LATEST MARINE ENGINEERING KNOWLEDGE MMD ORAL Q & A PART-10

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(a) Sketch and describe an exhaust valve used on two stroke slow speed large diesel engine.

(b) How to arrange valve cooling.

(c) State how they are cooled and modified to extend frequency of overhauling.

(a) Exhaust valve

  • Exhaust valve opened by oil pressure, closed by air spring action
  • When cam pushes actuator pump piston up, it displaces a specific volume of oil, into valve actuator.
  • Oil acts on hydraulic piston of exhaust valve, pushing down the valve to open
  • Oil volume discharged by actuator pump, adjust the opening stroke,
  • When actuator pump roller passes over cam peak, air pressure pushes up air piston and valve closed by air spring action
  • Valve opening oil pressure is about 160 bars and safety valve on the hydraulic oil line is set to open at 300 bars.

(b) Valve cooling arrangement

  • Exhaust valve cooling is most important because hot exhaust valve have a shorter life
  • Bottom piece or the valve seat and casing are bore-cooled with guided cooling water flow.
  • Cooling water space around exhaust valve housing is water-cooled from main engine fresh water.

(c) To extend frequency of overhauling

  • Air seal: small amount of closing air allowed to leak through annular space between valve stem and guide for sealing purpose to prevent deposition
  • Rotating spindle small vanes fitted to valve stem
  • Superior metal: Gives an exhaust valve longer life. The seat area is provided with required hardness and part of spindle stem is coated with a wear resistant mixture of metal carbide and super alloy.
  • New valve seat geometry: provides a wider contact area efficient heat transfers from valve to bottom piece.
  • Modification to the recess chamber type: gives prolonged overhaul interval and prevent valve leakage problems because reduces valve seat temperature and keeps the seat clean.

(a) Sketch 2/S large engine air starting valve.

(b) Explain valve operation. Describe how it operates in sequence and how to achieve correct timing.

(c) What are the causes of air starting valve leakages? Identify with reason those defects to which this valve more prone and how they are encountered.

(d) How to counter above problems?

(e) What can happen operating with leaky air starting valve?

(f) Describe materials of air starting valve and what heat treatment is applied them to withstand load exerted on them.

(a) 2/S starting air valve

(b) Operation

  • Starting air valve open around 5˚ after TDC with opening period around 130˚ of crank rotation.
  • During maneuvering, compressed starting air acts, down force on the valve and up force on guide piston, it is balance and spring pressure keep valve close.
  • If distributor allows pilot air to upper part of control piston, this pressure overcome spring pressure and push valve downwards to open.
  • Starting air admitted into cylinder.
  • After completed starting timing, distributor again vents air above control piston, spring pressure again close valve.

(c) Causes of starting air valve leakage

  • Foreign particles deposit on valve seat, prevent valve fully close.
  • Dirty control piston and guide piston, insufficient clearances of them.
  • Sluggish valve action prevents sharp opening and closing of valve.
  • Leakage starting air valve will damage valve seat and will lead to starting airline explosion.

(d) How to counter above problems

  • Keep good maintenance
  • Apply proper lubrication
  • Before maneuvering, turn valve spindle.
  • While engine running and in port, test valve leakage.
  • Overhaul in regular interval and when overhaul
  • Before assemble, clean valve parts.
  • When fitting, correct clearance.
  • Free movement of piston ring in grooves.
  • After overhaul, test tightness of valve.

(e) Defective air starting vlave

  • Starting airline explosion.
  • While maneuvering, dead spot happens

(f) Materials and treatment

  • Valve body is mild steel, spindle of high tensile or stainless steel, and valve and seat could have the contact faces stellited or hardened.

(a) What causes an air start valve to stick?

(b) What are the symptoms of a leaky starting valve?

(c) If engine cannot be stopped immediately. As a management engineer, what action will you do?

(d) How do you check cylinder starting valve for leakage when engine is stationary?

(a) Starting air valve stick

  • Caused by deposits or inadequate clearances between control piston and guide piston. formed by the oil and condensate carried over into the starting air system
  • Dirt and foreign particles from the starting air system caused by lack of lubrication.
  • Sticking air start valve cause malfunction air starting system

(b) Symptoms of leakage

  • While engine is in operation, indicated by overheating of the branch pipe connected to the air starting valve.
  • Heating occurs due to leakage of hot gases from engine cylinder into starting airline connected to starting air manifold.
  • During manoeuvring periods, check temperature of each branch pipe from main starting air manifold to starting air valve

(c) Engine cannot stop immediately

  • If engine cannot be stopped immediately due to leakage valve, fuel pump lifted (fuel rack zeroed, puncture valve operated or whatever) on the affected unit and informed bridge
  • Keep load at a minimum, as one unit is now out of operation. As soon as possible, engine stopped and replace air start valve
  • When engine is running, leaking of starting air valve checked temperature of air pipe adjacent to valve.

(d) Check leakage

(1) Obtain propeller clearance from the bridge

(2) Ensure turning gear is disengaged and

(3) No one is working in crankcase or on the engine.

(4) Keep indicator cocks open

(5) Close shut off valve for the distributor.

(6) Open main air stop valve and automatic valve

(7) Put manoeuvring handle to start position.

(8) Leaky v/v will shows that air is leaking from indicator cock.

