LATEST MARINE ENGINEERING KNOWLEDGE MMD ORAL Q & A PART-12

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(a) Define causes of cylinder and piston ring wear.

(b) Describe how cylinder liner wear is measured & recorded.

(c) Explain possible consequence of operation of M/E with excessive cylinder liner wear

(a) Causes of Liner Wear

1. Engine overload.

2. Incomplete combustion

3. Improper running-in during first few hours, 10 to 20 hours.

4. Misalignment of piston, or distortion of liner, preventing bedding-in of piston and liner.

5. Unsuitable liner material (quality), piston and ring materials.

6. Incorrect piston rings clearances.

7. Use of low Sulphur fuel oil

8. Incorrect Grade of cylinder oils, unsatisfactory arrangement of cylinder lubrication.

9. Contamination of lube oil by abrasive materials

10. Cylinder wall temperature too high or too low.

11. Scavenging air temperature too low

(b) Liner calibration is taken at every unit overhaul to know the wear down of the liner.

For slow speed engine, maximum wear down limit is 0.8 to 1 % of original bore diameter.

1. First clean liner thoroughly.

2. Liner calibration is taken by using inside micrometer with extension bar, calibrated against a master gauge.

3. Gauging taken at vertical positions (4 to 6) over swept area by piston rings. Measure fore & aft direction and in athwart ships directions.

4. Readings are taken at corresponding points, a template is used.

% of liner wear = (D last – D original) /D original *100

(c) If the cylinder is operated with excessive wear

1. The wear rate rapidly increase

2. Piston rings will distort and break

3. Piston slap may cause scuffing

4. Gas blow past will remove L.O film

5. Compression will be reduced, causing incomplete combustion

6. Carbon formed at exhaust ports and blown into scavenge space, cause scavenge fire. (In trunk type engine cause crankcase explosion)

(a) Explain how the piston rings are manufactured.

(b) State the initial and final clearances.

(c) State how its sealing action between the ring and the liner is achieved.

(d) Causes of piston ring failure.

(a) Piston rings manufacturing process

Piston rings are made of Pearlitic grey CI, having brinell hardness value of 175 ~ 185 and tensile strength of 280 MN/m2.

There are two methods of casting of piston ring depend upon sizes, Centrifugal drum casting, individual casting

The processes are as follows:

(1) Melt raw materials in the furnace

(2) Remove impurities with decarburizing method.

(3) Pour into the mould and slowly cool down.

(4) After take out from mould, rough machining and place in the storage yard for seasoning to relieve the stress.

(5) After seasoning, rings are cut according to the required joints and design, and then machined into exact required size.

(6) Outward-springing action is made by heat treatment method and by hammering method.

(b) Clearance of rings

(c) Piston ring sealing action

  • Piston ring outward springing effect achieved when manufacture. (1 ~ 1.5 bar for small engine and 0.05 ~ 0.5 bar for large engine).
  • When piston ring is squeezed into the liner, it presses against the liner wall and tends to initial sealing.
  • When engine operates, combustion or compression gas pressure pushes down the upper surface of ring and makes clearances at the top of piston ring.
  • Gas pressure enter to back of piston ring and pushes ring against liner wall assist outward pressure making firm sealing effect

(d) Causes of ring failure

(1) Poor material

(2) Incorrect ring and groove clearance (excess or insufficient)

(3) Excess wear of grooves

(4) Insufficient piston ring gap

(5) Insufficient cylinder lubrication

(6) Excess liner wear

(7) Excess diametrical clearance between piston and liner

(8) Depositing of incomplete combustion products in ring zone.

With reference to large slow speed diesel engines:

(a) State why it may be necessary to operate the engine with one unit out of service.

(b) Explain the procedure for hanging the piston of one unit to permit emergency operation of the engine.

(c) State a problem which may occur when running an engine with one unit out of service and the possible remedy.

(a) To hang up unit such as defective or damage of engine part at sea, unfortunate to repair due to

  • No spares available onboard -No time to complete overhauling
  • No weather permit -In river passage or traffic zone
  • Concern unit is consider to hang up to continue the voyage.

 (b) Procedure for hanging the piston of one unit

 When decision is made to be hung up concerning unit, first thing is clam down

 Discuss with captain about situation and inform to Head Office, follow maker’s instruction.

