The first step to extending the useful life of a lubricant is to understand the fundamental chemistry of its ageing process. This enables the equipment manufacturer to design equipment to minimize ageing and the user to operate his equipment for maximum fluid life.
Measuring the ageing process:
During normal usage, lubricants form decomposition (oxidation) products in small quantities. Total Acid number (TAN) measures the rate of decomposition by indicating the amount of acid present. It is expressed as the number of milligrams of potassium hydroxide (KOH) necessary to neutralize 1 gram of lubricant.
The standard Acid Number specification for most lubricants is less than 1 mg KOH/gm. Chemical breakdown of lubricants, known as hydrolysis, is greatly accelerated when moisture in the lubricants becomes too high or when the lubricant temperature becomes excessive.
In other words, the main points to keep in mind about the lubricant breakdown process are: · Water is needed and will accelerate the process of hydrolysis · Heat accelerates the process, and · The acid produced and/or some introduced contaminants can catalyze further hydrolysis.
Therefore the best way to prevent your lubricant from ageing is to keep the system as dry as possible, avoid unnecessary high temperature conditions and maintain a low acid number through a comprehensive fluid maintenance program.
The operating temperature of many industrial lubricants can be kept low with good design considerations. Providing sufficient buffer fluid for example, will allow the heat to dissipate. In the case of mobile equipment where there is limited space, the use of an appropriate heat exchanger is recommended.
Most functional fluids such as hydraulic oils are employed to convert electrical energy to mechanical energy. These fluids are not subjected to high heat stress and therefore will age very slowly. Heat can however be absorbed by the lubricant if the fluid has a high air content. The surging pressure experienced by the fluid will compress and expand the air, heating it, resulting in high lubricant temperature.
As a general rule, every 10oC increase in operating temperature over the "nominal" level would reduce the useful life of the lubricant by half.
Contamination in lubricants can either be introduced or self generated. The typical contaminants in lubricants include:
All new systems will contain some contaminant left during manufacture and assembly. This usually consists of fibrous material from rags, casting sand, pipe-scale, cast iron and other metal particles, jointing material and loose paint.
When a normal system has been run-in for a reasonable period, the majority of solid contaminants will be in the form of small platelets, created by bedding-in and the normal wear process, the bulk of which are between 5 and 15 microns in size. Because of their size and shape, they can take a long time to settle.
The other common form of self-generated contaminant is that local cold welding microscopic surface particles will be torn off when they move in relation to each other releasing wear particles.
Unless extreme care is taken in filling and topping up a system, considerable quantities of contaminant can be added during these processes. Many of these contaminants are likely to be abrasive.
A lubricating system can also be contaminated by ingression through the oil film on seals. Worn seals will increase this possibility. Contamination will be introduced if all reservoir openings are not fitted with air breather filters.
The other mechanisms that cause self-generating contaminants include: adhesive, abrasive, erosion, fatigue, de-lamination, corrosive, electro-corrosive, fretting corrosion, cavitation, electrical discharge and polishing wear. Each of these types of wear categories has its own mechanism and symptoms, however in practice they may occur singularly, combined or in sequence.
Nearly all lubricants contain some dissolved gases, and at atmospheric pressure hydraulic oil normally contains 8% of its volume as dissolved air, which in this state causes no problem. However, the presence of air bubbles in a system will cause erratic operation, and air bubbles in suction lines can damage some types of pumps.
It should be noted that entrained air in fluids, when compressed to 2000 psi or more, could become very hot locally. This generated heat causes the fluid surrounding the bubble to burn. As the products of combustion are both fluid and solid contaminants, more contamination can be generated reducing the life of the equipment.
Moisture and Chemicals
The detrimental effect that water and chemicals can have in hydraulic systems are, in certain systems, equal to or greater than operating a dirty hydraulic system.
There are two distinct phases of water that can be present in oil - free and dissolved water. Free water can also be present in the form of an emulsion - microscopic droplets of water distributed throughout the fluid.
