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Technical Information Part 2
Viscosity
The thick and thin of it
The thick and thin of it
Viscosity is defined as a measure of a fluid's
resistance to being deformed by either shear or extensional stress. It can also be thought of as a measure
of fluid friction.
A fluid's viscosity
can be shown in several different ways. Here at OCLS, we choose to report the fluid's kinematic viscosity.
The kinematic viscosity is the ratio of the viscous force to the
inertial force (or density). It can be shown by the following formula:
Where:
v = Kinematic
Viscosity (cSt [Centistokes])
µ = Absolute Viscosity
(cP [Centipoise])
r = Density (g cm-3 [grams per centimeter-cubed])
Simply speaking, the lower the viscosity, the thinner the oil. And conversely, a high viscosity value indicates a thicker oil. To put this into context, water has a viscosity of about 1 cSt and an average engine oil has a viscosity of about 100 cSt.
Viscosity
is probably the most important property of an oil. The viscosity determines an oil's ability to protect
the internal components of a machine by coating the parts and creating a thin film of oil between moving parts. Without
any oil, or with an oil whos viscosity is too low, these moving parts would be in direct contact with each other leading to
immediate heat generation and wear. An oil's viscosity can be affected by many factors and so regular monitoring
is essential.
Oil is referred to as a non-Newtonian fluid. Non-Newtonian fluids have viscosities that are not constant - they can't be described
by a single number. For example, a temperature increase of only 5°C can make
the viscosity of some fluids double!
Oil age, Oxidation, water contamination, overheating, oil transfer and fuel dilution are just some of the ways
in which the viscosity can be affected.
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| The latest model viscometer at OCLS |
Using the Wrong Viscosity Oil:
If the viscosity is too high, it can cause:
- Inadequate oil flow to components, e.g. bearings.
- A reduced energy efficiency.
- Excessive heat generation leading to varnish and sludge formation.
- Poor or no oil flow in cold-start conditions.
If the viscosity is too low, it can cause:
- Inadequate or no protective oil film between machine components.
- Excessive component wear.
- The oil film to fail at high temperatures or high loads.
- An increase in friction, resulting in heat generation and oil oxidation.
- Internal or external oil leaks.
TAN & TBN
This TAN has nothing to do with sunbeds...
This TAN has nothing to do with sunbeds...
TAN
stands for Total Acid Number
TBN stands for Total Base Number
Let's look
at them separately...
TAN
A common misunderstanding
is that a TAN test is used to find out the acidic strength of an oil; that is actually a pH test. In fact, a TAN
test is used to find out the amount of acidic components present within the oil, i.e. the acidic
concentration. To put this into context, a single molecule of animal fat would give exactly
the same TAN reading as a single molecule of hydrochloric acid, even though hydrochloric acid is by far the most corrosive
of the two. In fact, the acid present within a synthetic turbo oil is about the same strength as household vinegar!
TAN testing is really important in monitoring the mechanical integrity of equipment and to prevent possible internal
damage to components. An oil's TAN will increase naturally with the passage of time or if exposed to high running
temperatures - the oil gradually becomes oxidised (high temperatures cause oil molecules to react with the oxygen
within the air which degrades the oil). Oxidation severely affects an oil's ability to protect internal components
and can also adversely affect the viscosity which can lead to further problems.
A low TBN can also reduce the detergency of an oil. Detergency is a very important chemical property of an oil that helps it carry away contaminants to prevent them from settling on components. A low TBN can therefore lead to fouling within the crankcase.
As a general rule of thumb, if the TBN is measured at 2.0mg KOH g-1
or less, or if it's 50% of the virgin oil TBN, the oil is considered unfit for engine protection and there is a risk
that corrosion could take place.
The
use of a high sulphur fuel will decrease the TBN at a faster rate due to the increased formation of sulphuric acid.
In synthetic turbo oils hydrolysis (a chemical reaction involving water) can also cause the TAN to increase, especially when the oil is also subject to heat.
The TAN is defined as the weight (in mg (milligrams) of a standard base (e.g. potassium hydroxide, KOH) that's required to neutralise
all acidic components within the oil. The unit for the TAN is mg KOH g-1
(milligrams of potassium hydroxide per gram).
An initial decrease in TAN is no immediate cause for concern - some of the
lighter acidic compounds still present within the oil from the manufacturing process will evaporate away which will in turn
cause a decrease in the TAN. This is quite normal.
TBN
TBN is the opposite to TAN.
Oils are continually exposed to acidic compounds whilst being used which cause
the oil to turn more acidic. This is particularly true of crankcase oils. In an attempt to combat this problem,
manufacturers give their oils a 'reserve alkalinity' which is designed to 'cancel out' any
acidity which forms within the oil during use. The TBN 'fights' the acidity by neutralising it. But
there is only a limited amount of this reserve alkalinity available. A higher TBN means the oil has more reserve alkalinity
available which can be used to reduce the corrosive effects of acids on engine components.
A low TBN can also reduce the detergency of an oil.
