash lubrication. Nowadays ball bearing shaft supports are being used, but they have been reported as being quite noisy with a tendency to run hot.
A typical prop shaft white metal bearing with splash lubrication is shown here.
Propeller drop.
the propeller shaft in the after peak tank is provided with inboard and outboard seals.these seals contain nitrile rubber or viton lip seal which seals against the bronze liner shrunk fit around the cast iron propeller shaft.after a few years it creates grooves on them and naturally looses sealing and sea water can easily find its way inside.this reduces the lubrication effect and creates wear if the bronze liner.now as there is enough clearance the shaft will come down by certain amount because of the propeller weight.this drop in propeller shaft is termed as propeller drop and is measured by POKERS gauge.
Q7) EXPLAIN RUDDER CARRIER BEARING WITH DIAGRAM?
ANS) The rudder carrier bearing takes the weight of the rudder on a grease lubricated thrust face. The rudderstock is located by the journal, also grease lubricated. Support for the bearing is provided by a doublers plate and steel chock. Wedge type side chocks, welded to the deck stiffening, locate the base of the carrier bearing. The carrier is of meehanite with a gunmetal thrust ring and bush. Carrier bearing components are split as necessary for removal or replacement. Screw down lubricators is fitted, and the grease used for lubrication is of a water resistant type (calcium soap based with graphite).
Wear down
A small allowance is made for wear down, which must be periodically checked. This may be measured either between pads welded on top of the rudder and onto the rudder horn, or between the top of the rudder stock and a fixed mark on the inner structure of the steering gear flat. The latter generally involves the use of a 'Trammel gauge' which takes the form of a 'L' shaped rod made to fit the new condition of the gear. As wear down occurs it can easily be checked with this gauge. The rudder is prevented from jumping by rudder stops welded
onto the stern frame. Rudder movement stops
Rudder stops are arranged as follows; Angle from
centerline
Position of stop Note
35o On telemotor
system Normal limit
37o On steering gear Prevents rudder striking
external stops 39o External, on
stern frame Emergency stop to protect propeller These limits refer to rudders of traditional design and are governed by both the physical layout of the rudder and actuator but also due to the stall angles of the rudder. i.e. the angle at which lift ( turning moment ) is reduced or lost with increasing angle of attack. There are designs of rudder such as Becker flap which have increased stall angles up to 45o
Rudder wear down measurement: (Ram type Steering Gear ) At sea:
1)Jumping clearance or bouncing clearance, measured between swivel block and upper ram fork end. (limit is 19mm)
2)Wear down clearance, measured between swill block and bottom ram fork end. (limit is 12-19mm)
At docking:
1)Bouncing clearance: measured betwen top of ruddeR and jmpng bar. 2)Wear down clearance: beween the bottom of rudder and reference mark.
Read
more: http://wiki.answers.com/Q/How_is_the_rudder_drop_measured_in_a_ship#ixzz23nUkF 8Xr
Reasons for critical contouring of thrust face; I. For lubrication
ii. Conical in order to prevent sideslip and centralize rudder iii. Projected area gives greater bearing area allowing smaller diameter bearing
Rudder wear down refers to the measurements taken generally during a docking period to indicate excessive wear in the steering gear system particularly the rudder carrier. This wear down or rudder drop is measured using a special L shaped instrument called Tramel. When the vessel is built a distinct centre punch mark is placed onto the ruder stock and onto a suitable
location on the vessels structure, here given as a girder which is typical. The trammel is
manufactured to suit these marks As the carrier wears the upper pointer will fall below the centre punch mark by an amount equal to the wear down.
Rudder Clearance
Pads are welded to the hull and rudder. A clearance is given ( sometimes refered to as the jumping clearance). As the carrier wears this clearance will increase
Q8) WHAT ARE STABILIZERS? WHAT IS ITS PURPOSE? ON WHICH SHIPS THEY ARE REQUIRED MORE?
ANS) Ship stabilizers are fins mounted beneath the waterline and emerging laterally. In contemporary vessels, they may be gyroscopically controlled active fins, which have the capacity to change their angle of attack to counteract roll caused by wind or waves acting on the ship.
Location and diagram of retractable fin stabilizers on a ship.
Purpose
The purpose of cruise ship stabilizers is to reduce the rocking motion of the ship. They help a ship move more smoothly, which cuts down the chance of seasickness for passengers. When there is a great deal of movement, it can cause a discrepancy between what a person sees and what her inner ear senses. This is what causes
seasickness. The smoother the ride, the less chance for this to happen.
Function
Cruise ship stabilizers extend out below the water line on the port and starboard sides of the ship. They prevent it from rolling to the left and right as it moves through the water. They act much, as do airplane wing flaps, which can be adjusted to reduce turbulence. Although no stabilizers can prevent 100 percent of a cruise ship's movement, they can significantly reduce it. This is especially desirable in rough conditions when the waves are high or the wind is strong.
How Cruise Ship Stabilizers Work fin stabilizer
Q9) EXPLAIN FREEBOARD?
ANS) The distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship. In commercial vessels, the latter criteria measured relative to the Ship's load line, regardless of deck arrangements is the mandated and regulated meaning.
In yachts, a low freeboard is often found on racing boats, for weight reduction and therefore increased speed. A higher freeboard will give more room in the cabin, but will increase weight and may compromise speed. A higher freeboard also helps weather waves and reduces the likelihood of green seas on the weather deck. A low freeboard boat is susceptible to
swamping in rough seas. Freighter ships and warships use high-freeboard designs to increase internal volume, which also allows them to satisfy IMO damage stability regulations due to increased reserved buoyancy.
Graphical representation of the dimensions used to describe a ship. f is the freeboard
Q 10) WHAT IS SHEER?
curvature, in naval architecture.
The practice of building sheer into a ship dates back to the era of small sailing ships. These vessels were built with the decks curving upwards at the bow and stern in order to increase stability by preventing the ship from pitching up and down.
Q11) WHAT IS CAMBER?
ANS) The camber is a measure of lateral main deck curvature in naval architecture.
The practice of adding camber to a ship's deck originated in the era of small sailing ships. These vessels were built with the decks curving downwards at the sides in order to allow water that washed onto the deck to spill off.
Q12) WHAT IS TUMBLEHOME?
ANS)In ship designing, the tumblehome is the narrowing of a ship's hull with greater distance above the water line. Expressed more technically, it is present when the beam at the uppermost deck is less than the maximum beam of the vessel. A small amount of tumblehome is normal in many designs in order to allow any small projections at deck level to clear wharves (structure on the shore of a harbor where ships may dock to load and unload cargo or passengers)
Length overall (LOA) is the extreme length from one end to the other.
