Crankshaft Deflection3

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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 arehammered 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

Crankcase explosionsUnder normal conditions the atmosphere in the crankcase when the engine is runningcontains a large amount of relatively large oil droplets (200 micron) in warm air.Because of the droplets small surface area to volume ratio, the possibility of ignition bya heat source is very low.

Should overheating occur in the crankcase, say by failure of a bearing, then a hot spotis formed (typically over 400'C although experiments have shown two separatetemperature ranges, the other between 270 - 300'C>. Here lub oil falling on to thesurface is vaporised ( in addition some is broken down to flammable gasses such asHydrogen and acetylene ), the vapour can then travel away from the hotspot where itwill condense. The condensed droplets, in the form of a dense white mist, are verymuch smaller (6 to 10 microns) than the original and so have a high surface area tovolume ratio. Ignition 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

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vaporisation, is now a possibility.

Oil mists are formed at temperatures of around 350 o CIgnition occurs at under 500 o C

The white mist will increase in size and density until the lower flammability limit isexceeded (about 50mg/l is generally found in real situations ), the resultant explosioncan vary from relatively mild with explosion speeds of a few inches per second and littleheat and pressure rise. To severe with shock wave and detonation velocities of 1.5 to 2miles per second and pressures of 30 atmospheres produced. This is the extreme casewith pressures of 1.5 to 3.0 bar more normal raising to a maximum of 7.0 bar.

It can be seen that following the initial explosion there is a drop in pressure, if theinitial explosion is not safely dealt with and damage to the crankcase closure occurs, itis possible that air can be drawn in so creating the environment for a second andpossible larger explosion. The limiting factors for an explosion is the supply of fuel andthe supply of oxygen, the air as shown can be drawn in by the slight vacuum createdby the primary explosion. The supply of fuel may be created by the passage of theshockwave shattering the larger oil droplets into the small size that can readilycombust.

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

Vapour extraction fansThese generally take the form of a small electrically driven fan. They are fitted withflame traps on the exhaust side.Although 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 theremoval of flammable vapours and the reduction in oil leakage.

Crankcase doorsThese when properly designed are made of about 3mm thick steel with a dishedaspect and are capable of withstanding 12 bar pressure. They are securely doggedwith a rubber seal arrangement.

Crankcase relief door (setting 1/15bar)Due to the heavy force of momentum the gas shockwave is not easily deflected. Thusany safety device must allow for a gradual change in direction, and be of the non-return type to prevent air being drawn back into the crankcaseThe original design was of cardboard discs which provided no protection against the



ingress of air after the initial explosion, in addition it was known for these discs to failto rupture in the event of an explosion

The valve disc is made of aluminium to reduce inertia. The oil wetted gauzeprovides a very effective flame trap This reduces the flame temperature from1500'C to 250'C in 0.5 m. The ideal location for this trap is within the crankcasewhere wetness can be ensured. The gas passing from the trap is not normallyignitable. The gauze is generally 0.3mm with 40% excess clear areas over thevalve.

Specifically the regulations are;Non-return doors must be fitted to engines with a bore greater than

300mm, at each cylinder with a total area of 115sq.cm/m3 of grosscrankcase volume. The outlets of these must be guard to protectpersonnel from flame. For engines between 150 to 300mm relief doorsneed only be fitted at either end. Below this bore there is norequirement. The total clear area through the relief valve should notnormally be less than 9.13cm 2 /m 3 of gross crankcase volume

Lub Oil drain pipes to the sump must extend below the surface and multiengine installations should have no connections between the sumps

Large engines, of more than 6 cylinders are recommended to have adiaphragm at mid-length and consideration should be given to detectionof overheating (say by temperature measuring probes or thermalcameras) and the injection of inert gas.

Engines with a bore less than 300mm and a crankcase of robustconstruction may have an explosion door at either end

Means of detection of oil mist fitted.

Continuous extraction by exhauster fan may be used but this tends to be costly,flame gauzes must be fitted to all vents. Similarly a continuous supply of air can beused to reduce gas mist levels.

