The extension of service life of hydraulic fluids and lubricants can significantly reduce environmental pollution resulting from a reduction in fluid disposal quantities. The economic benefits include reduced fluid disposal costs as well as the resulting reduction in new fluid cost.
Particulate and water contamination can have adverse effects on the physical and chemical properties of hydraulic and lubricant fluids. These include accelerated oxidation of the fluid base stock and premature depletion of additives due to additive precipitation or chemical reactions involving additives. In many cases, the presence of free water in the fluid can be detrimental to proper system performance.
The importance of the removal of particulate and water contamination to fluid service life extension has been confirmed in laboratory studies and through experience in the field where life extension factors of 4 to 6 were realized. Fine particulate control filters, water absorbing filters, and vent dryers (to prevent ingression of moisture into the fluid system from the environment) in conjunction with fluid purifiers, can provide effective means to control particulate and water contamination accumulation in hydraulic fluids and lubricants.
Extending Service Life
The extension in service life of hydraulic and lubricant fluids has assumed prominence to many users in industry. A primary factor is the increasing emphasis on protecting the environment from industrial and hazardous waste. This has resulted in stringent U.S. federal and local regulations leading to higher costs and liabilities in the disposal of used oils and lubricants. There are substantial economic benefits associated with extended fluid service life, including reduced disposal costs and reduced new fluid purchases. Other benefits include reductions in maintenance costs and machine downtime which, in many applications, can result in significant economic gains to the user.
Particulate contamination and water can have serious adverse effects on the physical and chemical properties of the fluids. The loss of crucial fluid properties, central to useful service life of the fluid, can result in inefficient system performance and accelerated mechanical and chemical wear processes. In this article, attention is focused on the degradation of hydraulic and lubricant fluids in the presence of particulate and water contamination, and on the extension of fluid service life via removal of particles and water from fluids.
Degradation of Hydraulic Fluids and Lubricants
Hydraulic fluids and lubricants are carefully formulated and are comprised of a base stock and an additive package. The additive package consists of chemical compounds designed to protect the base stock and various system components, and to help ensure proper performance of the system. Typical additives include dispersants and detergents, antioxidants, corrosion inhibitors, antiwear and extreme pressure (EP) agents, and viscosity index improvers.
Particulate contamination and water affect both the base stock and additives adversely as discussed below.
Physical Properties of the Fluid
Water contamination alters lubricant viscosity, load-carrying ability and dynamic film thickness when present in the fluid. This can lead to greater surface-to-surface contact at sliding and rolling clearances, which leads to increase component wear.
The presence of free water in hydraulic systems that could experience freezing temperatures, can lead to degradation of system performance and malfunction due to “icing.”
The presence of water can also lead to the agglomeration or precipitation of particles to produce viscous sludge or solids. These byproducts of degradation increase system stress, particularly on pumps, and can lead to clogging of orifices, nozzles and jets.
Additionally, particulate contamination and water can promote chemical reactions involving the base stock and additives producing corrosive organic acids, gums and sludge. These chemical reaction byproducts adversely affect viscosity and other properties of the fluid.
Degradation of the Fluid Base Stock
Oxidation: A primary chemical degradation process in many applications is the oxidation of the fluid’s base stock. The oxidation process proceeds through a series of chemical chain reactions and is self-propagating, with the intermediate, reactive chemical species regenerating themselves during the process. The process results in the conversion of the fluid base stock to oxygenated compounds, such as organic acids in the case of hydrocarbon or poly-ol ester based base stocks, and eventually, high molecular weight polymeric compounds. The polymeric compounds are often insoluble and settle out of the fluid as gums, resins or sludges.
Oxidation is significantly accelerated by the presence of metals and water. Metals act as catalysts providing an alternate, favorable pathway for the reaction. In most cases the alternate pathway involves the participation of the metal surface. Fine metallic debris are ideal pro-oxidant catalysts, in this respect, due to their large, effective surface area.
Table 1. Effect of Metal Catalysts
and Water on Oil Oxidation
Table 1 summarizes data from tests carried out to determine the effect of metal catalysts and water on oil oxidation. The tests were conducted on turbine grade oil in pure oxygen according to the ASTM D943 oxidation test procedure. The acid number, tabulated in the last column, is a measure of the extent of oxidation.
The results show that the extent of oxidation is increased roughly 48-fold for iron/water and 65-fold for copper/water within 400 hours and 100 hours, respectively, compared to the baseline; Test Number 1, Table 1. In these tests, the metal catalysts were present in the form of strips as opposed to silt-size particles normally present in actual systems. The results would be expected to be even more dramatic in the latter case because of substantially increased metal surface area promoting oxidation.