Length at the waterline (LWL) is the length from the forward most point of the waterline measured in profile to the stern-most point of the waterline.
Length Between Perpendiculars (LBP or LPP) is the length of the summer load waterline from the stern post to the point where it crosses the stem.
Beam or breadth (B) is the width of the hull. (ex: BWL is the maximum beam at the waterline)
Depth or moulded depth (D) is the vertical distance measured from the top of the keel to the underside of the upper deck at side.
Draft (d) or (T) is the vertical distance from the bottom of the hull to the waterline.
Q13) EXPLAIN MOULDED BREADTH, MOULDED DEPTH, AND DRAUGHT?
ANS)Breadth (extreme):
The extreme breadth, recorded in meters to two decimal places. This is the maximum breadth to the outside of the ship's structure.
Breadth (moulded):
The moulded breath, recorded in meters to two decimal places. This is the greatest breadth at amidships from heel of frame to heel of frame. This will only be displayed when breadth extreme is not available.
Moulded Depth:
The moulded depth, recorded in meters to two decimal places. This is the vertical distance at amidships from the top of the keel to the top of the upper deck beam at side.
Draught:
The draft (or draught) of a ship's hull is the vertical distance between the waterline and the bottom of the hull (keel), with the thickness of the hull included; in the case of not being included the draft outline would be obtained. Draft determines the minimum depth of water a ship or boat can safely navigate. The draft can also be used to determine the weight of the cargo on board by calculating the total displacement of water and then using Archimedes' principle. A table made by the shipyard shows the water displacement for each draft. The density of the water (salt or fresh) and the content of the ship's bunkers have to be taken into account. The closely related term "trim" is defined as the difference between the forward and after drafts.
Draft marks on a ship's bow
Q14) WHAT IS RECENT AMENDMENT TO SOLAS WITH RESPECT TO MSDS, LIFEBOAT & ETA?
ANS) MSDS: MATERIAL SAFETY DATA SHEET: DATE OF ENTRY IN FORCE: 01-JULY-2009
Amendment to SOLAS chapter 6, to add new regulation 5-1 on material safety data sheet (MSDS) to require ships carrying MARPOL Annex 1 cargo (oil) & also marine fuel oils to be provided with material safety data sheet prior to loading such cargoes. The regulation refers to the Recommendations for material safety data sheet (MSDS) for MARPOL Annex 1 cargoes & marine fuel oils, adopted by the organization through resolution MSC 150 (77)
Prevention of accidents involving lifeboats: -
An amendment to SOLAS regulation III concerns provisions for the launch of free-fall lifeboats during abandon-ship drills. The amendment will allow, during the abandon-ship drill, for the lifeboat to either be free-fall launched with only the required operating crew on board, or lowered into the water by means of the secondary means of launching without the operating crew on board, and then maneuvered in the water by the operating crew. The aim is to prevent accidents with lifeboats occurring during abandon-ship drills. The amendment is expected to enter into force on 1 July 2008.
Q15) WHAT ARE THE SAFETY FEATURES IN AIR COMPRESSORS?
ANS)Every Air compressor on a ship is fitted with several safety features to avoid abnormal and dangerous operational errors of the equipment. If safety, alarms and trips are not present on the air compressor, abnormal operation may lead to breakdown of the compressor and may also injure a person working on or around it.
1.Relief valve: Fitted after every stage to release excess pressure developed inside it. The setting of the lifting pressure increases after every ascending stage.
2.copper Bursting disc: A bursting disc is a copper disc provided at the airside of the compressor. It is a safety disc, which bursts when the pressure exceeds over the pre-determined value.
3.Fusible plug: Generally located on the discharge side of the compressor, it fuses if the air temperature is higher than the operational temperature. The fusible plug is made up of material, which melts at high temperature.
4.Lube Oil low-pressure alarm and trip: If the lube oil pressure goes lower than the normal, the alarm is sounded followed by a cut out trip signal to avoid damage to bearings and crank shaft.
5.Water high temperature trip: If the intercoolers are choked or the flow of water is less, then the air compressor will get over heated. To avoid this situation high water temperature trip is activated which cut offs the compressor.
6.Water no-flow trip:If the attached pump is not working or the flow of water inside the intercooler is not enough to cool the compressor then moving part inside the compressor will get seized due to overheating. A no flow trip is provided which continuously monitor the flow of water and trips the compressor when there is none.
running or starting is very high then there is a possibility of damage to the motor. An overload trip is thus fitted to avoid such situation.
8.High Air Temperature Trip
Q16) PROCEDURE FOR OVERHAUL OF A/E?
ANS)D'carb of auxiliary engine is nothing but the carrying out
of certain routines at intervals prescribed by the manufacturer
or experience. Normally the following should be done during a marine decarb to free the engine from anomalies
Every 3000hrs
1. take out cylinder head, take the worn out mountings and/or over haul the mountings
2.All units cylinder head, piston, connecting rod, and
3.turbocharger to be overhauled
4.Clean sump tank and fill with fresh lube oil
5.Take crank shaft deflection before and after removal of bearings
6.Whatever actions taken should be recorded in the maintenance record book
D'carb preparation:-
1.Make sure the all stand by auxiliary engines are ready 2.Keep all the special tools and other tools ready
and adjustments
4.Put the display card "MEN AT WORK", "DON'T START" 5.Close air bottle valve to auxiliary engine and engine start
and stop valve
6.See that the turning bar is not in the flywheel and should be in place
7.Open the indicator cocks
8.If the main bearing is to be removed, check crank shaft deflections
9.Close lube oil, fuel oil, fresh water inlet/outlet valve, drain the cooling water line and remove connections
A) Removal of cylinder head:-
Drain the jacket water and watch the expansion tank level, it should not go down, if it is that means the valves are not holding.
Scavenge manifold, exhaust manifold , rocker arm, lube oil
drain connection from rocker arm, rocker arm tank and cover connection to be removed
Fuel oil high pressure connection from fuel pump to the injector, fuel valve cooling connections in and out (either diesel or water) to be removed
Remove the rocker arm assembly and the push rod. Remove all the mountings such as starting valve, indicator cock, relief valve and exhaust valve assembly
Remove the rocker cover and check any marking on cylinder head nuts and studs. If no torque spanner is available, note down the markings.
Open the cylinder head nut with box spanner and extension rod. Never use the torque spanner. With box spanner available note down the marking.