Crankcase oil mist detector (Obscuration)(set point 2.5% L.E.L)



Oil mists can be readily detected at concentrations well below that required forexplosions, therefore automated detection of these oil mists can be an effectivemethod of preventing explosionsShown above is the Graviner oil mist detector. This is in common use in slow speedand high speed engines. The disadvantage of this type if system is that there is a lagdue to the time taken for the sample to be drawn from the unit and for the rotoryvalve to reach that sample point. For this reason this type of oil mist detector is notcommonly used on higher speed engines.Modern detectors often have the detection head mounted in the probe, the probe isable to determine the condition of the crankcase and output an electrical signalaccordingly

The assembly consists of;Extraction fan -draws the sample from the sample points through the reference andmeasuring tubes via non-return valves.Rotary valve- This valve is externally accessible and is so marked so as to indicatewhich sample point is on line. In the event on exceeding the set point , the valveautomatically locks onto that point so giving a clear indication of the locality of thefault condition.Reference tube -measures the average density of the mist within the crankcase, asthere will always be some mechanically generated mist.Measuring tube- measures the opacity of the sample by means of a photoelectriccell as 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. Inthis 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. Thedifference is compared to a potentiometer varied setpoint which if exceed initiates analarm circuit. The alarm circuit, dependant on installation, will generally declutch thedrive to the rotary valve, give an output signal to the engineroom alarm monitoringsystem and an output to the engine protection system causing it to slowdown.

The rotary valve also has a position marked 'O' at which air is supplied to both tubes,and zero automatically (and manually if necessary) adjusted at each cycle. In additionat position 'L' an average sample of the crankcase is compared to air.

Crankcase oil mist detector (light scatter)The disadvantage of obscuration types is that they are generally slow to operate andsuffer from inaccuracies and false alarms caused by such things as a dirty lens.Light scatter do not suffer from these problems, are faster reacting and do not needto set zero during engine operations.



The relationship between the light landing on the sensor is nearly proportional to theoil mist density therefore the unit can be calibrated in mg/l.

It is possible to have the sensor and a LED emitter in a single unit which may bemounted on the crankcase. Several of these can be placed on the engine each witha unique address poled by a central control unit. The results of which may bedisplayed on the control room

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 suitablefor use with medium and high speed engines were otherwise detection would beimpossible.

Crankcase doors (non relieving)The older type consisted of doors lightly held by retaining clamps or clips. With doorsof this type a pressure of 0.5psi would give a permanent set of about 25mm, the doorswould be completely blown off by pressures of 2 to 3 psi Modern large slow speedengines have two types of crankcase door, a large securely held heavy mild steelsquare door which allows good access for heavy maintenance.A second smaller round dished aluminium door at around x-head height which allows



entry for inspection. Due to the curved design the door is able to withstand pressureswell above the setpoint for the relief doors.

Actions in the event of Oil Mist detectionThe consequences of a crankcase explosion are extremely serious and the greatestpossible caution in the actions taken should be exercised.

Should 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 alreadydone so. Should the bridge require manoeuvrability, and it is essential that the enginebe operated then consideration of evacuation of the engineroom should be made.Otherwise the engine should be stopped and turned on gear until cooled.The Graviner Oil Mist detector indicates via markings on the rotary valve which samplepoint has the high readings. By inspection of the graviner, and by viewing crankcase(or thrust, gearcase) bearing readings it is possible to ascertain whether a faultcondition exists.

Under no circumstances should any aperture be opened until the engine hassufficiently cooled, this is taken as normal operating temperatures as an explosioncannot occur when no part has a temperature above 270'C (Cool flame temperature)Once cooled the engine can be opened and ventilated (the crankcase is an enclosedspace).

An inspection should be made to locate the hotspot, the engine should not be run untilthe fault has been rectified.

Crankcase safety fittingFor the purpose of this Section, starting air compressors are to be treated as auxiliaryengines

Relief valvesCrankcases are to be provided with lightweight spring-loaded valves orother quick-acting and self-closing devices, of an approved type, to relieve

the crankcases of pressure in the event of an internal explosion and toprevent any inrush of air thereafter. The valves are to be designed to openat a pressure not greater than 0,2 bar.The valve lids are to be made of ductile material capable of withstandingthe shock of contact with stoppers at the full open position.The discharge from the valves is to be shielded by flame guard or flametrap to minimize the possibility of danger and damage arising from theemission of flame.

