LATEST MARINE ENGINEERING KNOWLEDGE MMD ORAL Q & A PART-11

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(a) Explain the causes for trunk engine piston seizure.

(b) Identify the part of the engine which will probably be subjected to greater stress and give reasons.

(c) What are the parts might be effected and how.

(d) What are the signs that a piston is about to seize.

(a) Causes of piston seizure

(1) Continue overload operation

(2) Misalignment of piston and distorted liner.

(3) Insufficient diametrical clearance

(4) High friction on liner due to failure of lubrication

(5) Incomplete combustion due to loss of compression

(6) Late ignition, over fuelling and valve damage.

(7) Excessive difference between Pmax and Pcomp (> 40 bar).

(8) Insufficient or failure of coolant supply and excessive deposits in cooling spaces

(b) Parts of the engine subjected to greater stress

  • Cause of piston seizure – overheating of piston produce high temperature across section, cause thermal stresses
  • Relative overheating of cylinder liner produce high temperature across section of liner and cause thermal stress.
  • In both cases the greater the temperature gradient, the greater the thermal stresses on the parts will occur.
  • Due to piston seizure, very large strain put on the cylinder running gear and piston or cylinder liner may break.

(c) Other effect parts

Due to piston seizure, parts affected might be

1. Shear stress on gudgeon pin boss, crankpin

2. Tensile stress on bottom end bolts

3. Compressive stress on bottom end bearings,

4. Bending in webs, torsion on journals, connecting rod bend due to serve unbalance force

5. Excessive bending stresses, broken crank shaft

6. Broken all the parts and total damage of crankcase

(d) Signs that a piston is about to seize

1. Reduce engine RPM

2. Knocking at both ends of the piston travel

3. High temperature = cylinder & piston cooling, exhaust gas

4. Smoky exhaust

5. Turbocharger surging

6. Oil mist detector alarm.

These facts indicate a hot piston working with high friction against the liner surface.

With reference to main slow speed engine safety system,

(a) List FOUR engine operating parameters which will initiate an automatic engine slowdown, indicating in each case why an automatic slowdown is necessary.

(b) List TWO engine operating parameters which will initiate an automatic engine shutdown, indicating in each case why an automatic shutdown

(c) Explain how the operation of each shutdown listed in Q (b) may be tested.

(a) FOUR engine operating parameters

(1) High oil mist in crankcase: Caused by hot spot due to wiped bearing or piston blow by etc.

  • Initiates a slowdown, reduced load on bearings, reduce hotspot temperature
  • Engine stopped as soon as possible. Crankcase explosion and bearing seizure may happen.

(2) High lubricating oil temperature: Caused by temperature control valve or cooler fault.

  • Increase LO temperature, reduce viscosity, lack of hydrodynamic film lubrication.
  • Increase oil oxidization. Pistons cooling; high temperature oil tend to carbonize on underside of piston crowns leading to poor heat transfer.
  • Slow down prevents piston and bearings damages.

(3) High cylinder cooling water outlet temperature: Caused by temperature control valve or cooler fault.

  • Initiates a slowdown to reduce temperature of cylinder liners, heads and valves and their damages.

(4) High exhaust temperature differential:

  • Caused by high exhaust temperature on one unit due to (1) overheating (2) Incorrect combustion (3) exhaust valve burning
  • Caused by low temperature on a unit due to unit not firing.
  • All of above result, uneven loading of crankshaft increase vibration and serious damage

(b) TWO engine operating parameters

(1) Low lube oil pressure:

  • Without sufficient LO supply, bearings overheat and wipe.
  • On trunk piston engines, liner lubrication reduced and lead to seizure.
  • Total damage of engine crankshaft and running gear happen.
  • low LO pressure alarm is in two stages with different pressure switches:
  • First level is low pressure, run stand by pump and engine slow down. Second level is low low pressure, engine shut down.

(2) Loss of air spring pressure:

  • Exhaust valve not have positive closing
  • Cause poor combustion, uneven running, damage to exhaust valves and seats and excessive stresses in crankshaft

(c) To test the LO shut down pressure switch:

  • With engine stopped, remove pressure switch and blank off tapping.
  • Connect the pressure switch to a hydraulic test rig and pump up to normal LO pressure.
  • Using a test meter across pressure switch connections, reduce pressure on test rig gradually until switch opens.
  • Note pressure and adjust pressure switch as necessary.
  • Pump up to normal LO pressure again and start engine with an assistant at engine controls, monitoring LO pressure.
  • Reduce pressure on hydraulic test rig, until engine trips and note the pressure.

To test loss of air spring shut down

  • Use a similar method above, except that a test air supply with a reducing valve used instead of a hydraulic pump.
  • On some engines, shut down trip tested by opening the drain from air spring cylinders with engine running.
  • not a test of pressure at which trip operate and should not be used.