Put the cylinder head lifting tool and before lifting make sure all the connections are removed. Also ensure that the liner is not removed along with the cylinder head Take out the copper joint between the head and the liner
CYLINDER HEAD BEFORE CLEANING
EXHAUST V/V BEFORE CLEANING
After lifting the head, check the liner surface for score marks, blow past etc. Crack remove the ridges or deposits if any on the top surface to avoid the lifting of liner along with the piston and breakage of piston rings while lifting piston
Open the crank case door and remove the bottom end bearing bolts after removing the lock arrangement and the remove the bolts
Remove the bottom half of the bottom end bearing
Bring the piston to TDC. Make sure the bolt holes on the piston top; lifting tool holes must be cleared from carbon deposits. Threads should also be checked and cleared
Put the piston lifting tools and tighten the bolts
Lift the piston and remove top shell of bottom end bearing Place the piston on the piston stand and cover the crankcase pin to avoid the foreign material damaging the crank pin.
PISTON WITH RINGS B4 CLEANINGPISTON WITHOUT RING B4 CLEANING
PISTON AFTER CLEANING CONNECTING ROD REMOVAL
Cleaning the carbon content on all the parts of engine:- Clean the piston rings, measure dimensions and keep them
in order
Clean the piston ring grooves thoroughly and measure the groove thickness at 3 different points
Check for the deposits on piston crown (Sulphur, carbon or thick vanadium deposits) and measure the dimensions Remove the gudgeon pin and clean the gudgeon lube oil
holes as well as the bush or small end bearing Check the bolts of connecting rod for any cracks
Every 20,000 hrs engine connecting rod bolt must be replaced
If new piston rings are going to be replaced, then there is no need for measurement
LINER B4 CLEANINGCOOLING WATER SIDE OF LINER (EXTERNAL VIEW)
COOLING WATER SPACE INSIDE ENGINE LINER AFTER HONING PROCESS
BOTTOM END BEARING BEFORE CLEANING BOTTOM END BEARING AFTER CLEANING
Assembly of the engine parts:-
First put the piston rings one by one and measure the butt clearance for all the rings
Then measure the axial clearance between piston rings & grooves
Place the piston guide on top of the liner and bring the particular crankshaft to TDC. Apply sufficient lube oil and start lowering the piston. Make sure that butt gap should not be in line it may cause blow past
Before engaging check the crankpin for any cracks or scratch
Check the bottom end bearing clearance and if needed measure the main bearing clearance as well
Taper clearance is checked
Check for any cracks in the water jacket and in the cylinder head
Replace all rubber joints and copper gasket to be put on the cylinder cover
Put the cylinder head gasket in the top of the cylinder
Anti-seizure coating or powder like molycote, copper slip should be used. It is applied to avoid any seizure mainly on the threads or joints and it will be easier while removal
Tighten the cylinder mounting according to torque specified as in manual and make all connection like lube oil, fuel, jacket cooling water connections etc
Fit the rocker arm back
DECARB IS DONE TO INCREASE THE EFFICIENCY OF ENGINE.
Q17) HOW WILL YOU DECIDE TO CHANGE THE PISTON RING?
ANS)
1. BY CHECKING THE BUTT CLEARANCE. IF ITS VALUE HAS BEEN INCREASED THAN THE NORMAL RANGE.
2. IF ITS AXIAL CLEARANCE HAS BEEN INCREASED THAN THE NORMAL RANGE.
Q18) WHAT ALL CHECKS TO BE DONE IN LIFTING GEAR? (E/R LIFTING CRANE)
ANS)
1. CHECK THE CONDITION OF WIRE ROPE & GREASE IT. 2. CHECK THE VISUAL CONDITION OF CHAIN.
3. CHECK THE LIMIT SWITCHES IN FORWARD, AFT, PORT & STBD DIRECTION ARE WORKING.
4. CHECK THE PROPER WORKING OF EMERGENCY BUTTON.
5. CHECK THE VISUAL CONDITION OF INSULATED COVER. 6. CHECK OVERLOAD TRIP WORKING SATISFACTORY.
7. CHECK VISUAL CONDITION OF CHAIN BLOCK, NO CRACKS SHOULD BE THERE.
8. CHECK THAT SAFETY LATCH IS THERE ON CHAIN BLOCK.
Wire rope, limit switches, chain, chain block, overloads trip, emergency button, and safety latch.
Q19) WHAT ALL CHECKS TO BE DONE ON PISTON?
ANS) Piston inspection on ships is part of the engine planned maintenance schedule (PMS) carried out to ensure the components is within the allowed tolerances. There are two methods of inspection: when the piston has been removed from the liner or inspection through the liner scavenges ports.
a)Piston Removed for Inspection:-
This examination will be under taken in a modular format, since the piston can be divided into various components.
Piston Crown
Check for any burning at top part of the piston.
Check any wear at the sidewalls of the crown and on ring grooves.
Check for any cracks at top due to the thermal and mechanical stress, check also for high temperature corrosion.
Check any signs of hot corrosion at the top surface and acidic corrosion at the lower part.
Piston Rings and Grooves
Check for the free movement of the piston rings. Check the ring clearance / groove clearance. Inspect for any wear, stepping and for scuffing. Piston Skirt and Side-wall
Check for any rubbing marks.
Inspect for any wear down of wear rings. Cooling Water Passage
Check for any scale due to poor water treatment. Choking due to high temperature.
Finally inspect the locking bolts; wires, studs and ‘O’ ring condition
b)Maintenance Schedule:-
Periodic inspection has to be done when the engine is not running. It can be carried out as above or by entering the scavenge space and inspecting the piston and piston rings through the scavenge ports, with the piston brought in line by rotating the engine via a turning gear.
Overhauling the piston as per Planned Maintenance Schedule (PMS).
Monitoring of the condition of the piston and the piston rings by the compression curve of the indicator diagram through process analysis.
The images shown below show examples of two means of inspection.
Piston removed for closer inspection
Emergency Repair of Piston Crown:-
Once the above checks have been carried out, what actions can be taken if some values or observations are out with the specifications? Given below is a list of common faults that might be found during inspection and means to make temporary emergency repairs.
Gauge piston crown and ascertain shape and wear-down. If it is beyond recommended limits, replace the piston if
there is a spare available. If not, rebuild the engine and proceed to the nearest port at reduced revolutions and arrange replacement. The crown head should not be welded except in a dire emergency- and even then only by an experienced welder. Remember that modern diesel engine pistons have a special lining of high temperature alloy on the top of the crown. This measure improves resistance to corrosion as well as to high combustion temperatures that the piston top is exposed to.
