Inspection provides useful information about the condition of cylinders, pistons and rings, at low expense. It consists of visually examining the piston, the rings and the lower part of the cylinder liner, directly through the scavenge air ports. To reduce the risk of scavenge box fire, remove any oil sludge and carbon deposits in the scavenge air box and receiver in connection with the inspection.
The port inspection should be carried out at the first stop after a long voyage, e.g. by anchoring if possible, to obtain the most reliable result with regard to the effectiveness and sufficiency of the cylinder lubrication and the combustion cycle (complete or incomplete).
A misleading result may be obtained if the port inspection is carried out after arrival at harbour, since manoeuvring to the quay and low-load running, e.g river or canal passage, requires increased cylinder oil dosage, i.e the cylinders are excessively lubricated.
During low load, the combustion cycle
might not be as effective and complete as expected, due to the actual fuel oil
qualities and service (running) condition of the fuel injection equipment.
Preparations:-
- Make sure that vessel as necessary permission from port authorities or bridge for main engine immobilization. Take control of the main engine to ECR. Put aux blowers on manual.
- Block the starting air supply to the main starting valve.
- Open the indicator valves. Engage the turning gear.
- Make sure engine is sufficiently cool before opening up the scavenge space door and under piston door.
- Complete all the paper work i.e. enclosed space permit, cold work permit, tool box talk and safe job analysis.
- Remove the inspection covers on the camshaft side of the cylinder frame, and clean the openings. Remove the cover(s) on the scavenge air receiver.
- After opening the door properly ventilate the space with portable blowers.
- Ensure pockets are empty and take in a flashlight, mirror, bucket of rags, turning gear operating control and a digital camera.
- Wear proper PPE.
Scavenge air receiver
Procedure:-
- Scavenge port inspections are best carried out by two men, the most experienced of whom inspect the surfaces, and state his observations to an assistant, who records them. The assistant also operates the turning gear.
- Keep the cooling water and cooling oil circulating, so that possible leakages can be detected.
- Begin the inspection at the cylinder whose piston is nearest BDC.
- Inspect the piston, rings, and cylinder wall.
- Wipe the running surfaces clean with a rag to ensure correct assessment of the piston ring condition.
- Use a powerful lamp to obtain a true impression of the details.
- Record the results.
- Measure the total clearance between the piston rings and the ring grooves.
- Continue the inspection at the next cylinder whose piston is nearest BDC, and so on according to the firing order.
- Check the non-return valves (flap valves/butterfly valves) in the auxiliary blower system for easy movement and possible damage.
- Remove any oil sludge and carbon deposits in the scavenge air boxes and receiver.
Observation:-
- Piston crown condition.
- Deposits on the top land and ring land.
- Ring breakage.
- Ring movement.
- Surface and lubrication condition of cylinder liner, piston rod, piston ring and piston skirt.
- Deposits in the scavenge box and scavenge receiver.
- Check for abrasion on the liner surface form remnants of mechanical wear, corrosion and combustion.
- Condition of the flaps and non-return valves.
- Check the scavenge relief valves and temperature probes. Check the security of grids and plates. Sight auxiliary blower fan impellors.
Piston Rings: In good Condition
The rings will move freely in the grooves and also be well oiled, intact, and not unduly worn. The ring edges will be sharp when the original rounding’s have been worn away, but should be without burns.
Piston Rings: Micro-seizure
Dry areas are formed on the cylinder wall, these areas and the piston ring surfaces will, by frictional interaction, become finely scuffed and hardened, i.e. the good “mirror surface” will have deteriorated.
A seized surface, which has a characteristic vertically-striped appearance, will be relatively hard, and may cause excessive cylinder wear.
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Piston Rings: Scratched
Scratching is caused by hard abrasive particles originating from the ring itself, or, usually, from the fuel oil.
Piston Rings: Sticking
The free movement of the rings in the grooves is essential, and can be checked either by pressing them with a wooden stick (through the scavenge ports) or by turning the engine alternately ahead and astern, to check the free vertical movement.
Piston Rings: Breakage/Collapse
Broken piston rings manifest themselves during the scavenge port inspection by:
· Lack of “elastic tension”, when the rings are pressed into the groove by means of a stick.
· Blackish appearance.
· Fractured rings.
· Missing rings.
Piston ring breakage is mostly caused by
a phenomenon known as “collapse”. Collapse occurs if the gas pressure behind
the ring is built up too slowly, and thereby exerts an inadequate outward
pressure. In such a case, the combustion gas can penetrate between the liner
and ring, and violently force the ring inwards, in the groove.
Slow pressure build-up behind the rings can be due to:
· carbon deposits in the
ring groove,
· too small vertical ring
clearance,
· partial sticking,
· poor sealing between
the ring and the groove floor,
· “clover-leafing” (see below)
· ring-end chamfers (see below)
· too large ring-edge radii, etc.
“Clover-leafing”, is a term used to describe longitudinal corrosive wear at several
separate points around the liner circumference.
Piston Rings: Blow-by
Leakage of combustion gas past the piston rings (blow-by) is a natural consequence of sticking, collapse or breakage. Blow-by is indicated by black, dry areas on the rings and also by larger black dry zones on the upper part of the liner wall.Deposits on Pistons
Usually some deposits will have accumulated on the side of the piston crown (top land). Carbon deposits on the ring lands indicate lack of gas sealing at the respective rings.
Lubricating Condition
All piston rings should show oil at the edges. White or brownish coloured areas may sometimes be seen on the liner surface. This indicates corrosive wear, usually from sulphuric acid, and should not be confused with grey-black areas, which indicates blow-by.
- No personal is left behind.
- Responsible person must check for the tools, rags etc. are not left behind.
- Account for all the nuts and bolts.
- Use correct tightening sequence and proper gasket to seal the door.
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