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How to Prevent Crankcase Explosion on a

Ship?

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OCTOBER 31, 2010 BY MOHIT2 COMMENTS

Crankcase explosion is one of the most dangerous reasons that can lead to massive accidents and

fires on a ship. It is therefore imperative to prevent all the reasons that can lead to crankcase

explosion on a ship. In this article we will learn about the various methods to prevent crankcase

explosion on a ship.

The first and foremost thing to avoid any type of explosion on a ship, it is necessary to take the

preventive steps right from the basic roots. In a main engine crankcase also there are safety features

provided to detect the causes of explosion.

There are two main features provided on the crankcase to prevent crankcase explosion. They are as

follows:

1. Oil Mist Detector

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The Oil mist detector takes continuous samples from the main engine crankcase and check whether

the sample concentrations of mist are well below the level at which a crankcase explosion can take

place. The oil mist is drawn into the instrument with the help of small fan which takes suction from

each crankcase through sampling tubes provided on each crankcase.

The oil mist detector consists of a small rotator with which it takes sample from one cylinder at a time

and the rotator then turns to the next after approximately 4 seconds. The sample from the rotator

goes to the measured cell and the reference cell takes sample from rest of the crankcase to evaluate

the difference in oil mist.

An overall mist density of the crankcase is also measured by comparing the samples with the fresh

air once every rotation of the sampling valve is done. A beam of light from a common lamp is

reflected through mirrors and output is measured from a photo cell.

Under normal conditions the output from the reference and measured contact is same and hence no

deflection is measured. However, a deflection in the output gives an alarm indication and the valve

rotator stops at position to know which chamber has high mist concentration.


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Some engines are even fitted with slowdown alarms so that when the oil mist alarm rings, the engine

automatically slows down to prevent crankcase explosion.

2. Crankcase relief doors

The Crankcase relief doors are also fitted to prevent any damage to the crankcase and ingress of

fresh air inside the crankcase.

The crankcase doors are spring loaded valves which lift up in case there is any rise of pressure

inside the crankcase. Once the pressure is released they re-seat to prevent any ingress of fresh air.

This helps especially in case of any ingress of air that can lead to a secondary explosion followed by

a lot of surge and damage to the crankcase.

The opening pressure and sizes of the valves are specified by different classification societies,

depending on the volume of the crankcase. The number of doors to be present also depends on the

bore of the cylinder.

Reference:Instrumentation and control engineering by G.J Roy, Operation and maintenance of

machinery in motor ship by N.E Chell

Search

Propulsion


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Home Knowledge

Diesel Engine General

Flexible couplingsWith a medium speed system an number of engines and other devices are connectedtogether. Flexible couplings are required to account for the slight misalignment whichcan exist. The claw type coupling as used with steam turbines allows for thismisalignment and provides for a large area of contact which keeps he stress limited.

The diesel engine drive is pulsating and normal face to face contact cannot be allowed.

Rubber blocks are therefore fitted between the claws. When the dive takes place theleading blocks are compressed allowing clearance at the trailing blocks which are

hammered because of the pulsating drive. This results in wear. Blocks must there forebe pre compressed so that trailing blocks can expand and maintain normal contact withthe drive

http://www.marineengineering.org.uk/index.htmlhttp://www.marineengineering.org.uk/index.htmlhttp://www.marineengineering.org.uk/index.html


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Crankcase explosionsnder normal conditions the atmosphere in the crankcase when the engine is runningcontains a large amount of relatively large oil droplets !"## micron$ in warm air.

Because of the droplets small surface area to volume ratio% the possibility of ignition bya heat source is very low.

&hould overheating occur in the crankcase% say by failure of a bearing% then a hot spotis formed !typically over '##() although experiments have shown two separatetemperature ranges% the other between "*# + ,##()-. ere lub oil falling on to thesurface is vaporised ! in addition some is broken down to flammable gasses suchasHydrogen and acetylene$% the vapour can then travel away from the hotspot where

it will condense. The condensed droplets% in the form of a dense white mist% are verymuch smaller !/ to 0# microns$ than the original and so have a high surface area to

volume ratio. 1gnition by a hot spot !generally of the flammable gasses which in turnignite the fine droplets in the mist$% which may be the same on that cause the original

vaporisation% is now a possibility.

2il mists are formed at temperatures of around ,3#o)


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1gnition occurs at under 3##o)

The white mist will increase in si4e and density until the lower flammability limit isexceeded !about 3#mg5l is generally found in real situations $% the resultant explosioncan vary from relatively mild with explosion speeds of a few inches per second and little

heat and pressure rise. To severe with shock wave and detonation velocities of 0.3 to "miles per second and pressures of ,# atmospheres produced. This is the extreme casewith pressures of 0.3 to ,.# bar more normal raising to a maximum of *.# bar.