Examine the crown for fractures or cracks, and if found the piston should be changed. If no spare is available these can be welded to manufacturer’s specifications; using the correct alloy welding rods, again as a means to proceed to the nearest port at reduced revolutions for a replacement.
Dismantled piston rings should be kept in sequential order so as not to interchange the rings when re-fitting to the piston.
Once repairs are complete, replace the piston rings and check for normal butt clearance.
If the butt clearance is more or less than the normal range, then replace the piston rings with new set of piston rings.
Note: It would be an extraordinary predicament to be in where as a Chief Engineer you sailed without main engine piston spares. However, strange things happen at sea, maybe the spares have been already used, and you're awaiting delivery of replacements.
If any of the above repairs are carried out, it is imperative that a close watch is carried out on the appropriate cylinder with the exhaust temperatures closely monitored as well as the piston cooling medium temperatures.
Q23) WHAT IS PURPOSE OF TAPPET CLEARANCE & HOW IT IS DONE?
ANS) Tappet clearance is a space between the top of the valve stem and the rocker arm. Its purpose is to allow for some mechanical expansion and lengthening of the valve stem and push rods as the engine warms up. This clearance is also called valve lash.
If insufficient (lower clearance) valve lash is set when the engine is cold the valves will not properly close when the engine warms up or early opening of the valve.. If too much lash is provided (additional clearance) then even after the engine warms up there will be some clearance, which will result in lost motion. Lost motion mean that as the cam tries to open the valve the push rod and rocker arm moves to first take up the clearance before touching the valve to open the valve. The result is late opening of the valve.
When checking tappet clearance on marine engines, we have to ascertain that the piston is at TDC. Though markings are provided on the flywheel, the marine engineer must know the other methods for this like inspection of the camshaft and the fuel pump window.
During the maintenance of a four stroke marine diesel engine there are times when we must know whether the particular
unit’s piston is at the top dead center or not. For example when checking the tappet clearances of the engine it is important to know which unit is at TDC.
Referring to the flywheel would indicate two units, but only one can be at injection TDC. So which one is it?
Flywheel Method: -
The flywheel is the simplest method to know which unit is at TDC. If the flywheel shows two units, simply open the bonnet covers and checks visually. The unit at TDC will have both the inlet and the exhaust valve closed and hence relaxed springs; the other unit would have both the arms of the rocker arm at different levels. In addition the push rods of the unit at TDC would be loose and can be turned by hand because of the release of the clearances. There is a word of caution however: this method is only useful in a working generator, which you have just stopped to check the tappet clearances. In case you have removed the rocker arms for any reason the spring height and the push rod freeness check would lead you nowhere and misguide you.
Dial Gauge Method: -
In this method the fuel injector is taken out and from the opening a dial gauge is put inside. Then the turning gear is engaged and the engine turned over. The pointer of the dial gauge will move in one direction and then stop and start in opposite direction. The moment the pointer of the dial gauge stops and changes its direction of movement is the TDC of the unit. This method is not normally used in day-to-day practice, but may be used in the calibration of the flywheel if it is not calibrated, or after some repairs.
Camshaft Method: -
The camshaft window of the engine can be opened up and the camshaft inspected. The cam of the engine has a base circle, and acceleration and dwell periods. If the roller of the follower is at the base circle, then the particular valve is closed by spring action. When both the exhaust valve and the inlet valve follower are on the base circle, then the unit is also at TDC. It must be remembered that as a four-stroke engine has two rotations of the crankshaft there is one injection TDC where the injection and the combustion take place. The second time the piston is at TDC is when the exhausting of the flue gases takes place. It is very important to identify the combustion TDC, as tappets have to be adjusted at that point.
Cam Profile
Crankcase Method: -
In this method the crankcase doors are opened up and the piston is visually checked whether is going up or down. This is the surest method, but a bit cumbersome. It should be used when you have a strong doubt about the other methods.
Valve Spring Method: -
This is not an independent method but is used in conjunction with the flywheel method. In this method if the flywheel is indicating two units, you can check the springs of both the units. The unit in which the springs are loose is the one at TDC. The caution is that this method is useful for an engine in use. If you have removed the rocker arms during the overhaul and thereafter you want to use this method then it can cause errors.
Push Rod Method: -
This method is like the spring method and you check that the push rods are free to turn. The unit at TDC will have loose springs. The care that must be taken is that it should be used along with the flywheel method and should be used in a working engine. By a working engine, I mean the engine that was running and has been stopped for tappets adjustment. Loosen the lock nut of the rocker arm.
TAPPET ADJUSTMENT:
Now adjust the tappet clearance between the rocker arm & valve stem by tightening or losing the nut below the lock nut. If tappet clearance is less:
I. Valve will open early & close late
ii. Air induced through inlet valve may leak out. So less air for combustion.
iii. Power will be reduced.
iv. Fuel consumption will increase, engine may become unbalanced, exhaust temp. will be very high.
v. In worst condition, valve may remain open; resulting in loss of compression pressure, burning of exhaust valve, T/C fouling will increase.
If tappet clearance is more:
I. Valve will open late & close early.
ii. Lesser heat energy to T/C, so reduction in scavenge air & hence power.
iii. No proper removal of gases.
Q24) what to check if Engine is not starting on air and fuel? ANS) Engine not starting on Air: -
* Low air bottle pressure or airline valve may be shut.
* Air bottle isolating valve or automatic valve or distributor not functioning.
* Control air valves faulty or less control air pressure. * Starting air automatic valve jammed.
* Turning gear engaged.
* Reversing has not taken place completely.
* Control valve for fuel or start is not in its end position. * Bursting diaphragm(disc) on start airline damaged. * Fuel lever on maneuvering stand not on remote mode. * Auxiliary blower not running or not on ‘auto’ mode. * Emergency stop has activated.
* Interlock is operated.
*Cylinder air start valve defective or sticky. *Piston not in firing mode.
Engine not starting on fuel: - *Less fuel in service tank. * Fuel Oil filter is chocked.
* Fuel Oil supply pumps not delivering required pressure. Or fuel pump tripped.
* Puncture valve still active. *Fuel oil temperature to low.
* Fuel level on local maneuvering stand, is not on remote stand. * Fuel rack stuck.
* Fuel pump malfunctioning, jammed plunger. * Injector nozzle needle sticking or holes blocked.
* Compression pressure is too low due to broken piston ring or exhaust valve not closing properly.
* Fuel pump relief valve leaking.
Q25) WHAT TO CHECK IF ENGINE IS NOT COMING ONLOAD?
ANS) * CHECK VOLTAGE OF BUS BAR & INCOMING
GENERATOR, BOTH SHOULD BE SAME.
