lubrication dilip

8/4/2019 Lubrication Dilip

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INTRODUCTION

All material surfaces, no matter how smooth they are, show many irregularities in the form of

peaks and valleys, which are large when considered on a molecular scale.

When these two solid surfaces are pressed over or slide over each other, a real contact between

these surfaces occurs that will cause friction and consequently the production of heat. During the

motion of the sliding surfaces, a considerable amount of frictional heat is evolved at the rubbing

surface. This results in high local temperature even under relatively light loads and speeds. This

friction also causes a lot of wear and tear of the surfaces of the moving parts.

Even under small load, the local pressure at the peaks of the metals may be sufficiently great to

cause appreciable deformation in ductile metals. If two materials of different hardness slide over

one another, the peaks of the softer metal get broken more easily than the peaks of the harder

metals.

Importance of Lubrication in Marine Diesel Engines

Lubrication reduces friction between the moving surfaces or rolling pairs. The lubricant also acts

as a coolant carrying heat away from the sliding surfaces, so proper lubrication of all the moving

parts is an important function in machinery or engine operation. By lubrication we keep the

moving surfaces separated by a fluid of some defined property. Friction is a necessary evil and it

opposes the relative movement between any two objects which leads to loss of energy and hence

wastage. Since there are lots of moving parts in any engine including marine diesel engines,

there needs to be a method to take care of the same. Friction cannot be eliminated completely but

can be reduced by applying appropriate lubrication techniques.

The main object of lubrication is to separate moving metallic parts by inserting a layer of fluid

between them in the form of a thin film which reduces friction. Before going on to study the

actual mechanism of lubrication system in a marine diesel engine, let us first see which all parts

of the engine need lubrication. It requires just an elementary knowledge of the engine

construction and a little forethought to imagine which all parts are moving relative to each other

and hence require lubrication.


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There are mainly four types of motions present in a marine engine and hence the lubrication can

be provided based on the type of motion. These types are as follows.

1. There is purely rotational motion between certain components and these include the

bearings of various parts such as the camshaft, crankshaft and so forth.

2. There is purely sliding motion between components such as the piston ring/cylinders,

crosshead guide.

3. There is oscillatory motion between parts such as the rocker arms , and

4. There is very small point or line type friction in certain parts such as meshing gear teeth,

chain and sprocket wheels and so forth.

Types of Lubrication

Considering the nature of motion between moving or sliding surfaces, there are different types of

mechanisms by which the lubrication is done. They are:

y Hydrodynamic lubrication or thick film lubrication

y Hydrostatic lubrication

y Boundary lubrication or thin film lubrication

y Extreme pressure lubrication

Hydrodynamic Lubrication or Thick Film Lubrication

Hydrodynamic lubrication is said to exist when the moving surfaces are separated by the

pressure of a continuous unbroken film or layer of lubrication. In this type of lubrication, theload is taken completely by the oil film.

The basis of hydrodynamic lubrication is the formation of an oil wedge. When the journal

rotates, it creates an oil taper or wedge between the two surfaces, and the pressure build up with

the oil film supports the load.


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Hydrodynamic lubrication depends on the relative speed between the surfaces, oil viscosity,

load, and clearance between the moving or sliding surfaces.

In hydrodynamic lubrication the lube oil film thickness is greater than outlet, pressure at the inlet

increases quickly, remains fairly steady having a maximum value a little to the outside of thebearing center line, and then decreases quickly to zero at the outlet.

Application of hydrodynamic lubrication

y Large plain bearings like pedestal bearings, main bearing of diesel engines.

Hydrodynamic Lubrication Sketch

Hydrostatic Lubrication

y Hydrostatic lubrication is essentially a form of hydrodynamic lubrication in which the

metal surfaces are separated by a complete film of oil, but instead of being self-generated,

the separating pressure is supplied by an external oil pump. Hydrostatic lubrication depends

on the inlet pressure of lube oil and clearance between the metal surfaces, whereas in


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hydrodynamic lubrication it depends on the relative speed between the surfaces, oil

viscosity, load on the surfaces, and clearance between the moving surfaces.

Application of hydrostatic lubrication

y The cross head pin bearing or gudgeon pin bearing employs this hydrostatic lubrication

mechanism.

In the cross head bearing, the load is very high and the motion is not continuous as the

bearing oscillation is fairly short. Thus hydrodynamic lubrication cannot be achieved.

Under such conditions, hydrostatic lubrication offers the advantage. The oil is supplied

under pressure at the bottom of bearing. The lube oil pump pressure is related to the load,bearing clearance, and thickness of the oil film required, but is usually in the order of 35-

140 kg/cm2

(3.5-14 bar).