(a) Why is it essential to provide crosshead guide and guide shoes in large two stroke engines?

(b) Explain how crosshead and guides are lubricated in modern large engines.

(c) How do you check the clearance between guide and guide shoe?

(a) In trunk type engines, no guide arrangement because ‘Strake/Bore’ ratio is around 1.

  • Side thrust from piston pin or gudgeon pin is transferred to liner though piston skirt.
  • Thrust is balanced by reaction from liner.
  • In large 2 stroke crosshead type engines, ‘Stroke/Bore’ ratio is more than 3.
  • Because of higher ratio, connecting rod angularity is much higher and results in high side thrust.
  • Cylinder liner cannot take up this side thrust, side thrust is taken up by guides & guide shoes

(b) Crosshead and guides lubrication

  • Lubricating and cooling oil is supplied to piston, crankpin, crosshead and guide shoes via a telescopic pipe mounted on crosshead.
  • Crosshead is provided with bores for distributing oil supplied from telescopic pipe,
  • partly as cooling oil for piston, partly as LO for crosshead bearing and guide shoes and through a bore in connecting rod for lubricating crankpin bearing.
  • Lower crosshead bearing shells have oil grooves machined to assist distribution of oil.
  • Oil is supplied to guide shoe rubbing faces from crosshead oil supply.
  • Guide shoes have oil grooves machined in them to assist spread of lube oil.
  • Piston cooling oil outlet is led through a discharge spout to a slotted pipe in the engine frame and through a control device for checking of temperature and flow before returning to the lube oil sump.
  • On some engines, crosshead is lubricated by bearing oil which has been boosted to a higher pressure about 12 bars.

(c) The crank pin stand in a horizontal position 90˚ towards fuel pump side.

  • Crosshead is automatically pressed by con-rod against the rails surfaces on exhaust side
  • Clearance is taken on the fuel pump side with feeler gauge.
  • Crosshead bears on one side fully.
  • Clearances taken on both exhaust and fuel pump sides.
  • One side should give a ‘zero’ value.
  • The clearance must not exceed 0.1mm than original. If so, the crosshead bearing must be disassembled for inspection

(a) State, with reasons, THREE safety devices incorporated in a direct drive main engine starting air system.

(b) State, with reasons, THREE causes of an engine failing to fire on fuel after successfully turning over on staring air

(c) Explain how the engine is transferred to local control in the event of failure of the main engine remote control system.

(a) Lloyd’s regulations state:

1. “Starting air piping system protected explosions by providing the isolating non return valve at starting air supply to each engine.” When explosion take place, to prevent a pressure wave/flame front going back to main air receivers.

2. “In direct reversing engines, bursting discs or flame arrestors installed at starting air line on each cylinder. The flame arrestor prevent flame from combusting in the cylinder

3. In addition, some engines fitted with a relief valve on air starting manifold to vent an excess of pressure.

(b) Three reasons for an engine turning over on air but failing to fire on fuel are –

1. Air signal prevents fuel injected during starting air sequence is not vented and not allowing fuel to injectors. In MAN B&WMC engine, signal operating puncture valves of fuel pumps on

2. Temperature of air in cylinders insufficient fuel combustion. Because start air pressure is too low, or poor compression due to leaking valves

3. The fuel injection system is excessively worn or mistiming

  • When operating with MDO, Excessive wear in fuel pumps reduce injection pressure especially
  • Worn or broken injector springs, worn nozzles and needles result in incorrect atomization

(c)

(1) The exact procedure varies from engine to engine.

(2) Possible to change-over from remote control to local control with engine running,

(3) if governor is faulty, easier to change over with engine stopped.

(4) On the MAN B&W MC engine,

  • Control of fuel pump shaft is changed from governor output to a local hand wheel manually operated cone clutch arrangement.
  • The starting and cam follower reversing local control controlled by switches and push buttons.

(5) On the Sulzer RTA engine,

  • The local control of fuel lever is change from governor output
  • Moving run/stop and direction control levers manual operation of pneumatic control valves allow air to distributor, automatic valve and reversing servomotors.

With reference to main engine crankshafts

(a) Explain the term axial vibration.

(b) Describe, with the aid of a sketch, how axial vibration may be controlled.

(c) State with reasons which bearing would be most at risk due to the effects of axial vibration?

(d) Describe how damage to the bearing due to the effects of axial vibration may be repaired.

(a)

  • Axial or longitudinal vibrations happen as the crank webs open and close due to gas loads in the cylinders.
  • These vibrations are more pronounced on large engines with high cylinder powers and long strokes and cause additional stresses in crankshaft.
  • The vibrations are transmitted through the thrust bearing to the ship’s hull where resonance with hull vibrations may occur.