Number of relief valvesIn engines having cylinders not exceeding 200 mm bore and having acrankcase gross volume not exceeding 0,6 m3, relief valves may beomitted.In engines having cylinders exceeding 200 mm but not exceeding 250 mmbore, at least two relief valves are to be fitted; each valve is to be locatedat or near the ends of the crankcase. Where the engine has more thaneight crank throws an additional valve is to be fitted near the centre of theengine.In engines having cylinders exceeding 250 mm but not exceeding 300 mmbore, at least one relief valve is to be fitted in way of each alternate crankthrow with a minimum of two valves. For engines having 3, 5, 7, 9, etc.,crank throws, the number of relief valves is not to be less than 2, 3, 4, 5,etc., respectively.In engines having cylinders exceeding 300 mm bore at least one valve isto be fitted in way of each main crank throw.Additional relief valves are to be fitted for separate spaces on thecrankcase, such as gear or chaincases for camshaft or similar drives, whenthe gross volume of such spaces exceeds 0,6 m3.

Size of relief valvesThe combined free area of the crankcase relief valves fitted on an engine isto be not less than 115 cm2/m3 based on the volume of the crankcase.The free area of each relief valve is to be not less than 45 cm2.The free area of the relief valve is the minimum flow area at any sectionthrough the valve when the valve is fully open.In determining the volume of the crankcase for the purpose of calculating



the combined free area of the crankcase relief valves, the volume of thestationary parts within the crankcase may be deducted from the totalinternal volume of the crankcase.

Vent pipesWhere crankcase vent pipes are fitted, they are to be made as small aspracticable to minimize the inrush of air after an explosion. Vents fromcrankcases of main engines are to be led to a safe position on deck orother approved position.If 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 25 mm of water.Lubricating oil drain pipes from engine sump to drain tank are to besubmerged at their outlet ends. Where two or more engines are installed,vent pipes, if fitted, and lubrication oil drain pipes are to be independentto avoid intercommunication between crankcases.

AlarmsAlarms giving warning of the overheating of engine running parts,indicators of excessive wear of thrusts and other parts, and crankcase oilmist detectors are recommended as means for reducing the explosionhazard. These devices should be arranged to give an indication of failure of the equipment or of the instrument being switched off when the engine isrunning.

Warning noticeA warning notice is to be fitted in a prominent position, preferably on acrankcase door on each side of the engine, or alternatively at the engineroom control station. This warning notice is to specify that wheneveroverheating is suspected in the crankcase, the crankcase doors or sightholes are not to be opened until a reasonable time has elapsed afterstopping the engine, sufficient to permit adequate cooling within thecrankcase.

Crankcase access and lightingWhere access to crankcase spaces is necessary for inspection purposes,suitably positioned rungs or equivalent arrangements are to be providedas considered appropriate.When interior lighting is provided it is to be flameproof in relation to theinterior and details are to be submitted for approval. No wiring is to befitted inside the crankcase.

Fire-extinguishing system for scavenge manifoldsCrosshead type engine scavenge spaces in open connection with cylindersare to be provided with approved fixed or portable fire-extinguishing

Reversing a slow speed engineFor an engine to be reversed consideration must be given to the functioning of thefuel pump, air distributor and exhaust valves. That is, there commencement andcompletion of operation in respect to the crankshaft position.

DistributorDue 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 of installation. The small distributor camshafts has seperate ahead and astern camsadjacent to each other. Reversing is by pulling and pushing the camshaft axially.During normal engine operation the pistons of the distributor are held off the cams,this simplifies the changeover of the camshaft. When starting air is required thepistons are first forced onto the cams. Starting air is emitted during the low pointsand stop and the majority high points.

Reversing methods

There are five solutions to reversal of the engine timing areReversing servos on all camshaft-such as older Sulzersseparate ahead and astern cams with axial movement to bring camsinto align with rollerstiming of fuel pumps, exhaust valve symmetricalfit air distributor as per doxford where reversal is performedinternally by air flowfit fuel pumps as per B&W new design where the follower isrepositioned relative to the camshaft and this re-times pump fornew direction.The Exhaust timing is symmetrical



Timed cylinder lubricationCylinder lubrication should be injected in carefully metered amounts. The injectionpoints 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 injection issuggested as being between the first and second rings. The difficulties in achievingthis are great, but injecting at TDC and to a lesser extent BDC assistsLubrication is of the total loss system i.e. the oil is expected to be completelycombusted without residue. The oil is injected through quills which pass through theliner wall.