Hydrostatic Lubrication Sketch

Boundary Lubrication or Thin Film Lubrication

Boundary lubrication exists when the operating condition are such that it is not possible to

establish a full fluid condition, particularly at low relative speeds between the moving or sliding

surfaces.


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The oil film thickness may be reduced to such a degree that metal to metal contact occurs

between the moving surfaces. The oil film thickness is so small that oiliness becomes

predominant for boundary lubrication.

Boundary lubrication happens when

y A shaft starts moving from rest.

y The speed is very low.

y The load is very high.

y Viscosity of the lubricant is too low.

Application of boundary lubrication:

y Guide and guide shoe in two stroke engine.

y Lubrication of the journal bearing in diesel engines (mainly during starting and stopping

of engine).

y

Piston rings and when cylinder liner is at TDC and BDC position when the pistondirection changes and if the relative speed is very slow.

Boundary Lubrication Sketch


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Extreme Pressure Lubrication

When the moving or sliding surfaces are under very high pressure and speed, a high local

temperature is attained. Under such condition, liquid lubricant fails to stick to the moving parts

and may decompose and even vaporize. To meet this extreme pressure condition, special

additives are added to the minerals oils. These are called extreme pressure lubrication. These

additives form on the metal surfaces more durable films capable of withstanding high loads and

high temperature. Additives are organic compounds like chlorine (as in chlorinated esters),

sulphur (as in sulphurized oils), and phosphorus (as in tricresyl phosphate).

Important Properties of Lube Oil And Checks

The following are the most common and required properties of the lube oil used for marine

machinery:

Alkalinity

The lube oil alkalinity plays an important part in marine engines. When fuel burns, the fumes

carry sulphuric acid which can cause acidic corrosion. For a trunk piston engine or four stroke

engines, the main lube oil is responsible for piston and liner lubrication; hence it comes directly

in contact with the combustible fuel. Therefore alkalinity of lube oil is important for controlling

acidic corrosion.

For two stroke engines, separate grade of lube oil is used as cylinder oil and its alkalinity

depends on the engine fuel grade (HFO or LSFO).

Oxidation resistant

Lube oil is always in contact with air and thus oxygen presence in oil is inevitable. Moreover, athigh temperature of the oil, the oxidation rate increases. After 85 degree C temperature, the

increase in every 10 degree C of oil oxidation rates doubles itself leading to sludge formation,

acid production and bearing corrosion. Hence additives are added to maintain keep these things

in check. Lube oil temperature is controlled by passing it through lube oil cooler.


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Load carrying capacity

It is also one of the important characteristics of lube oil which mainly depends upon the viscosity

of the oil. The load subjected to different internal parts of the marine engine is very high; hence

the load carrying capacity must be enough to withstand the pressure inside the engine. If this is

not achieved then oil will be forced out and metal to metal contact will result in wiping out and

wear down of the machine.

Thermal conductivity

The internal parts of marine engine are always in movement producing heat energy. This heat

energy has to be carried away or else it might lead to wear down due to thermal stresses. The

lube oil must cool down the internal parts to avoid such a situation and must have a good thermal

conductivity.

Detergency

Detergency of the oil is obtained by adding some metallic based additives which will prevent the

build up of small deposits in the metal surface. In two stroke engine, the cylinder oil detergency

is very important as it removes the deposits from the ring pack area and keeps the combustion

space as clean as possible

Disperency

It is the property of the lube oil which prevents impurities to mix up with itself and keeps them

suspended on the surface. This makes it easy for the separator or clarifier to remove it from the

oil.

High Flash Point

The flash point is the minimum temperature at which the oil vaporizes to give an ignitable

mixture of air. The flash point should always be on the higher side so that in case of increase in

temperature of the oil, fire hazard can be avoided. Normally for marine engine lube oils, the flash

point is always higher than 220 C.

Low Demulsification Number

It is not practically impossible to completely avoid contamination of oil with water. The low

demulsification number of the oil helps in easy separation of water from the oil in the separator

or when stored in the settling tank.

Volatility


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If the flash point of the oil is too low it might ignite with high temperature and therefore it should

normally be above 200 degrees Celsius.

Viscosity

This refers to the property which determines the ease of flow of the oil between small clearance

spaces. It should neither be too low nor too high for proper lubrication to take place. If the

viscosity is too low the oil will not provide proper boundary lubrication and if it is too less it will

not flow properly in all the places required, hence the need to maintain optimum viscosity.

