The marine diesel engine or ships main engine is the heart of the ship; responsible for its movement from place A to B. With all major loading and discharge schedule made in advance; any critical damage lead to huge loss of money and time.
With main engine accounting more than 30% of the total machinery cost in the engine room. It become more important to keep it in shape from monetary point of view; rather than satisfying ISM, IMO, flag and port state regulations. On board ship it is one of the work of the duty engineer; to perform inspection and maintenance of the main engine.
But against our common belief to overhaul complete unit or engine at a time; main engine parts and working parameters of different units are replaced and inspected based on their running time. They can be from daily inspection to the one needed after 8000 hours of operation.
Before we proceed and look into these inspection, checks and maintenance work. I would like you to revise the basic concept of an engine like the key parts of marine diesel engine and their function.
Main Engine Checks And Inspection Based On Running Hour’s
The Engine room management system look after the needs for timely preventive maintenance of the engine component; based on their running hours and past engine performance data. The program is installed on a dedicated system and get recent critical data such as past repairs, running hours, engine parameters, repairs overdue / in process / completed.
Although final maintenance schedule depends upon a number of factors such as, running hours, spares availability, past repairs, sudden irregularity in engine parameter, past alarms, ships position etc. The main factor which decide whether its needed to check or inspect a part of the system; depends solely on running hours.
Based on running ours all major checks and inspection are divided into six main types. These are daily checks and inspection, inspection and repair after 250hr, 1000 hr of operation, 2000, 4000 and 8000 hours of operation.
1 ) Daily Checks And Inspection Carried On Ships Main Engine
One of the best way to optimize engine maintenance is to take call of the running engine parameters. It is the duty of the watch keeping officer to take note of engine parameters. For ex: sump lube oil level, fuel oil level in settling tank, lube oil pressure; lube oil temperature and jacket water temperature.
Furthermore they also inspects lubrication of fuel rack assembly, lubrication of rocker arm ( if applicable ); air filters and exhaust gas temperature. Maintaining proper temperature of inlet and outlet jacket water temperature is important as; it ensures long life of the engine, reduce thermal stress and lower exhaust emissions.
Similarly keeping and eye on lub oil temperature and pressure helps detect any fault in the lube oil system. Having a functional lube oil system with good quality oil ensures the durability of the ships main engine.
2 ) Checks Carried Out On Engine After 250 Hrs Of Operation
After every 250 hours of ships engine operation the lube and fuel oil filters are inspected and cleaned. Furthermore the air filters of turbocharger inlet are also cleaned and inspected. The lube oil acts both as a coolant and lubricating medium in the ship’s main engine. The oil acts in between the two contact surface forming a protective oil film; that reduce friction and protect against wear and tear.
To ensure proper lubricating property it is required to have lube oil free of any abrasive particles. Thus filters are installed on both inlet and outlet side of the lube oil pump. These filters protect against the carryover of solid particles to the engine leading to abrasive wear. Thus it is required to inspect and clean these filters before they are fully clogged.
Much like that of the lube oil system; fuel oil filters are installed after the settling tank and viscosity regulator. These duplex and fine mesh filters are required to replaced by the one on standby and cleaned after each 250 hours of running operation. Similarly cleaning of air filters at the turbocharger inlet helps avoid surging of turbocharger.
3 ) Checks & Inspection Carried Out After 1000 Hrs Of Operation
After running of the engine for more than 1000 hrs its kind of mandatory to perform; crankcase inspection, check and maintain turbocharger oil level and inspect and maintain tappet clearance. The other key thing we do as an marine engineer after 1000 hrs of operation is to remove and dismantle fuel injector.
The fuel injector is dismantled, cleaned, pressure tested and installed again. In case there is slight deviation from the injection pressure we adjust the pressure to required pressure. In case of a four stroke engine we are also required to measure and maintain proper tappet clearance; between the the valve stem and rocker arm.
