Properties of Lubricating Oil

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 capacity to keep cold parts of an engine clean capacity to keep hot parts of an engine clean capacity to withstand temperature changes capacity to resist the action of oxygen capacity to contain wear capacity to preserve oil film even in the presence of high

pressures capacity to neutralize acids formed during combustion

or other sources thereby preventing corrosive wear capacity to separate contaminants capacity to withstand the action of water which can

affect additives capacity to pump at different temperatures capacity to separate insoluble elements capacity to resist rust, corrosive and foaming .

Properties for bearings moderate bearing loads improved heat transfer behavior corrosion protection cooling

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low friction good low temperature viscosity good high temperature viscosity

Properties for gear case corrosion protection cooling reduces friction good viscosity on low temperature good viscosity on high temperature sound damping properties with cushioning effects antifoam properties

Turbine oilCompromise between above two requirements

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

Anti-foaming properties important

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 and deterioration 2. To protect the mechanism from harmful combustion

products and malfunctioning lubricating oil 3. To improve existing physical properties and to create

new beneficial characteristics in the oil

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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.

Anti wear improvers-protects rubbing surfaces operating with this film

boundary lubrication.

Detergent-tend to neutralize the deposits before formation under high

temperature and pressure conditions, or as a result of using a fuel with

high sulphur content.

Dispersant-used to disperse or suspend the deposits forming

contaminants.

Alkaline agents-neutralizes acids, these form the TBN of the oil and

includes additives .

Rust inhibitors- protect against rust

Pour point improvers -improves low temperature viscosity

Oiliness agent-reduces friction seizure point and wear rates

Antifoam agents-prevents stable bubble formation

Viscosity Improvers-an additive that improves the viscosity .

Antiseptic-killed bacteria

Oxidation

Oxidation degrades the lube oil producing sledges, varnishes and resins.

Presence of moisture, and some metals particularly copper tend to act

as a catalyst. Once oxidation starts, deterioration of the properties of the

oil is rapid.

Recharging

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When recharging no more than 10 % of the working charge should be

topped up due to heavy slugging that can occur due to the heavy

precipitation of the sludge.

EMULSIFICATION

This occurs due to water contamination; also, contamination with grease,

fatty oils, varnish, paint and rust preventers containing fatty products can

also promote emulsification.

The presence of an emulsion can be detected by a general

cloudiness of the sample. Salt water emulsifies very easily and should

be avoided.

Water entrained in the oil supplied to a journal bearing can

lead to loss of oil wedge, rub and failure.

Fresh water contamination whilst not in itself dangerous can

lead to rusting. The iron oxides catalyses the oil to form sludge's. The

additives in the oil can leach out to change the water into an electrolyte.

Salt water contamination is very serious as it causes tin oxide

corrosion, and also leads to electrochemical attack on the tin matrix in

the white metal. The sea water act as then electrolyte.

A major problem of water within a lub oil is where the mix

enters a bearing, here it is possible for the water to be flashed off

collapsing the oil wedge.

Lube Oil requirements for Diesel Engines

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Crankcase

The oil has to serve two purposes; 1. reduce friction 2. Cool bearings

A good quality mineral oil will serve the purpose of reducing

friction to an acceptable level depending upon the metals involved and

other conditions such as temperature. All oils will oxidize and this

reduces its effectiveness as a lubricant. Oxidation will also cause

deposits which can block passage ways and coat working parts. The

rate of oxidation will depend upon temperature, the higher the

temperature the more rapid the rate. Anti oxidants are available which

reduce the rate, also additional properties can be achieved by the use of

additives.

Under high temperatures an oil is liable to thermal degradation

which causes discoloration and changes the viscosity. Additives cannot

change an oils susceptibility to this degradation. The refining process

can remove compounds which effect the thermal stability of the oil and

also those that lower oxidation resistance. Most of the chemicals found

in an oil will react more or less with oxygen, The effects of this oxidation

is always undesirable. Hence, a major objective of the refining process

of a mineral oil is to remove those hydrocarbons i.e. the aromatics, the

small amount of unsaturated together with molecules containing sulphur,

oxygen and nitrogen.

