Marine Diesel Engine Starting Air System

• Master valve is for commencing the operation of start command.
• Air supply is provided to engine by opening air supply valve from main air bottle.When this valve is open air reaches auto main start valve.
• Master valve when operated allows control air of 7 bar to pass through turning gear interlock, reversing direction completed interlock etc.
• To start M\E - First turning gear is to be disengaged.Ahead or Astern command is given.Once the interlocks are satisfied or deactivated air will  be supplied to automatic main air start valve.
• Control air acting on auto start valve which is generally a ball valve opens 90degree through a rack and pinion arrangement activated by a pneumatic piston or actuator.
• Air is now available at starting manifold and distributor.
• Distributor consist of a negative cam or inverse cam.Negative cam ensures positive closing of distributor and also it aids in overlapping of units.
• When a particular unit slide valve in distributor comes in the cam profile + air [pilot air for distributor] compresses the spring of that slide valve , air is supplied to operate the cyl head start valve of that unit.
• As the camshaft rotates, that unit air line is vented via distributor and cyl head start valve for that unit closes.
• The next unit distributor slide valve will be activated as per firing order.
• When command lever is on to the fuel ie, Run - Starting air system is completely isolated.

Safeties on starting air system: 

• flame arrestor 
• bursting disc
• relief valve
• main auto start valve
• thermal patches on start air line
• interlocks
• air bottle safety valve
• non return valve in starting line 

 Interlocks:

• turning gear interlock
• running direction interlock-prevents fuel from being supplied if engine's running direction doesn't match  telegraph order.
• reversing completed interlock-this prevents starting from taking place if distributor shifting has not been completed.
• Aux blower interlock- Ensures sufficient air in low speed running.
• Spring air interlock, safety air interlock, main l.o pressure interlock, jacket cooling water pressure interlock.

Starting air cam:

-ve cam ensures positive closing of air distributor slide valves and aids in overlapping of units.

 

Piston Rod Stuffingbox / Diaphragm Gland

Piston Rod Stuffing Box is used in large 2 stroke marine diesel engine to have a sealing between under piston scavenge space and crankcase. It prevents sludge and scavenge air from under piston space to enter crankcase and avoids crankcase oil getting into under piston space.

It is fitted in a diaphragm between crankcase and scavenge space.

Stuffing Box consist of 3 parts:

1. 1 set of

Upper scraper ring - 4 segments of brass held together by garter spring [Removes sludge from piston rod.Sludge scraped goes into scavenge drain tank] +

Pack sealing ring - 4 segments of brass held together by garter spring

      2. 2 sets of

Cover sealing ring - 4 segments of brass held together by garter spring +

Pack sealing ring - 4 segments of brass held together by garter spring

[ seals air from under piston space going into crankcase]

     3. 4 sets of

Oil scraper ring - 3 segments of steel held together by garter spring which has has two replaceable cast iron lamellas slid on to it [sludge scraped is drained to stuffing box drain tank and it scrapes down crankcase oil back to crankcase]

*Stuffing box inside the engine

*Split stuffing box during overhaul

*Understanding stuffing box

Reasons for stuffing box performance deterioration:

• vertical scratches on piston rod - reduces sealing
• seal rings getting worn out
• garter spring tension reduces due to heating over the period of time
• drainways or grooves getting choked
• reduction in butt clearance

If butt clearance reduce more than 50% of the gap, should replace the rings at the earliest.If no spares are available then grinding of rings can be done temporarily in emergency cases.

Clearances taken during maintenance:

• Vertical clearance between ring and housing groove [ high clearance causes hammering of rings in housing while engine working]
• Ring end clearance / butt clearance [ less clearance results in ineffective sealing]
• Tension of springs are checked using length of the spring at different loads as mentioned in manual and free length of the spring as given in manual.

Marine Diesel Engine Scavenge Fire

A fire which occurs in scavenge space or under piston space is called scavenge fire.

For a scavenge fire to begin there must be a combustible material,oxygen and heat source. In case of scavenge fire combustible material is oil. Oil can be cylinder oil drained down or crankcase oil carried upwards due to a faulty stuffing box. In some cases fuel oil can also be present due to poorly maintained combustion gear.Oxygen for combustion comes from scavenge air which is plentiful.Source of ignition could be piston blow by, after burning,late ignition etc.

Indications of scavenge fire:

• increase in exhaust temperature of affected unit as unit is starving from fresh air
• drop in rpm
• black smoke in exhaust
• discharge of black smoke,flame,spark through scavenge air box drain
• turbocharger surging
• local heating of scavenge air receiver
• jacket temp of affected unit rises

Causes of Scavenge fire:

• accumulation of cylinder oil due to excess lubrication
• unburnt fuel due to - leaky piston rings,prolonged blow by, worn liners, after burning
• accumulation of sludge

Actions to be taken:

• call all engineers alarm- inform wheel house- reduce engine rpm
• fuel pump of affected unit to be cut off
• rate of lubrication of affected unit to be increased

if fire is not subsiding

• stop engine
• stop aux blowers
• inject smothering gas [steam or co2]
• cooling system to be maintained
• after engine is cooled down, clean scavenge air receiver, cause of fire should be ascertained and rectified.