Cylinder lub oil propertiesThe type of cyl l.o. required will depend upon the cylinder conditions and the enginedesign e.g crosshead or trunk piston. However, the property requirements arebasically the same but will vary in degree depending upon the fuel and operatingconditions.

Normal properties required are;adequate viscosity at working temperature so that the oil spreads over the

liner surface to provide a tough film which resists the scrapper action of the piston rings

the oil must provide an effective seal between the rings and lineronly a soft deposit must be formed when the oil burnsalkalinity level (total base number or TBN) must match the acidity of the oil

being burntdetergent and dispersant properties are required in order to hold deposits in

suspension and thus keep surfaces clean

Behaviour depends upon the temperature of the liner, piston crown and piston rings.TBN and detergency are closely linked. This can have an adverse effect when runningon lighter fuels with lower sulphur content for any period of time. Coke deposits arecan increase.

Consequences of under and over lubricatingOver 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 of performance of Turboblowers as well as other plant further up the flue such as wasteheat recovery unit and power turbines.Under lubrication can lead to metal to metal contact between liners causingmicroseizure or scuffing. Excessive liner and piston wear as well as a form of wear notonly associated with under lubrication but also with inadequate lubrication calledcloverleafing



CausesInsufficient cyl l.oIncorrect cyl l.o.Blocked quillIncorrect cyl at each stroke.

The fine adjustment operates in such away that by screwing it in the stroke of eachpump may be accurately metered. Additionally it may be pushed into give a strokeenabling each p/p to be tested. The eccentric stroke adjuster acts as a coarseadjustment for all the pumps in the block. Additionally it may be rotated to operate allthe pumps, as is the case when the engine is pre-lubricated before starting. Correctoperation of the injection pumps whilst the engine is running can be carried out byobserving the movement of the ball

Electronic cylinder lubrication



Exact injection timing of cylinder lube oil is essential for efficiency. A move toelectronics for the control of this has been made by some large slow speed enginemanufacturers.

The system is based on an injector which injects a specific volume of oil into each

cylinder on each ( though more normally alternate) revolution of the engine. Oil issupplied to the injector via a pump or pumps. A computer, which is synchronised tothe engine at TDC each revolution, finitely controls the timing . Generally mostefficient period for lubrication is taken at the point when the top rings are adjacent tothe injection points.

The injection period is governed by the opening of a return or 'dump' solenoid whichrelieves system pressure.

Quantity can be adjusted by manually limiting the stroke of the main lubricator piston,by altering the injection period or by the use of multiple mini-injections per revolution.

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

Shown is the injector unit fitted to modern camshaftless slow speed engines. Themotive force is via a dedicated or common hydraulic system. The hydraulic piston actson multiple plungers one for each quill. At the dedicated time the electric solenoidvalve energises an allows hydraulic oil to act on the piston commencing oil injection.One or two pumps per unit may be fitted dependent on cylinder diameter and oil flowrequirements.

Precise control of the timing of injection 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 50%.

Pre- lubrication for starting may be built into the bridge remote control system orcarried out manually

Cylinder lubricator quill



Crankshaft DeflectionsTo see why crankshaft deflections are taken it is first necessary to look at one section i.e. two crankwebs, a crank pin and two journals.#

If a straight length of shafting I supported at either end is subjected to a central load the effect is forthe 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 theassembly at the journals and the point loading being effect by the weight of the piston and conrodassembly ( ignoring other loads found operational conditions such as combustion and centrifugal ).



Effect on Crankshaft

It can be seen that the effect is to increase the distance between the webs at top dead centre (TDC)and reduce the distance at bottom dead centre (BDC). This deflection is normally found in allcrankshafts although for smaller engines with very rigid cranks this may be very small.

A set of measurements taken from an engine will reveal this deflection which should be constantthrough each crank/piston unit. The caveat to this is that increase deflection is seen at the fly wheeland cam chain gear wheel sections due to the increased loading.