Hydrolysis: Fluid base stocks that are comprised of ester compounds, such as polyol ester gas turbine lubricants and phosphate ester hydraulic fluids, can undergo hydrolysis in the presence of free water under operating conditions in the system. This results in the formation of acidic compounds that can react with the materials of the system components and hence, corrode the components.
Additive Depletion: Depletion of additives can occur either by their physical removal from the fluid or by chemical reactions in which they are converted to nonfunctional products. Particulate contamination and water foster both types of processes.
The solubility of many additives is critically dependent on fluid composition. The presence of water can lead to the precipitation of these additives from the fluid.
A primary step in the action of additives such as corrosion inhibitors, antiwear and extreme pressure agents is their adsorption on metal component surfaces to form a protective layer. The presence of particulate contamination, especially metallic debris, provides an alternate, effective surface for adsorption and causes premature depletion of the additives.
Particulate contamination and water can also chemically react with additives creating sludge and solids as previously discussed, and at a minimum compromising additive effectiveness.
Contaminants may cause additives that protect the base stock to deplete more rapidly. A notable example is the depletion of antioxidants.
A summary of the effects of particulate contamination and water on hydraulic fluids and lubricants is presented in Table 2.
Table 2. Effect of Particulate Contamination and
Water on Hydraulic Fluids and Lubricants
Removal of Particulate and Water Contamination
Several methods are available for removing particulate and water contamination from fluids. The method of choice depends on the contamination level of the fluid and the specific area of application. Heavily contaminated fluids are best subjected to external purification prior to use. In-line contamination control in the system is conveniently achieved by in-line particulate filters and water-absorbing filters, and vent dryers to minimize ingression of water from the environment into the system.
Field Experience Experience
in the field demonstrates the importance of particulate and water removal in extending the service life of hydraulic fluids and lubricants. Some examples are enumerated below.
Pall Corporation conducted a study in 1982 on the effect of particulate and water control on the performance of hydraulic systems in ground-based airline cargo container handling equipment at the JFK International Airport, New York, in conjunction with United Airlines. Equipment studied included aircraft bridge/nose docks, container rotators and mobile cargo loaders.
The equipment is located outdoors and is exposed to dirt and weather extremes. Preliminary investigations revealed excessive contamination levels of particulates and water necessitating changes of hydraulic fluid every two to three months. In spite of this, equipment failure was common, resulting in aircraft delays and increased associated costs. A comprehensive contamination control program was then initiated that incorporated high-efficiency fine particulate control filters and portable oil purification units in the hydraulic systems. Results are shown in Table 3.
Table 3. Contamination Control in Hydraulic
Systems of Airline Ground Support Equipment
A marked increase in service life of the hydraulic fluid, factors of 4 to 6, was observed with the fluid replacement interval exceeding 11 months. The level of water in the fluid was consistently maintained below the manufacturer’s recommended level of 200 to 400 ppm. Associated benefits included improved system performance and sharply reduced system downtime.
Similar extensions in fluid service life have been achieved in many industrial applications. In one instance, frequent fluid replacements and machine downtime were severe problems in numerically controlled machine tools utilized in an aerospace manufacturing plant. Tests conducted in conjunction with Pall Corporation showed particulates to be a major contaminant in the hydraulic fluid.
Subsequent installation of fine, 3-micron rated filters resulted in annual fluid changes during maintenance work. In another instance, excessive particulate and water contamination was encountered in the lubricant used in the gearboxes of the mixing vats in a major pharmaceutical company. Incorporation of a fluid purification unit has led to a virtually indefinite extension in fluid life.
The extension of service life of hydraulic and lubricating fluids is gaining prominence due to the many definable benefits, including environmental pollution, conservation of natural resources and the economic benefits associated with extended service life.
Particulate and water contamination can degrade critical physical properties of fluids, promote oxidation/hydrolysis of the base stock, and induce depletion of additives. The resulting breakdown of fluid properties leads to increased chemical and physical wear processes.
The removal of silt and water is important in extending the service life of fluids. This is confirmed through laboratory tests and field experience. Typically, lifecycle extension factors of 4 to 6 are recognized.
The incorporation of fine particulate control filters, water-absorbing filters and vent dryers to prevent ingression of moisture into the fluid system from the environment, in conjunction with fluid purifiers, provide effective means to control particulate contamination and water accumulation in hydraulic fluids and lubricants.