* CHECK FREQUENCY OF BUS BAR & INCOMING GENERATOR, BOTH SHOULD BE SAME.
* POWER FACTOR IS OK.
* SYNCHRONISING PROCEDURE SHOULD BE CORRECT.
* ALWAYS BEFORE PARALLELING INCOMING GENERATOR’S PARAMETER SHOULD BE IN OPERATIONAL RANGE.
Q26) WHY IN UMS CLASS SHIPS THE GENERATOR ENGINE IS STARTED AUTOMATICALLY WITHOUT OPENING INDICATOR COCK GIVING A TRIAL START?
ANS) IN ENGINE ROOMS, WHICH HAVE WATER MIST FIRE
FIGHTING SYSTEM INSTALLED, THIS PROCEDURE IS NOT FOLLOWED BECAUSE WHEN THE ENGINE IS GIVEN A MANUAL KICK WITH OPEN INDICATOR COCKS, SMALL AMOUNT OF SMOKE COMES OUT OF THE HEADS WHICHCAN LEAD TO FALSE FIRE ALARM, RESULTING IN RELEASE OF WATER MIST IN THE SPECIFIED AREA.
Q27) WHAT ALL TRIPS & ALARMS ARE PRESENT IN AUXILIARY ENGINES?
ANS)The various trips and alarms are mentioned as follows Alternator bearing low oil level alarm & trip
Alternator bearing high temperature lube oil alarm &trip Low sump oil level alarm and trip
Lube low oil pressure alarm and trip Reverse current trip
Over speed trip Over load trip
High and low frequency trip
Jacket cooling water low-pressure alarm
Q28) WHAT ALL PRECAUTIONS SHOULD BE TAKEN TO START AUXILIARY ENGINE AFTER OVERHAUL?
ANS)
*Check any tools, objects should not be left inside the c/c.
* Turn engine by turning rod through flywheel for checking any restrictions.
* Blow through the engine before starting.
* Air to be removed from jacket water/fuel oil outlet line. *Water tightness to be checked.
* Run priming lube oil pump before starting the engine. * Check the lube oil level.
* Check the flow of lube oil.
*Check the crankcase temp. & other running parameters of the engine, they should be within the permissible limits.
Q29) HOW TO CHANGE PURIFIER IN TO CLARIFIER? ANS)Main Differences:
-The main difference between a clarifier and a purifier is the presence of a dam ring (gravity disc) in the latter. In a purifier, the interface or the line of separation between the oil and water is created using a dam ring. The position of the dam ring plays an important role in the generation of interface and thus
in the clarifying process. For example, if the diameter of dam ring is large, the interface moves out towards the periphery and as a result some oil is discharged with water from the water outlet. Also, if the diameter is small, the interface formed will be more inwards and water will be discharged with the oil from the oil outlet.
The diameter of holes in the dam rings also plays an important role in the creation of interface and purification process. If the diameter of the holes is more, the interface is formed towards the periphery and oil globules are found with water and sludge. If the diameter is less the oil-water interface moves inwards and water is released with the clean oil discharged. However, clarifiers do not have a dam ring but have a sealing ring which seals the water outlet. This prevents the impurities and water to remain inside the bowl unless opening the cleansing bowl discharges them automatically or manually. Also, the conical discs in a clarifier usually don’t have feed holes in them but if they do, then a disc without any holes is fitted at the bottom of the stack.
Another difference between a clarifier and purifier is that a purifier needs to be filled completely with water for the generation of a seal that prevents the oil to leave from the water outlet. Whereas a clarifier doesn’t needs to be filled up with water. Purifiers are used for filtering lubricating oil whereas clarifiers are not used for the same unless the oil is completely devoid(free) of water.
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Purifiers and Clarifiers differ only in that clarifiers are not set up to remove water. Their design are similar to the point that most purifiers found on board can be converted to use as a clarifier with simple alteration of the gravity disc
Q30) HOW TO SELECT DAMN RING FOR PURIFIER?
ANS)From the nomogram provided with manual, which is drawn with respect to viscosity of oil & which size damn ring to be used.
If nomogram not there, then
a. Chief Engineers experience will come into use. b. Hit & trial method to be used.
* First use the largest gravity disc and whether oil is overflowing, if so, and then use small size gravity disc and follow this process until oil stops overflowing.
Choosing Gravity Disc
The graph shown above is one typical of one found in a purifier instruction book for selecting appropriate gravity disc size. Shown on the diagram is an example of an oil of sg 0.93 at 0'C. The sg at 15'C for use with this graph is found by projecting along a horizontal line to 15'C. This step would be omitted if the sg at 15'C were already known. A line is then drawn parallel to the pre-drawn sloping lines. Where the drawn
sloping line cuts the appropriate oil supply temperature isothermal then this becomes the selection point for the disc. This is found simply by ascertaining which size band the point lies in.
Q31) WHAT TO CHECK IF PURIFIER IS OVERFLOWING? ANS)
* Size of gravity disc. * High throughput.
* Temperature of the oil.
* Operating water level in tank.
* Sealing water is not present in purifier. * Bowl is not closed properly.
* Seal ring is damaged.
* By mistake if bowl opening water is feeded.
* Increasing the specific gravity of the oil will tend to push the interface outlet and cause overflow from the heavy phase outlet until the equilibrium is restored.
Q32) How to stop Aux Engine if not stopping by stop handle?
ANS)
a. Pull the fuel rack to zero position. b. Operate any trip.
Q33) WHAT ARE ALL TRIPS & ALARMS ARE PRESENT IN PURIFIER?
ANS) Typical alarms and shut downs: -
The following gives a general list of alarms only some of which may be fitted.
o Back Pressure shutdown- this measures the
discharge oil pressure and alarms and initiates a shut down when below a set value.
o Heavy phase overflow. Oil has a much higher
viscosity than water. The heavy phase outlet is led to a small catchment tank contain a float. The outlet from the tank is restricted in such a way that water flows freely but oil tends to back up. This initiates an alarm and shut down
o Bowl not open- this may be done in several ways,
typically by a lever switch operated by the discharged sludge hitting a striker plate. The other method is by measuring the motor current, when the bowl opens the bowl speed is dragged down due to friction effects of the discharging sludge and water. The motor current rises until full speed is re-established. This is detected by a current sensing relay
o Water in oil- This found on modern designs which
have a detection probe mounted in the oil discharge
o High temperature alarm and shut down
o Low control/seal water pressure. Where control
water is supplied via a fixed small header tanks a float switch may be fitted.