Finding faultsAfter initial installation and alignment a set of deflections are taken. These then form the datum lineto which all other recordings are measured against.

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

Ambient temperatureEngine temperature vessel hull loading (hogging, sagging etc) vessel afloat, dry docked ( again vessel hull loading can cause effects even in drydock due tomovement of blocks, which tanks are full etc)

these effects are well known and an experienced engineer will take into account these factors whenlooking at a set of recordings

If a situation now occurs where a bearing becomes more worn than an adjacent one the effects willbe shown as a change in the pattern of deflections. When the cranks is turned from BDC to TDC theweight of the running gear causes the crank webs and crankpins to bend in such a manner that thedistance between the webs decreases, and continues to decrease until the bearing is no longer incontact with the journal<br> The deflection when the crankshaft is approaching TDC will then gofrom its normal positive reading to zero and then to negative readings at which point the assemblyis supporting the weight without the assistance from the lowered main bearing.



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

Taking Measurements

These are generally taken using a spring loaded dial gauge. The crank webs ar pock marked to ensure thatthe readings are taken in the same place each time. Five measuring points are taken- TDC, 90' either sideof TDC and 30' either side of BDC. The latter two measurements are required as it is not possible tomeasure at BDC due to the Con rod.

The measurements are always taken starting at the same starting point. In this case we will say Port sidenear BDC. The gauge is fitted and zeroed. The engine is rotated continuously and the readings read off during rotation. After the final reading the egine is rotated back to the start point. If the reading is not zerothen it indicates that the gaige is moved and the readings are re-taken.

ExampleThese readings were taken from a B&W 6K76EF (I bet you haven't sailed with one of them, it's the one withthe rocker arms and the self adjusting tappets that make you crap yourself when they fail)

Crank Position No1 Cyl No2 Cyl No3 Cyl No4 Cyl No5 Cyl No6 Cyl

Port near BDC (X) 0 0 0 0 0 0

Port Horizontal (P) 6 1 7 -9 -4 4

TDC (T) 12 3 13 -16 -12 5

Stbd Horizontal (S) 6 3 6 -7 -8 3

Stbd near BDC (Y) -1 2 -2 2 1 4

corrected BDC (X+Y/2=B)

0 1 -1 1 0 2

Vertical Alignement

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 adjustment.

the assistance from the lowered main bearing.


Vertical alignement [T-B=V] 12 2 14 -17 -12 7



Horizontal Alignement

<table class = "list"><tr><TD>Crank Position<TD>No1 Cyl<TD>No2 Cyl<TD>No3 Cyl<TD>No4 Cyl<TD>No5 Cyl<TD>No6 Cyl<tr><td>Horizontal alignement <b>[P-S=H]</b><td>0<td>-2<td>1<td>-2<td>4<td>1

</table>

Gauge reading Check

C & D should be practically the same, hence the readings from No6 Cyl may be suspect


Horizontal alignment [P-S=H] 0 -2 1 -2 4 1


T+B=C 12 4 12 -15 -12 3

P+S=D 12 4 13 -16 -12 7

Jacket Water System

Shown above is a typical cooling water circuit for a slow speed engine.Water is pump via one of two centrifugal pumps. One is normally in use with the



other stand-by. The water passes through to the distributing manifold on the engineside.

Jacket Water Heater In the line is a steam jacket water heater. When the engine isshut down steam heating maintains the engine in a state of readiness reducing thetime needed for starting. Attempting to start the engine without heating can lead topoor combustion, poor lubrication and thermal shocking. A modern variation on thisis the "blend" water from the stand-by auxiliary alternator engines into the mainengine circuit increasing plant efficiency

The water enters and leaves the engine via a series of cylinder isolating valves. Inthis way each cylinder may be individually drained to prevent excessive water andchemical loss. In addition dual level drains may be fitted which allow either fulldraining or draining of the head only. A portion of the water is diverted for cooling of the turbocharger.

Deaerator Was an essential part of engines incorporating water cooled pistons wereair was deliberately introduced in to the system to aid the "cocktail shaker" coolingaction. Air or gas entering the system can lead to unstable and even total loss of cooling water pressure as the gas expands in the suction eye of the circulatingpumps. In the event of gas leakage via the head or cracked liner rapid loss of jacketwater pressure can occur. The deaerator is a method to try to slow this processsufficiently to allow the vessel to be placed in a safe position for maintenance. Thissystem also allows the vessel to operate with minor gas leakage.