Water, in excess of the oil's saturation point, damages a system through accelerated abrasive wear, corrosion and fluid breakdown. Tests conducted at the Fluid Power Research Center at Oklahoma State University indicate that the presence of water significantly increases component sensitivity to abrasive wear from particulate contamination.
Most components exhibit much greater deterioration in performance when both dirt and water are present compared to dirt alone. Excessive water can impact on the system performance and fluid chemistry, adversely resulting in numerous problems such as: · Accelerated corrosion · Reduced bearing life · Thinner load-bearing oil film · Material fatigue · Accelerated oil oxidation · Change in viscosity · Deterioration of oil additives · Bacterial problems
Given the saturation point of water at 65oC is about 200ppm (0.02%), a moisture content exceeding 200 ppm will result in the formation of free water in the system - more free water in cooler regions of the system or when the system is not in operation.
This free water can react with products of lubricant oxidation and additives to form organic acid compounds and "sludge" that will compromise hydraulic performance.
MEASURING CONTAMINATION IN LUBRICANTS
There are several tests that either measure or give an indication of the amount of contamination in a lubricant, depending on the type of oil tested and its functional requirements.
Fluid Sampling and Analysis
Long before a lubricant is ready to be analyzed, provisions must be made to obtain the samples from the lubricating system and minimize the chance of additional contamination getting into the system. While many sampling methods are available this is not included in the scope of this article. More information on this topic is available from the author.
There have been several attempts to categorize degrees of contamination in fluids, including the ISO Cleanliness Code, SAE 749-1963, and the National Aerospace Contamination Limits NAS1638.
These different cleanliness level indicators can be compared in a Cleanliness Level Correlation Table.
The NAS-1638 classification shows the classification system issued by the National Aerospace Standards Committee. The system classes are numbered from 00 through 12. The particle contamination limits for new fluids and different type of hydraulic systems are also shown in the table below for comparative purposes.
Class No.6 (approx. NAS Class 9) indicates the particulate levels of hydraulic fluid as received from the refinery or oil supplier. An SAE Class No. 3 level or lower is preferred.
Class No.4 (approx. NAS Class 7) indicates the particulate level of hydraulic fluid required for most conventional hydraulic systems that do not include hydraulic pressure regulators or hydraulic servo-control valves.
Class No.2 (approx. NAS Class 5) indicates the particulate level of hydraulic fluid required for hydraulic systems when there are hydraulic pressure regulating valves and hydraulic servo-valves.
Maintenance of System Cleanliness
A new lubricant fill in a machine is kept clean by the action of filters and by the chemical action of the additive package in the lubricant. The types of filters used in most instances remove solid or gelatinous particles to the limits of the filter. These filters do not generally remove the liquid or gaseous contaminants.
Effective contamination control is not just a matter of filters. System planning, location of filters, heat exchanger capacity, etc. are but a few of the items that have been considered in a machine's design to reduce the generation of particulate contamination.
Barring isolated instances, it is generally recognized that by having clean lubricant, equipment will give better performance and more reliability. Changing filter elements at given regular time interval, is not desirable, nor necessary.
If installed, filter elements should be changed whenever the differential pressure across the filter exceeds the suggested maximum differential. The degree of filtration (micron rating) will depend on the type of equipment and /or the manufacturers recommendations.
Removing the contaminants present in a lubricant before they become a problem will prevent the fluid from deteriorating to the extent that it is rendered un-usable. This will extend the useful life of the lubricant and reduce its consumption.
The process involves routine testing of the fluid to determine the condition of the fluid and the system, and taking corrective action before the contamination becomes unacceptable.
The contaminants present can be removed from the fluid by various means.
These include: · Filtration · Separation · Centrifugation · Pasteurization · Vacuum dehydration · Ion exchange filtration · Coalescing filtration · Water Absorbing Element filtration · Others
While removing solid contaminants can be done relatively easily and that most lubricating systems have filtration units installed to manage this aspect, very few systems have the capability of removing moisture, chemicals and gases.