1t can be seen that following the initial explosion there is a drop in pressure% if the initialexplosion is not safely dealt with and damage to the crankcase closure occurs% it ispossible that air can be drawn in so creating the environment for a second and possiblelarger explosion. The limiting factors for an explosion is the supply of fuel and thesupply of oxygen% the air as shown can be drawn in by the slight vacuum created by the

primary explosion. The supply of fuel may be created by the passage of the shockwaveshattering the larger oil droplets into the small si4e that can readily combust.

By regulation%non returning relief doors must be fitted to the crankcase in order to relief the pressure of the initial wave but prevent a rapid ingress of air


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Vapour extraction fans

These generally take the form of a small electrically driven fan. They are fitted withflame traps on the exhaust side.

6lthough the fans keep the crankcase at a slight negative pressure thereby increasingthe risk of air being drawn in% this is seen to be more than compensated by the removalof flammable vapours and the reduction in oil leakage.

Crankcase doors

These when properly designed are made of about ,mm thick steel with a dished aspect

and are capable of withstanding 0" bar pressure. They are securely dogged with arubber seal arrangement.

Crankcase relief door (setting 1/15bar

7ue to the heavy force of momentum the gas shockwave is not easily deflected. Thus

any safety device must allow for a gradual change in direction% and be of the non+returntype to prevent air being drawn back into the crankcase

The original design was of cardboard discs which provided no protection against theingress of air after the initial explosion% in addition it was known for these discs to fail to

rupture in the event of an explosion


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The valve disc is made of aluminium to reduce inertia. The oil wetted gau4e provides avery effective flame trap This reduces the flame temperature from 03##() to "3#() in#.3 m. The ideal location for this trap is within the crankcase where wetness can beensured. The gas passing from the trap is not normally ignitable. The gau4e is generally#.,mm with '#8 excess clear areas over the valve.

&pecifically the regulations are9


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:on+return doors must be fitted to engines with a bore greater than ,##mm%at each cylinder with a total area of 003sq.cm5m, of gross crankcasevolume. The outlets of these must be guard to protect personnel fromflame. For engines between 03# to ,##mm relief doors need only be fittedat either end. Below this bore there is no requirement. The total clear areathrough the relief valve should not normally be less than ;.0,cm"5m, of

gross crankcase volume


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2il mists can be readily detected at concentrations well below that required for

explosions% therefore automated detection of these oil mists can be an effective methodof preventing explosions

&hown above is the @raviner oil mist detector. This is in common use in slow speed andhigh speed engines. The disadvantage of this type if system is that there is a lag due to

the time taken for the sample to be drawn from the unit and for the rotory valve toreach that sample point. For this reason this type of oil mist detector is not commonly

used on higher speed engines.?odern detectors often have the detection head mounted in the probe% the probe is

able to determine the condition of the crankcase and output an electrical signalaccordingly

The assembly consists of9Extraction fan+draws the sample from the sample points through the reference andmeasuring tubes via non+return valves.

'otary ale)This valve is externally accessible and is so marked so as to indicatewhich sample point is on line. 1n the event on exceeding the set point % the valve

automatically locks onto that point so giving a clear indication of the locality of the faultcondition.'eference tube+measures the average density of the mist within the crankcase% asthere will always be some mechanically generated mist.


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*easuring tube) measures the opacity of the sample by means of a photoelectric cellas with the measuring cell. To exclude variables in lamps a single unit is used withbeams directed down the tube by mirrors.

The photoelectric cell gives an output voltage proportional to the light falling on it. 1n

this way the opacity of the sample is measured% the voltages generated in the cell inthe measuring and reference tubes are compared in an electronic circuit. The differenceis compared to a potentiometer varied setpoint which if exceed initiates an alarmcircuit. The alarm circuit% dependant on installation% will generally declutch the drive tothe rotary valve% give an output signal to the engineroom alarm monitoring system andan output to the engine protection system causing it to slowdown.

The rotary valve also has a position marked (2( at which air is supplied to both tubes%and 4ero automatically !and manually if necessary$ ad=usted at each cycle. 1n additionat position (


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The relationship between the light landing on the sensor is nearly proportional to the oil

mist density therefore the unit can be calibrated in mg5l.

1t is possible to have the sensor and a 7 emitter in a single unit which may be

mounted on the crankcase. &everal of these can be placed on the engine each with aunique address poled by a central control unit. The results of which may be displayedon the control room


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having these heads mounted on the engine removes the need for long sample tubeswhich add to the delay of mist detection.This makes the system much more suitable foruse with medium and high speed engines were otherwise detection would beimpossible.

