What is Lubricity ???[Defination of Lubricity]
The lubricity is defined as the ability of fluid to minimize friction between and damage to surfaces in relative motion under load. The study of lubrication and wear mechanisms is called tribology. The test for lubricity is very important because it protects the wear and tear part of the internal parts of the engine which are directly exposed to fuel contact. If the lubricity is not at an adequate level, then various core engine components including fuel pumps and injectors are disposed to excessive wear and metal damage. This type of wear and tear can affect the performance of equipment, increase the maintenance cost, and shorten the service life of machinery.
|Use better Lubricity fuel to protect wear of engine parts|
The lubricity of fluid cannot be measured directly as it is not a material property. For the purpose of measurement of lubricity, the test is accomplished to quantify the performance of lubricants on a specific system. It is generally determined in terms of how much wear scar is caused by the fluid on the specific surface in a particular time period. Other factors like temperature, pressure, surface size, etc. also affect the wear scar, and the effect of these factors must be considered while performing the lubricity test. Hence the lubricity is correlated with unit wear scar on the specific surface, lesser the wear scar value better is lubricity.
Here wear scar is the average diameter of a worn and scuffed area, measured in two specified directions. It is generally measured in ml or micrometer unit
The lubricity may be different for two fluids with the same viscosity, and the fluid having lesser wear scar is considered to have greater lubricity. Hence lubricity is also known as anti-wear property of a substance
|Fuel with higher lubricity increase engine life span|
The lubricity of fuel can be measured by various methods based on different types of wear mechanisms, such as vehicle test method, pump rig test method, and bench test method. The bench test method is most popular because these test methods allow rapid and relatively inexpensive measurements of fuel lubricity
Various type of bench tests used for measurement of lubricity is as follows:-
Lubricity Test-1: The High-Frequency Reciprocating Rig (HFRR)
The High-Frequency Reciprocating Rig is versatile computer-controlled equipment for evaluation of the co-efficient of friction and wear properties of fuels and lubricants under dry & lubricated conditions It is a rapid and easy screening method, and can produce a wide range of wear mechanisms depending on the fuel being tested. In the case of diesel fuels, it provides an absolute value in terms of wear scar and for lubricants. Even it is also used for ULSD diesel fuel lubricity conforms to the EN 590 specification.
It has become the industry standard test for diesel fuel lubricity and conforms to ASTM D6079, D7688; ISO 12156; IP 450; EN 590; CEC F-06-A , EN 590, and JPI-5S-50.
Lubricity Test-2: The Ball-on-Cylinder Lubricity Evaluator (BOCLE):
BOCLE was established for aviation jet fuels (ATF). It is principally convenient for assessing the effects of fuels and additives on oxidative wear—an important wear mechanism in aviation fuel systems. This test method includes two procedures, one using semi-automated equipment and the second is fully automated equipment.
Test standard compliance: ASTM D 5001
Lubricity Test-3: The Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE) :
SLBOCLE instrument was developed in the mid-1990s for carrying out lubricity testing of diesel (middle distillate) fuel. In this test method, the lubricity of a fuel is measured in the aspect of friction produced between the stationary ball and rotating ring-fuel wetted by Appling minimum applied load in grams. It is similar to the BOCLE test but with modifications to make it less sensitive to oxidative wear and more sensitive to adhesive scuffing
Test standard compliance: ASTM D 6078.
Lubricity Test-4: The Ball on Three Disk (BOTD) :
The Ball on Three Disk (BOTD) Test instrument is a light weight, compact, dedicated system for assessing wear properties of low viscosity fluids such as diesel. The Ball on Three Disk (BOTD) envisages a diesel fuel’s ability to prevent wear of moving parts in fuel injection systems. This system incorporates an upper ball rotating against three disks in a tetrahedral orientation. Recently, the same test principle has been developed to evaluate minor differences in anti-wear properties of aqueous emulsions. This method is fairly recent and is still in the development phase.
Note: Since the tests are based on different types of wear mechanisms, and no correlation between methods is developing yet.
Lubricity Correlation with Sulfur in Fuel:
There is a misunderstanding that sulfur provides the lubricity to fuel oil, but that idea is only indirectly correct, there are other factors also which contribute the lubricity of fuel.
The crude oil would contain various bonds of hydrogen (H) and carbon (C) atoms which chains of various length by developing linkage together in hydrocarbon systems. Sulfur is present in crude oil as an impurity, content can range from as low as 500 ppm (sweet crude) up to as much as 10 foldings (sour crude), depending upon the crude oil source.
Earlier, it was considered that most of the distillate fuels have sufficient lubricity naturally to protect against the wear of the engine component. (Exception Kerosene), Hence There was no reason established test method to measure a lubricity value.
Though, everything has been changed after EPA regulation implementation, as the sulfur must be removed from the fuel before it can be sold because sulfur emission increases the air pollutant oxides, which are hazardous to health and long term corrosive acid rain damage
For the effective implementation of EPA regulation, the refinery starts the desulfurization process via hydrotreating, in which crude oil is treated with hydrogen gas under extreme temperatures and pressures. Hydrogen gas combine with sulfur and form Hydrogen sulfide which can be further converted into elemental sulfur by oxidation methods.
However, during desulfurization process yield is a satisfactory low sulfur diesel fuel, but also one that is unsatisfactorily low in lubricity, but unfortunately destroyed critical polar and organic aromatic compounds inherent to the fuel, it is mainly due to loss of nitrogen and oxygen-based polar trace compounds- identified as responsible for providing significant lubricity quality to the fuel.
Understanding this correlation, ASTM launched a task force to consist of fuel lubricity requirements as part of it’s, and to determine an effective laboratory test method for quantifying the lubricity values.