KNOCKING SOUND SHORTLY AFTER PISTON OVERHAUL
Have you encountered slight knocking sound shortly after piston overhaul? We have attended several troubleshot services due to knocking sound and most of the time reason found due to new piston cleaning ring.
Below, we look into function and maintenance of piston cleaning ring (PC ring) on the guidance of MAN B&W Service letters; SL00-382/JSB, SL2016-620, SL2019-684 and our service experiences.
The piston cleaning ring is installed to protect against excessive deposit build-up on the piston crown top land by scraping off the deposits.
Deposits on the top land may cause polishing of the liner surface and also lead to a lack of lubrication between the piston rings and liner, which eventually can result in piston ring or piston skirt scuffing.
A deposit build-up can also affect the position of the piston inside the cylinder and cause high friction between the liner and piston skirt. This situation may lead to severe damage of the liner and skirt.
In order to prevent undesired consequences caused by PC ring, we can summarize below points to be carefully evaluated during the piston unit overhaul:
- The PC ring and cylinder liner are designed to always follow each other which means assumed liner and PC ring wear in same manner, therefore it is very important to mark exact position of the PC ring related to liner and to be installed exactly same liner and same position. It is key point to prevent knocking sound after piston overhaul.
- Since the PC ring is worn to the same extent as the liner, it is not necessary to replace the PC ring during overhaul. Most common mistake during overhaul is replacing the PC ring without evaluating of the wear rate of cylinder liner unit. In case liner wear in the piston skirt TDC position is more than 0.1% of the bore diameter, new PC ring should not be installed. (Note, that the evaluation of the liner wear must NOT be based on the liner maximum wear but on the wear measured in the the piston skirt TDC.) PC-ring should be removed, thoroughly cleaned, inspected, and measured. The radial thickness should be measured at four or eight equally spaced positions, including the four positions corresponding to the Fore, Aft, Exhaust and Camshaft sides. Furthermore, the general condition should be noted, with special attention to deformations or cracks and to be re-used if found intact.
- When installing a new liner, it is mandatory to install a new PC ring too.
- In case liner wear at skirt area more than %0.1 of the bore diameter, it might be necessary to install oversize or dummy PC ring. Refer to engine specific maintenance manual or service letter SL2016-620 for more information.
- For engines from 26-cm bore to 50-cm bore equipped with a slim-type cylinder liner, PC ring is part of the sealing surface between cover and liner, so, it must be ensured that a standard-size dummy ring is available on board at all times for emergency operation with worn-out liners for the mentioned engine types (part numbers of the dummy ring can be found in service letter SL2016-620
- Operation without a PC ring cannot be recommended and is only allowed in emergency situations when no PC ring is available. The condition should be rectified as soon as possible
Rule-1: Position of the PC ring should be marked before removal and to be installed back to
exact marked position.
Rule-2: PC ring to be removed, cleaned, inspected and reused if intact.
Rule-3: For the 26-50 MC bore slim type cylinder liners, dummy rings to be kept on board as
spare part.
During sailing you have started to hear knocking sound, but you did not carry out any piston overhaul. Do you wonder why?
In case abnormal condition occurs in the cylinder liner such as bad fuel, water in the cylinder liner, insufficient cylinder lubrication, incomplete consumption etc., liner wear rate will be faster than PC ring and eventually piston crown will start to touch PC ring and create knocking sound especially while vessel rolling.