Q34) HOW TO CHARGE THE GAS IN REFRIGERATION SYSTEM?
ANS) READ from notes.
After leak test , evacuation, drying out.
*make sure vaccum exists &all stop valve in circuit are open. *weigh bottle, check for proper refrigerant, connect charging line & purge. Close the receiver outlet valve and collect the gas in the receiver.
* Open bottle stop valve open charging v/v slowly.
* Once all gas is collected in receiver then shut the compressor suction valve.
Check the liquid level; if it is below L/3, Charging is reqd. Check the weight of the refrigerant bottle & keep it
upright.
Connect the charging line to the connecting point and
keep it loose.
Open the bottle valve slightly and purge the line into the
collecting cylinder and then tighten the connection.
Open the charging valve and fully open the bottle valve. Check the liquid level in the sight glass and make sure no
air bubble present in the system.
Close the charging valve and the bottle valve.
Open the receiver outlet valve & start the compressor. Carry out leak detection test.
Check the suction pressure & discharge pressure.
Q35) WHAT IS PROPERTY OF IDEAL REFRIGERANT? ANS) Required Properties of Ideal Refrigerant:
1) The refrigerant should have low boiling point and low freezing point.
2) It must have low specific heat and high latent heat. Because high specific heat decreases the refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the refrigerating effect per kg of refrigerant.
3) The pressures required to be maintained in the evaporator and condenser should be low enough to reduce the material cost and must be positive to avoid leakage of air into the system.
4) It must have high critical pressure and temperature to avoid large power requirements.
5) It should have low specific volume to reduce the size of the compressor.
6) It must have high thermal conductivity to reduce the area of heat transfer in evaporator and condenser.
non-corrosive.
8) It should not have any bad effects on the stored material or food, when any leak develops in the system.
9) It must have high miscibility with lubricating oil and it should not have reacting properly with lubricating oil in the temperature range of the system.
10) It should give high COP in the working temperature range. This is necessary to reduce the running cost of the system. 11) It must be readily available and it must be cheap also. Important Refrigerants:
Properties at -150C
(1) Ammonia (NH3)(R-717) Latent heat = 1312.75 kJ/Kg Specific volume = 0.509 m3/kg
(2) Dichloro–Difluoro methane (Freon–12) (R-12) [C Cl2 F2] Latent heat = 162 kJ/Kg
Specific volume = 0.093 m3/kg
(3) Difluoro monochloro methane – or Freon-22 (R-22) [CH Cl F2]
Latent heat = 131 kJ/Kg
Specific Volume = 0.15 m3/kg.
36) EXPLAIN THE PROPERTY OF LUBRICANT USED IN REFRIGERATION SYSTEM?
ANS) For satisfactory performance, all refrigeration lubricants – mineral oil or synthetic – must be compatible with the refrigerant in the system and have the following requirements: 1. Good immiscibility and insolubility to assist in good oil return to the compressor, where it belongs.
2. Chemical stability to resist chemical reaction with the refrigerant or other materials present in the system.
3. Thermal stability to eliminate excess deposits at compressor hot spots.
4. Low wax content to prevent separation of flocculent wax from the oil mixture at the low temperature points in the system.
5. Low pour point to prevent separated lubricant from congealing and restricting flow.
6. Proper viscosity, even when diluted with refrigerant, to ensure high film strength at elevated operating temperatures and still provide good fluidity under coldest operating conditions.
8. No contamination to prevent scarring of bearing surfaces, plugging of lines or oil ports and general deterioration.
Some major compressor manufacturers prefer alkyl benzene refrigeration oil for some applications with HCFC refrigerant blends such as R-22, R-123 and R-401A. However, alkyl benzene refrigeration oil with the proper viscosity can be used with most CFC and HCFC refrigerants as well as hydrocarbons and ammonia in most refrigeration and air-conditioning applications.
The benefits of high-quality alkyl benzene lubricants are high miscibility, low foaming, excellent thermal stability, very low flock points and good compatibility:
1. High miscibility: Miscibility is the ability of the refrigerant and lubricant to stay together as one homogeneous solution.
Alkyl benzene has excellent miscibility with CFC and HCFC refrigerants, resulting in the oil and refrigerant remaining as one mixture at a wide range of temperatures and pressures. 2. Low foaming: The low foaming quality of alkyl benzene reduces carryover at compressor startup and subsequent oil loss from the crankcase.
3. Excellent thermal stability: Alkyl benzene can enhance the life of refrigeration systems by providing better thermal stability in the presence of CFC and HCFC refrigerants. It resists change under high temperatures, reducing problems with sludge, acids and copper plating.
4. Very low flock points: The flock point is the highest temperature at which wax-like materials precipitate from the oil in the refrigeration system. Because alkyl benzene is a synthetic lubricant, it contains little or no paraffin or wax, which can plug up parts of a system. This can be very desirable in low-temperature applications.
5. Good compatibility: Alkyl benzene can be blended with mineral oil of the same viscosity. It will not affect motor insulation and is compatible with most elastomers and additives often used to improve lubricity.
Preventing contamination problems is extremely critical in the refining and handling of all refrigeration oils. Great care must be used to assure that refrigeration oil is free of moisture and other contaminants. Service technicians must ensure that oil remains clean and dry.
Q37) EXPLAIN THE PROCEDURE OF CHARGING THE OIL IN TO REFRIGERATION PLANT?
ANS) Mostly ships have hand p/p provided which develop more pressure than the inside pressure
Q38) WHAT DO WE CHECK IF TEMPERATURE OF ANY ONE ROOM IS NOT COMING DOWN?
ANS) 1. IF ROOM DOOR IS NOT CLOSED PROPERLY. 2. PARTICULAR ROOM’S INSULATION IS BAD.
3. PARTICULAR ROOM’S FAN IS NOT RUNNING. 4. EVAPORATOR OF THAT ROOM IS FROSTED.
5. EXPANSION VALVE FOR THAT ROOM IS BLOCKED. 6. SOLENOID IS NOT WORKING FOR THAT ROOM.
Q39) WHAT ALL THINGS TO BE CHECK IF ALL ROOM’S TEMPERATURE IS NOT COMING DOWN?
ANS) 1. COMPRESSOR IS NOT RUNNING WELL.
2. PRESENCE OF MOISTURE IN SYSTEM & DRIER IS NOT WORKING PROPERLY DUE TO THIS EXPANSION VALVE OF ALL ROOMS ARE GETTING BLOCKED.
3. LESS REFRIGERANT IN SYSTEM.
Q40) WHAT TO DO IF DOMESTIC REFRIGERATION PLANT IS SHORT CYCLING?