These tappet clearance allows for positive valve operation at all times; providing sufficient space for thermal expansion. A valve that close early or too late will have negative consequences on the engine. Thus adjusting tappet clearance after 1000 hrs of operation helps maintain normal operation of marine diesel engine.
Similarly crankcase inspection is done to ensure the condition of bearings; journals, grating and crosshead for signs of cracks or damage. Furthermore crankcase is looked for signs of bacterial contamination, metal particles or any foul smell.
A ) Maintenance And Checks On Fuel Injector
The fuel injectors which ideally run on heavy oil are taken out of the engine and tested with diesel oil; when its stopped while its still hot. Caring checks right after the engine stops ensure the injector shows correct performance. Using the same test while its cold would have flushed the system.
In majority of case we only need to check for the spray pattern; but in some we also need to verify the injection pressure. In case the valves show optimum test results; they should just be cleaned from outside and put back to the system. But in case there is any abnormality we need to dismantle, inspect and repair the component as per the manufacturer instruction.
We then clean the needle with fresh diesel oil and check for its freeness within its needle guide. The needle should go through the entire length under its own weight; falling freely pushing oil out through the small clearance. Similarly we inspect and check packing, nozzle spring and the seating in between valve and nozzle body.
B ) Things To Look in Ships Main Engine Crankcase Inspection
Crankcase inspection is a lot off work that it needs proper clearance and paperwork before proceeding. Proper immobilization certificate is to be taken and engines to be stooped. Placards to be placed at designated place with “Men At Work” in writing. To ensure safety the main starting air valved to be shut and tagged, stop associated pumps, open indicator cocks and engage turning gear.
In most ship it is also required to fill enclosed space checklist prior to taking the enclosed space permit. Once the crankcase door is opened and ventilated for quite some time; its required to get in and inspect a few things.
Look for any foul smell or sticky wax like substance on the surface to find the microbial degradation of lube oil. Checks the cross head, bearings, guides, gratings, pipelines and connecting rod for signs of wear, cracks or damage. Also check for the possible oil leaks from stuffing box area.
Check for the overall quality of the sump oil looking for signs of metal particles and discoloration. At last look for any loose connection of pipes and signs of insufficient lubrication; in the form of dark spots.
C ) Maintenance And Checks On Rocker Arm ( Tappet Clearance )
A rocker arm is an oscillating lever fitted in between the push rod and the yoke of the inlet and exhaust valves. To compensate for deviation in rocker arm position w.r.t rotation angle due to thermal expansion; a clearance is given in between yoke and rocker arm. It is the one among many duties of the watch keeping officer; to take measurement of tappet clearance.
The measurement is taken with the help of filler gauge; by trying passing each sheet in between the gap of valve steam and rocker arm. It need a much steady hand and a bit of experience; to find the exact size of metal that first pass through without much resistance. To take measurement the particular unit must be at the TDC ( Engine stopped ) and cylinder cover opened to access rocker arm.
Any adjustment in these clearance will result in late or early operation of the inlet and exit valves. You can go through one of my old post named “Tappet Clearance – Requirement, Measurement & Adjustments“; if want to learn in details its significance and how to take and adjust this clearance.
4 ) Checks Done After 2000 Hours Of Ships Engine Operation
Only three major checks are done prior to the 2000 hours of engine operation; crankshaft deflection, stuffing box overhaul and scavenge space inspection. While crankshaft deflection test helps us check the alignment of the shaft and so does the engine. It is role of the engine crew / officer to check and maintain crankshaft alignment close to its initial condition.
Similarly overhauling stuffing box required a skilled set of hand and quite some time; otherwise you have to lift complete piston out of the cylinder. After 2000 hrs of operation the rings of the stuffing box starts to stick and deteriorate. Moreover the springs which holds it tight on to the piston rod deteriorate too.
Such deterioration in spring tension with damage or sticking rings leads to poor performance of the stuffing box. If not fixed this will results into the contamination of scavenge space and crankcase oil; or loss of scavenge oil all together over time.
On another hand inspecting the scavenge space allows to inspect for the condition of cylinder liner; piston ring and piston itself without removing all of them together.
