Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
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118 Fuels and Lubes: Chemistry and Treatment
carbon deposits or prevent their formation; dispersants help to maintain
particles (such as combustion residues) in suspension in the oil until
they are eliminated via filters and centrifuges.
l Anti-wear and HP additives contribute to the maintenance of satisfac-
tory lubrication under the most severe wear conditions.
l Anti-oxidation additives retard damage due to thermal and oxidation
phenomena at high lubricant pressures and hence extend the life of the
lubricant.
Other specific roles are performed by anti-foaming, anti-corrosion, anti-rust
and anti-freeze agents.
Environmental legislation has also impacted on the composition of lubri-
cants. A reduction in the sulphur content of fuels, for example, influences
TBN levels and calls for a new balance of detergent and dispersant additives.
Measures taken to reduce NOx emissions from engines—such as delayed fuel
timing, the injection of water into the cylinder, emulsified fuel and selective
catalytic reduction (SCR) systems based on ammonia—also dictate attention to
lubricant detergency, resistance to hydrolysis and compatibility with catalysts.
All additives are created from a number of ingredients performing diverse
functions. One part of the chemical molecule of ‘overbased’ detergents, for
example, cleans the engine and another neutralizes acids produced by combus-
tion. Additives can interact with each other either positively (synergy) or nega-
tively (antagonism). The art of the lubricant formulator is to select the best base
oils and optimize the proportions of the various additives for the specific duty.
Whether or not the engine is a two-stroke or a four-stroke design, operating
on distillate or residual fuel, the function of the cylinder lubricant is the same
as follows:
l To assist in providing a gas seal between the piston rings and the cylin-
der liner
l To eliminate or minimize metal-to-metal contact between piston rings,
piston and liner
l To act as a carrier fluid for the functional alkaline additive systems, par-
ticularly that which neutralizes the corrosive acids generated during the
combustion process
l To provide a medium by which combustion deposits can be transported
away from the piston ring pack to keep rings free in grooves
l To minimize deposit build-up on all piston and liner surfaces.
Formulating an ‘ideal’ lubricant to perform all these functions for all
engines on the market is difficult and will always be subject to experience and
judgement. First, there are varying requirements depending on engine type
(single or dual level lubrication, oil injection position in the liner and oil feed
rate applied), operating condition (power and rotational speed set points) and
fuel quality used. Secondly, in many cases the requirements are contradictory;
for example, the high viscosity necessary to yield good load-carrying perform-
ance will adversely affect spreadability.
Cylinder oils
The cylinder conditions in the latest designs of low-speed crosshead engines
impose a higher maximum pressure, higher mean effective pressure and higher
cycle temperature than those of previous generations. In addition, the combined
effect of a longer piston stroke and lower engine rotational speed exposes the
cylinder oil film to higher temperatures for a longer period of time.
The diversity of engine designs, operating conditions and worldwide bun-
ker fuel qualities means there can never be a single truly optimum cylinder oil
grade. In certain special circumstances other grades may perform better or at
least more cost-effectively.
Modern crosshead engine cylinder oils are formulated from good quality
base oils with an additive package, which performs various functions within
the cylinder. The product must possess adequate viscosity at high working tem-
peratures, yet be sufficiently fluid to spread rapidly over the working surfaces;
it must have good metal wetting properties and form an effective seal between
rings and liners; and it must burn cleanly and leave a deposit which is as soft
as possible.
The results of shipboard trials and other investigations by a leading lubri-
cating oil company indicated that a 70-BN/SAE-50 cylinder oil, formulated
with the right technology, is the most cost-effective way of providing cylinder
lubrication for the majority of low-speed engines.
Alternative cylinder lubricants can be considered for special applications or to
counter specific problems, or if the ship involved is engaged in a trading pattern
where the sulphur level of the bunkers is consistently high or consistently low.
Shell introduced a higher TBN cylinder lubricant to complement its Alexia
Oil 50 product. The Shell Alexia X Oil is specially blended at 100 TBN for
more highly rated low-speed engines consistently burning residual fuels with a
sulphur content greater than 3.5 per cent.
Castrol Marine’s Cyltech 80 TBN crosshead engine cylinder lubricant has
reportedly proven its ability in service consistently to achieve significant reduc-
tions in both liner and piston ring wear. Lubricant costs have also been reduced
through lower consumption secured by the combination of a high TBN and
effective anti-wear (AW) properties.