If running gear of respective unit no longer operate, immobilize in following manner

Depending on damaged component parts, piston hang up procedure for Man B&W-MC engine is as follows;

If damage to piston assembly, At first cut-out below

1. Fuel system

2. Piston cooling system

3. Starting air system

4. Cylinder lubricating system

For piston suspension case

1. Remove connecting rod

2. Support with special device-complete piston with crosshead

3. Cut out fuel system

4. Cut out exhaust system

5. Close exhaust valve

6. Open oil inlet to exhaust valve actuator

7. Open air for spring air

8. Blank starting valve

9. Suspended crosshead

10. Blank oil inlet to crosshead

11. Remove crankpin bearing

12. Cylinder lubricator set to ‘0’ delivery

After complete hanging up, warm up engine and test run with running of two aux. blower.

(c) When one unit cut out, no longer possible to maintain normal engine speed.

Precaution observed and reduce engine speed limit to avoid

1. Too high exhaust temperatures

2. Turbocharger surging

3. Vibrations

4. Abnormal running condition

5. dead spot for starting

Write short notes on the adverse effects that fuel high contents of the following may cause,

(a) Conradson Carbon Residue

(b) Vanadium and sodium

(c) Sulphur

(a) Water

(a) Conradson Carbon Residue

  • Fuel oil with high CCR values cause unburnt carbon deposits and increased smoky emission.
  • Values of CCR obtained by weighing carbonaceous residues left after burning a sample of fuel under vacuum condition.
  • CCR values over 20% by weight commonplace with modern fuels.
  • High CCR values over 10% cause fouling of combustion chamber and scavenge space.
  • Resulting smoke emission and trumpet formation at nozzles.
  • Use of Combustion Improver, increase combustion performance reduce unburnt carbon particles
  • The use of Asphaltene Dispersant improved the combustion of high CCR.

(b) Vanadium and Sodium

  • Fuel oil with high Vanadium and Sodium contents cause hot corrosion.
  • Cause hot corrosion especially to exhaust valve, turbocharger turbine blades and nozzle ring.
  • When combustion temperature is about 530˚C, formed corrosive products as Sodium Vanadate compound,
  • stick to the metal and attack the protective coating of the surface,
  • Being this attack, by-pass passage is produced on seat surface and blown through by combustion gas, tend to leak and burn.

(c) Sulphur

  • During combustion, sulphur in fuel oil reacts with oxygen and become SO2 and SO3.
  • Combine with the water vapor produced by combustion of hydrogen in the fuel and become sulphuric acid vapor.
  • When metal temperature is lower below 120 ~ 160 ˚C (acid due point) , this acid vapor become Sulphuric acid
  • Result corrosion attack piston rings, Liners, exhaust v/v housing, T/C water cooled casing, exhaust valve cage etc.called cold corrosion.

It may also cause

1. High temperature corrosion to exhaust valve

2. Adhesion of black lacquer on the liner inner surface and

3. Promote L.O acidity condition.

Sulphur attack is greater than vanadium attack.

(d) Water

More than 1 % of water content may cause

1. Loss of energy content and quantity of fuel

2. Poor combustion, reduce output and T.C surging

3. Misfiring of engine and stop in extreme case

4. Corrosion on fuel pump and injector components tends to seizure.

5. Emulsification of fuel leading to sludge formation

6. Promote microbial degradation

7. Fuel foaming in mixing column

Explain the Following terms regarding with marine Lubrication,

(a) Oxidation

(b) Emulsion

(c) Viscosity Index

(d) Detergent/Dispersant

(e) EP oil

(a) Oxidation

All petroleum based oils react chemically with oxygen and oxidizes.

Oxidation of oil take place at all time, increased when oil is agitated and heated in present of air and moisture.

Rate of oxidation depend upon:

(1) Type of oil

(2) Temperature

(3) Degree of contact with oxygen

(4) Presence of certain metals

  • Particularly copper acts as catalyst and accelerates oxidation.
  • Rate of oxidation is double for every 10° C rise in temperature and increases rapidly above 82° C.
  • Oxidation slow down by use of anti-oxidant additives.

(b) Emulsion

  • Emulsion is mixture of water and oil, very small drops of oil are suspended in water or very small drops of water suspended in oil, stabilized by third component such as soap.
  • Oil is contaminated will not easily separate from water and cause an emulsion, in whole or in part,
  • Emulsification is associated with precipitation of sludge, such sludge are formed from increasing of resins and asphaltenes.
  • The presence of water promote thickening of oil and formation of an emulsion.