Crankcase doors (non relieingThe older type consisted of doors lightly held by retaining clamps or clips. With doors of

this type a pressure of #.3psi would give a permanent set of about "3mm% the doorswould be completely blown off by pressures of " to , psi ?odern large slow speed

engines have two types of crankcase door% a large securely held heavy mild steel squaredoor which allows good access for heavy maintenance.6 second smaller round dished aluminium door at around x+head height which allowsentry for inspection. 7ue to the curved design the door is able to withstand pressureswell above the setpoint for the relief doors.

,ctions in t+e eent of "il *ist detectionThe consequences of a crankcase explosion are extremely serious and the greatestpossible caution in the actions taken should be exercised.

&hould the oil mist detector activate an alarm condition% then personnel should takesteps to ascertain if the fault is real. They should initially assumed that it is% the bridgeshould be informed and the engines slowed if the oil mist detector has not already done


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so. &hould the bridge require manoeuvrability% and it is essential that the engine beoperated then consideration of evacuation of the engineroom should be made.2therwise the engine should be stopped and turned on gear until cooled.The @raviner 2il ?ist detector indicates via markings on the rotary valve which samplepoint has the high readings. By inspection of the graviner% and by viewing crankcase !orthrust% gearcase$ bearing readings it is possible to ascertain whether a fault condition

exists.

nder no circumstances should any aperture be opened until the engine has sufficientlycooled% this is taken as normal operating temperatures as an explosion cannot occurwhen no part has a temperature above "*#() !)ool flame temperature$2nce cooled the engine can be opened and ventilated !the crankcase is an enclosedspace$.

6n inspection should be made to locate the hotspot% the engine should not be run untilthe fault has been rectified.

Crankcase safety fitting

For the purpose of this &ection% starting air compressors are to be treated as auxiliaryengines

Relief valves• )rankcases are to be provided with lightweight spring+loaded valves or other

quick+acting and self+closing devices% of an approved type% to relieve the crankcases of

pressure in the event of an internal explosion and to prevent any inrush of air

thereafter. The valves are to be designed to open at a pressure not greater than #%"

bar.

• The valve lids are to be made of ductile material capable of withstanding the

shock of contact with stoppers at the full open position.

• The discharge from the valves is to be shielded by flame guard or flame trap to

minimi4e the possibility of danger and damage arising from the emission of flame.

• :umber of relief valves

• 1n engines having cylinders not exceeding "## mm bore and having a crankcase

gross volume not exceeding #%/ m,% relief valves may be omitted.

• 1n engines having cylinders exceeding "## mm but not exceeding "3# mm bore%

at least two relief valves are to be fitted9 each valve is to be located at or near the ends

of the crankcase. Where the engine has more than eight crank throws an additional

valve is to be fitted near the centre of the engine.


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• 1n engines having cylinders exceeding "3# mm but not exceeding ,## mm bore%

at least one relief valve is to be fitted in way of each alternate crank throw with a

minimum of two valves. For engines having ,% 3% *% ;% etc.% crank throws% the number

of relief valves is not to be less than "% ,% '% 3% etc.% respectively.

• 1n engines having cylinders exceeding ,## mm bore at least one valve is to be

fitted in way of each main crank throw.• 6dditional relief valves are to be fitted for separate spaces on the crankcase%

such as gear or chaincases for camshaft or similar drives% when the gross volume of

such spaces exceeds #%/ m,.

• &i4e of relief valves

• The combined free area of the crankcase relief valves fitted on an engine is to be

not less than 003 cm"5m, based on the volume of the crankcase.

• The free area of each relief valve is to be not less than '3 cm".

• The free area of the relief valve is the minimum flow area at any section through

the valve when the valve is fully open.

• 1n determining the volume of the crankcase for the purpose of calculating the

combined free area of the crankcase relief valves% the volume of the stationary parts

within the crankcase may be deducted from the total internal volume of the crankcase.

• Aent pipes

• Where crankcase vent pipes are fitted% they are to be made as small as

practicable to minimi4e the inrush of air after an explosion. Aents from crankcases of

main engines are to be led to a safe position on deck or other approved position.

• 1f provision is made for the extraction of gases from within the crankcase% e.g.

for oil mist detection purposes% the vacuum within the crankcase is not to exceed "3

mm of water.

•


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• Where access to crankcase spaces is necessary for inspection purposes% suitably

positioned rungs or equivalent arrangements are to be provided as considered

appropriate.

