MAN B&W has witnessed (A Service letter issued in the year 2012/2013) seen a number of incidents with high top ring wear on small bore engine types 26 to 50. For this reason, MAN B&W has successfully tested a new piston top ring, which they now introduce as part of the standard ring pack configuration for two-stroke MAN B&W 26-50 bore engines.
On the small bore engines, the smaller depth of the controlled leakage (CL) grooves reduces the wear potential of the piston top ring compared to the large bore engine types.
In the cases observed, premature wear out of the controlled leakage (CL) grooves on the top ring running surface can be seen. As a result, the pressure drop over the top ring increased further and caused increased wear on the top ring, as well as a high liner wear rate and an increased ring groove wear.
To remove the negative influence from worn out (CL) grooves, MAN B&W has tested an alternative ring type to replace the CPR-CL ring. The new ting type is called CPR-POP (Port-On-Plane).
The CL grooves on the running side have been omitted and replaced with a number of ports milled into the lower side of the piston ring.
These passages have been configured with a 90 degree narrowing, causing the ring to increase the bypass area as the ring wears, instead of reducing it when the minimum depth is reached, as is the case with the CL grooves. Thereby, the pressure drop decreases instead of increasing.
To measure the wear of the ring, the width of the leakage passage can be measured using a feeler-gauge-type measuring tool.
The Table lists the maximum allowable width of the leakage gap for our small bore engines.
All ring packs from 26 to 50 bore with the new CPR-POP top ring have alu-coat on the running surface.
The new piston top ring has been tested on the various engine types. The test results have shown improved TBO in all cases, compared with the previous CPR-CL piston top ring.
It is important that operators experiencing worn-out top rings of the CPR-CL type on small bore engines note that there is a risk of liner surface hardening when running with a worn out top ring. In a number of cases, ship operators have seen a high increase in liner and ring wear rates after installing new rings in a liner suffering from a hardened surface due to running with worn-out piston rings. To ensure normal wear rates and eliminate surface hardening, the liner surface must be machined by honing or grinding.
If the oil film between the cylinder liner and the piston rings is damaged, adhesive contact will occur, followed by temporary high temperatures on the surface and, subsequently, cooling, with hardening of the running surfaces as the result. These areas of hardened surface must be removed to facilitate the running-in of new piston rings.
Source: MAN B&W