Having studied about the cylinder liners of a marine diesel engine and their design and construction, it is now time to take a look at the issues involving inspection and wear of these liners. A liner can be inspected whenever a piston has been removed for changing the rings or replacement of the piston itself.
Inspection of the Cylinder Liner
What do you expect when you remove a piston and take a look at the inner surface of the cylinder liner? Well if you see a mirror sort of dry finish then do not be happy about it because ideally the surface should appear worn-bright and a bit oily, which gives an indication that all is well within the liner. The mirror finish described earlier is an indication of trouble and means that local lubrication has failed in that region and severe galling action has occurred.
If you see black patches on the liner it indicates that the piston rings are leaky and should be changed accordingly. Since the piston moves in a to and fro fashion, there might be a ridge or sharp edges at the extreme points and/or ports and these need to be removed by proper grounding. Any other scoring marks and ridges should also be removed using grind stones or oil stones as appropriate.
Apart from the physical signs of wear, the actual wear is measured using an inside micrometer length gauge using a reference strip to ensure that the readings are taken each time from the same positions; the readings being taken in the fore-aft as well as athwartship directions. The maximum allowed wear is nearly 0.6% for large bore engines and a max of 5mm for smaller engines. Apart from wear an ovality of 1mm could be allowed. Given below is a picture which shows a person inside a liner taking these readings.
The readings taken over a period of time should be recorded and then they should be presented in the form of a graph which shows liner wear as a function of time, and there should be an independent graph for each cylinder liner of the engine. A typical graph of such a nature is shown below in the sketch. The time is shown in hours along the x-axis whilst the wear in terms of mm/1000 hours is shown on the y-axis. The grey dots show the wear as they progress along the red line. Remember that the hours referred to here are running hours of the engine.
The wear also varies with the type of fuel used and is nearly four to five times more for an engine running on heavy fuel oil or residue oil, as compared to that running on distillate fuel or diesel oil. This is so because the heavy fuel oil being of inferior quality consists of several abrasive residues which increase wear of the piston rings and liner surface. Moreover the wear of piston rings could be as much as ten times more than the liner wear.
Another cause of more wear in case of residue oil is the presence of sulphur which gets oxidized and then condenses to form a weak acidic solution which eats up the liner surface.