184.ME Piston removal

SAFETY PRECAUTIONS

Stopped engine
Shut off starting air supply – At starting air receiver
Block the main starting valve
Shut off starting air distributor/distributing system supply
Shut off safety air supply – Not ME engines
Shut off control air supply
Shut off air supply to exhaust valve – Only with stopped lubricating oil pumps
Engage turning gear
Shut off cooling water
Shut off fuel oil
Stop lubricating oil supply

1     Removing a Piston from the unit after the Cylinder head

      Inspection of the liner and piston crown soon after the cylinder head has been removed   one of the important tasks during overhaul of a unit.

      First bring the piston to TDC and have a look at the piston crown for the quantity and   presence (condition) of carbon deposits, oil or water, and any abnormality.

     Take the piston to mid-way stroke and go inside the cylinder with a light and inspect the liner for carbon deposits at the upper part, ridges at the end of piston ring stroke, condition of liner with respect to scuffing, vertical scoring marks, oiliness or dryness.

      Take the piston to BDC and check the condition of ports and carbon deposits.

      Note / record all observations made.

1.2     Preparation before lifting a Piston (general)

a)   Bring the piston to mid-stroke and remove carbon deposits from the upper section of the liner. Using an emery paper, polish ridges at the upper section such that the piston rings can slide easily when the piston is being lifted out of the liner.

b)   Clean up all carbon deposits fallen around the piston circumference so that the piston will not get seized at the upper end.

c)   Clean the threads of the holes for fixing ring for lifting up the piston. If necessary, use No. 2 tap to clean carbon deposits from the threads. Remove carbon deposits from the bottom of the holes before screwing in the bolts with lifting jig.

d)   Apply some oil around the circumference of the upper end of the liner to facilitate movement of piston when it is being taken out.

d)   From inside the crankcase, bring the crosshead to a convenient location to work on the piston rod nut at the crosshead.

2   Lifting out a Piston together with the Stuffing box.

 

The description given below has been taken from the maintenance manual MITSUI  MAN B&W S50MC engine. However, procedures for removal of pistons of other engines will differ slightly due to difference in design of components. 

Dismantling of Piston

  

1.   Turn the crosshead down far enough to give access to the piston rod stuffing box, the tightening screws for the piston rod, and to the telescopic pipe.

2.   Release the stuffing box by removing the innermost screws with locking devices from the stuffing box flange.

3.   Remove the screws from the piston rod.

Mount the two distance pieces on the piston rod foot for protecting the lower scraper ring and for guiding the stuffing box.

4.   Dismount the cylinder cover, as explained in the previous unit.

Carefully smooth out any wear ridges at the top of the cylinder liner by using a hand grinder.

Note: Before removing piston, carefully remove any carbon deposits at the top of the cylinder bore.

5.   Loosen and remove the four screws, which tighten the telescopic pipe to the bend.

Mount the special tool for suspending the telescopic pipe to stuffing box.

Turn the crosshead to TDC, while checking that the guide pins of the distance pieces enter the holes in the stuffing box.

Suspend the telescopic pipe from the distance piece by screwing a nut on to the special stud.

When turning downwards, remove the distance pieces from the bend.

Note: When a piston is removed, never turn the engine without first dismantling the telescopic pipe from the crosshead.

6.   The top of the piston is now free of the cylinder liner.

Remove one or two cylinder cover studs.

Clean the lifting groove of the piston crown and mount the lifting tool in such a manner that the two "fixed" claws of the tool fit into the lifting groove of the piston. Fit the third adjusting claw of the tool to the lifting groove and tighten it up against the stop of the tool.

Note: The bolt (A), which is provided specially for the purpose of tightening the removable claw, must be firmly tightened7.            Lift out and land the piston with stuffing box in the support placed beforehand over one of the openings in the platform.

8.   Place a cover over the opening for the piston rod stuffing box in the bottom of the cylinder unit. Clean, measure and recondition the cylinder liner.

While working on the piston, ensure that the crankcase opening is properly covered to prevent ingress of water or anything else from falling in. 3     Cleaning and preparing a Piston

2.1     Checks to be made soon after a piston           

           is removed from the unit

Piston must be inspected before removing piston rings and commencing its cleaning.

·         Check piston crown for signs of burning or cracks

·         Check piston rings for stickiness and breakage

·         Check condition of piston body and skirt.

·         Check surfaces that make contact with liner

·         Check condition of piston rod for any abnormal signs of wear and abrasion (vertical).

·         Check condition of telescopic pipes (if applicable) 

·         Record the observations made.

·               ·               ·                                              

3     Cleaning and preparing a Piston

3.1     Removing Piston rings

      Using the tool "ring expander", carefully remove one ring at a time starting from the top ring.

      See the markings on ring (if any) otherwise it is advisable to make punch mark or scratch with file to indicate upper side of the ring. Prepare identification tag for each ring and tie it with each ring such that it does not fall off while handling the rings.

      Measure and note axial height (thickness) and radial width (depth) of each ring at three places.

      Assess wear down of each ring and check the limit for reusing provided by the maker.

      It is advisable to retain some old rings while replacing others with new ones.

3.2     Cleaning a Piston

·       Clean piston crown with a wire brush and remove loose carbon. Should there be hard deposits, remove them by means of a scraper.

·       Clean each piston ring groove. Remove all carbon deposits. Using a sharp marker, clean small grooves at upper and lower surfaces towards the inner corners end.

·       Remove carbon from the piston skirt.

