What affects piston rings durability and service life? Which counter measures could be implemented to avoid breaking of piston rings?
19 Jun '17, 13:20
I. Primary contributory factors from quality of fuel
1) The quality of the fuel, Separation standards of fuel (Treatment, Centrifuging and Filtering), Maintaining the parameters like Viscosity, pressures and temperatures and further in old vessels the tracing steam systems in high pressure pipes in order to maintain the fuel temperature are the factors need attention.
2) The excessive wear caused by the Catalytic fines of the fuels can be separated mainly by the purification and fine filtering. If water mixes in the fuel, Sea water, the separation of catalytic fines while centrifuging makes difficult. These catalytic fines are embedded and appear shining metallic white spots promotes uneven high wear at top piston ring TDC position. These vertical strong scratches the newly fitted piston rings tend to wear rapidly and eventual failure too. It is necessary to remove the embedded cat fines in the liner by honing machine before fitting the new rings.
3) Asphalt is hardly combustible and promotes poor combustion is liable to occur the piston ring sticking or breaking and increase deposits in the combustion chamber, fouling in exhaust manifold and surging of turbochargers. Only additives and intensive fuel injection maintenance are the available solutions.
II. Reasons and cases of failures analyzed by makers observations
1) Cylinder oil: Insufficient feed rate causes piston ring scuffing, excessive wear and in worst case the piston seizure and excessive feed rate causes hard carbon deposits and sticking the piston ring in the groove. It is necessary to maintain the makers recommended value between 1.1 to 1.3 gm/ Kw h Note: Do not reduce the feed rate instantly if found excessive. Reduce gradually with observations continued on piston ring status.
2) Temperature of charge air at piston underside: If a particular cylinder temperature is observed more than 3C above the average value of other cylinders, it is considered that blow by is happening in this cylinder. In these cases, the blow by and ring breakage happened and prompt inspection to be carried out for clear assessment of piston ring status.
3) Excessive wear of ring grooves: This increases the stress in the piston ring is the major cause of piston ring breakage. Whenever a piston is overhauled measure the grooves and replace with spare piston crown as necessary.
4) Removal of wear collar: The step at upper boundary of the liner surface, Just above the top piston ring upper edge is called “wear ridge or Collar.” Remove the wear ridge before pulling out the piston with the special tool. CAUTION: Do not use the hand grinding without positioning device, as there is risk of damaging the liner surface.
5) Cautions when fitting new ring: Use the special tool with minimum enlargement of ring gap. If the ring gap enlarged too much at fitting, the stress at the opposite of the ring gap becomes too high and cause plastic deformation of this area, which can cause the breakage of the piston ring.
6) Running In After Overhauls: Makers recommended the RUNNING-IN process to be followed when Piston rings and liners renewed.
7) Good combustion: Knocking, improper atomization, defective combustion filling carbon deposits causes early failure of piston rings.
8) Wear pattern of the Cylinder Liner: when the liner limits of wear approaching and the liner replacement is justified to improve the performance.
9) Prelubrication: At departure ports conduct hand lubrication with slow turning of engine only, over lubrication fills ring grooves, hindering the ring free movement causing fatigue cracks.
10) Regular Inspection for Fouling: The residues building up between the ring grooves facilitating ring cracks, whenever, ring inspected check for fouling.
11) Check after every long passage, the condition of piston rings/cylinder liners through scavenge ports, once broken or deformed piston ring found should be replaced at the earliest. Repeat the procedures when recurrent failures observed.
12) Temperatures and Pressures: Jacket cooling temperatures maintained above 80 0C, all times.
13) Scavenge air temperature & Air Cooler differential: Scavenge/ charge air temperature to be 50 oC and at maneuverings heaters should be effectively used, if provided. Charge air cooler drop, maximum allowed 300mm wp. Scavenge spaces drain condensate lines to be kept always clear.
14) Liner wall temperature recorders (If fitted): Whenever cylinder liner wall temperature rise observed, check the piston rings and replace at the earliest.
15) Scuffing of liners: Scuffing can happen by many reasons. Some of the reasons are, High sulfur content of fuel, due to loss of lube film and sludge formation, temperature rises to melting point and results breaking of ring, liner and even crowns.
III. Some other observations leading to Piston rings failure
1) Heavy firing thump or knocking at Maneuvering or Reverse maneuver: The reasons for this occurrence may be the some of the following mentioned reasons. Fuel is injected too early before TDC, too low temperature of fuel oil, Start fuel limiter and control failure, Water or fuel oil accumulations in Combustion chamber or bad fuel injectors are the reasons attributed to such situation. This heavy firing thump is another cause to collapse or even break the piston ring and this situation to be corrected immediately.
2) Piston ring Landing: Sections of piston rings may have highly localized wall pressure on the liner during the initial period of bedding, leading to oil-film break down in the region of highly loaded piston ring part and possible subsequent scuffing.
3) Lubrication quills: Insufficient quantity of lubricating oil supply to liner or partly clogged liner quill status and leaking cylinder oil supply lines.
4)Piston and Liner Clearance: Excessive or uneven diametrical clearance between piston crown and cylinder.
5) Piston ring gap: Piston ring gap is too small. This usually leads to ring breakage and could lead to very serious consequences.
6) Piston ring end: Incorrect preparation of ends of piston ring adjacent to gap with relatively with wider port locations.
7) Radius at Scavenge ports: Radius at top and bottom of exhaust and scavenge ports in cylinder inadequate, the ring then receiving a shock when sliding past the port edges.
19 Jun '17, 13:46