Describe the two-stroke Main Engine piston overhaul procedure

asked 20 Apr, 15:39

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KnowledgeBase
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Before commencing the overhaul:

• Shut off starting air and lock it off.
• Open the indicator cocks.
• Cool down the Engine sufficiently to allow LO pumps to be shut down.
• Isolate fuel and cooling water.
• Work discussion - Toolbox meeting must be carried out to ensure that all participants read manuals, familiar with the procedure, safety precautions and maker recommendations.
• Ensure that each person knows his tasks.
• Confirm required spares are on board.
• Check that lifting tools, strops, shackles etc are within certification and in good order.
• Check that jacks are in good condition with no leaks.
• Ensure permission for immobilisation is given.
• Carry out risk assessment.
• Ventilate crankcase and obtain permit to work.
• Ensure that no-one else is working elsewhere on the engine and could be potentially harmed (or harm others).
• Check that no-one is working in the vicinity of the shafting system and propeller, and that the bridge have confirmed that it is safe to turn
the engine.

Overhaul Procedure

• Drain cylinder cooling water from respective cylinder. NOTE: It is not advisable to just drain down to liner level in case the
 liner is disturbed when lifting the piston.
• Remove cylinder head and associated pipework.
• Turn engine to BDC and remove carbon and wear ridge from cylinder liner using a grinder.
• Unbolt the piston rod from the crosshead.
• A two piece spacer tube is bolted to the piston rod foot to protect the piston rod gland when lifting the piston.
• Unbolt the gland housing. The piston rod gland is removed with the piston rod.
• Turn the crank to TDC. The spacer tube will push the piston rod gland out of its seating.
• Clean out the threaded holes in the piston crown with a tap.
• Bolt on the lifting bracket.
• Lift the piston, rod and gland with the engine crane and lower the assembly into the special cradle for maintenance.

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• Once mounted in the cradle, the piston should be cleaned and all deposits removed.
• The piston rings are removed using the correct
ring expanding tool. Inspect the ring grooves for wear on the bottom landing surface.
• A profile gauge is used to check that the 
crown has not burnt excessively which would thin and weaken it. If the burning is excessive the cause is often poor fuel 
injection - investigate and replace the crown.
• The crown of the piston must be inspected very carefully for evidence of cracking. Cracking is caused by excessive thermal stressing due to poor cooling and carbon build up or
excessive mechanical stressing due to overloading.
• The contact faces of the rings and the skirt are inspected for signs of abrasion
and scuffing which indicates poor filtration of fuel or air and poor cylinder lubrication. Excessive wear on the skirt may indicate
misalignment or excessive clearance of crosshead guides.
• Piston rings should be gauged, and a decision based on manufacturers recommendations made as to whether to replace them.
• The piston is dissembled from the rod and the cooling spaces inspected once it reaches overhaul interval stated by Maker. On oil cooled pistons look for build up of carbon deposit s
which could indicate overheating of the piston crown or contamination of hib oil by used cylinder oil. On a water cooled piston 
look for evidence of scale build up due to incorrect water treatment.
• After reassembly of the piston, check that the cooling space is tight by applying a hydraulic test, (do not over pressurize).
• If new rings are to be fitted check the axial clearance in the ring grooves and the circumferential clearance by placing the ring into 
an unworn part of the cylinder liner.
• During the piston overhaul the stuffing box normally overhauled as well. Segments must be inspected for wear, clearances and springs to be measured according to the manual.
• Clean, measured and possibly overhauled piston with stuffing box inspected or overhauled is ready to be installed.
• Prior installing the piston, measure and record the liner wear.
• Apply rings guiding tool and sufficient cylinder oil when lowering the piston.
• Bolt the connecting rod to the crosshead.
• Secure stuffing box.
• Install the cylinder head and associated pipework.
• Fill the cooling space with water and check for any leakages from the outside and inside the engine.
• Start preheating the Engine.
• Start LO circulation.
• Pre-lubricate and turn the engine with the turning gear.

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answered 20 Apr, 16:14

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cheng
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