A ) Maintenance And Overhaul of The Stuffing Box
In two stroke marine diesel engine the crankcase is separated from the cylinder space using the diaphragm plate. The stuffing box is then installed to completely isolate each other with set of rings; tightly held together preventing oil and gas to escape to the other side.
It prevent the accumulation or ingress of crankcase oil in scavenge space; prevent dirty cylinder oil being carried downward and leak of scavenge air to the crankcase.
Basically it scrap off the oil on either side of the piston rod preventing it to pass on to another side. The stuffing box divided into two main rings the upper one responsible for preventing cylinder oil and air; while the lower one responsible to prevent crankcase oil from moving upward. These two set of rings are fitted together with a tell tell space in between the upper and lower rings.
These tell tell holes can latter be used to check the condition of the upper and lower rings. If the air or shoot particle came though the line it confirms the upper rings have worn down; while large amount of oil is the indication of bottom rings failure. Ideally at each 2000 hrs the condition of stuffing box rings are checked and noted; and the overhaul is scheduled next with the overhaul of the piston.
In such case the complete piston assembly is needed to be taken out with the stuffing box. The stuffing box separated and then inspected for damage replacing old rings with the new ones. On another hand skilled officer tends to do it through the scavenge space opening; operating the scrapper rings after separating stuffing box into its two halves.
B ) Scavenge Space inspection Checks
The scavenge space inspection allows for a quick check of the condition of various engine parts; the piston ring, cylinder liner wear and stuffing box. This helps detect any leaks from cylinder liner, jacket, piston ring wear and more in the form of oil and shoot deposits. The scavenge space allows for the inspection of scavenge port and a large part of under piston area.
Proper cleaning and inspection of the scavenge space helps avoid and scavenge fire. Due to high risk of fire from oil, shoot deposit and high pressure air its one of the key checks in maintaining safe engine operation.
Enclosed space permit are taken prior to performing any check or inspection in the scavenge space. Once scavenge space door is opened it need to be well ventilated for half an hour prior to entry. Its common practice to take note of the scavenge space condition; to ensure its free of any sludge or major oil deposit.
Check whether there is major shoot deposit inside scavenge space and clean with rags if any. Ensure the blower inlet flaps are free to operate and close. Check for any signs of cracks or wear on piston and liner surface for the part accessible.
Using the filler gauge take measurements of the piston rings clearance. Check for signs of water to detect cracks in liner and oil accumulation near stuffing box; to account for the health of stuffing box rings and holding spring.
C ) Checking Alignment of The Engine Using Crankshaft Deflection
The continuous operation of ships engine lead to vibration and uneven wear of the main bearing. They together lead to an upward or downward force on the crankshaft; leading it to bend slightly up or down across its entire length.
While the deflection may not be seen from naked eyes it can easily exceed dangerous level’s. Thus to avoid fatigue failure of the crank webs the crankshaft deflection is taken at each 2000 hrs of engine operation. Furthermore it can happen also due to; grounding, hull deflection, damaged foundation block, slack or broken tie rod and crankcase explosion.
Before checking the alignment of the crankshaft the engine is stopped and allowed to cool. All associated pumps stopped, turning gear engaged, starting mechanism halted, indicator cock opened & starting air valve closed. Furthermore enclosed space permit has to be taken, Man at work placard be placed and crankcase area ventilated in advance.
The misalignment of the crankshaft is measured using a dial gauge fitted deliberately in between the two crank webs. To ensure proper deflection reading on each event; the dial gauge is placed on the same position as the last time. This is done by using marker or a light punch.
Ideally the dial gauge reading should be taken for each 90 degree rotation on complete cycle; but due to some practical restriction on bottom two readings are taken. The average of these two reading give the reading for the bottom position for BTDC.
The dial gauge reading for the four points TDC, 90 degree off the TDC on both sides and 30 degree off on both sides the BDC; then used to plot deflection graph to match it with idle graph. On dial gauge the closing in of the crank throws is considered as negative while opening out as positive.