Jacket Water Cooler The hot water leaving the engine passes to a temperaturecontrol valve were a portion is diverted to a cooler. Temperature is controlled usingboth a feedback signal (temperature measured after the cooler) and a feed forwardsignal (temperature measured at outlet from the engine). In this way the systemreacts more quickly to engine load variations.

Evaporator Increases plant efficiency by utilising heat in jacket water to producefresh water. Modern systems sometimes rely on the evaporator to supplement areduced size main cooler. Expansion or header tank Maintains a constant head on the circulation pumpsreducing cavitation at elevated temperatures. Allows the volume of water in thesystem to vary without need for dumping. Acts as a reserve in the event of leakage

HT/LT systems

Scaling of Jacket Water SystemScale and deposit formation

In areas of deposit formation, dissolved solids, specifically Calcium and magnesiumhardness constituents can precipitate from cooling water as the temperatureincreases. Deposits accumulate on the heat transfer surfaces as sulphates andcarbonates, the magnitude of which is dependent on the water hardness, thedissolved solid content, local temperatures and local flow characteristics. Temperaturesolubility curves for CaSO4



Scales can reduce heat transfer rates and lead to loss of mechanical strength of component 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 by;System temperaturesAmount of leakage/makeupquality of make upquality of treatment

Calcium CarbonateAppears as a pale cream, yellow deposit formed by the thermal decomposition of calcium bi-carbonateCa(HCO3)2 + Heat becomes CaCO3 + H2O + CO2Magnessium SilicateA rought textured off white deposit found where sufficient amounts of Magnesium arepresent in conjunction with adequate amounts of silicate ions with a deficiency onh OHalkalinityMg2+ + OH - becomes MgOH +H2SiO3 becomes H + + HSiO3 -MgOH + + HSiO3 - becomes MgSiO3 + H2SO4Silicate deposit is a particular problem for systems which utilise silicate additives forcorrosion protection. Thi sis typical of systems with aluminium metal in teh cooling

system. The silicate forms a protective barrier on the metal surface. A high pH (9.5 -10.5) is required to keep the silicate in solution. In the event of sea watercontaimination or some other mechanism that reduces the pH the silicate is rapidlyprecipitated and gross fouling can occur.

Iron Oxides- Hematite (Fe2O3)

Is a loose red /brown deposit and is indicative fo active corrosion within a system

CopperThe prescence of copper within a cooling system is very serious ast it can leadto agressive corrosion through galvanic action. Specific corrosion inhibitors arecontained with cooling water system corrosion inhibitors.

Effects of scale depositionThe effects of scale deposition can be both direct or indirect,typically but notspecifically

Insulates cooling surfaces leading to;increased material temperatures as the temperature gradient mustincrease to ensure maintain heat flow.Loss of efficiency as exhaust gas temperatures form cylinders increasesIncreased wear due to lubrication problems on overheated surfaces

Indirectly;



Lead to caustic attack be increasing the OH - ion concentration

Corrosion inhibitors used in Jacket Water SystemJacket Cooling water systemIn order to maintain mechanical strength the components surrounding thecombustion zone must be cooled. The most convenient cooling medium is water, the

use of which could lead to possible problems of corrosion and scaling if not properlytreated.Within the jacket water system a number of corrosion cells are available but the twomost common and most damaging are due to dissimilar metals and differentialaeration. In both types of cell there exists an anode and a cathode, the metals whichform part of the jacket system, and an electrolyte which is the cooling water. Therate at which corrosion takes place is dependent upon the relative areas of thecathode and the anode and the strength of the electrolyte. It is the anode thatwastes away. Corrosion due to temperature differences is avoidable only by the useof suitable treatments. Dissimilar metals-a galvanic cell is set up where two differentmetals and a suitable liquid are connected together in some way. All metals may beplaced in an electro-chemical series with the more noble at the top . Those metals atthe top are cathodic to those lower down. The relative positions between two metalsin the table determined the direction and strength of electrical current that flowsbetween them and hence, the rate at which the less noble will corrode