• When interior lighting is provided it is to be flameproof in relation to the interior

and details are to be submitted for approval. :o wiring is to be fitted inside the

crankcase.• Fire+extinguishing system for scavenge manifolds

• )rosshead type engine scavenge spaces in open connection with cylinders are to

be provided with approved fixed or portable fire+extinguishing arrangements which are

to be independent of the fire+extinguishing system of the engine room.

'eersing a slo- speed engineFor an engine to be reversed consideration must be given to the functioning of the fuelpump% air distributor and exhaust valves. That is% there commencement and completionof operation in respect to the crankshaft position.

Distributor

7ue to the differing requirements for the change in angle between the distributor andthe Fuel and exhaust cams% two camshafts are fitted although this adds to the cost ofinstallation. The small distributor camshafts has seperate ahead and astern camsad=acent to each other. Reversing is by pulling and pushing the camshaft axially.7uring normal engine operation the pistons of the distributor are held off the cams% thissimplifies the changeover of the camshaft. When starting air is required the pistons arefirst forced onto the cams. &tarting air is emitted during the low points and stop and

the ma=ority high points.

'eersing !et+ods

There are five solutions to reversal of the engine timing are

• Reversing servos on all camshaft+such as older &ul4ers

• separate ahead and astern cams with axial movement to bring cams into align

with rollers

• timing of fuel pumps% exhaust valve symmetrical

• fit air distributor as per doxford where reversal is performed internally by air flow

• fit fuel pumps as per BW new design where the follower is repositioned relative

to the camshaft and this re+times pump for new direction.The >xhaust timing issymmetrical


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.i!ed cylinder lubrication)ylinder lubrication should be in=ected in carefully metered amounts. The in=ectionpoints should be spaced around the periphery in such a way as to ensure adequatecoverage when the piston passes the feed points. The best timing for in=ection is

suggested as being between the first and second rings. The difficulties in achieving thisare great% but in=ecting at T7) and to a lesser extent B7) assists


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detergent and dispersant properties are required in order to hold deposits insuspension and thus keep surfaces clean

Behaviour depends upon the temperature of the liner% piston crown and piston rings.TB: and detergency are closely linked. This can have an adverse effect when running

on lighter fuels with lower sulphur content for any period of time. )oke deposits are canincrease.

Conseuences of under and oer lubricating

2ver lubrication will lead to excessive deposit build up generally in the form of carbondeposits. This can lead to sticking of rings causing blowpast and loss of performance%build up in the underpiston spaces leading to scavenge fires% blockage and loss ofperformance of Turboblowers as well as other plant further up the flue such as wasteheat recovery unit and power turbines.nder lubrication can lead to metal to metal contact between liners causingmicrosei4ure or scuffing. >xcessive liner and piston wear as well as a form of wear notonly associated with under lubrication but also with inadequate lubrication called

cloverleafing


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)auses

• 1nsufficient cyl l.o

• 1ncorrect cyl l.o.

• Blocked quill

• 1ncorrect cyl at each stroke.

The fine ad=ustment operates in such away that by screwing it in the stroke of each

pump may be accurately metered. 6dditionally it may be pushed into give a strokeenabling each p5p to be tested. The eccentric stroke ad=uster acts as a coarse

ad=ustment for all the pumps in the block. 6dditionally it may be rotated to operate allthe pumps% as is the case when the engine is pre+lubricated before starting. )orrect


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operation of the in=ection pumps whilst the engine is running can be carried out byobserving the movement of the ball

Electronic cylinder lubrication

>xact in=ection timing of cylinder lube oil is essential for efficiency. 6 move toelectronics for the control of this has been made by some large slow speed enginemanufacturers.

The system is based on an in=ector which in=ects a specific volume of oil into eachcylinder on each ! though more normally alternate$ revolution of the engine. 2il issupplied to the in=ector via a pump or pumps. 6 computer% which is synchronised to the


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engine at T7) each revolution% finitely controls the timing . @enerally most efficientperiod for lubrication is taken at the point when the top rings are ad=acent to thein=ection points.

The in=ection period is governed by the opening of a return or (dump( solenoid which

relieves system pressure.

Cuantity can be ad=usted by manually limiting the stroke of the main lubricator piston%by altering the in=ection period or by the use of multiple mini+in=ections per revolution.

The high degree of accuracy with this system allows for lower oil consumption rates.

&hown is the in=ector unit fitted to modern camshaftless slow speed engines. Themotive force is via a dedicated or common hydraulic system. The hydraulic piston acts

on multiple plungers one for each quill. 6t the dedicated time the electric solenoid valveenergises an allows hydraulic oil to act on the piston commencing oil in=ection. 2ne or

two pumps per unit may be fitted dependent on cylinder diameter and oil flowrequirements.

