Piston Inspection On Ships

We have talked a lot of marine diesel engine pistons and their various aspects such as the importance of piston clearance, removal and renewal of piston, cooling of pistons and so forth. We have also discussed about various types of piston defects and ways to rectify them but in this article we will take a fresh look at the various ways in which a piston can be inspected when taken out for maintenance or even when it is within the cylinder liner. This will be done in a modular format which will be helpful for a marine engineer to know instantly at a glance what all to look for in such a situation.

Since the piston can be divided into various parts, they will be taken up one by one as follows

In piston crown the following inspections have to be carried out:

1. See any burning at top part of the piston
2. Check any wear at the side walls of the crown and on ring grooves
3. Check for any cracking at top due to the thermal and mechanical stress along with high temperature corrosion
4. Check any signs of hot corrosion at the top surface and acidic corrosion at the lower part

In piston rings & grooves:

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

In piston skirt and side wall:

1. Check for any rubbing marks
2. Inspect for any wear down of wear rings

In cooling water passage:

1. Check for any scalings due to poor water treatment
2. Choking due to high temperature

Finally inspect the LOCKING BOLTS & WIRES, STUDS and 'O' ring condition

Periodic inspection has to be done when the engine is not running. It can be carried out by just opening the scavenge space and inspecting the piston and piston rings through the scavenge ports.

Overhauling the piston as per PLANNED MAINTENANCE SCHEDULE (PMS).

Condition monitoring of the piston and the piston rings by the compression curve of the indicator diagram through process analysis

The images shown in the above section show some of these types of inspections

We have seen various checks and inspections above, but what should you do if some values or observations arent as expected or desired. Given below is a list of common faults that might be found during inspection and means to mend them.

1. Gauge piston crown and ascertain shape and wear down if it is beyond recommended limits, replace the piston
2. Examine for fracture or cracks, piston rings grooves and general condition
3. The crown should be welded with proper weld metal to correct shape and height above datum
4. Dismantled piston rings should be kept in sequential order so as not to inter change the rings when fitting back
5. Piston rings to be welded and examined after welding is completed
6. Put back the piston rings and check for the normal butt clearance
7. If the butt clearance is more or less than the normal range, then replace the piston rings with new set of piston rings
8. If any irregularities found on the piston, piston-crown and grooves to be machined