Inspection of Cylinder Liner and Piston Assembly on Ships
Complete inspection of cylinder liner and piston assembly will require all components to be removed from the engine and fully dismantled. The initial inspection should be carried out before the components are cleaned to monitor fouling rates, areas of concern such as very dirty liner wall.
Once the components have been cleaned, gauging and measurements can be carried out before the final inspection is done.
Specific areas of inspection are:
- Liner wall
- Coolant side of liner
- Lubricator quills
- Piston crown
- Coolant side of piston
- Piston ring area
- Piston rod
- Piston internals
Possible Faults and Causes
- Excess wear on liner due to insufficient lube oil, abrasives in the fuel, liner too cool
- Cracks on liner wall due to fuel impingement (fuel injector fault or fuel oil too cold)
- Deposits on liner coolant side due to insufficient water treatment or oil in water
- Lubricator quills condition such as poor or nil operation due to supply system defect
- Burning and cracking of piston crown due to fuel impingement or insufficient coolant
- Deposits on piston coolant side due to fresh water treatment insufficient or LO additive depletion
- Wear of piston rings and grooves due to insufficient lube oil, abrasives in the fuel, poor combustion (causes excessive carbon build-up)
- Scoring of piston rod due to heavy dirt in scavenge space, excess spring tension
- Cracking of the piston mating surfaces caused by incorrect tensioning procedures
Preventive Measures
- Ensure adequate LO supply, clean fuel using separator at low through-puts, operate FW coolant at 80 degrees
- Regular inspection and testing of fuel injector, correct setting of FO viscosity control to 12cSt
- Weekly checks of FW condition for nitrate reserve and low level of chlorides
- Regular check of flow rate from cylinder LO unit, plus daily consumption
- Check piston coolant flow alarms
- Use LO analysis to ensure LO condition maintained correctly
- Regular checks of combustion efficiency by maintained fuel injection equipment
- Regular cleaning of scavenge space. and monitor condition of stuffing box drain
- Check the internals of the piston and correctly tension piston studs and apply fastening devices
What is meant by thermal stress and persistent burning in pistons ?
Thermal stress is caused when the free expansion of the surface metal is prevented by the relative cool metal of the main structure. This is caused when the piston surface is exposed to the hot temperatures of combustion and the outer skin of the piston expands. However this expansion is prevented by the main piston body, which leads to compressive thermal stress in the piston skin, and tensile thermal stress in the piston body.
Persistent burning of the piston crown is due to localised excess temperatures in that region. This could be due to either excessive external skin temperatures due to fuel impingement (fuel injector defect or incorrect fuel viscosity), or excessive internal skin temperatures due to fouling/scaling of the coolant surfaces.