Drecise control of the timing of in=ection allows oil to be delivered into the ring pack%something which has proved impossible with mechanical means. This has reduced oilconsumption by as much as 3#8.


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Dre+ lubrication for starting may be built into the bridge remote control system orcarried out manually

Cylinder lubricator uill


23/40


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Cranks+aft DeflectionsTo see why crankshaft deflections are taken it is first necessary to look at one section i.e. two crank webs% a

crank pin and two =ournals.E

1f a straight length of shafting 1 supported at either end is sub=ected to a central load the effect is for the

shaft to sag with the upper material in compression and the lower in tension

This effect is applicable to the section of crankshaft described above with the bearings supporting the

assembly at the =ournals and the point loading being effect by the weight of the piston and conrod assembly! ignoring other loads found operational conditions such as combustion and centrifugal $.

Effect on Cranks+aft

1t can be seen that the effect is to increase the distance between the webs at top dead centre !T7)$ and

reduce the distance at bottom dead centre !B7)$. This deflection is normally found in all crankshaftsalthough for smaller engines with very rigid cranks this may be very small.

6 set of measurements taken from an engine will reveal this deflection which should be constant througheach crank5piston unit. The caveat to this is that increase deflection is seen at the fly wheel and cam chaingear wheel sections due to the increased loading.

Finding faults6fter initial installation and alignment a set of deflections are taken. These then form the datum line towhich all other recordings are measured against.

it should be noted that changes in circumstances will effect the deflections are not indicative of faults. Theseinclude9

• 6mbient temperature

• >ngine temperature

• vessel hull loading !hogging% sagging etc$

• vessel afloat% dry docked ! again vessel hull loading can cause effects even in drydock due to

movement of blocks% which tanks are full etc$

these effects are well known and an experienced engineer will take into account these factors when looking

at a set of recordings

1f a situation now occurs where a bearing becomes more worn than an ad=acent one the effects will beshown as a change in the pattern of deflections. When the cranks is turned from B7) to T7) the weight ofthe running gear causes the crank webs and crankpins to bend in such a manner that the distance between

the webs decreases% and continues to decrease until the bearing is no longer in contact with the =ournalbr-The deflection when the crankshaft is approaching T7) will then go from its normal positive reading to 4eroand then to negative readings at which point the assembly is supporting the weight without the assistancefrom the lowered main bearing.


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Thus% any changes from natural deflections can be related to main bearing misalignement and isproportional to the differences in height of the bearings

.aking *easure!entsThese are generally taken using a spring loaded dial gauge. The crank webs ar pock marked to ensure that

the readings are taken in the same place each time. Five measuring points are taken+ T7)% ;#( either side of T7) and ,#( either side of B7). The latter two measurements are required as it is not possible to measureat B7) due to the )on rod.

The measurements are always taken starting at the same starting point. 1n this case we will say Dort sidenear B7). The gauge is fitted and 4eroed. The engine is rotated continuously and the readings read off


26/40

during rotation. 6fter the final reading the egine is rotated back to the start point. 1f the reading is not 4erothen it indicates that the gaige is moved and the readings are re+taken.

Exa!pleThese readings were taken from a BW /G*/>F !1 bet you haven(t sailed with one of them% it(s the one with

the rocker arms and the self ad=usting tappets that make you crap yourself when they fail$


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)rank Dosition :o0 )yl :o" )yl :o, )yl :o' )yl :o3 )yl :o/ )yl

Dort near B7) !H$ # # # # # #

Dort ori4ontal !D$ / 0 * +; +' '

T7) !T$ 0" , 0, +0/ +0" 3

&tbd ori4ontal !&$ / , / +* +I ,

&tbd near B7) !J$ +0 " +" " 0 '

corrected B7) !HKJ5"LB$ # 0 +0 0 # "

Vertical ,ligne!ent

These figures may now be used to draw a misaligement curve similar to the one below and may be analysedto see which bearings are in need of ad=ustment.

the assistance from the lowered main bearing.


Aertical alignement MT+BLAN 0" " 0' +0* +0" *

Hori0ontal ,ligne!ent

table class L OlistO-tr-T7-)rank DositionT7-:o0 )ylT7-:o" )ylT7-:o, )ylT7-:o' )ylT7-:o3 )ylT7-:o/ )yl

tr-td-ori4ontal alignement b-MD+&LN5b-td-#td-+"td-0td-+"td-'td-0

5table-


28/40

Gauge reading C+eck

) 7 should be practically the same% hence the readings from :o/ )yl may be suspect


ori4ontal alignment MD+&LN # +" 0 +" ' 0


TKBL) 0" ' 0" +03 +0" ,

DK&L7 0" ' 0, +0/ +0" *

acket 2ater 3yste!