(b)

  • In order to control axial vibration, vibration damper is fitted to the free end of the crankshaft.
  • Usually consists of damper piston moving in a cylinder bolted to the No (1) transverse girder.
  • It is sealed between cylinder and piston, and cylinder and crankshaft, two oil spaces are formed either side of damper piston.
  • These two oil spaces are connected by a throttling valve and are fed from the main engine oil supply via non return valves.
  • (The adjustment of the throttling valve limits the oil transfer between the two spaces).
  • When crankshaft vibrates axially, movement of damper piston causes oil to be forced through throttling valve and so that resistance to movement is set up.
  • The resistance damps out the vibration.

(c) Axial vibrations are transferred to ship’s hull through thrust bearing.

  • Axial vibrations create reaction force in thrust bearing.
  • Thrust bearing most at risk due to additional loading caused by axial vibration.
  • Additional loading may squeeze the hydrodynamic oil film causing damage to the bearing surface.

(d)

(1) If thrust bearing becomes damaged due to axial vibration, the thrust pads are dismantled as per maker instruction.

(2) Then the thrust collar and bearing pads are examined and any minor damage can be stoned smooth.

(3) If necessary, new thrust pads may be mounted.

(4) Thrust clearance should be measured (approximately 1 mm on large 2 stroke engine, but refer to manufacturers data).

(5) Oil supplies checked before running the engine.

(6) Major damage may call for re-machining after emergency repairs.

(a) With reference to starting air system of main engine, sketch and describe each of the followings, and state reasons why each is fitted.

(1) Bursting cap

(2) Flame trap

(3) Air start manifold relief valve

(b) To reduce the possibility of starting airline explosion, state the maintenance requirements.

(a) (1) Bursting cap

  • Safety cap consists of a bursting disk enclosed by a perforated cylinder & a perforated cover in order to protect any bystanders, in the event of a burst.
  • Cover is fitted with a tell tale, which shows if the bursting disc has ruptured due to excessive pressure in the starting air line.
  • If a new disk is not available, or cannot be fitted immediately, the cover turned to blank off the holes in the perforated cylinder, in order to reduce the leakage of starting air.

(2) Flame trap

  • The flame trap made from brass or aluminium alloy which both have a high specific heat capacity.
  • A number of holes are bored through the thick circular form to allow the air to pass through.
  • Fitted in main air line immediately before air start valve to restrict the risk of a flame in cylinder propagating back to main air start manifold, by dissipating the heat energy in the flame.
  • A reversing engine must have a flame trap or bursting disk fitted before each start air valve.

(3) Air start manifold relief valve

  • The sketch shows a relief valve fitted to the air start manifold of Sulzer RTA 2/S engines.
  • Its purpose is to relieve excess pressure in the starting air manifold.
  • It consists of a spring loaded valve disk which locates on a valve seat which is bolted to the end of the air start manifold.
  • When the force exerted on the disk due to excessive pressure, the valve will open against spring force.

(b) Maintenance required to reduce the possibility of starting air line explosion

1. Drained and periodically inspected air bottle, main starting air line and all other high pressure lines

2. Open high pressure air from the bottle to the lines slowly.

3. Maintain starting air valves, check for any leakage during port stay as per maker instruction.

4. Checked and regularly maintained flame arrester, bursting discs or caps, non return v/v

5. Maintain main air compressors as per maker instruction so that oil discharge from the compressors is kept to a minimum.

(a) As a management engineer how to instruct your crew to avoid air line explosion?

(b) Explain the procedure to be followed while manoeuvring.

(i) If a hot start air pipe is discovered

(ii) If a safety device has ruptured due to the start air line explosion.

(a)

As a management engineer, the following must be instructed to E/R crew to avoid air line explosion

6. Drained and periodically inspected air bottle, main starting air line and all other high pressure lines

7. Open high pressure air from the bottle to the lines slowly.

8. Maintain starting air valves, check for any leakage during port stay as per maker instruction.

9. Checked and regularly maintained flame arrester, bursting discs or caps, non return v/v

10. Maintain main air compressors as per maker instruction so that oil discharge from the compressors is kept to a minimum.

(b)

(i) If a hot start air pipe is discovered:

1. Lift the fuel pump on the affected cylinder

2. Inform the bridge

3. Reduce the load on the engine if necessary

4. Run out fire hose and cool the pipe. (It could be a source of ignition for an external fire).

5. If engine is subsequently stopped, do not restart engine until pipe is cool.

6. Change air start valve as soon as ship is in secure position.

(ii) Safety cap consists of a bursting disk enclosed by the perforated cylinder & a perforated cover in order to save any bystanders, in the event of a burst.

  • Cover is fitted with a tell tale, which shows if the bursting disc has ruptured due to excessive pressure in the starting air line.
  • If a new disk is not available, or cannot be fitted immediately, the cover turned to blank off the holes in the perforated cylinder, in order to reduce the leakage of starting air.