As the oil is used to lubricate the engine its properties deteriorate over a period

of time due to addition of impurities which could include un-burnt fuel, water, acid, suspended

particles and so forth. Purification and filtration of the oil is carried out during its use but still

there is a limit to which the same oil can be used and at a certain stage it will need to bereplenished.

Since lubricating oil performs a useful task and undue deterioration in its

properties could damage the engine components, a close watch needs to be kept on the quality of

the oil. Various checks include physical examination, blotting paper test, water content test and

so on. Lubricating oil test kits are available on board ships which are used by duty engineers to

check for the quality of the oil. Apart from that the oil should be renewed after a specified

number of running hours depending on the exact engine in question.

Composition of lubricating oils

Lubricating oil fractions extracted from crude oil are a widely varying mixture of straight and

branched chain paraffinic, napthenic aromatic hydrocarbons having boiling points ranging from

about 302o

to 593oC. Some specialty lubricants may have boiling point extremes of 177 and

815oC. The choice of grade of lubricating oil base is determined by the expected use.

General capabilities expected from an engine lubricant

y Dispersivity or capacity to the cold parts of an engine cleany Detergency or capacity to keep hot parts of an engine clean

y Thermal strength or capacity to withstand temperature changesy Anti-oxidant or capacity to resist the action of oxygen

y Anti-wear or capacity to contain wear


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y Anti-scuffing or capacity to preserve oil film even in the presence of highpressures

y Alkalinity reserve or capacity to neutralise acids formed during combustion orother sources thereby preventing corrosive wear

y Demulsibility or capacity to separate contaminantsy

Resistance to hydrolysis or capacity to withstand the action of water which canaffect additivesy Pumpability

y Centrifugibility and filterability or capacity to separate insoluble elementsy Anti-rust,corrosive and anti-foam are just some of the other properties required

Properties ideal for bearings

y Soluble for high speed fluid film hydrodynamic lubrication, hence, low viscositywith reduced oil film friction.

y moderate bearing loadsy

improved heat transfer behaviory corrosion protectiony cooling

y low frictiony good low temperature viscosity

y good high temperature viscosity

Properties ideal for gear case

y high film strength to prevent metal to metal contact. Hence, high viscosityadhesive to resist sliding and centrifugal forces

y corrosion protectiony cooling

y reduces frictiony good low tempo viscosity

y good high tempo viscosityThe thicker the oil film the greater the cushioning against shocks. Also less

tendency for pit formation by hydraulic action in cracks,y sound damping properties with cushioning effects

y antifoam properties

Turbine oil

Compromise between above two requirements

y Generally a good quality refined mineral oil derived from paraffanic base stockused with various additives including EP additives for highly loaded gearing.

y Anti-foaming properties important


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Additives

Improvements in lubricating oil over the last twenty years have come about almost entirely from

the use of additives.

These are added for three main reasons;

1. to protect the lubricant in service by limiting the chemical change anddeterioration

2. To protect the mechanism from harmful combustion products and malfunctioninglubricating oil

3. To improve existing physical properties and to create new beneficialcharacteristics in the oil

Typical additives are; Barium, calcium, phosphorus, Sulphur, chlorine, zinc,

oxidation inhibitor-increases oil and machinery life, decreases sludge and varnish on metal parts

Corrosion inhibitor-protects against chemical attack of alloy bearings and metal surfaces.

Antiwear improvers-protects rubbing surfaces operating with this film boundary lubrication.

One such antiwear ( and oxidation inhibitor) chemical is Zinc dithiophosphate or ZDDP

Detergent-tend to neutralise the deposits before formation under high temperature and pressure

conditions, or as a result of using a fuel with high sulphur content. The principle detergents are

soaps and alkaline metals, usually calcium ( often referred to as 'matallo-organic compounds').

They are usually ash forming and spent additive will contribute to the insolubles loading of a

used oil. It should be noted that additives which do not burn cleanly without ash tend to be

avoided for use with Cylinder Lubricating Oils.

Dispersant-used to disperse or suspend the deposits forming contaminants. Typical dispersants,

such as polyesters and benzlamides, are usually clean burning. The molecules have a polar

charge at one end which attracts and holds the deposits

Alkaline agents-neutralises acids, htese form the TBN of the oil and includes additives such as

the above dispersants and detergents. An excess of acid neutralising alkalis are present in the oil

and these help to keep parts clean. Failure to keep an oil alkaline can lead to damage to bearings

due to acidic attack as well as increased liner wear.