Cyltech 80AW was introduced to provide stronger protection against wear
and scuffing through enhanced resistance to oil breakdown at high tempera-
tures. Extra protection is also promised from the AW properties if oil break-
down occurs under extreme conditions, resulting in metal-to-metal contact.
Apart from reducing wear rates under normal and extreme operating conditions
and increasing safety margins against scuffing, the lubricant is said to foster
operation at minimum oil feed rates, thus reducing annual lubrication costs.
Excellent cleanliness in piston ring zones and extra protection against corro-
sive wear are also reported.
Cyltech 70, introduced at the same time and replacing Castrol Marine’s
S/DZ 70 lubricant, is described as a superior 70 BN cylinder oil, benefiting from
the same resistance to breakdown at elevated temperatures as Cyltech 80AW.
Lubricating oils 119
120 Fuels and Lubes: Chemistry and Treatment
It is offered as a 70 BN solution to operators not requiring the higher protec-
tion of Cyltech 80AW and where a higher base number (BN) is less critical.
Investigations in the 1990s by Castrol Marine included shipboard trials
with 130 TBN and 30 TBN products to monitor the performance of cylinder
lubricants under differing conditions. The 130 TBN lubricant was evaluated for
3 months in the Sulzer 6RLB90 low-speed engine of a container ship, which
operated at 85–90 per cent maximum continuous rating. The aims of the trial
were to increase Castrol’s experience with higher TBN cylinder lubricants and
improve its knowledge of the mechanisms of liner corrosive wear. The 30 TBN
cylinder lubricant was evaluated in a two-stroke engine vessel operating in the
Baltic and burning low-sulphur 180 cSt fuel. The main aim was to determine
an optimum between acid neutralization (corrosive wear) and detergency (pis-
ton cleanliness).
FAMM (Chevron/Texaco) notes that most commercial cylinder lubricants
are 70 TBN/SAE 50 grade but warns the operator to be aware that the different
products have a different chemical composition. The latter affects the corrosive
wear protection of the liner. The group’s R&D laboratory conducted Bolnes
engine tests on a series of different 70 TBN/SAE 50 cylinder lubricants, with
the focus on measuring corrosive wear (the most dominant wear factor in low-
speed engines). Differences of up to 100 per cent in the maximum liner wear
were found. FAMM advises that corrosive wear can be adequately minimized
by a number of measures:
l Keeping the liner temperature above the sulphuric acid dew point
l Maintaining proper water shedding of the intake air, especially under
tropical conditions; the water shedding operation of the charge air
should be regularly inspected
l Applying a sufficiently high oil feed rate to guarantee a satisfactory oil
film thickness, minimizing adhesive wear and providing good alkalinity
retention
l Using a cylinder lubricant with superior corrosive wear protection.
Specialists stress that lubricant feed rate is critical and should be carefully
balanced between cylinders. Any downward departure from the enginebuild-
er’s recommended feed rate is potentially dangerous, while an excessive rate is
generally not cost-effective.
BP Marine’s improved Energol CLO 50M 70BN cylinder lubricant prom-
ises cost-effectiveness with the highest performance levels. Lower operational
costs result from reduced wear and cleaner operation, underwriting a higher
margin of safety for low oil feed rates, extended time between overhauls and
better control against scuffing.
A new generation cylinder oil from ExxonMobil, Mobilgard 570, is
claimed to secure maximum protection from adhesive and corrosive wear at the
higher operating temperatures and pressures imposed by the latest two-stroke
engines. Such conditions reduce the viscosity of the lubricants and increase
the loads it must withstand. In addition, longer piston strokes have extended
the amount of surface to be protected and the amount of time the lubricant
must resist the severe cylinder temperatures and corrosive sulphur acids. New
additives with a greater thermal stability and an optimum viscosity (21 cSt at
100°C) foster lubricant distribution and film retention. Superior ring and liner
protection is reportedly demonstrated by Mobilgard 570 at the 70 TBN alkalin-
ity level, along with cleanliness under sustained operation with fuel sulphur
levels from less than 0.5 up to 5 per cent.
