Wear edge is formed in the top position of the upper piston ring when the cylinder liner is worn after many thousands of hours of service or from abnormal running conditions, such as scuffing or corrosion.
With heavily worn cylinder liners, it is a necessity to remove the edge prior pulling out the piston, otherwise the piston will not pass and in most cases will lift the cylinder liner. If this happens, will then have to pulled the cylinder liner in order to renew the O-rings.
Some of the engine manufacturers, recommends that these wear edges to be removed when replacing the piston rings and a groove to be created by milling or grinding in the upper part of the cylinder liner, as this will prevent the build-up of a new wear ridge.
Moreover, it has been noticed quite severe conditions when newly fitted piston rings are run in a cylinder liner with an existing wear edge, where the upper piston ring hammers against the wear edge at every piston stroke until the ring becomes chamfered at the top and collapses.
MAN Diesel in cooperation with Chris Marine from Sweden, have developed a milling procedure for the reconditioning of the cylinder liners of large bore diesel engines by using a VKS type Chris Marine milling machine.
The job it will take around 1 hour or more, depending on the operator experience, and the most time consuming is the assembling and calibrating of the equipment as it must be in a perfect centered position. In approximately 15 minutes, the tool cuts a fine groove in the cylinder liner exactly in the desired position, but attention must be paid as the upper piston ring should not extend more than 2 mm above the lower groove edge.
However, not every vessel is equipped with such a tool and if such equipment is not available, an angle grinder with a coarse stone wheel of adequate thickness can be used. In this case, it should be noted that only a cylindrical grinding of 5 mm in height, just above the wear ridge, must be performed. The rest of the area, to the tp of the cylinder liner, will then act as a scrape down ring. The result will be good running conditions of the piston rings as the coke on the top land never touches the cylinder liner surface.
Efforts should be made to create the groove as specified before.
When hand grinding is applied with an angle grinder, it is recommended that an old piston ring is placed on top of the piston crown, and that the piston crown is turned to a position that enables the grind stone to rest on the piston ring while removing the wear ridge.
This method is highly recommended as although angle grinder is a very efficient tool, unfortunately it is impossible to control it vertically. This means that the cylinder liner running surface below the wear ridge can be easily damaged and often to the extent that the upper piston ring does not stand a chance of lasting the running-in period. If the piston ring collapse, gas blows by the ring, destroys the oil film and creates further blow by of the entire ring package and a scavenge fire could be the result.
MAN Diesel introduced the so-called Controlled Pressure Relief (CPR) top piston ring which highlighted the importance of removing wear ridges. The reason behind it is that the gas-tight seal of the CPR ring is susceptible to breakage in the event that a wear ridge is not removed when the piston is overhauled.
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