29/40

&hown above is a typical cooling water circuit for a slow speed engine.Water is pump via one of two centrifugal pumps. 2ne is normally in use with the otherstand+by. The water passes through to the distributing manifold on the engine side.

Packet Water eater 1n the line is a steam =acket water heater. When the engine is shutdown steam heating maintains the engine in a state of readiness reducing the timeneeded for starting. 6ttempting to start the engine without heating can lead to poorcombustion% poor lubrication and thermal shocking. 6 modern variation on this is theOblendO water from the stand+by auxiliary alternator engines into the main enginecircuit increasing plant efficiency

The water enters and leaves the engine via a series of cylinder isolating valves. 1n thisway each cylinder may be individually drained to prevent excessive water and chemical

loss. 1n addition dual level drains may be fitted which allow either full draining ordraining of the head only. 6 portion of the water is diverted for cooling of theturbocharger.

7eaerator Was an essential part of engines incorporating water cooled pistons were air

was deliberately introduced in to the system to aid the Ococktail shakerO cooling action.6ir or gas entering the system can lead to unstable and even total loss of cooling waterpressure as the gas expands in the suction eye of the circulating pumps. 1n the event of


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gas leakage via the head or cracked liner rapid loss of =acket water pressure can occur.The deaerator is a method to try to slow this process sufficiently to allow the vessel tobe placed in a safe position for maintenance. This system also allows the vessel tooperate with minor gas leakage.

Packet Water )ooler The hot water leaving the engine passes to a temperature controlvalve were a portion is diverted to a cooler. Temperature is controlled using both afeedback signal !temperature measured after the cooler$ and a feed forward signal!temperature measured at outlet from the engine$. 1n this way the system reacts morequickly to engine load variations.

>vaporator 1ncreases plant efficiency by utilising heat in =acket water to produce freshwater. ?odern systems sometimes rely on the evaporator to supplement a reduced si4emain cooler. >xpansion or header tank ?aintains a constant head on the circulation pumps reducing

cavitation at elevated temperatures. 6llows the volume of water in the system to varywithout need for dumping. 6cts as a reserve in the event of leakage

T5


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1n areas of deposit formation% dissolved solids% specifically )alcium and magnesiumhardness constituents can precipitate from cooling water as the temperature increases.7eposits accumulate on the heat transfer surfaces as sulphates and carbonates% themagnitude of which is dependent on the water hardness% the dissolved solid content%local temperatures and local flow characteristics. Temperature solubility curves for)a&2'

&cales can reduce heat transfer rates and lead to loss of mechanical strength ofcomponent parts% this can be exacerbated by the presence of oils and metal oxides.

The degree and type of scaling in a cooling water circuit are determined by9

• &ystem temperatures

• 6mount of leakage5makeup

• quality of make up

• quality of treatment

Calciu! Carbonate

6ppears as a pale cream% yellow deposit formed by the thermal decomposition ofcalcium bi+carbonate)a!)2,$" K eat becomes )a)2, K "2 K )2"


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*agnessiu! 3ilicate

6 rought textured off white deposit found where sufficient amounts of ?agnesium arepresent in con=unction with adequate amounts of silicate ions with a deficiency onh 2alkalinity?g"K K 2+ becomes ?g2K

"&i2, becomes K K &i2,+?g2K K &i2,+ becomes ?g&i2, K "&2'&ilicate deposit is a particular problem for systems which utilise silicate additives forcorrosion protection. Thi sis typical of systems with aluminium metal in teh coolingsystem. The silicate forms a protective barrier on the metal surface. 6 high p !;.3 +0#.3$ is required to keep the silicate in solution. 1n the event of sea watercontaimination or some other mechanism that reduces the p the silicate is rapidlyprecipitated and gross fouling can occur.

4ron "xides) He!atite (Fe#"

1s a loose red 5brown deposit and is indicative fo active corrosion within a system

)opperThe prescence of copper within a cooling system is very serious ast it can lead toagressive corrosion through galvanic action. &pecific corrosion inhibitors are containedwith cooling water system corrosion inhibitors.

Effects of scale deposition

The effects of scale deposition can be both direct or indirect%typically but not specifically1nsulates cooling surfaces leading to9

• increased material temperatures as the temperature gradient must increase to

ensure maintain heat flow.