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Rust inhibitors-

Pour point depressants-improves low temperature viscosity

Oiliness agent-reduces friction seizure point and wear rates

EP additives-increases film strength and load carrying capability

Antifoam agents-prevents stable bubble formation

Viscosity Improvers-an additive that improves the viscosity index of the oil. I.e. reduces the

effect of temerpature of=n the viscosity of the oil. Shear stability property is measured indicating

the effect of high rates of shar on the VI improver as the improver molecules are broken down

into smaller molecules

Metal deactivators-prevent catalytic effects of metal

Antiseptic-bactericide.

Recharging

When recharging no more than 10 % of the working charge should be topped up due to heavy

sludgeing that can occur due to the heavy precipitation of the sludge.

Stresses on Lube oil

The main stresses experienced by Lube oils in diesel engines operating on heavy fuel oils are

expressed as follows

Acid Stress- Caused by sulphuric and oxidation acids. This leads to increased

corrosive wear, deposits, reduced Base Number and shorter oil life.Rapid depletion of the BN is

the clearest sign of oil stress

Thermal/Oxidative stress-This caused by elevated temperatures leading to

increased rates of thermal/oxidative breakdown of lubricant and fuel. This leads to increased

levels of deposits, sludges, corrosive wear of bearing material, oil thickening and reduced oil life.


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In addition deposits on the under crown side of the piston can lead to increased hot corosion on

the piston.

Asphaltene Stress-

This caused by fuel contamination of the lube oil and can lead to increased levels of deposits,

sludges, lacquers, oil thickening and reduced oil life. In addition deposits on the under crown

side of the piston can lead to increased hot corrosion on the piston.

Crankcase Lubricating Oil System

The system consists of a big reservoir at the bottom of the engine known as the oil sump which

contains a sufficient quantity of lubricating oil all the time. There is a level indicator whichshould be frequently checked and oil should be replenished if required. Also routine lubricating

oil tests are carried out to test the various properties of oil to see if it requires to be changed

otherwise it can be changed after the specified interval of time.

There are normally two parallel pumps which take suction from the lubricating oil sump and

usually only one pump is used at a time while the other is rested. This also provides time for

repair if anything goes wrong with one pump and the other one can be used in the meantime.

There are also lubricating oil filters in the line which should be periodically cleaned and replaced

as and when required.

The oil from the sump gets distributed into various places and finally drains back into the sump

after fulfilling its purpose of lubrication and cooling. Obviously the oil also gets contaminated

during this cycle as it also gathers many impurities on its way. The system of taking care of this

is different depending on whether we are talking about smaller 4 stroke engines which are used

for power generation on big ships or the main propulsion engine. In case of large engines there is

normally a separate circuit from the sump which purifies the oil continuously by passing it

through a lubricating oil purifier and passes it back to the lubricating oil sump. In case of smaller

engines the oil is simply replenished frequently and changed at the required intervals. The

lubricating oil purifier or a centrifuge is a subject matter of separate discussion and we will take

it up in a different article later on.


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In case of large engines, there is an oil cooler which cools the oil as it returns back to the sump

after absorbing heat which keeps the oil at the required temperature. The cooler is located in such

as position that it can be easily accessed for cleaning at regular intervals. The bulk of fresh

lubricating oil is stored in separate tank known as storage tank on the ship and whenever fresh oil

is required for replenishment or renewal, it is taken from this tank.

Cylinder Lubricating Oil System

We know that a piston moves to and fro within a cylinder and there are several piston rings

inserted in the grooves on the piston which perform multiple tasks including sealing of pressure

inside the combustion chamber lest it leaks from below. The speed of rubbing between these

piston rings and the cylinder liner is quite high and apart from that there are extreme conditions

of temperature, pressure and corrosive gases inside the combustion chamber to which any

lubricating fluid between the liner and the piston would be subject to.

If you remember the 3 types of lubrication conditions we had studied in a previous article, the

type of lubrication found inside the cylinder is of the thin film or boundary type of lubrication for

most parts of the piston motion except for the upper and lower extremes of motion where this

changes to imperfect lubrication as the speed reduces to zero at these points and direction of

motion is reversed.

It does not require much reflection to imagine that lubricating oil used inside a cylinder liner

must be able to withstand conditions of high temperature, pressure and should have good

corrosion resistance as well. Just to give you a broad idea, typical lubricating oil used inside the

liner should have viscosity in the range of 115-150 cst at temperatures of 50 degrees Celsius and

this should not vary too much even at higher temperatures. Special additives could be added to

the oil in order to increase its oiliness or stickiness. Apart from that the amount of oil supplied

should be ideal because if too little oil will cause the surfaces to tear apart rapidly, too much of

oil will also be a problem as it would interfere with normal combustion inside the chamber and

also cause damage to various parts such as valves, valve seats and the exhaust pipe.


