What is in this article?:
Today's emphasis on pollution prevention and waste stream minimization has made the benefits of contamination control programs for hydraulic fluids even more compelling.
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.
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.
For several decades, a product called Air Cleaner Fine Test Dust (ACFTD) served as the standard solid-particle contaminant for a number of purposes in the area of hydraulic contamination measurement and testing. The irregularly shaped ACFTD particles - ranging in size from roughly 0 to100 mm - were very similar to the contaminants found in typical hydraulic systems. In the particle-size distribution defined by ISO Standard 4402, ACFTD was used to set the electronic threshold levels that establish the particle sizes measured in automatic particle counters (APCs). The dust also was added to fluids in filter-performance testing to measure both the efficiency and dirt-holding capacity of filter media. In addition, ACFTD was used to test the contaminant sensitivity of hydraulic components.
The AC Spark Plug Div. (later the AC Rochester Div.) of General Motors Corp. manufactured ACFTD by collecting dust - primarily silica - from a certain area in Arizona, then ball milling and classifying it into a consistent particle-size distribution. But in 1992, GM announced that it would discontinue production of ACFTD.
As a result, ISO Technical Committee TC 22 and the Society of Automotive Engineers (SAE) went to work to find suitable test dusts to replace the old standard. Their efforts produced a new standard, ISO 12103-1, 1997, which defines and designates four new test dusts. Powder Technology, Inc. (PTI), Burnsville, Minn., manufactures these dusts from the same silica-based material used by AC Rochester so that their chemical characteristics are similar to the AC Test Dusts. In a slightly different production method, PTI processes the Arizona dust with a jet mill and then classifies it. Of the four new dusts, ISO Medium Test Dust (ISO MTD) has a particle-size distribution closest to ACFTD and, therefore, has been selected as the replacement dust for APC calibration and filter-testing purposes.
While it is very similar to ACFTD, ISO MTD produces test results that are somewhat different. Therefore, results of both automatic particle counting and laboratory filter performance testing (including filter efficiency and dirt-holding capacity) can be significantly affected. Note that this is an artifact of the testing only; filter performance and actual contamination levels in the field will remain the same as before.