Previous year oral question on PISTON.

Q)HOW WILL YOU DECIDE TO CHANGE THE PISTON RING?

ANS)

1.BY CHECKING THE BUTT CLEARANCE. IF ITS VALUE HAS BEEN INCREASED THAN THE NORMAL RANGE.

2.IF ITS AXIAL CLEARANCE HAS BEEN INCREASED THAN THE NORMAL RANGE.

3.BY CHECKING THE VISUAL CONDITION OF PISTON RING.

Q) WHAT ALL CHECKS TO BE DONE ON PISTON?

ANS)Piston inspection on ships is part of the engine planned maintenance schedule (PMS) carried out to ensure the components is within the allowed tolerances. There are two methods of inspection: when the piston has been removed from the liner or inspection through the liner scavenges ports.

a)Piston Removed for Inspection:-

This examination will be under taken in a modular format, since the piston can be divided into various components.

Piston Crown

Check for any burning at top part of the piston.

Check any wear at the sidewalls of the crown and on ring grooves.

Check for any cracks at top due to the thermal and mechanical stress, check also for high temperature corrosion.

Check any signs of hot corrosion at the top surface and acidic corrosion at the lower part.

Piston Rings and Grooves

Check for the free movement of the piston rings. Check the ring clearance / groove clearance. Inspect for any wear, stepping and for scuffing.

Piston Skirt and Side-wall

Check for any rubbing marks.

Inspect for any wear down of wear rings.

Cooling Water Passage

Check for any scale due to poor water treatment. Choking due to high temperature.

Finally inspect the locking bolts; wires, studs and ‘O’ ring conditionb)Maintenance Schedule:-

Periodic inspection has to be done when the engine is not running. It can be carried out as above or by entering the scavenge space and inspecting the piston and piston rings through the scavenge ports, with the piston brought in line by rotating the engine via a turning gear.

Overhauling the piston as per Planned Maintenance Schedule (PMS).

Monitoring of the condition of the piston and the piston rings by the compression curve of the indicator diagram through process analysis.

The images shown below show examples of two means of inspection.

Inspection of piston and rings through the scavenge port 

Emergency Repair of Piston Crown:-

Once the above checks have been carried out, what actions can be taken if some values or observations are out with the specifications? Given below is a list of common faults that might be found during inspection and means to make temporary emergency repairs.

Gauge piston crown and ascertain shape and wear-down. If it is beyond recommended limits, replace the piston if there is a spare available. If not, rebuild the engine and proceed to the nearest port at reduced revolutions and arrange replacement. The crown head should not be welded except in a dire emergency- and even then only by an experienced welder. Remember that modern diesel engine pistons have a special lining of high temperature alloy on the top of the crown. This measure improves resistance to corrosion as well as to high combustion temperatures that the piston top is exposed to

Examine the crown for fractures or cracks, and if found the piston should be changed. If no spare is available these can be welded to manufacturer’s specifications; using the correct alloy welding rods, again as a means to proceed to the nearest port at reduced revolutions for a replacement.

Dismantled piston rings should be kept in sequential order so as not to interchange the rings when re-fitting to the piston.

Once repairs are complete, replace the piston rings and check for normal butt clearance.

If the butt clearance is more or less than the normal range, then replace the piston rings with new set of piston rings.

Note: It would be an extraordinary predicament to be in where as a Chief Engineer you sailed without main engine piston spares. However, strange things happen at sea, maybe the spares have been already used, and you're awaiting delivery of replacements.

If any of the above repairs are carried out, it is imperative that a close watch is carried out on the appropriate cylinder with the exhaust temperatures closely monitored as well as the piston cooling medium temperatures.