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The circular section shows the top view of a cylinder liner with 8 lubricating quills fitted alongits circumference which contained lubricated oil under pressure

Crosshead Lubricating Oil system

The type of motion of the crosshead of a marine diesel engine is oscillatory in nature wherein it

oscillates to a few degrees on either side of the center of oscillation. Apart from that the speed of

oscillation is not very significant which in turn means that it will not generate a pumping action

for lubrication unlike the case of the fast rotating shaft in a bearing.

Due to the nature of the two stroke cycle both in the normal 2 stroke marine diesel engines as

well as supercharged 2 stroke marine diesel engines, the force acting on the crosshead bearing is

always acting in the downward direction throughout the cycle without any such time period for

which this load might be relieved. This certainly presents a challenging situation for effectivelubrication to take place.

Difficulties in Cross head Lubrication:

To overcome these challenges it is important not only to find a suitable lubrication technique but

also to improvise on the design of the bearings so that lesser lubrication effect is required. Some

of the steps which can be taken in this regard are as follows.

y The connecting rod design can be modified in a manner which provides better support for

the crosshead pin.

y Apart from that the pin is also made thicker that what is required from calculations of

load so that the large surface area of the thicker pin reduces load per unit area.


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y The pin is also finished to a high surface finish and made more wear resistant so that the

oiliness of the oil keeps it sticking to the surface

y Bearing shells are lined with a special kind of antifriction white metal that helps it to

attain higher strength.

The hand drawn sketch below shows a typical arrangement of the crosshead bearing.

Apart from taking the above mentioned measures the lubricating oil is pumped at high pressure

to the loaded region using high pressure pumps. This is necessary since the slow oscillatory

motion of the pin is not sufficient to produce oil pressure that would keep the surfaces separate

from each other.

Sources of Lube Oil Contamination

CONTAMINATION BY UNBURNT FUEL:

y In case of a trunk piston engine, the unburnt fuel may leak into the crankcase due to

blow-by, because of which there is a reduction in flashpoint, viscosity, and load carrying

capacity of the oil.

CONTAMINATION BY CARBON:


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y There are minute carbon particles present. Due to the local overheating on the bearing

surfaces, carbon particles are formed. Carbon may also be due to blow past gasses

leaking past the piston into the crank case. These carbon particles also increase the

viscosity, flash point, and affect the load carrying capacity of the lube oil.

CONTAMINATION BY WATER:

y This is a common problem encountered as a result of leakage from the glands, seawater

or freshwater leak from the coolers, from the piston cooling water system, etc. The

presence of water thickens the lube oil and there are chances of formation of emulsion.

y If it is seawater, because of presence of the salts, the bearing surfaces and journal pins get

roughened.

y The corrosion due to water reduces its load carrying capacity and lube oil properties.

CONTAMINATION OF LUBE OIL BY ACID:

y Sulphur from the fuel oil upon oxidation forms sulphur trioxide and mixes with any

water, forming sulphuric acid upon oxidation.

y Sulphuric acid is extremely corrosive. This acid mixes with excess lube oil and finds

access to the crankcase through a defective gland. This occurs in the case of a two stroke

engine.

y In case of a trunk piston engine, due to leaky piston rings and scrapper rings, the

combustion gasses bearing sulphur derivatives may contaminate the lube oil.

CONTAMINATION BY SOLID IMPURITIES:

y The lube oil may be contaminated by the solid impurities. These may be due to the wear

down of the white metal bearing, journal surfaces, corroded parts, sludge from the lube

oil storage tanks, and ash from the products of combustion due to the blow past.

y To remove these solid impurities the lube oil is purified by a method called continuous

purification.


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ME Lubrication Oil System MD12


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General:

y The lubrication oil from the main engine sump is collected m a sump tank below the

engine

y The LO pumps are protected by a pressure relief valve which opens when the pressure

rises over a preset value. These valves are not modeled in detail and are not available

from the variable list.

y The service tank oil can also be circulated by the LO purifier.

y New oil is supplied by a make-up pump with flow directly to the sump tank.

y The lubrication oil is cooled in two LT fresh water cooled LO coolers and is then passed

through an automatic back flush filter or a standby conventional filter before it enters the

main engine The LO temperature is controlled by a PI controller, which regulates a by-pass valve for the LO coolers.

y The LO filters must be checked regularly to avoid pressure/flow reduction.

y The sump tank oil level will gradually decrease due to oil consumption and possible drain

sludge discharge from the purifier.

y The level is unstable in poor weather and if the level is low. there may be false alarms

shut downs.

