„ Viscosity:
The thick and thin of it...
Viscosity is a measure of the resistance
of a fluid to being deformed by either shear stress or extensional stress. It can also be thought of as a measure of
fluid friction.
There are many different types of ways of expressing a fluid's viscosity.
Here at OCLS, we choose to calculate the fluid's kinematic viscosity.
The kinematic viscosity is the ratio of the viscous force to the inertial
force (or density). It can be defined as:
Where:
v = Kinematic Viscosity (cSt [Centistokes])
µ = Absolute Viscosity (cP [Centipoise])
r = Density (g cm-3 [grams per centimeter-cubed])
|
| Viscosity Comparison |
Simply speaking, the lower the viscosity
value, the thinner the oil will be. Which follows that a high viscosity value indicates a thicker oil.
For example, water has a viscosity of about 1 cSt. An average engine oil has
a viscosity of about 100 cSt.
Viscosity is a very important property of oil that allows
it to protect the internal workings of a machine by creating a thin film of oil between moving parts. Viscosity can
be affected by many factors and so regular monitoring is essential.
Oil is a non-Newtonian fluid, which means that its viscosity is not
constant - it cannot be described by a single number - it varies. For example, a temperature increase by
only 5°C can cause the viscosity of some fluids to double! OCLS check all oil viscosities at 40°C.
Oxidation, water contamination, oil age, overheating, fuel dilution and oil transfer are some of the ways
in which the viscosity can be affected.
|
| The latest model viscometer at OCLS |
Using the Wrong Viscosity Oil
Viscosity
Too High:
- Inadequate
flow to components, e.g. bearings.
- Energy
consumption losses occur.
- Excessive
heat generation causing varnishes and sludges.
- Poor
oil flow in cold-start conditions.
Viscosity Too
Low:
- Lack
of protective oil film between components.
- Excessive
wear.
- Oil
film fails at high temperatures or high loads.
- Increase
in friction, resulting in heat generation.
- Internal
or external oil leaks.
„ TAN & TBN:
What are they and why are they important?
TAN stands for Total
Acid Number
TBN stands for Total
Base Number
Let's look at them separately...
TAN
A common misconception is that a TAN test is used to determine the acidic strength
of an oil. Actually, 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 the same TAN reading as a single molecule of hydrochloric acid, even though hydrochloric acid
is by far the most corrosive of the two. Indeed, the acid present within a synthetic turbo oil is about the same
strength as household vinegar!
TAN testing is crucial to maintaining the mechanical integrity of equipment
and to prevent internal damage to components. An oil's TAN will increase with the passage of time or if exposed to high
running temperatures - the oil becomes oxidised (high temperatures cause oil molecules react with the oxygen within
the air). Oxidation severely affects an oil's ability to protect internal components and can also affect the viscosity.
In synthetic turbo oils, hydrolysis (a chemical reaction involving
water) can also cause an increase in the TAN, especially when the oil is subject to heat.
The TAN is defined as the
weight (in milligrams) of a standard base (e.g. potassium hydroxide, KOH) that's required to neutralise all of the acidic
components within the oil. Its unit is mg KOHg-1
(milligrams of KOH per gram).
An initial decrease in TAN is no cause for concern - some of the lighter acid
compounds present within the oil when it was manufactured will evaporate away which will in turn reduce the TAN.
TBN
Oils are continually exposed to acidic compounds
which cause the oil to turn more acidic. This is particularly true of crankcase oils. In an attempt to combat
this problem, manufacturers give the oil a 'reserve alkalinity' which is designed to 'cancel out' any acidity
which forms in the oil during use. The TBN determines how effective the battle
will be against any acids formed during the combustion process. A higher TBN means the oil has more reserve alkalinity
available which can be used to reduce the corrosive effects of acids.
A low TBN can also reduce the detergency of an oil and can
therefore lead to fouling within the crankcase.
As a general rule of thumb, if the TBN is measured at 2.0mg
KOHg-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
Titration:
What is it & how is it useful?
Karl Fischer moisture titrations are used to determine the
water content of your oil samples. This method is particularly useful as it can be used to detect both high moisture
contents and trace moisture contents. The process was invented by a German chemist named Karl Fischer in 1935.
|
| The Volumetic Karl Fischer at OCLS |
The process involves a chemical reaction between water and Iodine within a
reagent. Iodine is dispensed into the sample in small amounts until the reaction endpoint is reached. The amount
of Iodine used in the reaction is directly proportional to the amount of water within the sample. The following reaction
takes place (in the presence of a solvent mixture):
I2 + 2H2O
+ SO2 ® 2HI + H2SO4
Karl Fischer titrations are extremely accurate and can give results ranging
from 1ppm to 100%. Another significant advantage is that unlike the conventional Loss On Drying method (LOD),
this process is not adversely affected by the presence of other volatiles - the LOD method detects the loss of any
volatile substance, not just water.
|
| Water contamination in diesel fuel causing microbial growth |
The effects of water contamination can be disastrous and can ultimately lead
to catastrophic failure of vital equipment.
Some of the effects of water contamination in your oil:
Rust - when water comes into contact
with iron or steel surfaces, 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 then rust. This rate can increase exponentially
if left unchecked.
Corrosion - water can pair
up with acids present in the oil and can go on to corrode metal surfaces.
Erosion - if free water comes
into contact with hot metal surfaces, it can undergo flash evaporation and can cause pitting in the area it came into contact
with.
Cavitation - when any water
vapour bubbles are exposed to extreme pressures, e.g. in a pump or high-load zone, the water vapour bubble implodes and simultaneously
converts back to its liquid form. This water droplet can impact
any metal surface in the form of a high-pressure, needle-like jet which can cause surface fatigue.
Microbe Growth - free water
contamination in diesel fuel will separate out into two insoluble layers. Microbes will grow at the fuel-water
interface, the point where the two insoluble liquids meet (see picture above-right). Microbes can block fuel filters.
„ Flashpoint:
What's the point?
|
| The Latest Flashpoint Tester at OCLS |
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 the case of oil analysis is diesel or petrol - fuel which has contaminated the
oil.
All flammable liquids have a vapour pressure. The vapour pressure is
related closely to the liquid's temperature. So, as the temperature goes up, 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 ignites.
| The Flashpoint Test |
|
|
| Click for a better view |
Diesel Engine Oil Testing
The oil in your diesel engine can be badly affected by fuel contamination.
Diesel fuel acts as a thinner to your engine oil and as a result the viscosity can drop dramatically. As we pointed
out 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
cause of fuel dilution 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.