•


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electro+chemical series with the more noble at the top . Those metals at the top arecathodic to those lower down. The relative positions between two metals in the tabledetermined the direction and strength of electrical current that flows between them andhence% the rate at which the less noble will corrode

Galanic ,ction

)orrosion within cooling systems can occur if the coolant% i.e. water% has not beenproperly treated. The corrosion can take the form of acid attack with resultant loss ofmetal from a large area of the exposed surface% or by 2xygen attack characterised by

pitting. 6 primary motive force for this corrosion is @alvanic action

.+e Galanic 3eries$

2r >lectromotive series for metals)athode

@old and DlatinumTitanium

&ilver&ilver solder

)hromium+:ickel+1ron !Dassive$)hromium+1ron !Dassive$&tainless &teel !Dassive$)opper?onel*#5,# )upro+:ickel/*+,, :ickel+)opperydrogenleadTin"+0 Tin lead &olderBron4es

Brasses:ickel&tainless+&teel 0I+I !6ctive$&tainless &teel 0I+I+, !6ctive$

)hromium 1ron !6ctive$)hromium+:ickel+1ron !6ctive$

)admium1ron

&teel)ast 1ron

)hromiumQinc

6luminium6luminium 6lloys?agnesium6nodeThe metals closer to the anodic end of the list corrode with preference to the metalstowards the cathode end.6 galvanic cell can occur within an apparently omogeneous material due to severalprocesses on of which is differential aeration where one area is exposed to more oxygenthan another. The area with less oxygen becomes anodic and will corrode.


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Galanic action -it+in !etal

@alvanic action due to temperature gradient


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This situation can exist in cooling water systems with complex layout of heatexchangers and passage ways within the diesel engine. &ystems containing readilycorrodible metals such as 4inc% tin and lead alloys can complicate and intensifyproblems by causing deposit formations.

Differential ,eration

+Where only a single metal exists within a system corrosion can still take place if the

oxygen content of the electrolyte is not homogenous. &uch a situation can occur readilyin a =acket water system as regions of stagnant flow soon have the oxygen level

reduced by the oxidation of local metal. The metal ad=acent to water with reducedlevels of oxygen become anodic to metals with higher oxygen content electrolyte in

contact with it.. @enerally% the anodic metal is small in comparison the cathode i.e. thearea of stagnant flow is small compared to the area of normal flow of electrolyte% and

high rates of corrosion can exist. 2ne clear case of this is the generation of deep pitsbelow rust scabs.

3olutions

2ater treat!ent

To remove the risk of corrosion it is necessary to isolate the metal surface form the

electrolyte. 2ne method would be by painting% but this is impractical for engine cooling


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water passages. 6 better solution would be a system which not only searched out baremetal coating it with a protective barrier% but also repaired any damage to the barrier.

• for corrosion to occur four conditions must be met9

• There must be an 6node

• There must be a cathode

•

6n electrolyte must be present• 6n electron pathway should exits

Corrosion 4n+ibitors

• )orrosion inhibitors are classified on how they affect the corrosion cell and are

placed into three categories9

• 6nodic 1nhibitors

• )athodic 1nhibitors

• )ombination inhibitors5organic inhibitors

Co!!on Corrosion 4n+ibitors

• Drincipally 6nodic 1nhibitors

• )hromate• :itrite

• 2rthophosphtae

• Bicarbonate

• &ilicate

• ?olybdenate

• Drincipally )athodic 1nhibitors

• )arbonate

• Dolyphosphate

• Dhosphonates

• Qinc

• Both 6nodic and )athodic 1nhibitors

• &oluble 2ils

• ?ercaptoben4othia4ole !?BT$

• Ben4otria4ole !BQT$

• Tolytria4ole !TTQ$

,nodic 4n+ibitors

:itrite !:2"+ $+ These are the most commonly used form of treatment and operate byoxidising mild steel surfaces with a thin% tenacious layer of corrosion product !magnetiteFe,2'$. Relatively high volumes of treatment chemical are required so this method isonly viable on closed systems

&odium :itrite+ !sometimes with Borate added$+effective with low dosage%concentration non+critical. 1t is non toxic% compatible with anti free4es and closedsystem cooling materials. 1t does not polymerise or breakdown. owever protection fornon+ferrous materials is low. 6n organic inhibitor is thus required. 6lthough will notcause skin disease it will harm eyes and skin. 6pproved for use with domestic freshwater systems.


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&odium :itrite is a Dassivator% a passivator will act chemically to produce an insulatinglayer on the metal surface. Whenever corrosion takes place the corrosion productsincluding bubbles of gas% are released from the metal surfaces. Dassivating chemicalsact on the corrosion products preventing release from the metal surface and thus

stifling further corrosion. 1f the insulating layer becomes damaged% corrosion begins again and the passivator acts on the new products to repair the layer.