Galvanic ActionCorrosion 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 of metal from a large area of the exposed surface, or by Oxygen attack characterised bypitting. A primary motive force for this corrosion is Galvanic actionThe Galvanic Series.Or Electromotive series for metalsCathodeGold and PlatinumTitaniumSilverSilver solder

Chromium-Nickel-Iron (Passive)Chromium-Iron (Passive)Stainless Steel (Passive)CopperMonel70/30 Cupro-Nickel67-33 Nickel-CopperHydrogenleadTin2-1 Tin lead SolderBronzesBrassesNickelStainless-Steel 18-8 (Active)Stainless Steel 18-8-3 (Active)Chromium Iron (Active)Chromium-Nickel-Iron (Active)CadmiumIronSteelCast IronChromiumZincAluminiumAluminium AlloysMagnesiumAnodeThe metals closer to the anodic end of the list corrode with preference to the metalstowards the cathode end.A galvanic cell can occur within an apparently Homogeneous material due to severalprocesses on of which is differential aeration where one area is exposed to moreoxygen than another. The area with less oxygen becomes anodic and will corrode.

Galvanic action within metal



Galvanic action due to temperature gradient

This situation can exist in cooling water systems with complex layout of heatexchangers and passage ways within the diesel engine. Systems containing readilycorrodible metals such as zinc, tin and lead alloys can complicate and intensify

problems by causing deposit formations.Differential Aeration-Where only a single metal exists within a system corrosion can still take place if theoxygen content of the electrolyte is not homogenous. Such a situation can occurreadily in a jacket water system as regions of stagnant flow soon have the oxygenlevel reduced by the oxidation of local metal. The metal adjacent to water withreduced levels of oxygen become anodic to metals with higher oxygen contentelectrolyte in contact with it.. Generally, the anodic metal is small in comparison thecathode i.e. the area of stagnant flow is small compared to the area of normal flow of electrolyte, and high rates of corrosion can exist. One clear case of this is thegeneration of deep pits below rust scabs.

SolutionsWater treatmentTo remove the risk of corrosion it is necessary to isolate the metal surface form theelectrolyte. One method would be by painting, but this is impractical for enginecooling water passages. A better solution would be a system which not only searchedout bare metal coating it with a protective barrier, but also repaired any damage to



the barrier.for corrosion to occur four conditions must be met;

There must be an AnodeThere must be a cathodeAn electrolyte must be presentAn electron pathway should exits

Corrosion InhibitorsCorrosion inhibitors are classified on how they affect the corrosion cell and areplaced into three categories;

Anodic InhibitorsCathodic InhibitorsCombination inhibitors/organic inhibitors

Common Corrosion InhibitorsPrincipally Anodic Inhibitors

ChromateNitriteOrthophosphtaeBicarbonateSilicateMolybdenate

Principally Cathodic InhibitorsCarbonate

PolyphosphatePhosphonatesZinc

Both Anodic and Cathodic InhibitorsSoluble OilsMercaptobenzothiazole (MBT)Benzotriazole (BZT)Tolytriazole (TTZ)

Anodic Inhibitors

Nitrite (NO2 - )- These are the most commonly used form of treatment and operateby oxidising mild steel surfaces with a thin, tenacious layer of corrosion product(magnetite Fe3O4). Relatively high volumes of treatment chemical are required sothis method is only viable on closed systems

Sodium Nitrite- (sometimes with Borate added)-effective with low dosage,concentration non-critical. It is non toxic, compatible with anti freezes and closedsystem cooling materials. It does not polymerise or breakdown. However protectionfor non-ferrous materials is low. An organic inhibitor is thus required. Although willnot cause skin disease it will harm eyes and skin. Approved for use with domesticfresh water systems.

Sodium Nitrite is a Passivator, a passivator will act chemically to produce aninsulating layer on the metal surface. Whenever corrosion takes place the corrosionproducts including bubbles of gas, are released from the metal surfaces. Passivatingchemicals act on the corrosion products preventing release from the metal surfaceand thus stifling further corrosion. If the insulating layer becomes damaged,corrosion begins a gain and the passivator acts on the new products to repair thelayer.