Methods of Water Removal
The separation of moisture by means of centrifugation utilizes centrifugal force developed by rotating the oil at high speed. Solid particles and water are thrown outward and are continuously drained. Dry oil leaves the centrifuge from the center of the separation bowl. Whilst a centrifuge is an excellent piece of equipment for removal of bulk free water, it will not remove dissolved or emulsified water and is generally expensive to run with high maintenance and operating costs.
This is a process through which tiny water droplets come together and form one large droplet. This form of water removal from oil uses special cartridges, which combine small dispersed water droplets into larger ones. The large water drops are retained within a separator screen and fall to the bottom of the filter while the dry oil passes through the screen. Coalescers are greatly affected by the properties of the carrier liquid, including interfacial tension and viscosity, but are a simple means of removing a small percentage of free water.
Water Absorbing Elements
Water absorbing filters are usually a non-woven polymeric medium containing an immobilizing water absorbing polymer. This polymer has been modified to retain its integrity as it chemically bonds water. The water retention capacity of these elements is dependent upon the oil viscosity and flow rate. These elements are an effective means of removing and retaining only small quantities of free and emulsified water, which are not considered economical for larger system or those that suffer continual water ingress.
Vacuum dehydration purifiers employ the principles of mass transfer to achieve high efficiency removal of water and gases. To achieve an efficient mass transfer the processed fluid must have a large surface area. This is produced by a variety of techniques including spinning discs, distribution rings and spray nozzles.
Free and dissolved water and gases are removed by exposing the contaminated fluid to a low relative humidity atmosphere, which is obtained by maintaining a chamber vacuum. When air is drawn into the vacuum chamber, it expands to about five times it former volume resulting in five times reduction in relative humidity. Water and gas molecules are attracted to the lower vapour pressure produced in the chamber and are exhausted along with the airflow.
Vacuum dehydration will achieve reductions of up to 100 percent of free water and gases and most of the total dissolved water and gases - to approximately 100 parts per million.
There are several options available to equipment owners to reprocess their lubricants. These include on-site and off-site reprocessing.
There are many advantages in processing lubricants on-site. Some on-site oil reprocessing companies are able to provide this service while the equipment is still running. The advantages of this process include:
1) No machine down time There is no need to shut down the machine while the fluid is being reprocessed. There is therefore no loss in production.
2) No mess no fuss There is no need to arrange drums or suitable storage tanks, transfer pumps etc., and to organize the transferring of oil from the system tank into these storage facilities. Often, the transferring of fluid from the system tank to temporary storage facilities can cause spills resulting in massy clean up exercise.
3) Bonus System Flush Since the system will be running throughout the process, newly reprocessed fluid will flush out contaminants trapped in the pipe-works, the valves and other system components. This process can often produce a system that is cleaner than when it was newly installed.
4) Little disruption to operation There will be minimal disruption to customer operation. The only potential interference are hoses that run from and back to the reprocessing vehicle and the presence of the reprocessing vehicle.
Oilclean on-site side-stream reprocessing offers:
· State of the art dehydration facilities that are very effective in removing moisture from oil. · State of the art filtration systems. · Skilled operators backed up by engineers and laboratory support. · Allows your machine to continue operating while the fluid is being brought back to specification. · Flushes the system with cleaned fluid while it is running. · Extend its useful life of lubricants. · Reduce its cost associated with waste disposal. · Reduce its consumption of new oil.
The oil is removed from site to a recycling facility for reprocessing. This option is becoming less attractive due to the continuous tightening of environmental regulations governing transportation of prescribed waste.
The oil is removed from site to be disposed of as prescribed waste. Similar to the above, this option is becoming more costly due to the continuous tightening of environmental regulations governing both transportation and disposal methods.