Types of Cylinder wear
Cylinder lubricants are an essential component in the control of liner and pis-
ton ring wear in low-speed crosshead engines. High wear rates can be caused
by corrosive, abrasive and adhesive wear, the conditions prevailing in the com-
bustion space being the key influence on the type of wear experienced. In older
engine designs, Castrol Marine explains, the wear was predominantly corro-
sive, but in the latest engine designs it is now largely adhesive wear.
Traditionally, cylinder oils had high alkalinity to provide protection against
corrosive wear: 70 BN was the industry norm, with higher BN oils available
to secure even greater protection. The increase in engine performance, and in
particular the rise in cylinder liner temperature, effectively engineered out high
corrosive wear. Engines designed and built in the late 1980s and early 1990s
had liner temperatures sufficiently high enough to prevent corrosive wear; and
the use of a 70 BN lubricant was often the most economical means of provid-
ing cylinder lubrication and protection.
Once liner temperatures are raised above 250°C, the performance of stand-
ard cylinder oils becomes marginal in providing protection against increased
liner and piston ring wear. Under these conditions, the type of wear is largely
adhesive and the performance of cylinder oils has to be assessed by their abil-
ity to minimize such wear. (Corrosive wear, however, is a problem that may
not have been completely eliminated. Theory, as well as data published on test
engines with very high peak pressures, points to an increased corrosion poten-
tial as more of the sulphur is converted to sulphur trioxide. If the engine charge
air contains enough water, then its combination with sulphur trioxide leads to
the formation of sulphuric acid in sufficient quantities to tip the wear balance
towards corrosion.)
Adhesive wear, sometimes referred to as ‘scuffing’, occurs when the sur-
face asperities on the liner and piston ring have metal-to-metal contact. When
contact takes place, local welding and metal deformation result, with the
amount of contact determining the rate of wear. The rougher the surfaces, the
greater the likelihood of adhesive wear occurring (which is why it is usually
experienced during the engine running-in period).
Basic separation of the contact surfaces is provided by the lubricant film,
and the higher the viscosity the thicker the oil film. Because of the high liner
temperatures experienced in modern engines, the oil viscosity on the liner sur-
face can be as low as 2 cSt near top centre, the resultant film thickness being
1 m or less. Even under these conditions, adhesive wear will not take place,
Lubricating oils 121
122 Fuels and Lubes: Chemistry and Treatment
provided the surfaces are not rough. The roughness of the liner and ring sur-
faces has a significant influence on the load at which adhesive wear starts; and
the rougher the surfaces, the less the load-carrying ability. Rougher surfaces
also require thicker oil films (or higher viscosity oils) to carry the same load.
The term ‘boundary lubrication’ describes the condition where the peaks
of one rough surface (asperities) come into contact with those of another rough
surface, and the lubricant film between the surfaces is not thick enough to pre-
vent metal-to-metal contact. Full separation is achieved only when the ratio of
film thickness over the mean roughness value (or mean height of asperities) is
greater than three. A typical surface finish for the cylinder liner of a modern
engine in service is around 0.5 m, while the oil film thickness of three times
this value (1.5 m)—required to prevent asperity contact—may not be estab-
lished until some distance down the stroke from top dead centre. The condi-
tions for adhesive wear are therefore present at around top dead centre.
Control of the onset of adhesive wear calls for an adequate oil film thick-
ness, which can be enhanced by chemical films from lubricant additives. Such
films have lower shear strengths than the metal surface and hence are removed,
partially or fully, during sliding. Additives that are effective at reducing friction
chemically react with the metal surfaces to form relatively thin planar layers of
low shear strength. Replacement of these films during the out-of-contact time
of the surfaces is therefore essential to the control of wear.
Additive effectiveness is strongly temperature dependent: as the metal sur-
face temperatures increase, the chemical bond between additive and surface
is broken and the additive de-absorbs from the metal surface. Only additive
systems that absorb at elevated temperatures (250°C) are effective in provid-
ing the chemical film that can give added protection against the onset of adhe-
sive wear. Additionally, the calcium detergents used to provide the alkalinity of
the cylinder oil have the effect of lowering the coefficient of friction. The long
chain natural acid present in such detergents is slowly released during neutrali-
zation and migrates to the metal surface to form a friction-modified surface.
Consequently, higher base number cylinder oils will provide higher load-car-
rying performance before the onset of adhesive wear, and then provide some
anti-wear performance to reduce the amount of adhesive wear.