)hromate(s+the first passivator product was &odium )hromate which was an excellentinhibitor. 1nexpensive% effective and concentration easily tested. )orrosion may increaseby incorrectly dosing% dangerous to handle% poisonous and can cause skin disease. :otallowed where domestic water production is in use !Packet water heated evaporators$.nfortunately it was also highly toxic% a severe pollutant and staining agent% wasincompatible with antifreee4es% and will attack 4inc and soft solder slightly. 7ue to itstoxicity is sometimes used as a biocide in such places as brine in large Reefer plants.

&ilicates+ react with dissolved metal ions at the anode. The resultant ion5silicatecomplex forms a gel that deposits on anodic sites. This gel forms a thin% adherent layerthat is relatively unaffected by p in comparison to other inhibitors. The inhibitingproperties increase with temperature and p% normal p levels are ;.3 to 0#.3.)are should be taken with the use of silicates% which are often used for the protection of system containing aluminium. 1n the event of boiling increased concentrations and leadto aggressive corrosion due to the high p.2rthophosphate Forms an insoluble complex with dissolved ferric ions that deposit atthe anodic site. 1t is more adherent and less p sensitive than other anodic inhibitors.The film forms in p of /.3 to *.#. 7osage is typically 0#ppm in neutral water

Cat+odic 4n+ibitors

6olyp+osp+ate+ Forms complexes with )alcium% Qinc and other divalent ions% thiscreates positively charged colloidal particles. These will migrate to the cathodic site andprecipitate to form a corrosion inhibiting film. The presence of calcium is required at atypical minimum concentration of 3#ppm.

>xtreme variations in p can upset the film and a reversion to orthophosphate willoccur with time and temperature.

Dositively charged 4inc ions migrate to the cathodic site and react with the free hydroxylions to form a 4inc hydroxide stable film at p *.' to I.". 1f the water is too acidic thefilm will dissolve and not reform. 1f it is too alkaline the 4inc hydroxide will precipitate inbulk and not at the cathodic site.6+osp+onates. 1nitially introduced as scale inhibitors to replace polyphosphates% theyexhibit absorption at the metal surface especially in alkaline hard water. @enerally usedwith other inhibitor types


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7ot+ ,nodic and Cat+odic 4n+ibitors

Ben4otria4ole and Tria4ole &pecific corrosion inhibitor for copper. They break theelectrochemical circuit by absorbing into the copper surface.They are generally added to standard treatments.

&oluble and dispersible oils. Detroleum industry recognised that emulsifying cutting oils

!erroneously called soluble oils$ were able to reduce corrosion on metals by coating thesurface. There were disadvantages though% if the coating became too thick then it could

retard the heat transfer rate. 6dherent deposits form as organic constituentspolymerise or form break down products which can accumulate and disrupt flow. ?6:+

BW recommend it not to be used.

1t is effective in low dosages% safe to handle and safe with domestic water production.>ffectiveness is reduced by contamination with carbon% rust% scale etc. 7ifficult to check

concentration% overdosing can lead to overheating of parts2ils are classed as a barrier layer type inhibitor. The surfaces being coated in a thin

layer of oil.

*odern treat!ent

:itrite+Borate treatment is most effective with a high quality water base. This treatmenthas no scale prevention properties and its effectiveness is reduced by high quantities ofdissolved solids.

6 modern treatment will be a :itrite +Borate base% with a complex blend of organic andinorganic scale and corrosion inhibitors plus surfactants% alkali ad=usters% dispersantsand foam suppressers. 6 high quality water supply is still strongly recommended.

.+e 8se of 3acrificial ,nodes

+>lectrolytic protection for the whole system by the use of sacrificial anodes isimpractical. Darameters such as water temperature% relative surface area of anode and

cathode% activity of metals in system and relative positions in galvanic series come intoplay. 6nodic protection has become out of favour for cooling water systems as it can

lead to local attack% causes deposits leading to flow disturbance and it has no scaleprotection


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6reparation for cooling -ater treat!ent

+6ll anodes should be removed and the system inspected. :o galvanised piping is to beused !old piping can be assumed to have had the @alvanising removed$. igh qualitywater should be used and chemicals measured and added as required. 6 history log

should be kept

*icrobiological Fouling

nder certain conditions bacteria found in cooling water systems can adapt to feed onthe nitrite treatment.This can lead to rapid growth% formation of bio+films% fouling and

blockages.Typical evidence is a loss of nitrite reserve but a stable or rising conductivity level asthe nitrate formed still contributes to the conductivity%Droblems of this sort are rare due to the elevated temerpatures and p levels. &hould it

occur treatment with a suitable biocide is required.


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