Unfortunately these same molecules are those that improve

the boundary lubrication performance. Hence, a careful balance must be

struck. The use of anti-oxidants make a slightly better balance although

there usefulness is limited.

Tin based white metal is susceptible to hardening as an oxide

layers from on the surface.

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These tin oxides are a grey -black in appearance and are

extremely hard. There formation reduces the bearing clearance as the

oxide layer is thicker than the original white metal material from which it

formed. The oxide has a lower coefficient of friction than the original

white metal but it will cause problems if it brakes up as fragments will

become embedded edge on in the white metal and can score the pin.

Contamination

Water Water from,

1. bilge's 2. Jackets 3. Sea via coolers 4. leaky seals or washing in purifiers 5. Condensation

Problems caused by water contamination, Water leads to corrosion especially if there is sulphur

present due to fuel contamination forms emulsions which are not capable of withstanding

high loads removes water soluble additives when centrifuged out leads to possible bacterial attack

Fuel

May be heavy residual or light diesel/gas oil and can be sourced to faulty

to cylinder combustion or faulty seals on fuel p/ps.

Problems Increases viscosity for hfo, reduces viscosity for D.O. reduces flashpoint Introduces impurities such as sulphur

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dilutes lub oil when in large quantities

Solid impurities

i.e. carbon from the cylinder combustion process, particularly of

importance with trunk piston engines but also for crosshead engines with

inefficient diaphragm. The carbon can lead to restrictions and blockages

of oil ways causing bearing failure. Straight mineral oils hold 1% carbon

in suspension, dispersant oils hold about 5%.

Bacterial attack

Certain bacteria will attack oil but water must be present. The bacteria

may exist in a dormant state in the oil but water is required if they are to

reproduce.. The bacteria digest the oil causing breakdown emulsions to

be formed, acidity increases, dead bacteria block filters and corrosive

films form on working surfaces.

In summary their must be three essential conditions for microbiological growth;

There must be a source of carbon- present in the oil There must be some bacteria or fungal spores present-

these are almost universally present in the atmosphere There must be free water present

Two other factors which encourage the growth are a slight acidity in the

water (pH 5 or 6) and a slightly raised temperature (20 to 40oC) which

can lead to rapid growth.

Biocide additives are available but they are not always

compatible with other desired additives and can lead to large organic

blockages if treated in the machinery. The best solution is to avoid the

presence of water. If mild attack takes place the oil may be heated in the

renovating tank to above 90oC for 24hrs before being returned to the

sump via the centrifugal separator. For a severe attack the only solution

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is complete replacement of the charge followed by sterilization of the

system. It may be noted that on replenishment the bacteria may be

present in a dormant state in the new charge.

Test results of crankcase oils

Viscosity-Increases due to thermal degradation or hfo contamination,

reduces with diesel oil contamination, corrective action needed if it

increases by 25% from new oil.

Water content-Corrective action required at 1%

Insoluble Sediments-basically the result of wear and oxidation,

corrective action at 1% by weight

Ash-a measure of incombustibles in the oil sample, corrective action at

0.13% by weight

TAN-Total acid number consists of the strong acids (mainly sulphuric

acid) formed in the combustion process and weak acids resulting form

oxidation of the lub oil.

SAN-Strong acid number, the oil should be renewed if any is detected

TBN-Total base number indicates the alkaline reserve particularly

important for trunk piston engines

Closed flash point-highlighted fuel contamination, corrective action if

reduces by 30oC from new

Cylinder lub oil

The type of cyl l.o. required will depend upon the cylinder conditions and

the engine design e.g crosshead or trunk piston. However, the property

requirements are basically the same but will vary in degree depending

upon the fuel and operating conditions.