Detergency is a very important chemical property of an oil that helps it carry away contaminants to prevent them from settling
on components. A low TBN can therefore lead to fouling within the crankcase.
As a general rule of thumb, if the TBN is measured at 2.0mg KOH g-1 or less, or if it's
50% of the virgin oil TBN, the oil is considered unfit for engine protection and there is a risk that corrosion could take
place.
The use of a high sulphur fuel will
decrease the TBN at a faster rate due to the increased formation of sulphuric acid.
Karl Fischer Moisture Titrations
Is your oil
drowning in water?
Karl Fischer moisture titrations are
used to determine the water content of an oil sample. The Karl Fischer method is useful in that it can be used
to detect both high moisture contents and trace moisture contents very accurately. The process was invented in 1935
by a German chemist named Karl Fischer (pictured right).
The titration process involves a chemical reaction between the water in the sample and iodine within a reagent. Iodine is dispensed into the sample in small, controlled amounts until the reaction is complete. The amount of iodine dispensed during the reaction is directly proportional to the amount of water present in the sample. The Karl Fischer apparatus is pictured left.
The following reaction takes place (in the presence of a solvent mixture):
I2 + 2H2O
+ SO2 ® 2HI + H2SO4
Karl Fischer titrations can accurately report results ranging from 1ppm to
100%. Another significant advantage is that, unlike the conventional Loss On Drying method (LOD), this process
isn't adversely affected by the presence of other volatiles within the sample - the LOD method detects the loss of
any volatile substance, not just water and so can overestimate the water content.
The effects of water contamination can be disastrous and will ultimately lead to catastrophic failure of a machine or its components.
Some of the symptoms of water
contamination in your oil include:
Rust - when moisture
comes into contact with iron or steel surfaces within a machine, a chemical reaction takes place producing a red/brown oxide
(rust). Rust particles are very abrasive and can go on to expose fresh metal surfaces which can also rust. This
rate can increase exponentially if left unchecked producing a catastrophic result.
Corrosion - any moisture can pair up with gasses and form acidic compounds present
within the oil. These can go on to corrode metal surfaces.
Erosion -
if free water drops come into contact with hot metal surfaces, they can undergo instant flash evaporation which can cause
a phenomenon called pitting in the area it came into contact with. This creates small holes in the metal surface of
components which can lead to surface fatigue.
Cavitation -
when any water vapour (steam) bubbles are exposed to extreme pressures, e.g. in a pump or high-load zone, the pressure causes
the water vapour bubble to implode and simultaneously convert back to its liquid water state. This water droplet
can then impact any metal surface in the form of an extremely high-pressure, needle-like jet
which can cause surface fatigue.
Microbe Gowth - when free water comes into contact with diesel fuel it will separate out into two insoluble layers (if left undisturbed). If the conditions are right, microbes can begin to grow at the fuel-water interface, the point where the two insoluble liquids meet (see picture, right). This area provides optimum living conditions for bacteria. Microbes can cause blockages to fuel filters or injectors.
Flashpoint
What's the point?
All flammable liquids have a flashpoint.
It is defined as the lowest temperature at which the liquid can form an ignitable
mixture in air. The flammable liquid we are referring to in this case is diesel or petrol,
i.e. fuel which has contaminated the oil in an engine. We can also test for diesel fuel which has been contaminated
with petrol.
All flammable liquids have a vapour pressure. The vapour pressure is closely related to
the liquid's temperature. So, as the temperature increases, so does the vapour pressure. When the vapour pressure
increases, the concentration of evaporated flammable liquid in the air increases. It is therefore clear that the temperature
determines the concentration of evaporated liquid at equilibrium.
In essence, the flashpoint is the lowest temperature at which enough fuel vapour
exists that it will ignite.
The
oil in a diesel engine can sometimes become contaminated with diesel fuel. The fuel acts as a thinner to your engine
oil and as a result the viscosity can drop dramatically. As indicated in the viscosity section, an oil's
viscosity is one of the single most important defences against abnormal wear and/or equipment failure.
Should the flashpoint indicate the presence of fuel, this may suggest that fuel is entering the crankcase by way of the combustion chamber. This is called blow-by. Another way fuel can dilute the oil is by raw fuel entering the crankcase due to dripping faulty injectors.
The flashpoint test works hand-in-hand with the viscosity test and together they can help us tell the difference between an oil thinning due to oil transfer and an oil thinning due to the presence of fuel.
Dean & Stark Distillation
You can thank
Mr Dean and Mr Stark for this one...
The Dean and Stark distillation procedure was formulated by E. W. Dean and D. D. Stark back in 1920 to determine the water content of samples of petroleum. This process is particularly useful for determining the water content of heavy fuel oils.
The sample is mixed with a solvent and heated in a glass
vessel. The mixture boils and the vapour rises and is converted back to a liquid in a condenser. The equipment
is fairly specialised and, judging by the 1920 science article shown on the right, Dean and Stark already knew that their
test method could be marketed!
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