LUBRICANTS
A good lubricant generally possesses the following characteristics:
· A high boiling point and low freezing point (in order to stay liquid within a wide range of temperature)
· A high viscosity index
· Thermal stability
· Hydraulic stability
· Demulsibility
· Corrosion prevention
· A high resistance to oxidation
Please go through this link for to know in detail about Lubrication.
" Beyond boundaries: Exploring the secret of Lubrication for Optimal performance"
Properties of lubricants:
1. Keep moving part apart.
2. Reduce friction.
3. Transfer heat.
4. Carry away contaminents and debris.
5. Transmit power.
6. Protect against wear.
7. Prevent corrosion.
8. Seal for gases.
9. Stop the risk of smoke and fire.
Formulation
Typically lubricants contain 90% base oil (most often petroleum fractions, called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefin, esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc.
Additives
· Pour point depressants are compounds that prevent crystallization of waxes. Long chain alkyl benzenes adhere to small crystallites of wax, preventing crystal growth.
· Anti-foaming agents are typically silicone compounds which lower surface tension in order to discourage foam formation.
· Viscosity index improvers (VIIs) are compounds that allow lubricants to remain viscous at higher temperatures. Typical VIIs are polyacrylates and butadiene.
· Antioxidants suppress the rate of oxidative degradation of the hydrocarbon molecules within the lubricant. At low temperatures, free radical inhibitors such as hindered phenols are used, e.g. butyrate hydroxytoluene. At temperatures >90 °C, where the metals catalyze the oxidation process, dithiophosphates are more useful. In the latter application the additives are called metal deactivators.
· Detergents ensure the cleanliness of engine components by preventing the formation of deposits on contact surfaces at high temperatures.
· Corrosion inhibitors (rust inhibitors) are usually alkaline materials, such as alkyl sulfonate salts, that absorb acids that would corrode metal parts.
· Anti-wear additives form protective 'tribofilms' on metal parts, suppressing wear. They come in two classes depending on the strength with which they bind to the surface. Popular examples include phosphate esters and zinc dithiophosphates.
· Extreme pressure (anti-scuffing) additives form protective films on sliding metal parts. These agents are often sulfur compounds, such as dithiophosphates.
· Friction modifiers reduce friction and wear, particularly in the boundary lubrication regime where surfaces come into direct contact.
Properties of Lubricating Oil
§ The quality of a lubricating oil is tested for the following various properties to evaluate its suitability and merits for certain service conditions.
Viscosity | Viscosity is a measure of the flow ability of oil under a particular temperature and pressure. |
Viscosity Index
| The viscosity index (VI) is an arbitrary, unit less measure of the change of viscosity with temperature, mostly used to characterize the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the viscosity is affected by changes in temperature. The viscosity index can be calculated using the following formula:[citation needed] Viscosity Index = 100[(L-U)/(L-H)] Where U is the oil's kinematic viscosity at 40 °C (104 °F), and L and H are values based on the oil's kinematic viscosity at 100 °C (212 °F). L and H are the values of viscosity at 40°C for oils of VI 0 and 100 respectively, having the same viscosity at 100°C as the oil whose VI we are trying to determine. These L and H values can be found in ASTM D2270. |
Specific Gravity | Specific gravity of lubricating oil varies considerably and hence should not be regarded as the main indication of its lubricating property. |
Dynamic coefficient of friction | It is the ration of the tangential force required to overcome friction. |
Flash Point | The lowest temperatures at which the oil flashes and fires, known as flash and fire points. These two temperatures must be sufficiently high for any lubricating oil to avoid flash or burn during use. |
Pour Point | The lowest temperature at which the oil pours is called its pour point. Below this temperature the oil becomes plastic, so it does not produce hydrodynamic lubrication and therefore cannot be used below this temperature. |
Carbon Residue | Lubricating oils being the chemical compounds of carbon and hydrogen, when burnt deposit carbon on the engine parts. This should be as low as possible for lubricating oil. |
Corrosiveness | It is the tendency of the oil to oxidize due to the presence of oxygen in high-temperature gaseous surroundings. |