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Normal properties required are; a. 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

b. the oil must provide an effective seal between the rings and liner

c. only a soft deposit must be formed when the oil burns d. alkalinity level (total base number or TBN) must match

the acidity of the oil being burnt e. detergent and dispersant properties are required in

order to hold deposits in suspension and thus keep surfaces clean

Additives

All oils for all purposes can be designed to give particular properties

through the careful use of additives to the base mineral oil stock.

Common additives are; Antioxidants-these are used in all oils to reduce the rate

at which oxidation occurs and are especially useful were the lub oil cools the piston

Extreme pressure agents these are compounds of phosphorus, Sulphur or Chlorine which increase the strength of the oil film under conditions of high temperature or pressure.

Dispersants or detergents-found in trunk piston engine oils and cyl l.o. these keep surfaces clean by holding deposits in suspension.

Viscosity index improvers- these prevent excessive changes in viscosity with change in temperature

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Other additives can be defined by name such as anti-wear, anti-corrosion, anti-bacteria, anti-foaming etc.

When running in, the cylinder lube oil injector pumps may be

filled with a straight mineral oil without anti-wear properties- typically the

crankcase oil- once this small reserve of oil is exhausted, running in

carries on with normal cylinder lube oil. The flow of oil is increased to

carry away metallic particles.

Problems caused by stuffing box leakage oil entering crankcase

Low speed engines are particularly at risk from crankcase lubricant

contamination caused by cylinder oil drainage past the piston rod gland

and combustion products. This can lead to severe damage of engine

crankcase components and reduction of life of oil which is normally

expected to last the lifetime.

There has been a general increase in the viscosity and Base number of

crankcase oils over recent years particularly for engines built since the

early 1980's. Increased alkalinity, viscosity and insoluble, fuel derived

elements such as vanadium and oil additive derived elements such as

calcium, suggest that the contamination is from the cylinder oil drainage.

Deterioration of the crankcase oil has led to the expensive necessity of

replacing up to 50% of the sump, this is particularly of concern as it is

often only a temporary measure.

Four causes are put forward, 1. New crankcase oil contaminated with new cylinder oil-

unlikely 2. Cylinder oil draining being recycled and returned to the

sump-very likely as it is a common practice to purify oil

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leaking through the gland, tests done on this purified oil found high amounts of insoluble

3. Leakage past rod gland- very likely, high pressure scavenge air can blow cylinder oil and dirt past the top scrapper ring and sealing rings into the piston rod drain tank, and even possibly directly into the sump. A problem that worsens with age and wear.

4. leakage of exhaust valve lubrication system-unlikely

From above the suggestion is the most likely cause for

contamination is leakage past the piston rod. It is seen that maintenance

of the stuffing box is of the utmost importance. Tell tales and drainage

lines should be proved free and use of oil drained from the uppermost

drain should not be allowed even after purification due to the high level

of contamination which can destroy the properties of the oil in the sump

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 samples are generally tested every 3 to

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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 Main supply line inlet or outlet from l.o. cooler Outlet from main l.o. pump

Incorrect standpipes purifier outlet purifier direct sump suction

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 provide optimum 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

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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.

Metals Content

Indicates the presence of metal element composition and identifies

additive and contaminant levels.

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

engine oils, hydraulic oils and gear oils, to enhance anti wear and over

properties of the oil

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

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 catalytic 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.

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

Sulphur(S)- May indicate the presence of greases

Sodium(Na)- With Mg indicates the presence of sw contamination,

possible coolant system and fuel derivative

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

Alkalinity and acidity

TBN-TOTAL BASE NUMBER- measure of alkaline additives available

for the neutralization 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: 10 x fuel sulphur content (%) for trunk piston engines

(10mgKOH/g) 20 x fuel sulphur content (%) for cyl oil in x-head engines

(20mgKOH/g)

Purple:Good level of TBN Green:Borderline 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.

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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

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