(c) Viscosity Index (V.I)

  • The viscosity of oil changes with temperature.
  • Rate of change of viscosity of oil in relation to temperature change is indicated by Viscosity Index.
  • Viscosity Index is numerical value, measures the ability of oil to resist viscosity change when temperature changes.
  • High VI oil has smaller change in viscosity. V.I scale is from 0 to 100 and highest oil has 100 VI.
  • With improved methods of refining, highest VI of mineral oil is about 115. Using special additives, VI raised to 160 with

(d) Detergent / Dispersant Oil

  • In trunk type engine, incomplete combustion products from both fuel and LO deposit as tiny solid particles on piston, ring zones and exhaust ports.
  • Detergent additive prevents such depositions and washes away from one part, but deposit in another part of engine.
  • By adding Dispersant additive, tiny particles carried in a colloidal suspension, dispersed evenly throughout bulk of oil.
  • Detergent/ Dispersant oils- lubricant containing additives, specially to keep piston and ring zone free from deposits

Function of detergent / dispersant oil is

(1) Neutralization of acid.

(2) Prevention of lacquered deposit formation

(3) Dispersion of oil soluble

The detergent oil cannot be water-washed in a centrifuge due to its water-soluble/oil soluble properties

(e) E.P oil (Extreme Pressure Oil)

  • EP Oil-Lube oil imparts increased load-carrying capacity to rubbing surfaces under severe operating condition
  • E.P oil added with suitable mineral base oil has great compressive strength.
  • E.P additives reduce friction and wear on the bearing surfaces and used mostly for gear oils.

While operating a heavy weather, the main engine losses power and misfire.

Investigation shows considerable quantities of water in the fuel.

(a) As Chief Engineer Officer explain immediate action which should taken to ensure safe operation of engine.

(b) State with reasons, the possible places where water could enter the fuel system.

(c) As Chief Engineer Officer, write the standing instructions that would be issued, respect to the operation of the fuel system is order to prevent major problems due to water In the fuel

(a) Due to heavy weather, vessel movement disturbed water lying in bottom of service tank, mixed with oil in lower part of service tank.

  • If the vessel is equipped with two day tanks, and change over.
  • If not, service tank has a low and high suctions, change to high suction.
  • Not equipped with both of these, change DO service tank
  • If D.O is used, steam inlet to fuel heaters shut off to prevent gassing up of system.
  • Return line from the engine led back to settling tank instead of mixing column.
  • Mixing column is isolated from fuel oil supply pump suction.
  • Operate back flushing filters continuously.
  • Do not reduce engine load, if possible increase revolutions.
  • If engine stalls, difficult to restart it without flushing the whole fuel system.

(b) On older vessel, a holed tank top due to corrosion

  • On new vessel, bilge water leak into double bottom tanks due to incomplete construction or faulty welding.
  • During heavy weather, sudden ingress of water into service tank due to shearing off or corrosion of vent pipe at deck level.
  • Water enter into fuel tank due to wave breaking on deck.When bunkering, water contaminated
  • Main cause water enters system is from purifier, due to bowl sealing water has not been shut, gravity disk incorrect
  • Steam coil leakages from fuel tanks or purifiers heater or main engine fuel heaters.

(c) The standing instructions to be issued.

Whilst bunkering sample test for water content.

If water content is above 0.5%, fuel, settle for at least 24 hours in settling tank, desludging before purifier operation

To prevent major problems due to water in the fuel, the following standing instructions are to be issued.

1. When heating bunker tank, monitor the observation tank (cascade tank) for any trace of oil.

2. The settling and service tanks de-sludge regularly every watch by the duty engineer.

3. If any water is detected in the service tank, the low suctions should not be used.

4. Maintained proper temperature of settling tank heating for separation

5. A daily sample is taken from the booster pump discharge and tested for water.

6. The purifier operated correct gravity disc size and correct through put

7. The operation of the purifier and automatic de-sludge cycle checked by duty engineer.

8. Once service tank is full, change the purifier to Service-tank to Service-tank until the service tank is about 66% full, Change back to Settling-tank to Service-tank.

9. Running condition of main engine, auxiliary engines and its operation parameters are to be closely monitored.

10. If any abnormal found, inform the chief engineer immediately