5 ) Maintenance And Overhaul At 4000 Hours Of Operation
A 4000 hours of engine running operation is quite significant in the maintenance point of view. With that long hours of continuous running its time to overhaul a few major part of the engine. The 4000 hour maintenance because of its significance is thus also called as half decarbonization; as the exhaust and inlet valves ( 4 – Stroke Medium Speed Engine ) are overhauled.
Furthermore the Camcase is inspected for any sign of wear or damage to the roller surface. It is inspected and checked whether the guide of the roller follower is tight; if not they are vowel to rectify. Any signs of major oil deposit is to be checked to ensure the camcase drains are not clogged.
Any signs of black spot or scratch on the cam can be result of burn or wear; negatively affecting the cam operation. Similarly damage to roller follower can also be inspected in the form of cracks or pitting. The camcase inspection is one of the job primarily associated with 4000 hrs maintenance; but are also part of monthly inspection routine in a majority of merchant ships.
Maintenance And Inspection of Exhaust Valve
Large two stroke marine diesel engine ( Ships engine ) have exhaust valve installed in the center of cylinder head. These pneumatic / hydraulic controlled valves work round the clock under severe temperature and thus need proper overhaul and decarburization. The exhaust valves can be lifted out of the cylinder head; by removing the studs and lock nut using the hydraulic tool.
Stop the engine, disconnect all the associated pumps and starting mechanism. Allow for the engine to cool for half an hour and then start operating; following proper safety measures. Disconnect all the connections to the exhaust valves including hydraulic oil line and high pressure pipeline; directing exhaust air towards the turbocharger.
Remove and overhaul one unit of the ships engine at a time. Once the studs and nut slacked lift the valve from the lifting nut and place to a secure area. At first remove the oil cylinder by disassembling nuts and safety handle an then lifting its part out. Then the spindle and the air piston assembly is being removed.
Now we will inspect and clean the bushings, spindle and the sitting surface for signs of wear. The wear of the guide bushing can be find by measuring its diameter at its top and bottom. O-rings in the bushings are replaced with the new one and outer seating are being lapped before reassembling the parts of exhaust valve.
Any part including the bottom piece if exceeds its allowed wear limit; then is to be replaced with the new one before boxing up the system.
6 ) Maintenance Done At 8000 Hours of Ships Main Engine Operation
Eight thousand hours of running operation for ships main engine accounts for complete decarbonization. It include the disassembly of the cylinder head, exhaust valve and piston assembly. The maintenance include to inspect and clean cylinder head assembly, replace piston rings, clean and measure piston wear and liner inspection.
At first the engine is stopped and all starting mechanism blocked including associated pumps for fuel, lube and jacket water. All the valves to be shut; indicator valve opened and turning gear engaged. At first isolate and remove the pipes carrying high pressure oil before complete de assembly.
Dismantle all the high pressure fuel oil, oil return, air line and pipelines to the exhaust receiver. Now using the hydraulic tool set on top of the studs. Dismantle all the valves, studs and the pipelines on top of the cylinder head. Unscrew the last few screws holding the jacket water system to cylinder head; and then lift.
Overhauling Piston assembly
Now to remove the piston open the crankcase door and unscrew the nuts from below the stuffing box on crankcase side. Sometimes the ships engine itself had to be turned; so that the crosshead is much down to provide access to the stuffing box. Low carefully remove the screws and locking wire off the piston rod crosshead assembly.
Now using a lifting tool pull the piston out of the cylinder block using engine room crane. Make sure while you lift and move your piston it does not should hit the liner surface. Put the piston along with the piston rod securely in upright position; once done its all set for inspection and maintenance.
Check for any broken or sticky piston ring, measure the piston ring groves and then inspect piston crown for signs of burn or wear. Clean the piston of any carbon deposits and foreign particle. Measure the liner diameter at top and bottom comparing it with the recommended wear limit.
Once done the piston is reassembled with all the cylinder head mountings in ships engine units; along with those high pressure lines we remove the first.
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