Chromate's-the first passivator product was Sodium Chromate which was anexcellent inhibitor. Inexpensive, effective and concentration easily tested. Corrosionmay increase by incorrectly dosing, dangerous to handle, poisonous and can causeskin disease. Not allowed where domestic water production is in use (Jacket waterheated evaporators). Unfortunately it was also highly toxic, a severe pollutant andstaining agent, was incompatible with antifreeezes, and will attack zinc and softsolder slightly. Due to its toxicity is sometimes used as a biocide in such places asbrine in large Reefer plants.

Silicates- react with dissolved metal ions at the anode. The resultant ion/silicatecomplex forms a gel that deposits on anodic sites. This gel forms a thin, adherentlayer that is relatively unaffected by pH in comparison to other inhibitors. Theinhibiting properties increase with temperature and pH, normal pH levels are 9.5 to

10.5.Care should be taken with the use of silicates, which are often used for theprotection of system containing aluminium. In the event of boiling increasedconcentrations and lead to aggressive corrosion due to the high pH.Orthophosphate Forms an insoluble complex with dissolved ferric ions that deposit atthe anodic site. It is more adherent and less pH sensitive than other anodicinhibitors. The film forms in pH of 6.5 to 7.0. Dosage is typically 10ppm in neutral



waterCathodic Inhibitors

Polyphosphate - Forms complexes with Calcium, Zinc and other divalent ions, thiscreates positively charged colloidal particles. These will migrate to the cathodic siteand precipitate to form a corrosion inhibiting film. The presence of calcium isrequired at a typical minimum concentration of 50ppm.

Extreme variations in pH can upset the film and a reversion to orthophosphate will

occur with time and temperature.

Positively charged zinc ions migrate to the cathodic site and react with the freehydroxyl ions to form a zinc hydroxide stable film at pH 7.4 to 8.2. If the water is tooacidic the film will dissolve and not reform. If it is too alkaline the zinc hydroxide willprecipitate in bulk and not at the cathodic site.Phosphonates . Initially introduced as scale inhibitors to replace polyphosphates,they exhibit absorption at the metal surface especially in alkaline hard water.Generally used with other inhibitor types

Both Anodic and Cathodic Inhibitors

Benzotriazole and Triazole Specific corrosion inhibitor for copper. They break theelectrochemical circuit by absorbing into the copper surface.They are generally added to standard treatments.

Soluble and dispersible oils. Petroleum industry recognised that emulsifying cuttingoils (erroneously called soluble oils) were able to reduce corrosion on metals bycoating the surface. There were disadvantages though, if the coating became toothick then it could retard the heat transfer rate. Adherent deposits form as organicconstituents polymerise or form break down products which can accumulate anddisrupt flow. MAN-B&W recommend it not to be used.

It is effective in low dosages, safe to handle and safe with domestic waterproduction. Effectiveness is reduced by contamination with carbon, rust, scale etc.Difficult to check concentration, overdosing can lead to overheating of partsOils are classed as a barrier layer type inhibitor. The surfaces being coated in a thinlayer of oil.

Modern treatmentNitrite-Borate treatment is most effective with a high quality water base. Thistreatment has no scale prevention properties and its effectiveness is reduced by highquantities of dissolved solids.

A modern treatment will be a Nitrite -Borate base, with a complex blend of organicand inorganic scale and corrosion inhibitors plus surfactants, alkali adjusters,dispersants and foam suppressers. A high quality water supply is still stronglyrecommended.

The Use of Sacrificial Anodes-Electrolytic protection for the whole system by the use of sacrificial anodes isimpractical. Parameters such as water temperature, relative surface area of anodeand cathode, activity of metals in system and relative positions in galvanic seriescome into play. Anodic protection has become out of favour for cooling watersystems as it can lead to local attack, causes deposits leading to flow disturbanceand it has no scale protection

Preparation for cooling water treatment

-All anodes should be removed and the system inspected. No galvanised piping is tobe used (old piping can be assumed to have had the Galvanising removed). Highquality water should be used and chemicals measured and added as required. A

history log should be kept

Microbiological FoulingUnder certain conditions bacteria found in cooling water systems can adapt to feedon the 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,Problems of this sort are rare due to the elevated temerpatures and pH levels.Should it occur treatment with a suitable biocide is required.

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