Probable places where damage might occur:

• stuffing box
• piston rod
• cylinder liner surface
• Tie rods near fire to be retensioned

Preventive maintenance:

• periodic inspection of scavenge air receiver
• scavenge drains should be blown regularly
• piston rings and liner to be properly maintained
• correct rate of cylinder lubrication to be maintained
• proper maintenance of fuel gear
• cylinder liner wear rate is to be checked and tracked on

*Well maintained scavenge space

Safety devices:

• scavenge air temperature sensors
• pressure relief valves
• fire extinguishing medium[co2 or steam]

Starting Air Line Explosion

Starting air line explosion occurs when oil accumulated in starting air manifold or piping gets ignited under high pressure air.

Sources of ignition can be:

• A leaky air start valve - whilst the engine is running hot gases produced in cylinder as fuel burns [at above 1200degree celsius] leak past a valve which has not seated properly.Branch pipe to manifold gets heated to red hot.If engine has stopped and restarted before cooling down, oil vapour in the air can be ignited leading to explosion.
• Fuel leaking into the cylinder whist engine is stopped, gets ignited when piston compresses the air inside the cylinder in starting sequence.When air start valve opens as the piston comes to TDC, the pressure in the cylinder is higher than start air pressure.Burning combustion gases passes into the air start manifold and ignite oil entrained resulting in explosion.
• Oil deposited on inner surface of start air manifold can get auto ignited because of high temperature created by inflow of high pressure air. At about 400 degree celsius, oil deposits in manifold self ignite resulting in explosion.

*Starting air pipe to cylinder exploded

To minimize the risk of explosion:

• Oil carryover from compressor to be minimum.
• Air intake mist catcher, oil seperator of compressor to be properly maintained and cleaned.
• Non return valve at starting of air supply to engine[incorporated in auto start valve]
• As per IACS, In direct reversing engines with bore dia >230mm should have flame arrestor, bursting disc between manifold and cylinder start valve.A relief valve may be fitted in manifold where flame arrestors are fitted.

*Bursting disc

*Bursting disc MAN B&W

Safety devices:

• Flame trap
• Bursting disc
• Relief valve
• Thermal patch at manifold
• Non return valve at air inlet to engine

Marine Diesel Engine Crankcase Explosion/Fire

Causes of explosive atmosphere in crank case:

• A source of heat or ignition[vaporize oil into fine vapor] -

A hotspot is that source due to lack of lubrication, improper bearing clearances, too tight stuffing box, scavenge fire,misalignment of crankshaft,heat transmitted from outside due to fire.

• Correct air to fuel ratio -

Heat causes lub oil in contact to vaporize into fine explosive vapor which forms an oil mist due to condensation in relatively cooler area of crankcase.

• Fine oil vapors have large surface area in contact with air ; ready to explode.

*Exploded crankcase

Primary Explosion:

When lub oil falls on hot spot, because of high temperature it vaporizes into fine oil vapors.This vaporized oil moves in crankcase into a low pressure and relatively cold area and condense to form white mist.

This white mist is a perfect mixture of oil and air.This white oil mist circulates in crank case and when it comes in contact with hotspot gets ignited and BOOM - Primary explosion occurs.

Secondary Explosion:

Pressure waves created by primary explosion break open the crankcase.Thereby vacuum get created.Because of this air from outside rushes into crankcase and cause secondary explosion which is highly destructive than primary explosion.

Minimizing or Avoiding Hazardous condition:

By design and equipment-

• Bearing material chosen should have strength to carry maximum load and minimum generation of heat.Material shouldn't readily deteriorate.
• Adequate lubricating oil supply to bearings
• Thermometers to be provided in strategic locations
• Bearing temperature indicators to be installed
• Oil mist detectors to be installed
• Crankcase relief door to be installed

By operating personnel-

• Use of good quality oil
• Proper treatment and purification of oil
• Regular lub oil analysis for quality check
• lub oil purifiers to be always running
• lub oil temp shouldn't go high
• regular crankcase inspection[pay attention to wiping of metals,overheating]
• regular check of bearing clearances and crankshaft deflection
• ensure all warning devices and safety devices are working properly
• checking of oil lub oil filters for evidence of bearing material.

*Crankcase relief doors

Regulations concerning crankcase safety:

• All engines with bore dia > 300mm should have crankcase relief doors for all units
• Crankcase relief door fitted to be greater than 115sq.cm for 1cub.m of crankcase
• For engines between 150mm to 300mm bore dia, crankcase relief doors to be fitted at either ends of the engine
• All engines greater than 2250KW or bore dia > 300mm should have oil mist detector unit [OMD]

Indications of overheating other than Oil Mist Detector:

• High bearing temperature
• High lub oil temperature
• Unusual noise
• Smell of hot carbonizing oil
• White smoke from crankcase breather
• Feel over the crank case

Actions to be taken in the event of oil mist detector alarm actuation:

As per MAN B&W:

