Piston rings are to be diagnosed at regular intervals on the basis of the wear pattern. Piston rings have to be replaced after they have been use for time periods given in the maintenance schedule. Replacement will also be necessary if interim checking reveals damage to running surface or excessive clearances.
Different Piston Rings:
The rings being in use are compression rings with chromium-plated, Plasma-coated or (the most recent solution) chromium/ceramic-coated running surfaces, and chromium-plated oil scraper rings.
|MAN B&W 9L 58/64||MAN B&W 18V 51/60 DF|
|Groove 1||Compression Ring (Packing Ring)||Plasma-Coated||Groove 1||Compression Ring||Packing Ring|
|Groove 2||Compression Ring (Tapper Ring)||Chromium-Coated||Groove 2||Compression Ring||Tapper Ring|
|Groove 3||Compression Ring (Tapper Ring)||Chromium-Coated||Groove 3||Oil Scraper Ring||Tapper Ring|
|Groove 4||Oil Scraper Ring||Chromium-Coated|
Wear appearance of plasma-coated, chrome-plated or chromium/ceramic-coated piston rings
Due to its shape, the running face of new compression rings does not have contact across its full width but only the width of the actual contact surface A. With the progress of wear, the actual contact surface width will increase.
Figure 2. System of cracks on chromium/ceramic-coated piston rings (time in use approx. > 8000 hrs)
At the end of the useful life, the actual contact surface extends up to the chamfers or radii of the ring sides, please see Figure 3.
The chamfers or radii are still distinctly visible on the piston ring shown in Figure 3. Coated compression rings have to be replaced when the actual contact surface extends over the entire piston ring width, i.e. when the chamfers or radii are hardly visible any more. In this condition, the residual thickness of the coating will not safely endure the subsequent maintenance interval.
Burns (C) produce local discoloration of the piston ring face. Such rings have to be replaced. (See Figure 4).
Coated compression rings and oil scraper rings have also to be replaced if the coating has worn through at one spot.
Figure 5 shows a chrome-plated compression ring. Turn chromium layer has completely worn spot wise at (D).
Broken-off particles or cracks in the coating of a piston ring also call for its replace.
Figure 6 shows a plasma-coated compression ring. Parts of the coating have broken off at (E).
Chromium/ceramic-coated compression rings have to be replaced when the continuous, micro-cracked surface has decayed to a crazed crack appearance. The cracks will then have extended down to the base material, which involves the danger of particles breaking away.