Answer MEP Question 15
Q. A. State the circumstances owing to which it may be necessary to renew a cylinder liner;
B. Explain how the cylinder liner is removed and fitted back;
C. State the important checks to be made on the engine before after fitting to which it may be necessary to renew a cylinder liner
Cylinder liners are usually renewed if it is cracked, heavily damaged, calibration reveals more wear, etc., such damages are usually caused by the following defects:
The wear in the cylinder liner is mainly because of the following reasons: -
1) Due to friction.
2) Due to corrosion.
3) Abrasion
4) Scuffing or Adhesion
Frictional Wear:
The frictional wear depends upon various factors like speed of movement between the surfaces, materials involved, temperature, load on the engine, pressure, maintenance, lubrication, and combustion efficiency.
Usually, in most cases, the failure is due to the lack of lube oil or failure of lube oil to the cylinder. Cylinder lube oil plays a major role in frictional wear.
Corrosion Wear:
The wear due to corrosion is caused due to the burning of heavy fuel oil in the combustion space. This happens because heavy fuel oil contains high sulphur content. During combustion, acids are formed inside the space which should be neutralized by cylinder oil which is alkaline in nature.
The production of acids will be more if sulphur content is more, leading to the formation of sulphuric acid. Sulphuric acid is formed due to the absorption of the condensate or moisture present inside the combustion space.
Sulphuric acid corrosion is found more in the lower part of the liner as the temperature of jacket water is very low. Corrosion due to sulphur will be high due to the presence of water in fuel and condensate in the air. This wear is generally seen between the quills. The wear near the quills enlarges and gives a characteristic of the cloverleaf shape to the wear pattern. This phenomenon is called clover leafing.
Abrasion
This type of wear is due to the hard particles present and formed during combustion. Catalytic fines in the fuel and the ash formed during the combustion cause abrasive wear.
Adhesion or Scuffing
This is a form of local welding between the particles of piston rings and the liner surface. As the piston is moving inside the liner, the welding which has occurred breaks and leads to the formation of abrasive material. The abrasive material will increase the rate of wear of the liner. This is generally caused by insufficient lubrication due to which a large amount of heat is produced and microscopic welding of rings and liner surface takes place. Due to this type of wear the liner loses its properties to adhere cylinder oil to the surface. One more reason for this phenomenon is the polishing of the surface caused by scuffing, giving liners a mirror finish.
Cylinder liners may crack because of poor cooling, the improper fit of the piston or pistons, incorrect installation, foreign bodies in the combustion space, or erosion and corrosion.
Improper cooling, which generally results from restricted cooling passages, may cause hotspots in the liners, resulting in liner failure due to thermal stress. Scale formation on the cooling passage surfaces of liners may also cause hot spots; wet liners are subject to scale formation. Proper cooling of dry liners requires clean contact surfaces between the liners and the cylinder block. Particles of dirt between these Surfaces cause air spaces, which are poor conductors of heat.
Films of oil or grease on these mating surfaces also resist the flow of heat.
Distortion, wear, or breakagé“may result if a liner is ñot properly seated. Causes of improper liner seating may be metal chips, mice, or burrs, or improper fillets.
An oversized sealing ring may cause improper positioning of the liner. As the sealing ring is over-compressed, the rubber loses its elasticity and becomes hard, which may cause the liner to become distorted.
If the manufacturer’s technical manual specifies the distance from the cylinder deck to the upper surface of the liner flange, use this dimension to check on the seating of the liner.
Obstructions in the combustion chamber may be destructive not only to the liner but also to the cylinder head and other parts. Erosion and corrosion may take place in a few isolated spots and weaken a liner, sufficiently to cause cracks.
B. Explain how the liner is removed
Before commencing work for which permission will have to be granted from the owners and from the port. Risk assessment to be carried out. All personnel involved in the overhaul shall familiarize themselves with the job and know the specific tasks that they are expected to carry out.
A permit to work, in the crankcase, will have been obtained, and all the necessary tools assembled and checked. Lifting tools and equipment will be checked for condition and certification. Jacks will be checked for leaks.
Sufficient spares must be on board to carry out the overhaul. Not just the replacement liner, but new piston rings; joints and seals. Because the piston, rod and stuffing box are to be removed it may be advantageous to carry out a stuffing box and piston overhaul at the same time.
Preparations for the removal of a cylinder liner:
2. Remove piston together with piston rod gland.
3. Remove the screws, which locate cylinder liner on support ring.
4. Separate all cylinder lubricating oil connections from the lubricating quills. Remove all passengers for lubricating quills as well as their protecting bushes before pulling out any cylinder liner.
5. Usually the liner is jacked from its seating before attempting to lift it from the engine. On some engines, the engine is turned to TDC and a jack mounted on the crosshead pin, which then locates across a support beam is turned to TDC and a jack mounted on the crosshead pin, which then locates across a support beam bolted across the bottom of the cylinder liner.
In the case of the ZA40, the jacking device is bolted to the crankpin bearing and the crankpin is turned to TDC. The hydraulic pump connected to the jacks is operated so that the jack locates in the bottom of the liner.
The liner is jacked upwards until the liner is moved off its seat.
In the case illustrated above, an 'I' section support beam is lowered down through the liner and placed on the piston rod gland seating of the cylinder Jacket in the longitudinal direction of the engine. The lifting beam is bolted onto the top of the cylinder liner as shown. Using the engine room crane the liner is pulled fully out of the engine and landed at a safe location and secured. Remove hydro jacks and supporting beam from the engine. Clean and inspect all guide areas and sealing and seating surfaces on the cylinder liner.
C. Explain how a new liner is renewed:
Pre-checks
The jacket cooling space around the liner is inspected for an overall condition that can indicate the effectiveness of the cooling water treatment. The guiding bores in the entablature and O-ring seating's are cleaned and examined for evidence of Corrosion/erosion and the landing face for the cylinder liner is cleaned and examined.
The new liner is cleaned, inspected and gauged to ensure it is within limits specified by the engine manufacturer.
Landing and sealing faces are inspected to ensure they are free from damage.
Lubrication drillings are blown through with compressed air to ensure they are clear. The lifting gear is attached and the liner is died in the entablature without 0 rings to ensure that it fits without binding.
Procedure:
Fit lifting beam, lift the liner and suspend over the engine in position, spread lube oil around the O-ring to lubricate. Apply silicone sealing compound to the seating surface of guide ring and collar of the liner.
Lower liner ensuring that the locating pin which ensures correct positioning of the liner is lined up with the hole on the supporting ring.
Fit cylinder head and tighten down using four nuts. Remove cylinder head and fit the screws which locate cylinder liner on support ring.
Gauge liner, and record readings. Fit and test cylinder lubricators.
D. State the important checks to be made after fitting
Gauge the cylinder liner bore and record the readings.
Connect lubricators and check for operation.
Fill the cooling space up to the top of the liner and check for leaks. (Once the cylinder head has been fitted and the system filled and pressurized, this check is repeated).
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