(a) Explain how to change Large Slow Speed Main Engine’s timing Chain with aids of sketches with important precaution points.

(b) Explain how to change when you found one linkage of chain is need to be renewed.

(a) Procedure to change timing chain

  • In a two stroke engine, two timing chains fitted.
  • Chains replaced one at a time.
  • Remove top chain case cover. The chain to be renewed is slackened off at the tensioner.
  • One of the chains is split at the point indicated.
  • New chain attached to old chain with a temporary link, and engine is turned, old chain is pulled out of engine and new chain is fed in.
  • Once the new chain has joined, the process is repeated for the second chain.
  • After replacing new chain, chain retention, camshaft retimed.

(b) Procedure for replacing one chain link

  • Turn engine until replace link, slacken off chain.
  • Free ends of the chain lashing with thin wire round rollers link, near from disassembling point.
  • Wire is lightly tight by a pull block, grind off the riveted over metal on the pins
  • To remove one roller link, remove two pin links.
  • Using special tools, new links fitted with new pins and side plates.

(a) Sketch any roller chain system of a camshaft drive for a marine engine.

(b) Describe above; state how adjustment is made and how required tension is achieved

(c) Sketch one complete length of chain and state elongation allowance.

(a) Roller chain system of camshaft drive

(b)

  • Sketch shown is B&W engine chain drive system.
  • Camshaft drive fuel pumps, exhaust valves. Crankshaft drive camshaft.
  • The chain is passed over two intermediate wheels.
  • On one wheel, chain tightener fitted to adjust the chain tension.
  • On shaft of other intermediate wheel fitted with small wheel drive starting air distributor, governor, cylinder lubricators.
  • Guide bars with vibration damper fitted along chain length at a distance about 1/2 ” from chain.
  • The chain is lubricated by oil sprayer jet with continuous stream of L.O onto chain.
  • Chain wrap around at least 120° on both intermediate wheel
  • Chain have high factor of safety (more than 25) to prevent stretching.

Chain adjustment

Tensioning device adjust, chain tension locate on slack side of chain.

For adjustment, maximum two chain links can remove.

Check before 4000 running hours check.

(1) The engine turned the slackness of the chain is on the tightener side.

(2) Loosen nuts A,B,C and D to free tightener bolt

(3) Tighten nut (C) until free length of spring measured, continue tightening until required tightness obtained.

(4) Tighten nut (B) slightly against pivot shaft .

(5) Tighten nut (A) and lock in nut (B)

(6) Tighten nut (C) until thrust disc bears against distance piece, according to maker instruction

(7) Tighten lock nut (D) on nut (C).

Chain tension require

  • Excessive tension cause high loads, possible damage and breakage of the chain system.
  • Excessive slackness cause vibration and leading to eventual failure.

(c) Elongation

  • Maximum elongation not exceed 2% and attention when elongation 1.5 %.
  • Sprocket having less than 60 teeth – 1.5% longer than original length.
  • Sprocket having between 60 and 80 teeth -1.2% longer than original length.
  • Sprocket having between 80 and 100 teeth -1.0 % longer than original length.

(a) State causes and effects of increased chain length and the method of assessing percentage increase in length

Explain the causes of the increasing chain slackness in service, using sketches to illustrate your answer

State each of following:(i) effects of increased chain length (ii) method of assessing percentage increase in length:

(b) State the reasons why a limit is placed on percentage chain elongation, and how to correct the timing of chain. Describe how the effects of elongation are corrected;

(c) State what a measure you, as CE, would consider necessary after the elongation correction procedure has been completed and the engine is running. State why a limit is placed on the percentage chain elongation;

(a) Causes of increased chain length

  • Timing chains elongate in services, this is sometimes referred to as stretch,
  • Material of chain (nickel chrome molybdenum steel) does not stretch; the safety factor is too high,
  • Wears take place between pins and bushings which cause the chain to get longer by effectively increasing the pitch.

(i) Effects of increased chain length

  • Chain gets longer, slack on free side, causing vibration, wear on wheel teeth and damage to guide bars.
  • Excessive slackness damage oil sprayers, change camshaft timing
  • A chain tensioner is fitted to adjust correct chain tension.

(ii) Method of assessing percentage increase in length

  • Chain elongation decide by measuring chain length about 10 links with the chain tight.
  • Compared with new chain length of same number of links
  • Percentage increase = ( increase in length / original length ) x 100

(b)

Chain elongation causes timing of camshaft, fuel pumps, exhaust valves and air start distributor to be altered.

Small differences in fuel pump timing adjust by the fuel pump’s VIT,

when degree of mistiming reached a limit set by manufacturer and done to check limit by use of a pin gauge, camshaft retimed.

1. Engine is turned to No 1 unit TDC .

2. A hydraulic pump is connected to coupling between camshaft drive wheel and camshaft.

3. A special “C” type spanner is located in flange on camshaft and connected to a chain block.

4. Hydraulic coupling is filled with oil to a predetermined pressure.

5. Camshaft is turned using chain block and spanner until pin gauge locate correctly in punch marks in camshaft and housing.

6. Hydraulic pressure is relieved and timing is rechecked.

7. On an engine with a chain in middle, this procedure repeated for other drive coupling.

(c) After camshaft retiming, while engine is running, , take power cards and draw cards

  • To correct fuel injection and engine units not overloaded, check exhaust temperatures and fuel pump rack settings