Chemically active surface components give further protection against the
onset of adhesive wear by providing a low shear film between the surface
asperities. In terms of wear protection, therefore, this addition is equivalent
to the use of a higher viscosity cylinder oil: a 19 cSt oil of 80 BN and con-
taining high-temperature anti-wear additives is equivalent to a 23 cSt standard
70 BN oil.
Once the conditions for adhesive wear are established then measurable and
rapid wear will take place. If the conditions are particularly severe the rate of
wear can accelerate, leading to severe damage and seizure. When the metal
surface asperities come into contact, local temperatures are generated that are
much higher than the bulk temperature of the oil or the metal surface tempera-
tures. The local temperatures (or flash temperatures) for cylinder lubrication
are typically between 400°C and 500°C. The only way to control the rate of
adhesive wear is to limit the local welding and deformation of the asperi-
ties, and this can be achieved by using high-temperature high performance
additives.
Special tests were conducted on a modified test engine in which the liner
temperatures were raised to almost 300°C. Under these conditions, severe
adhesive wear took place within 100 h with a standard 70 BN cylinder oil.
Using an 80 BN oil incorporating high-temperature anti-wear additives, how-
ever, minimized the amount of adhesive wear. Other tests showed that this
combination of high BN and anti-wear can reduce the amount of adhesive wear
by up to 50 per cent.
Severe adhesive wear will increase the surface roughness on the rings and
liners, with the result that the load to the onset of adhesive wear is reduced. For
example, if an engine which experiences adhesive wear at a certain operating
load is shut down (perhaps to examine the wear), then the increased roughness
of the surfaces means that when the engine is run again adhesive wear will
start at a lower load. If the conditions that resulted in adhesive wear are now
removed (by operating at a much lower load), then it may be possible that the
surface damage to liners and rings may be polished out to give acceptable run-
ning surfaces.
anti-wear performance summary
l Viscosity has a major influence on the load at which adhesive wear
starts; higher viscosity index (VI) cylinder oils provide more protection
than lower VI oils.
l The rougher the surfaces, the lower the onset threshold for adhesive
wear or the higher the viscosity required to prevent adhesive wear.
l The load at which adhesive wear starts is also influenced by the anti-
wear performance of the alkaline detergent and the use of high-
temperature anti-wear additives.
l Only a standard BN cylinder oil with a viscosity of 23 cSt at 100°C will
provide similar protection against the onset of adhesive wear as an 80
BN oil with high-temperature anti-wear performance at 19 cSt at 100°C.
l The practical limit on the viscosity of cylinder oils (monogrades) for
reasons of pumpability, handling and distribution over the liner surface
is 21 cSt at 100°C with a VI of 100.
l The load at the onset of adhesive wear is 30 per cent higher from an 80
BN cylinder oil with high-temperature anti-wear performance compared
with a 70 BN oil.
l True anti-wear performance is a measure of the ability to reduce the rate
of wear once the conditions for adhesive wear are established. A reduc-
tion of up to 50 per cent can be achieved by using an 80 BN cylinder oil
with high-temperature anti-wear performance compared with a standard
70 BN oil.
Lubricating oils 123
124 Fuels and Lubes: Chemistry and Treatment
medium-speed engine system oils
Lubricating medium-speed trunk piston engines is considered much more
of an exact science than lubricating low-speed crosshead engine cylinders.
Sulphurous by-products from the combustion of high-sulphur fuel are coun-
tered by increasing the amount of over-based additives in the formulation to
give an oil of higher alkalinity, as measured by TBN. But this is not quite as
simple as merely adding alkaline additives. Research is carried out to estab-
lish a balanced blend of the many functionally different additives, for example,
anti-oxidants, anti-wear agents, corrosion inhibitors, detergents, dispersants,
anti-foam agents and pour-point depressants.
The performance of a lubricating oil in a marine medium-speed engine
tends to be measured in terms of its ability to maintain an acceptable level of
TBN in service. TBN retention is influenced by factors such as fuel sulphur
content, combustion quality, lubricating oil make-up, piston ring blow-by and
water contamination. With regard to the oil itself, however, retention is directly
related to additive balance and the quality of the additives used.
Although marine lubricants will continue to rely largely on the types of
additives that have ensured success so far, formulators suggest that more
sophisticated and effective versions can be expected. The search for more pow-
erful synergisms between additives of different species will intensify, and lube
oils that can outlast the life of an engine could be possible in the future.