y If the purifier is operated with ""broken" water seal oil is continuously discharged to the

sludge tank and there is a risk of emptying the LO sump tank completely. The oil

pressure after the pumps will be reduced towards zero as the LO sump tank runs dry.

y The oil temperature in the sump tank is affected by the return oil flow' temperafure from

the main engine, the oil flow temperature from the purifier and the heat loss to the

surroundings. If all inlet flows stop, the temperature will gradually approach ambient air

temperature. Low oil temperature gives reduced flow at main engine

Cylinder Lubrication

y A simple cylinder lubrication model is included. The day tank is refilled by pump

from the storage tank. There will be a steady consumption of cylinder oil dependent on

main engine speed.


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y The cylinder LO tank must be refilled periodically. At low cylinder LO tank level there

will be ME slow down/shut down.

Cam Lubrication

y The lubrication oil from the main engine cam shaft is collected in a cam shaft LO tank

y The LO pressure is controlled after the two cam LO pumps by a pressure control valve

with return flow to the cam LO tank

OIL ANALYSIS

Regular testing of crankcase lub oil is important to ensure that deterioration has not taken place. The

results of in service deterioration could be a reduction in engine protection or actual attack on

working points by corrosive deposits.Oil analysis may be done in three parts. They are:

1. Properties of the lubricating oil

2. Condition of the lubricating oil (to tell whether it can be used further or not)

3. Fuel dilution, dirt contamination, excessive bearing wear, etc.,

Oil samples are generally tested every 3 to 4 months depending on the system and experience.

Shipboard testing is taking a rising prominence to allow monitoring of oil condition between

testing.To ensure good representation, care should be taken where the sample is drawn

Correct

o Main supply line

o inlet or outlet from l.o. cooler

o Outlet from main l.o. pump

Incorrect

o standpipes

o purifier outlet

o purifier direct sump suction


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Samples should be drawn over a period of several minutes

Viscosity

The viscosity is the most important property of the oil. Oil of correct viscosity will provideoptimum film strength with minimum friction losses and leakage.

The viscosity of a L.O. may fall due to fuel dilution if running on gas oil, and rise if running on

heavy f.o. Viscosity may also increase due to heavy soot loading if purifiers and filters not

operating efficiently. Oil ageing caused by oxidation and thermal degradation increases

viscosity.

A simple shipboard test is the Mobil flow stick where drops of new and used oil are placed in

separate channels on an inclined 'stick'. The rate the oil flows down the stick is proportional to its

viscosity.

Water content

Initially determined by 'crackle' test. The presence of Na and Mg in a 4:1 ratio indicates salt

water contamination.

Limits are laid down by the manufacturer, but as a rule of thumb a limit of 0.2% should cause

investigation into source and remedial action at 0.5%

Gross contamination can be remedied by placing the charge in a separate tank and heating to

70oC and circulating through purifier.

Spectrometry

Indicates the presence of metal element composition and identifies additive and contaminantlevels.

Zinc(Zn),Phosphorus(P)- are components of many oils such as diesel engine oils, hydraulic oils

and gear oils, to enhance antiwear and over properties of the oil


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Calcium(Ca)- primarily a component of engine oils, provides detergency,alkalinity and

resistance to oxidation. Residual fuel engine oils have higher Ca levels

Nickel(Ni)- Bearings, Valves, gear plating, fuel derivative

Barium(Ba)- Multi purpose additive, declining importance

Magnessium(Mg)- as for Ca, may also be due to sea water contamination if found in Ratio of

1:4 of Na

Chromium(Cr)- Piston rings, hydraulic actuator cylinders

Manganese(Mn)- Cylinder wear

Aluminium(Al)- generally comes from wearing piston skirts, levels rise where new piston fitted

to old engine. Typically 10ppm, but rises during bedding in. May also indicate the presence of

catylytic fines in residual fuels.

Iron(Fe), Molybdenum(Mo), Chromium(Cr)- metals alloyed for piston ring etc, a rise in level

may indicate ring pack/liner wear.

Copper(Cu), Lead(Pb) , Tin(Sn), Silver(Ag) - soft metals used in the overlay of shell bearings,

and phosphor bronze gears.Note that high copper content can also occur when samples are drawn

from copper pipes which have not been flushed as well as gear wear.

Silicon(Si)- Indicates poor air filtration, possible fuel derivative

Sulphur(S)- May indicate the presence of clay based (bentonite) greases

Sodium(Na)- With Mg indicates the presence of sw contamination, possible coolant system andfuel derivative

Vanadium(V)- Usually indicates the presence of fuel oil


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Alkalinity and acidity

TBN-TOTAL BASE NUMBER- measure of alkaline additives available for the neutralisation

of acids from combustion products and oxidation. Level governed by type of fuel.