ANS)REASONS: -
* L.P Cut out is defective.
* L.P Cut out setting not correct, too low difficult for Cut In. * Lesser gas flow
* Less gas in system. * Drier Choked.
* Expansion valve filter choked or Expansion valve Malfunction.
* Evaporator Choked.
* Compressor valves leaking. Actions: -
a. Check L.P. cut out setting, cut out pressure OK.
b. Check flow of gas by seeing sight glass, which should show full flow of refrigerant.
c. If no full flow- either less gas or drier chocked, change the drier.
d. Check level in receiver, if low, then charges gas. e. Expansion valve filter choked, then clean it. f. Expansion valve malfunctioning- Change it.
g. Evaporator choked- Blow-thru evaporator with nitrogen.
Q41) HOW WILL YOU OVERHAUL A CENTRIFUGAL PUMP? ANS) Centrifugal pumps have been used in industry for a hundred and fifty years or more. They are used to convert the energy from the pump driver to kinetic and potential energy into the fluid, via the impeller. They are used onboard ships to circulate seawater and freshwater cooling for the main engine.
A ship's engine room contains several different types of pumps
including centrifugal pumps.
Removal of Pump for Inspection and Maintenance: -
1 Isolate pumps electrical circuit breaker on main switch board and attach a warning notice. (Do Not Operate-Men at Work). 2. Switch off and lock pump supply at its local supply panel. Attach a warning notice to pump local supply panel.
3. Close suction and discharge valves, chain and lock hand wheels.
4. Open pump suction and discharge pipe drain valves to bilge and when water ceases to flow; crack open the pipes / pump flange joints carefully to ensure that pump has drained off and is safe for opening.
5. Fix a shackle to lifting pad eye above pump and hang chain block; ensuring SWL of block, slings and shackles are satisfactory.
6. Use a center punch to match/mark coupling and casing, then remove the coupling bolts.
7. Disconnect, fix i/d tag and remove motor supply cables; taping over bare ends with insulating tape.
8. Connect shackle and sling to motor eyebolt and lift motor clear of pump using overhead chain block. Lay motor on its side out of harm’s way, protecting machined surfaces on both pump and motor coupling halves against damage. (Cardboard and masking tape is quick and efficient method.)
9. Disconnect all external fittings from pump casing e.g. cooling pipe, pressure gauge, oil reservoirs and air cock.
10. Remove bolting from top cover and remove cover. Scrape off old gasket and check mating surfaces, and renew gasket on assembly. (Light smear of grease on gasket / faces)
11. The pump shaft with impeller can be lifted out of casing. 12. Dismantle the impeller, and remove the wear ring.
13. Remove the gland packing and disregard; replacing it on rebuild. Remember to cut ends of packing at 45° and stagger joints when repacking gland.
Inspection Procedure for Pump and Motor: - Pump: -
1. Impeller, pump shaft and internal volute/casing can now be inspected for erosion, pitting and wear.
2. If required rectify pitting or erosion in the impeller and casing with two-part alloy epoxy putty. (See my article in the Reference section)
3. Check main drive shaft bearings and thrust bearings for wear and replace if required.
4. Check wear ring clearance using feeler gauges; in my day at sea it was general practice is to replace with new rings at major overhaul.
5. Check impeller / shaft key and keyways for damage and undue wear, Unscrew impeller shaft securing nut and check threads are in satisfactory condition; retighten to manufacturers torque settings.
6. Give all parts a good clean removing any dirt/ medium residue before re- assembly using new parts as required.
7. Enter date of overhaul and parts renewed in the pump maintenance record card.
Drive Motor
1. Grip motor drive shaft /coupling firmly and check for excess axial and longitudinal movement. Rotate shaft at speed by hand, allowing it to run to a stop whilst listening for excess noise from bearings. Any doubt on either count, the bearings should be replaced.
2. Megger check motor windings to ensure no dampness is present and windings are in good condition. Any suspect readings indicate a full motor strip to check condition of rotor and stator.
3. If these checks are satisfactory, grease bearings as required. Some bearings are now sealed for life and will not require greasing.
Procedure to Start the Pump: -
1. Unlock and remove chains from inlet/outlet valve wheels and open both valves full.
2. Open air cock and expel air from line and pump while checking for any leaks
3. Turn the shaft coupling and ensure shaft is free to rotate. 4. Reconnect motor.
5. Remove danger notices from pump power supplies and reinstate breakers.
6. Start and record current drawn by the motor under starting and running conditions. Check and record the discharge pressure.
Q42) WHAT IS PURPOSE OF BILGE INJECTION VALVE?
ANS) we have been talking about various types of emergency
situations on board a ship. Needless to say some of the most
dangerous situations arise not due to grounding or collision of ships (though they are risky too) but mainly could be due to those situation, which either involve a fire or flooding.
Both these types of emergencies (fire and flooding) involve the use/role of seawater. If there is a fire, seawater is the biggest resource of water available in the sea. Similarly if it involves
flooding of the engine room, cargo spaces or any other place on
the ship for that matter; you would again require pumping the seawater out of the ship. In both these cases you require pumps.
We have studied a lot about seawater pumps, marine bilge
pumps and piping arrangement on ships including various
types of valves.
So as you must have noticed, there are two valves in close proximity namely main injection valve and bilge injection valve. Both of them have their own independent controls. The diameter of the bilge injection valve is kept nearly 66% of the main valve diameter, which draws water directly from the sea through the grid. This is a legal requirement that the diameter of this injection valve is at least 2/3 times the main suction, though it can be more also.
Hence the injection valve is an arrangement where the main sea chest can be bypassed in case of emergency so that instead of the sea, water gets drawn from within the ship itself.
There is a strainer attached to the bilge injection valve and the pump used for this valve is normally the largest seawater pump (or pumps) available in the engine room. Hence this valve is used to suck seawater from one of the lowest points in
the engine room, which you can also see from the sketch. This basically means that when you need to remove a lot of water from the ship, you simply need to open this valve and run the big pump/s.
REFERENCE:
http://www.brighthub.com/engineering/marine/articles/485 81.aspx#
Checks and Precautions: -
Emergency situation can arise anytime (that’s why is called emergency) so it would not be a good idea to find out that your valve is stuck due to rust or non-operation. Hence it is a good practice to check for the operation as a matter of routine.
The space near the injection valves should be kept clear of all obstacles since normally one would rush to open the valve in an actual emergency, and hence should be minimal obstacles in the space around the valve.
Not only should the valve be easily approachable and operational, but it also needs to be checked regularly for actual suction and operation. This can be done occasionally by actually running the pump and trying to draw out water from the bilge spaces uses this valve.