LineR LaCQueRing
In recent years a number of highly rated medium-speed engines of certain types
have experienced problems with lacquer formation on cylinder liners. This
build-up of lacquer in the honing grooves can result in a smoothed or glazed
liner surface that leads to a significant increase in the rate of oil consumption.
If left unchecked, lacquering can also be accompanied by hard carbonaceous
deposits, which lead to scoring or polishing of the liners and increased engine
operating costs.
A number of common factors link examples of engines in which liner lac-
quering has been found: large variations in load (e.g. frequent and long periods
of idle followed by full load operation); high mean effective pressure medium-
speed designs; and low-sulphur, mainly distillate, fuels. Lacquering can typi-
cally occur in the propulsion engines of offshore supply vessels and short-sea
ships. Lacquer characteristically contains both organic and inorganic (metal
salts) materials, the colour varies from very light amber to dark brown, and
there is uneven distribution over the liner.
Modern, highly rated engines exploit much higher fuel-injection pressures
and shorter injection times. The total injection, ignition and combustion proc-
ess has to take place in a few degrees of crank angle under conditions in which,
besides complete combustion, thermal cracking of fuel components can be
expected. If these cracked components reach the relatively cool cylinder liner sur-
face, they will condense, concentrate and start the formation of resinous lacquer
by a process of polymerization and evaporation of the light ends (see Figure 4.7).
Fuel quality has not suffered a step change in recent years that would
account for the incidences of liner lacquering problems. However, the com-
plexity of refining processes has increased during the past decade and there-
fore the complexity of components in both distillate and residual fuels has also
increased. Experiments have shown that different gas oils produce different
distributions of deposits between pistons and liners under conditions known to
promote lacquer formation. While the fuel clearly influences liner lacquer for-
mation, however, the link to the sources of gas oil is not clear.
An insight into how fuel contributes to liner lacquering was derived from
service experience with a ship running on residual fuel rather than distillate
fuel. The ship bunkered a highly cracked, high-sulphur residual fuel before
each outward voyage leg and a straight-run, low-sulphur residual fuel before
each return leg on a regular sailing schedule. This practice always resulted in
an increase in sump oil top-up and building of liner lacquer during the return
voyage. Although distillate fuels do not show the same sensitivity as this exam-
ple, it does show that the fuel composition is a factor in lacquer formation.
Evidence from liner lacquering in the field as well as in laboratory engines
suggests the following mechanism for lacquer formation, characterized by the
following steps:
l Fuel droplets injected at HP begin to burn rapidly.
l Rapid burning leads to incomplete combustion of some larger fuel drop-
lets but intense heating of droplet cores.
l Thermal cracking of some fuel components occurs within larger
droplets.
Lubricating oils 125
Fuel injector
Cylinder head
Heat from
compressed air
Injection
Heating
Ignition
Combustion
products
Oxygen
Evaporation
Combustion
of volatile
components
Thermal cracking
Liner
Condensation
polymerization
Soot & carbon
formation
Development
of flamefront
Piston crown
FiguRe 4.7 Formation of resinous lacquer on the cylinder liner (shell marine)
126 Fuels and Lubes: Chemistry and Treatment
l Some droplets impinge on the cylinder liner where thermal cracking
reactions continue to form unsaturated reactive hydrocarbon products.
l Condensation and polymerization of unsaturated products on the liner
forms resinous organic polymeric deposit.
l Sticky resinous deposit is spread over the liner by piston ring movement.
l Lubricant, including metal-containing additives, becomes entrained in
the deposit.
l Continual exposure of the deposit to combustion, evaporation of light
ends and rubbing by piston movement forms hardened lacquer deposit.
l Build-up of lacquer fills honing grooves and leads to increased rate of
oil consumption.
Current understanding suggests that any fuel will contribute to liner lac-
quer formation under engine conditions that promote incomplete combustion
and thermal breakdown of the fuel. However, the combination of particular
fuel compositions combined with such engine conditions is the principal cause
of liner lacquer formation in medium-speed engines.
