Marine Lubricating Oil
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Transcript of Marine Lubricating Oil
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
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
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
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
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
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
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 .
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.
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.
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
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
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
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.
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.
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
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
dilutes lub oil when in large quantities
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%.
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
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.
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
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
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
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
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
4 months depending on the system and experience. Shipboard testing is
taking a rising prominence to allow monitoring of oil condition between
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
The viscosity is the most important property of the oil. Oil of correct
viscosity will provide optimum film strength with minimum friction losses
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.
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.
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
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 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
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
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