• Call all engineers alarm
• Inform wheel house.Reduce speed if auto slowdown has not activated
• Stop the engine at the earliest
• Keep blowers, lub oil pump and all cooling on
• Evacuate the engine room
• After 1hr crankcase should cool down
• Feel the crankcase and stop l.o pump
• Open crankcase keeping door as shield while opening
• Fire extinguisher to be ready
• Enter crankcase, investigate and rectify problem
• Turn engine with lub oil ON for 30 mins
• Start engine and slowly raise to full load.feel over the crankcase.
• After stop for a while and inspect again

Actual actions or response to be taken:

• Call all engineers alarm
• Inform wheel house and reduce rpm if auto slow down has not activated
• Check O2 and span setting of OMD
• If alarm is still coming follow MAN B&W actions
• If alarm has vanished then problem is with OMD

Severity of crankcase explosion can be reduced by crankcase relief valves, general housekeeping of ER,purifier rooms enclosed [will reduce fire spread]

*Understanding crankcase relief door valve -designed to open at a pressure not more than 0.2bar[0.02–0.1bar]. valve disc material is Aluminium.

Oil Mist Detector:

• Based on light scatter principle
• Individual sensors are fixed at each crank throw space and chain case
• A suction fan draws sample through each detector
• In detectors[ D1,D2….Dn] Light is transmitted at one end.Directly opposite to transmitter is compensating/check receiver
• A measurement or alarm receiver picks up scattered light produced by oil mist particles
• Monitor compares received signal against a set point[2.5% LEL]. Alarm condition is reached then alarm is set out.Auto slowdown is initiated.
•  

Picture credits: www.brighthubengineering.com, www.dhtd.co,www.ship-technology.com

Main Engine Chain Drive

A chain drive is used to transmit motion from crankshaft to camshaft of the engine.This drive is known as timing chain. It is responsible for the rotation of camshaft which governs the fuel pump and exhaust valve timing.

In 2 stroke engine rotational speed of camshaft is same as crankshaft. In 4 stroke engine rotational speed of camshaft is half that of crankshaft. An intermediate wheel serves as a guide and can be connected to a separate chain to drive governor,lubricators etc.This wheel is also called as idler wheel. An adjuster wheel serves to adjust the chain tension.Chain rollers run over chain sprocket wheels, one being bolted to crankshaft and other to camshaft.Chain are run between guide bars with rubber shock absorbing pads.

Adjuster wheel is mounted on levers which can be held at steady measured tension.Chain rollers rolling on wheels are amply lubricated by lubricating nozzles.

Chain details and material:

Link plates - Chromium Molybdenum steel

Pin - Hardened steel

Rollers - Alloy Steel

Reasons for chain elongation in service:

Chains have a very high factor of safety to prevent stretching. Any loss in tension in chain is due to wear while in use.Elongation is due to

• wear between pins and bushing
• wear between roller and bushing
• wear between roller and sprocket wheel teeth

Maximum elongation allowed is 2%. At 1.5% of elongation renew the chain.Elongation changes camshaft position with respect to crankshaft. As fuel and valve timings depends on camshaft position, chain elongation has adverse effect on timing.

Chain Elongation also results for greater shock loading,vibration,cyclic stresses and possible fatigue failure.Too tight chain results in overloading of chain wheel and bearings, wear of rollers,links and bearings, cracking of links.

Chain Tightness checking [small engines]:

• Turn the engine to slacken the longest free length of chain
• At the middle of longest face length, pull the chain by hand away from the guide bar
• Limit of slackness is half to one chain pitch length.If too slack or tight , adjustment is done for 1 chain pitch link length.

Chain inspection:

• Chain tightness is checked
• Inspect and check lub oil pipes and jet nozzles
• Check rubber track on the guide bars for condition
• Place a short straight edge plate and cover two points A and B on chain sprocket wheel tooth.Measure wear at point 1

• Check for cracks on rollers or side plates.Check free running of rollers for one complete revolution

Chain tightening procedure: As per MAN B&W MCC engines

 

• Open tab washers for nut A,B,C,D.
• Loosen the nuts A,B,C,D to free the chain tightener bolt
• Turn the engine so that slack side of the chain is on the same side as tightener wheel
• Make sure balance weight of moment compensators if fitted are hanging downwards
• Now tighten nut B until there is clearance of 0.1mm between the shaft and the nut. After tighten nut B to prescribed tightening angle as per manual.12 sides of hexagon nut for 6S60MCC engine. 12*60=720degrees
• Tighten nut C hard against contact face of the shaft
• Tighten nut D and lock nuts C&D with tab washer
• Tighten nut A and then lock nuts A&B with tab washer.
• Measure distance X

• If chain is worn ie, X>165mm.Repeat the whole tightening procedure.But tighten nut B on a reduced tightening angle of 600degrees[10 sides of hexagon].
• If still the X distance exceeds maximum distance of 165mm.Find and eliminate the cause of abnormal chain elongation.

Readjustment of camshaft angular position would be necessary after repeated chain tightening procedure as this causes camshaft position to be altered with respect to crankshaft.Maximum limit is 2degrees increase in lead angle.