For crosshead engines the TBN will tend to rise due to contamination by liner lubrication, it

should not be allowed to raise more than twice that of the new charge.

As a guide, the TBN of fresh oil should be at least:

o 10 x fuel sulphur content (%) for trunk piston engines (10mgKOH/g)

o 20 x fuel sulphur content (%) for cyl oil in x-head engines (20mgKOH/g)

A simple shipboard go,no-go test is available for measuring the TBN, it involves the addition of

an indicator and acid reagent to a 30ml sample. The quantify of acid reagent added is determined

by the required level of TBN, for TBN2.5 0.5ml are added, for TBN20 4ml is added. After three

minutes the colour is checked against a chart

o Purple: Good level of TBN

o Green: Borderline

o Yellow: Low level of TBN

TAN-TOTAL ACID NUMBER-measure of organic acid and strong acid content of oil. Where

SAN is nil, the TAN represents the acidity in the oil due to both the acids in the additives and the

oxidation of the hydrocarbons in the oil. The TAN of fresh oils varies with oil type, and tends to

climb with age. A high TAN may indicate that an oil should be changed or freshened by top up.

A high TAN may be accompanied with increased viscosity.

SAN-STRONG ACID NUMBER-indicates the prescience of strong, highly corrosive

(inorganic) acids, usually formed from combustion products. If SAN is non zero the oil should

be changed immediately

Oil cleanliness

IC-INDEX OF COMBUSTION-measures soot loading of oil


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MD-MERIT OF DISPERSANCY-Ability of an oil to disperse contaminants, such as soot,

wear debris and water and thereby carry them away from the critical areas. Measured by oil blot

test and should not be allowed to fall below 50

DP-DEMERIT POINTS- combination of IC and MD: the lower the value, the healthier is the

condition of the oil

Shipboard water content test

1. The flask is filled to mark 'A' with kerosene2. A capsule of reagent (calcium hydride) is added. Any water in the kerosene will react with

the calcium hydride and any gas vented off.

3. The container is topped to mark 'B' with sample oil4. The screw valve and cap are closed.5. The flask is inverted and shaken6. After 2 minutes the screw valve is opened. The hydrogen produced by the reaction

between the reagent and water exerts a pressure which forces the kerosene through the

open valve into the graduated cylinder. The amount discharged is proportional to the water

content in the oil sample.

7. If the water content is greater than 1.5% then the test should be repeated this time using asmaller sample by filling only to mark 'C'. The second scale on the graduated cylinder

should then be used.

8. If water is detected its type, sea or fresh , should then be determined by use of a specialreagent the water

The oil analysis can increase the machinery's life, decrease failure, and reduce money paid

for repair. Due to the severe operating load, speed, temperature effect and the introduction of

foreign substances into the lube oil system, there are chances that the system gradually

lowers the properties and hence it becomes harmful for the engine operation. Therefore it is

always good to identify and analyze the possible sources of contamination and monitor them

from time to time.


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Forces on Bearings

The bearings in diesel engines are subject to various kinds of forces which includeforces due to gas pressure plus inertial and centrifugal forces due to differentreciprocating and rotating motion of the engine parts. Hence the bearings should bedesigned keeping these forces in mind. Moreover the bearing material should betolerant towards minor surface irregularities.

In case of marine diesel engines the lubricating oil is used along with bearings atseveral places and there is a possibility that the oil has small abrasive particles whichcan lodge themselves within these bearings, hence they should also be capable towithstand such particles without getting seized.

Materials Used for Construction

The above forces and circumstances are taken care of by using special white metalalloys which are normally tin or lead based alloys. The only disadvantage of these alloysis their weak mechanical strength which is also necessary apart from the good anti-friction properties; hence steel is used to provide the required backup of strength. The

bonding between the alloy and the back up material is either through mechanicalanchorage or chemical bonding. Steel is preferred over cast iron since it provides betterbonding between the two materials.

Main Engine Bearing Arrangement

It is time to take a look at the main engine bearings and their arrangement in a typicalmain propulsion engine now. I would suggest you take a close look at the diagrambelow to understand how the arrangement is made. The sketch is fairly self descriptiveand all the parts are labeled clearly on the picture.


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The left hand side shows the front view while the sketch on the right hand side showsthe side view of the arrangement, along with various parts such as the thrust bolts, tierod, crankshaft and so forth. As you must have noticed the bearings used here are ofshell type which is constructed in two parts for ease in fitting and operationalconvenience and the bearing is force lubricated.