The lubricant can affect lacquer formation. Research by Shell Marine has
shown that a lubricant formulation must have the correct balance of detergency,
dispersency, alkalinity and anti-oxidancy characteristics to prevent lacquering
from occurring. Lacquering will occur most frequently when the engine is run-
ning under very high load conditions. Good maintenance is essential, and the
following are dictated to prevent lacquer formation: injection timing should be
correct; injectors should not be worn; and liner and inlet air temperatures should
not be too low. Shell’s investigations showed that high performance lubricants
can contribute to minimizing lacquering and thus reducing lube oil consumption.
black sludge Formation
A problem in medium-speed engines may arise from the contamination of lube
oil by the combustion blow-by gases and/or by direct raw fuel dilution. Higher
fuel pump pressures in modern engines have contributed to increased fuel leak-
age: analyses of used lube oils from engines running on heavy fuel oil have
indicated an average HFO contamination of 2 per cent; in some cases, levels of
up to 15 per cent were detected.
Most heavy fuel oils supplied today emerge from cracking installations.
The cracked asphaltenes, an inherent part of such bunkers, do not dissolve
in engine lubricants but instead coagulate and form floating asphalt parti-
cles, 2–5 m in size. These particles are very sticky and form black deposits
on all metal surfaces of the engine, including the cambox and the crankcase.
The admixture of residual fuels and some lubricants may lead to coagulation
of asphaltenic particles causing sludge formation in engines, clogging the oil
scraper ring and increasing piston ring groove deposits. Sludge may overload
the lube oil separators and the filters may become clogged.
This problem posed by so-called black sludge or black paint—a sticky,
viscous mixture of fuel and combustion-derived deposits—stimulated leading
lube oil suppliers to develop new system lubricants able to disperse high con-
centrations of cracked asphaltenes. Products were developed to meet the chal-
lenge posed by engine blackening, undercrown deposits, piston head corrosion,
purifier heater fouling, increased lube oil consumption, base number depletion,
oil scraper ring clogging and increased piston deposits. Additives prevent the
contaminants causing black sludge from depositing in the engine, the lube oil
holding the impurities in suspension so that they can be removed by centrifugal
separation. The lubricants are claimed to disperse the highest level of cracked
asphaltenes, resulting in cleaner engines (especially excellent piston cleanli-
ness), reduced lube oil purifier cleaning intervals and filter consumption, and
the near elimination of deposit formation in purifier heaters.
Lube oil stress
Nick Topham, fleet general manager of Reederei Blue Star, suggests that a
relatively new term has emerged in association with lube oil performance in
recent years—oil stress—which may not be so familiar to shipboard engineers
in the front line of efficient engine operation. There are three basic categories.
Acid stress is related to the sulphur content of the fuel. If the additives in
the lube oil cannot neutralize the products of combustion, then acidic attack
can occur on cylinder liners, piston rings and grooves; the resulting wear
reduces component life and reliability. The presence of acid stress is easily
noticed from depletion of the oil BN, which shortens the oil life.
Thermal stress is caused by exposure of the oil to high temperatures. This
leads to breakdown of the oil molecules and oxidation, which in turn fosters
sludge and lacquer formation in the cooler areas of the engine. In addition,
thermal stress can cause deposits in piston ring grooves and on the internal sur-
face of the piston crown. Deposits in ring grooves lead to a decrease in the
pressure build-up behind the ring, causing a reduction in the sealing pressure of
the ring against the liner. Deposits on the internal surface of the piston crown
can lead to overheating of the crown as the heat transfer is reduced.
Piston undercrown deposits reduce the life of the crown; ring groove
deposits shorten the life of liners and piston rings and reduce reliability; and an
increase in the viscosity of the oil shortens its life.
Asphaltenes stress: HFO contains asphaltenes (very large hydrocarbon
molecules), which become mixed with the lubricant: fuel contamination of the
lubrication system is virtually impossible to prevent, especially in trunk piston
engines. As the asphaltenes do not dissolve in the lubricant, they settle out in
the form of sludge in the crankcase and cam case, and can also lead to fre-
quent clogging of the oil filter. Fuel pump sticking can also be attributed to the
precipitation of asphaltenes from the lubricant if the latter cannot cope with
asphaltene stress.
To summarize, oil stress in whatever form can result in the shortening
of lubricant life, lower lifetime of key components, reduced reliability and
increased downtime. All of these effects raise operational costs with respect to
spare parts consumption, lube oil renewal and labour costs, but can be avoided.
Lubricating oils 127