Emulsification | A lubricant when mixed with water tends to separate. The emulsification number is an index of the tendency of any oil to emulsify with water. |
Foaming | It is the undesirable phenomenon of the oil mixing with air resulting in cavitations and overheating. |
Oxidation at High Temperature | Lubricating oils may break down at high temperature due to oxidation producing hard carbon and varnish, which deposits on the engine parts. Therefore, lubricants must resist oxidation. |
Evaporation | Evaporation test is conducted to find the quantity of oil that may evaporate at high temperatures. Lubricating oil should have a low evaporation characteristic. |
Sulphur Content | Sulphur in a corrosive form is detrimental in lubricating oil. Thus its presence should be avoided. |
Sediments | Sediments are the particles formed due to wear and carbon. Their maximum allowable content is 1.5%. They cause clogged oil filters and purification problems |
Neutralization value
| It is the measurement of the acidity or alkalinity of the oil. |
Total acid number (TAN) | It is a measurement of acidity that is determined by the amount of potassium hydroxide in milligrams that is needed to neutralize the acids in one gram of oil. The TAN value indicates the potential of corrosion problems to the crude oil refinery and to determine oxidation and the subsequent corrosion risk to machines. It is usually the naphthenic acids in the crude oil that cause corrosion problems.
|
Strong acid number (SAN) | It is the measure of the inorganic acids which are formed due to contamination by the acidic products. |
Total base number (TBN) | It is the measure of alkalinity of alkaline oil. Base number is the capacity of the oil to neutralize the sulfuric compounds which are formed, especially in modern engines burning sulfur-rich residual fuel. TBN generally ranges from 6–80 mg KOH/g in modern lubricants, 7–10 mg KOH/g for general internal combustion engine use, 10–15 mg KOH/g for diesel engine operations and 15-80 mg KOH/g for marine grade lubricant. |
Cloud | The low temperature at which the lubricant changes from liquid state to a plastic or solid state is called cloud point. In some cases the oil appears to be cloudy at the start of solidification. |
Dilution of Crankcase Oil | Petrol vapor may escape past the piston rings during the compression stroke, which mixes with oil and affects its lubricating property. The test, which determines the amount of dilution in crankcase oil, indicates the suitability of such oil. |
Cracking stability | It is the property of the oil to be stable and resist cracking at high temperatures. Cracking is the breakdown of molecules into smaller sizes at high temperatures. |
Wear Particle Analysis
Wear particle analysis is a powerful technique for non intrusive examination of oil weted part of a machine.The particles contained in the lubricant carry detailed and important information about the condition of the machine.Oil analysis is the best non destructive technology (NDT) for detecting surface wear on a wide array of oil lubricated machines like engines, turbines, gear boxes, compressors etc. The parameters measured using oil analysis that provide those clues are:
- · Wear material,
- · Particle shape,
- · Particle size and
- · Quantity of debris.
There are many different particle analysis technique. The most common techniques today are-
- · Ferrography,
- · Patch filter analysis
- · Element analysis,
- · Particle count,
- · SEM/EDXRF,
- · Ferrous index/PQ and
- · LaserNet fines laser imaging particle analysis.
1. 1. Direct reading ferrography instrument:
PLP = Percentage Large Particles
2. The most deterministic particle analysis tool is the Scanning Electron Microscope with Energy Dispersive X-ray analysis (SEM/EDX). This technique can identify each particle’s material, shape and size. Due to the very high cost and low throughout, it is mainly used as a failure analysis tool, instead of a routine oil analysis tool.
3. Elemental analysis performed with Optical Emission Spectrometers (OES) such as ICP or RDE. It provides concentrations of wear metals by elements. It gives clues of the potentially problematic components and widely used in oil labs.
4. A sample oil analysis report of Turbine oil is attached for reference.
Introduction
Maintaining proper oil cleanliness is critical to the success of any industrial operation. Oil contamination from particulate matter accelerates the rate of component wear and can lead to premature component failure. In fact, oil contamination is linked to more than 75 percent of all industrial equipment failures.