The valves should be clearly marked since more often than not, people do get confused in emergency situations and you certainly don’t want to be opening some wrong valve at such a critical time
Q43) BRIDGE INFORMS LOT OF SMOKE COMING FROM FUNNEL. WHAT ALL THINGS WE SHOULD DO?
ANS)
Reduce load on engine.
Check purifier operating alright/ reduce throughput to have better purification.
Drain water from settling & service tank.
Check scavenge air temperature & adjust if required. Check the boiler uptake temperature.
Soot blow the economizer.
Ensure, fuel oil end heater outlet temperature proper corresponding to attain viscosity at the point of injection. Check, if any particular Exhaust temperature is higher
than others, if so, then stop the engine, Change the injector with a spare overhauled injector.
Check all fuel pump timings are correct or not. Dismantle and carry out overhaul of T/C.
Reasons: -
Improper combustion.
Burning of carbon particles collected at EGE. Boiler uptake fire.
Overloading of engine.
Lots of smoke is also seen in scavenge fire. Exhaust valve is defective.
Fuel valve is defective.
Purifier not working efficiently.
Fuel oil quality is bad or water in fuel.
Q44) FLOODING IN ENGINE ROOM, WHAT WILL BE YOUR ACTION?
ANS)
Inform bridge & Chief engineer.
Raise engineer’s call/emergency alarm.
Before starting bilge pump note down the position of vessel & time of starting.
Other engineers will in between try to locate the hole or burst of pipe and repair.
If ingress of water very high, start another pump. Reduce the engine r.p.m.
Change over main seawater suction to emergency bilge suction.
If level is still coming up try to protect the motor from short-circuiting,
If situation is not coming in control, prepare lifeboat for lowering.
Q45) HOW WILL YOU TEST & OVERHAUL THE DEFECTIVE FUEL INJECTOR?
ANS)Safety Precautions: -
* Check whether all tools and spares are available or not. * If so, then start the Stand by generator.
* Check all parameters are normal.
* Now share the load with the help of synchroscope.
* Again check all the parameters are within normal range. * Put full load on the Stand by generator.
* Stop the generator on which work has to be carried out. * Put MEN AT WORK tag.
* Shut the air-starting valve, fuel oil inlet & outlet valves and isolates the system.
* Let lube oil-priming pump run for half hour after then stop it. * Remove the lock nut of the high-pressure pipe.
* Now, remove the high-pressure pipe. * Take out the fuel injector using it tool. * Put it on the testing kit.
* Check the lifting pressure, atomization, pressure falling steadily, and dripping of oil.
* Now, take out the injector from the testing kit, put in diesel oil & clean it.
* Make sure the workshop table should be clean, no rags or jute to be there.
* Put the injector on the vice and tighten it. * Loosen the lock nut of the injector.
* Now loosen the compression nut to release the spring pressure, and then take out the spring.
* Open the cap nut and take out the needle and guide. * Put the parts on the cleaned table.
* Check the condition of spring by dropping on the floor plate, it should jump and also check it by tightening in the vice and then releasing. The difference in the length, no cracks to be there.
* Check visually needle, there shouldn’t be any scoring marks because it is made of Nitrite material.
* Try to insert the needle inside the guide; the needle should go on its own weight.
* Check the size of injecting holes by using Go or No go gauge. * If go gauge is going then hole size is OK.
* If no go gauge going, then it means the size has increased, then nozzle needs to be changed.
* Now assemble the injector and do the lifting pressure setting on test kit by adjusting the compression nut.
* After this check the injector again for its lifting pressure, atomization, steady fall of pressure and dripping.
Q46) WHAT IS BUMPING CLEARENCE IN AIR COMPRESSOR, HOW TO MEASURE IT & HOW TO ADJUST IT?
ANS) The adjustment of Bumping Clearance is a very critical adjustment of the clearance volume. If more the volumetric efficiency of the compressor suffers and if less the unloaded piston may hit the cylinder head and damage both. In this article we discuss the need of this clearance and its adjustment.
What is Bumping Clearance?
Bumping clearance as the name signifies is a clearance given so that the piston of the marine reciprocating compressor would not bump into its cylinder head. In new compressors the manufacturers adjust this clearance and the marine engineers are blissfully unaware of its importance. However the ship does not remain new forever and every machine demands overhauling and that is where the problems start. Even routine jobs like lifting the cylinder head to change the low pressure or
first stage valves can change the bumping clearance if the correct thickness gaskets are not used or if the head is over tightened thus squeezing out the gaskets. Many engineers miss this vital adjustment during overhaul of the compressors and efficiency and free air delivery of the compressor suffers.
Bumping Clearance Changes over Time
The bumping clearance in a new machine is set properly by the manufacturers during construction but over a period of time the clearance changes because of the following reasons:
Wear at the crankpin bearing. The crankpin bearing wears down due to use and this clearance can travel right up to the piston and an unloaded piston can hit the cylinder head. This type of wear can be recognized when the compressor makes impact sounds running unloaded at the starting and stopping operations. This type of wear would also be accompanied by a slow decrease in oil pressure over a period of time.
Opening up of cylinder heads. In certain types of reciprocating compressors the cylinder head have to be removed for the changing of the first stage suction and discharge valves. When the cylinder head is put back the correct thickness of the cylinder head gaskets should be used otherwise it would change the bumping clearance. Wear on the main bearings. Over all wear on the main
bearings would lower the crankshaft and would thus lower the piston and increase the bumping clearances.
The bumping clearance must be adjusted properly otherwise there is risk of damage and loss of efficiency. If the bumping clearance were less the volumetric efficiency would increase but there is risk of the piston hitting the cylinder head, especially when the compressor is unloaded during start and stopping.
On the other hand to play safe, the engineer gives few millimeters of extra clearance, the volumetric efficiency of the compressor would decrease, the free air delivery will fall and there will be a fall in pressure. The extra clearance would result in a small volume of air being re-expanded every time causing increase in air temperature, fall in efficiency and overheating of the compressors. This would endanger the ship during maneuvering by sudden loss of propulsion.
How to Check Bumping Clearance: -
The bumping clearance can be checked by the following methods:
In case a suitable opening is available the piston can be barred to the top dead centre and then feeler gauges can be put inside and the clearances checked at two three points.
The more convenient method is to take lead wire from the engine store and make a small ball based on the expected clearance and put it between the piston and the head from the valve opening. Then the piston is slowly turned to the top dead centre with the help of a Tommy bar. After that the piston is again turned down