Hydraulic contamination - part 1

Hydraulic contamination - part 1

Editors note: This is part one in a two-part series on contamination. Part 2 focuses on NIST certification and calibration, filter performance, and reporting.

Fig. 1. Portable particle counter operates in three sampling modes: high pressure (from 50- to 5,000-psi sources), low pressure (sources less than 50 psi), and from bottles. Typical analysis consists of three counting runs; instrument then displays cleanliness codes and average particle counts for seven micron-size ranges, and can produce reports from its built-in printer.

Fig. 2. Water sensor mounts in fluid line or hydraulic reservoir. LED display instantly indicates percentage of saturation and temperature, also flashes at 90% saturation to warn operator.

Fig. 3. Oil purifier removes free and dissolved water as well as free and dissolved gases. Nozzles spray water-contaminated fluid into its vacuum chamber; air with low relative-humidity passes through chamber to pick up water vapor and gases, then is discharged to atmosphere.

Concern for the environment has become a fact of life for most companies. For the companies that use hydraulic equipment, that concern is reinforced by stringent federal and local regulations requiring pollution prevention and governing disposal of used industrial fluids. Higher costs (and greater potential liabilities) for fluid disposal are a direct result of these regulations. This adds another economic item to the list of benefits derived from controlling contamination to extend the service life of hydraulic fluids. The list now includes:

• lower cost for fluid needed for replacement and replenishment
• more consistent hydraulic system performance
• less component wear, and
• reduced fluid disposal costs.

Contamination's detrimental effects

It is an established fact that particulate contamination and water in hydraulic fluids can have serious adverse effects on the fluids' physical and chemical properties. The loss of crucial fluid properties, which are central to useful service life, can result in inefficient system performance and accelerated mechanical and chemical wear processes.

Hydraulic fluids are carefully formulated for specific areas of application. They usually are comprised of a base stock and an additive package. The additive package consists of chemical compounds designed to protect the base stock - as well as the components in the hydraulic system - and to ensure proper performance of the system. Typical additives include dispersants and detergents; anti-oxidants; anti-corrosion, anti-wear, anti-foaming and extreme pressure (EP) agents; and viscosity-index improvers. Particulate contamination and water adversely affect both the base stock and the additives.

Water is a poor lubricant, and significant concentrations of water in hydraulic fluids can decrease their viscosity and load-carrying ability, as well as hydrodynamic-film thickness. This can lead to greater surface-to-surface contact at sliding and rolling dynamic clearances, and hence, increased component wear. The presence of free water in systems that could be exposed to temperatures below the freezing point of water can lead to icing, which will degrade system performance and can produce malfunctions.

The presence of both water and particulate contamination can lead to the formation of insoluble precipitates, and viscous sludges and gels. These materials induce excessive stress on system components - especially pumps - and can clog orifices, nozzles, and jets.

Degradation of fluid base stock

Oxidation of the fluid base stock is a primary chemical-degradation process in many hydraulic fluids. 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 result is the formation of oxygenated compounds (notably acidic compounds in the case of hydrocarbon, polyol-ester, or phosphate-ester base stocks) and, eventually, high-molecular-weight polymeric compounds. All of these compounds often are insoluble; they settle out of the fluid as gums, resins, or sludges.

Oxidation is significantly accelerated in the presence of metals and water. Metals act as catalysts, and fine metallic wear debris, commonly found in hydraulic systems, are especially active, due to their large effective surface area.

Table 1 (below) summarizes data from tests that were carried out to quantify the effect of metal catalysts and water on oil oxidation. The tests were conducted on turbine-grade oil in a pure oxygen atmosphere, according to the ASTM/D-943 oxidation test procedure. The neutralization number, tabulated in the last column, is a measure of the extent of oxidation. The results show that the extent of oxidation is greatly increased: roughly 48-fold for iron/water and 65-fold for copper/water within 400 and 100 hours, respectively - compared to the baseline test with no water and metal catalysts. Even with only a single contaminant present (either water or a metallic catalyst), the neutralization numbers increase.

Hydrolysis - the alteration or decomposition of a chemical substance due to the presence of water - is another potential problem in hydraulic fluids. Fluid base stocks that are comprised of ester compounds, such as polyol esters and phosphate esters, can undergo hydrolysis under typical hydraulic-system operating conditions. The acidic compounds that may form during hydrolysis can react with materials of the hydraulic-system components, leading to corrosion and insoluble corrosion products.

Additive depletion

Depletion of additives can occur either by their physical removal from the fluid or by chemical reactions which convert them to non-functional products. 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. In addition to being rendered non-functional, the precipitated additives contribute to the particulate contamination level in the fluid.

Additives that protect the base stock can be depleted rapidly due to the enhanced degradation of the base stock in the presence of both particulate contamination and water. A notable example is the depletion of antioxidants. A summary of the detrimental effects of particulate contamination and water is presented in Table 2. (below)

Monitoring contaminants

How can you check on the status of the hydraulic fluid in an active system? Not too long ago, you had to take samples of fluid from the system, send them to a laboratory, and wait for a report. Today, a variety of on-line monitors is available for in-house measurement of particulate levels and water concentration. Some monitors can be interfaced with fluid-purification devices for automated operation. When the monitor detects that preset threshold concentration limits for particulates and/or water have been reached, the contaminated fluid is directed into the purifier - with no operator intervention required.

On-line particle counters or contamination monitors are used most commonly to measure particulate levels in fluids in installed systems. Particle counters provide an estimate of the particle size distribution, i.e., particle size vs number of particles, for several preset size ranges (usually six to seven). They operate on the principle of light obscuration by particles in the fluid-sample stream. Portable models, Figure 1, can be moved to multiple locations or installed at a single location for continuous on-line sampling. The contamination level typically is quantified in terms of the fluid cleanliness code (ISO 4406) or fluid cleanliness classes (NAS 1638 or ISO 11218).