Ensuring oil cleanliness can help mitigate these issues and lead to longer equipment life, less unscheduled downtime and reduced maintenance costs.
What is oil cleanliness?
Oil cleanliness is a measure of the level of particle contaminants in the oil, including both insoluble and hard particles. Acceptable oil cleanliness levels are often determined by Original Equipment
Manufacturer (OEM) recommendations and can be controlled through proactive maintenance methods.
It is particularly important to maintain oil cleanliness in applications with tight clearances (such as equipment with servo valves) or harsh operating conditions (such as extreme temperatures, pressures or speed).
Factors that contribute to oil contamination
There are two main sources of oil contamination:
1. External sources: these include foreign particles such as dirt, dust and other particulate matter that enter into the system.
2. Internal sources: these include wear particles that contaminate the oil as a result of mechanical wear. There are four main types of mechanical wear: abrasive, fatigue, adhesive and erosive. The table below provides more detail on each type of wear and outlines their effect on equipment performance.
Type of wear | Effects |
1. Abrasive: particles in the clearance between moving surfaces remove material from the surface | Dimensional change |
Leakage | |
Lowering efficiency | |
2. Adhesive: two metals rub together – leading to an instantaneous welding of the surface – and the continuous motion leads to the break of the welded points, causing generated wear metals | Metal-to-metal contact points |
Cold welding | |
Adhesion and shearing | |
3. Fatigue: repeated stress caused by the clearance surface particles trapped by the moving surfaces | Deterioration of finish surface |
Leakage | |
Cracks | |
4. Erosive: particles impinge on a component surface of edge and removing material due to momentum effects | Slow response |
Spool jamming | |
Burnout |
How do we measure oil cleanliness?
There are many international standards are available to measure the Oil cleanliness. This codes quantifies particulate contamination levels per milliliter of oil sample. The first step in measuring oil cleanliness is counting the particulate matter using one of several particle counting methods.
Some of them are discussed below:
ISO 4007 : 2002
This International Standard specifies methods for determining the level of particulate contamination in liquids used in hydraulic systems by counting the number of particles deposited on the surface of a membrane filter using an optical microscope. It includes particle counting by two manual methods and image analysis, using either transmitted or incident lighting systems.
Particle sizes greater than or equal to 2 µm can be sized and counted by this method, but the resolution and accuracy of the results will depend upon the optical system used and the capabilities of the operator.
This method was one of the original methods used for particle counting, but it is extremely time-consuming and is rarely used today.
ISO 11500:2008
This International Standard specifies an automatic particle-counting procedure for determining the number and sizes of particles present in hydraulic-fluid bottle samples of clear, homogeneous, single-phase liquids using an automatic particle counter (APC) that works on the light-extinction principle.
It is applicable to the monitoring of the cleanliness level of fluids circulating in hydraulic systems, the progress of a flushing operation, the cleanliness level of support equipment and test rigs and the cleanliness level of packaged stock. Because the test is fully automated, results are processed quickly.
ASTM D7647 - 10
Standard Test Method for Automatic Particle Counting of Lubricating and Hydraulic Fluids Using Dilution Techniques to Eliminate the Contribution of Water and Interfering Soft Particles by Light Extinction.
This test method covers the determination of particle concentration and particle size distribution in new and in-service oils used for lubrication and hydraulic purposes. Particles considered are in the range from 4 µm (c) to 200 µm (c) with the upper limit being dependent on the specific automatic particle counter being used. The test method is specific to automatic particle counters that use the light extinction principle and are calibrated according to the latest revision of ISO 11171.
Determining the level of contamination
Once operators have accurately measured the particle count, they should then use the ISO 4406
classification to determine the contamination level.
As mentioned previously, the ISO Cleanliness Code quantifies particulate contamination levels per milliliter of fluid at three sizes: 4, 6 and 14 µm. The code is expressed in three numbers, which represent the respective contaminant level code for the correlating particle size. The code includes all particles of the specified size and larger.
Turbine Oil test Report
Pl
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