Forces on the Crankshaft

Due to its nature of operation, there are several types of forces which come to act uponthe crankshaft of engines used in marine propulsion. You will get a better idea aboutthese forces if you take a close look at the image shown below which shows the varioustwisting and bending forces.

As can be seen from the figure, these forces are due to a variety of factors including butnot limited to the weight of the pistons, combustion loads, the axial load from thepropeller which is immersed in the sea, compressive loads of webs on journals and soforth.

Most of these forces have alternating patterns which gives rise to fatigue and thematerials used for construction need to have substantial Ultimate Tensile Strength.

Apart from that the other properties required in the material of a crankshaft are wearresistance, tensile strength, and ductility.

The material for construction also depends on the speed on the engine and slow speedmarine diesel engines have crankshafts fabricated out of plain carbon steel with apercentage of carbon lying between 0.2 & 0.4%, while the alloy steels are used forengines having a relatively higher speed.


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A stress diagram of a particular crankshaft would also help to show the stresses in aweb fillet in a Sulzer RND 10 crankshaft as follows

Fabrication of Crankshaft

Crankshaft manufacture is a complex and elaborate process and the exact procedurewould vary with the type and size of the crankshaft under consideration but a few things

would be good to know

y Fully-built Crankshafts are those in which all the various components areshrink-fitted after separate fabrication

y Semi-built Crankshafts are those in which several parts such as crank-throw

and pins are case out of a single piece.

y Welded Crankshafts are those in which the crank-shaft is made by welding

case web crank pins and half journal units.

y Flanged Coupling Crankshafts are made out in two pieces joined together byflanged couplings

Ship Board Lub Oil Test

Qualitative oil test carried out in board ship do not give a complete and accurate picture of thecondition of lube oil .This could be obtained in a laboratory.

However they do give good enough indication of the oil to enable the engineer to decide whenthe oil should be replaced or if some alteration in the cleaning procedure is considered necessary.

Test forAlkalinity,Dispersiveness,contamination ,water and viscosity are usual.

samples of oil should be taken from the main supply line just before entry into engine since it isthe condition of the oil being supplied to the engine that is of the greatest important.


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Possible sources of contamination are by the unburnt fuel, carbon, water, acid, solid

impurities, etc.

3.

. 2

PORTABLE FLUID ANALYZER

ABSTRACTA portable fluid analyzer that combines LaserNet Fines for debris analysis, Fluid Scan for oil conditionanalysis and a COTS viscometer is described. Examples of instrument operation and results of

application to

shipboard equipment are given.

1.0 INTRODUCTION

We describe a portable, comprehensive, oil analysis instrument, called the Portable Fluid Analyzer, thatprovides immediate analysis of the physical properties of the oil and analysis of debris for the presence of

mechanical wear and its source or for particulate contamination. The instrument combines the LaserNet

Finesoil debris monitor, the Fluidscan oil condition monitor and a commercial viscometer into a single portableinstrument controlled by a notebook computer. It was developed to address the need of fleet support

personnel for immediate, actionable information during shipboard maintenance operations.

Comprehensive oil analysis, including both fluid condition and lube oil debris, is an important part ofmachinery maintenance. In combination with other diagnostics such as vibration analysis it is an

importanttroubleshooting tool for fault diagnostics and machinery health assessment. Several major fault modes fordiesel engines can be identified from effective oil analysis, including faults in fuel injectors, governors,

andturbochargers, and bearing wear. Currently, analysis of lube oil from shipboard engines is done bydrawing asample and sending it to a shore based lab. There can be significant delays in getting the analysis results

to

the user. In addition, the routine tests used by the Navy for debris analysis are often inadequate forreliable

identification of impending catastrophic problems.


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ONR has conducted aggressive programs to address these limitations. The result has been the

development ofadvanced technologies for analysis of oil fluid condition and debris. These technologies are the Foster

MillerFluid Scan oil condition monitor and the LaserNet Fines oil debris monitor.

The Fluidscan (FS) oil condition monitor has been developed by Foster Miller under ONR support for on-

lineapplications. It measures the infrared transmission of the oil sample at several wavelengths that have beenassociated with important chemical species in the oil. From these measurements, quantitativedetermination

of total water, soot, total base number (TBN), byproducts of chemical reactions by the oil additives such

assulfation, nitration and oxidation, contaminants such as glycol, and antiwear additive depletion are made.

Infrared spectral analysis can replace standard lab analysis tests and allows immediate determination ofoil

properties on site.

lubrication dilip

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