Contamination monitors operate on the principle of mesh blockage by particles in the fluid-sample stream. They provide an estimate of the fluid cleanliness level in terms of cleanliness codes or cleanliness classes. Contamination monitors are the preferred choice for systems where the fluid properties prevent the use of light-obscuration particle counters: dark fluids that transmit light poorly or fluids containing air bubbles or emulsions.

Typical on-line water monitors, Figure 2, are adaptations of devices that measure relative humidity. They indicate the percent saturation of water in the fluid - free water is 100% saturation - and the temperature. Note that the saturation level for water in hydraulic fluids depends on the specific fluid composition (both base stock and additive package), the actual condition of the fluid, and its temperature. The same type fluid, formulated by different manufacturers, could differ in water saturation levels. Likewise, the saturation level of the same fluid could differ over a period of time. Thus, correlation of the absolute water concentration, i.e., ppm concentration, with percent saturation, requires determination of the absolute water concentration in the specific fluid in question. In view of the above, it is most convenient to specify water concentration limits in terms of percent saturation.

Removal of water and particulate

Several methods also are available to remove particulate contamination and water from hydraulic fluids. The choice of method depends both on the contamination level of the fluid and its specific area of application. Heavily contaminated fluids are best cleaned by removing them from the operating system and purifying them externally prior to re-use. Subsequently, in-line particulate filters and water-absorbing filters can provide contamination control.

Although a variety of equipment is available to remove free water (e.g. centrifuges, coalescers, and water-removal cartridges), only fluid purifiers offer the ability to remove free, emulsified, and dissolved water. In addition to removing water and particulate contamination, fluid purifiers also take out volatile solvents and dissolved gases.

Two common types of purification devices are flash-distillation and vacuum-dehydration systems. In flash-distillation systems, fluid is heated and then introduced into a vacuum chamber so that free and dissolved water, gases, and solvents are distilled off, thus dehydrating the fluid. In vacuum-dehydration systems, the fluid is exposed to a low-humidity atmosphere in a partial vacuum chamber, resulting in the transfer of free and dissolved water, solvents, and gases from the fluid to the atmosphere in the vacuum chamber. To facilitate the transfer, the surface area of the fluid should be maximized.

In one purifier design, Figure 3, the contaminated fluid is introduced into the vacuum chamber through fine spray nozzles to form a conical, thin film through which a flow of low-humidity air is directed. This arrangement results in a large fluid-surface area that allows for more efficient transfer of water from the fluid film to the air stream. The air then is exhausted through a de-mister filter to remove any residual fluid it may be carrying.

In the final stage of the purifier, de-aerated and dehydrated fluid exits the vacuum chamber through a particulate contamination-control filter. These fine filters have high efficiency particle-removal characteristics, especially in the smaller size ranges. For example, their particle-removal efficiency is 99.5% or higher for particles greater than 3 ∝m in size, i.e., β₃ >200. These filters also exhibit a high dirt-retention capacity.

Read on to Part 2 for more on NIST certification and calibration, filter performance, and reporting, and more.

At work in the real world One U.S. airline studied the impact of contamination on performance in the hydraulic systems of its aircraft ground-support equipment, such as mobile cargo loaders, container rotators, aircraft bridges, and nose docks. This equipment operates outdoors and is exposed to dirt and weather extremes. Initial investigations revealed high particulate levels - 20/16 on the ISO 4406 scale - and water contamination in excess of 1,000 ppm. These conditions required fluid changes every two to three months. In spite of these relatively frequent changes, equipment failure was common. The airline initiated a comprehensive program of contamination control. It included installing high-efficiency fine filtration and the use of portable fluid purifiers. The result: hydraulic-fluid service life was extended to more than eleven months. The water concentration in the fluid was held consistently below the manufacturer's recommended 200- to 400-ppm level. Particulate levels were reduced to 13/11 on the ISO 4406 scale. The associated benefits were improved performance and less downtime.

Dealing with acidic components Phosphate-ester fluids are particularly susceptible to hydrolysis during service, resulting in an accumulation of acidic hydrolysis products. A new trend in fluid purifiers is the incorporation of ion-exchange resin cartridges to remove acidic components from phosphate-ester fluids. If acidic components are a problem, these purifiers provide a double benefit: they remove water from the fluid to reduce the possibility of hydrolysis, and they adsorb any acidic hydrolysis products that already exist in the fluid to minimize acid buildup.

Table 1. Effect of metal catalysts and water on oil oxidation
Catalyst	Water	Hours	Final neutralization number
None	No	3,500+	0.17
None	Yes	3,500+	0.90
Iron	No	3,500+	0.65
Iron	Yes	400	8.10
Copper	No	3,000	0.89
Copper	Yes	100	11.20

Table 2: Effect of particulate contamination and water on hydraulic fluids
Fluid breakdown	Cause	Effect on system
Physical properties	a. Agglomeration and precipitation of particulate contamination b. Oxidation/hydrolysis products - gums and sludges c. Reactions involving additives - sludges and solids d. Free water	component wear clogging of jets, nozzles, and orifices; valve jamming system malfunction due to icing of free water
Base-stock degradation	a. Oxidation b. Hydrolysis	corrosion and surface degradation of components
Additive depletion	a. Precipitation of additives b. Adsorption by particulates c. Reactions involving additives d. Abnormal degradation of base-stock	loss of component protection increased component wear and corrosion