·           ·           ·                                              

3.3     Preparation of Piston for mounting back

Checking of Piston and Piston Rings

1.   Measure the radial width D-2.

2.   Measure the piston ring gap D-3.

3.   Insert the piston ring either in a new cylinder liner or in the              bottom of a used cylinder liner during the measuring.

4.   Clean the piston crown and check the burn-away by means of the template indicating the maximum permissible burn-away, see D-4. Check the burn-away on the whole circumference of the piston crown.

5.   Clean the ring grooves and check these for burn marks or other deformation. Measure the ring grooves by means of a dial gauge or a caliper gauge. See D-5.

6.   Fit the piston rings (alternately right-hand and left-hand cuts with the ring gaps staggered 180o and with the 'TOP' mark upwards) by means of the ring expander. The ring expander prevents unintended deformation of the rings.

2.   Worn rings to be renewed if radial width D-2 is less than 14 mm.

New Piston Ring      D-1 = 17 mm.

Worn Piston Ring    D-2 = 14 mm.

3.   Effect checking of ring gap in new cylinder or at bottom of used liner.

Minimum ring gap    D-3 = 3.5 mm.

4.   Maximum permissible burn-away of piston top 

      D-4 = 12 mm.

Clearance in piston ring grooves.

5.   The maximum vertical height in a worn ring groove must not exceed D-5 :

Groove Nos. 1 and 2 = 13.1 mm

Groove Nos. 3 and 4 = 10.1 mm.

The groove is also worn out if there is no chromium layer.

.               Mount the piston rings using the tool.

7.   Vertical clearance, new piston ring and new or reconditioned ring groove :

                 D-6 = 0.30 - 0.35 mm

4     Cleaning and inspection of Cylinder Liner

·         Remove all carbon deposits and clean scavenge ports and exhaust ports (as the case may be).

·         In a liner that has worn down much during its past operations, it is advisable to dress up ports by filing the sharp edges to produce a chamfer on the edges.

·         Clean grooves provided for flow of cylinder oil. In case of a well-used liner, the grooves may have completely worn out. Grooves will need to be cut with the help of a small round tip chisel.

·         Turn cylinder lubricator with hand to check that oil flows out of every hole.

·     ·     ·                                              

Take liner measurements as suggested below :

Inspection, measuring and reconditioning of Cylinder Liner

1.         Dismount the cylinder cover and the piston.

Clean the cylinder liner and scavenge air ports.

2.         Position the cylinder liner measuring rod in the cylinder liner.

Measure the cylinder liner according to the points indicated on the measuring rod.

Take measurements in the fore-and-aft and athwartships directions (see data)

3.         Check the condition of the cylinder liner for wear, corrosion, cracks and overheating.

            Carefully grind away any scores or marks (or

            wear ridge in the bottom) on the cylinder liner

            running surface by means of a grinding stone.

Check the lubricating points of the cylinder by

manually pumping cylinder oil to each individual

lubricating point.

Clean any blocked lubricating duct.

Mount the piston and cylinder coverNote: In cases where shims have been mounted between the piston rod and the crosshead, the

length of the measuring rod must be adjusted in accordance with the thickness of the shims.

5     Mounting a Piston

With all parts cleaned and inspected, the piston can now be lowered into the liner.

1.   Arrange the piston rings alternately with right-hand and left-hand cuts so that the ring gaps are staggered 180o.

       Note: Arrange all piston rings so that surfaces with                             "TOP Mark" are located upper.

      Coat the piston rings and cylinder liner with cylinder oil.

2.   Mount the lifting tool on the piston crown.

3.   Ensure that the stuffing box is correctly positioned over the guide pins in the distance pieces, which are mounted on the piston rod foot.

            Coat the O-rings of the stuffing box with oil.

4.   Dismount the protecting shield from the crosshead bearing cap. Dismount the cover from the piston rod stuffing box opening in the bottom of the cylinder unit.

Mount the two guide screws (tools) for the piston

rod on the crosshead.

5.   Turn the crosshead to a position 45o from TDC.

Mount the guide ring for piston rings on the

cylinder liner.

6.   Lower the piston into the cylinder liner while guiding the piston rod foot through the cut-out in the stuffing box flange until the piston rings are inside the liner, corresponding to the lifting tool protruding about 30 mm from the guide ring.

7.   Mount the distance tube on the bend.

Turn the crosshead to TDC while checking that the

guide screws on the crosshead enters the hole in

the piston rod.

8.   After turning the piston fully into TDC, unscrew the loose claw of the lifting tool and pull the lifting tool free of the lifting grooves of the piston.

Remove the lifting tool and the guide ring for

piston rings.

Release the telescopic pipe from its suspension

and land it on the distance tube.

Remove the lifting tools for the telescopic pipe.

9.   Turn the crosshead with the piston rod and the telescopic pipe down far enough to permit tightening the screws for the telescopic pipe.

Remove the distance pieces from the piston rod

foot.

Mount and tighten the screws for the piston rod.

Lock them by means of a locking wire.

The locking wire is to be mounted in such a way

that the wire is tightened if a screw is turned left.

10. Tighten down the piston rod stuffing box by means of the screws through the inner holes in the stuffing box flange, and lock the screws by means of locking device.

11. Mount the cylinder cover studs by means of the stud setter.

Carefully clean the surfaces around the base of

the mounted studs and refill the roots properly

with Permatex 3.

12. Smear the piston rod with molybdenum disulphide, and then turn the crankshaft a couple of revolutions.

       Proceed with mounting of cylinder cover.

ASSIGNMENT:

1.   Make a report of the work done on a piston of the main engine during overhaul of a unit.