Contaminants Enter a Hydraulic System in a Variety of Ways. If Not Properly Flushed out, Contaminantsfrom manufacturing and assembly operations will be left in the system. These contaminants include dust, welding slag, rubber particles from hoses and seals, sand from castings, and metal debris from machined components. Also, when fluid is initially added to the system a certain amount of contamination comes with it. Typically this contamination includes various kinds of dust particles and water.
During system operation, atmospheric contaminants enter through breather caps, imperfect seals and any other openings. System operation generates internal contamination by wear debris and chemical byproducts from fluid and additive breakdown. Such materials then react with component surfaces to create even more contaminants.
The first line of defense against fluid contaminants is to prevent their entry into the system. In most hydraulic and lubrication systems, the vast majority of harmful contamination is ingressed from the atmosphere. It is therefore most economical and efficient to prevent contaminants from entering a system. Once in the system, this contamination can cause component damage and prove difficult and expensive to remove.
Equipping a reservoir with an efficient breather is a simple, yet effective technique to prevent contaminants from entering a system, and to provide a cleaner, safer working environment. An effective breather provides a large volume of silica gel to absorb water as it passes through the unit. This hygroscopic agent provides a high removal volume and indicates its condition by a color change. An activated carbon element effectively removes exhaust vapors created in the system before they enter the work environment. Particulate matter is removed from incoming air by efficient filter pads that are regenerated as air exhausts. The air filter element rating should be at least 98.7 percent efficient at 3 micrometers.
In addition to breathers, several other techniques are effective in reducing contaminant ingress:
- Fit the reservoir with baffles and return-line diffusers to prevent churning that can whip air into the fluid.
- Make sure all fittings are properly tightened to prevent airborne contaminants from being drawn into the system, as well as fluid leakage.
- Flush the system thoroughly before installation and start-up.
- Prefilter fluid before filling the reservoir. Most fluids are contaminated by particulates and water during processing, mixing, handling or storage. Drums should be stored indoors and positioned horizontally to minimize contaminant build-up around drum bungs.
- Make sure filter indicators are installed and functioning.
- Use boots or bellows to protect cylinder rods and seals.
- Replace filter elements before the filter bypass valve opens.
- Replace worn seals and hoses.
- Practice good housekeeping whenever a system is opened for maintenance.
The next step in an effective total fluid system care program is maintaining the necessary cleanliness level of an operating system.
Once in the fluid, contamination may be reduced by settling, outgassing (in aerated fluids), filtration/separation, and fluid replacement. For settling to occur, a contaminant must have a density greater than the fluid transporting it. The lower the density of a contaminant particle, the more buoyant it will be. The flow rate of the fluid also helps determine how quickly a contaminant will settle.
A reservoir with baffles and return-line diffusers will reduce fluid velocity to enhance settling of larger particles. On the other hand, contaminants must remain in suspension if they are to be transported to a filter for removal. This is particularly important in fluid lines and components where particle settling can cause unpredictable contaminant removal rates or silting interference between parts. For this reason, system designers want a reasonable degree of turbulence in a hydraulic or lubrication system so smaller particles remain in suspension.
Outgassing can be thought of as the reverse of settling. If fluid turbulence is low enough to prevent mixing action, dissolved air can come out of suspension and rise to the surface of a liquid. The lower the turbulence in the reservoir, the more likely it is that a contaminant will leave the fluid through outgassing or settling.
Natural mechanisms, such as settling or outgassing, cannot by themselves reduce contamination to an acceptable level. In the absence of filtration and separation devices, frequent fluid replacement may be necessary. There is an economic trade-off between the cost of buying, installing, and servicing filters and separators and the cost of periodic fluid replacement.
The function of a properly sized, installed and maintained filtration system is to remove contaminants, but its purpose is to reduce operating costs. Appropriate fluid filtration increases mean time between component failures. Still, this benefit has to be balanced against the cost of purchasing filters, replacing elements, and maintaining filtration equipment. Careful filtration system design and component selection will help minimize these costs. The best way to optimize the benefit/cost trade-off is to follow sound practices for the selection of filters, elements and filter media (Figure 1).
Suction filters, located before the inlet port of the pump (Figure 2), serve to protect a pump from fluid contamination. Some may be simply inlet strainers, submersed in the fluid. Others may be externally mounted. In either case they utilize relatively coarse elements due to cavitation limitations of pumps. For this reason, they are not used as primary protection against contamination. Some pump manufacturers do not recommend the use of a suction filter. Always consult the pump manufacturer for inlet restrictions.
Pressure filters are located downstream of the system pump (Figure 3). They are designed to handle the system pressure and sized for the specific flow rate in the pressure line where they are located. Pressure filters are especially suited for protecting sensitive components directly downstream from the filter, such as servo valves. Located just downstream from the system pump, they also help protect the entire system from pump-generated contamination.
Return filters may be the best choice when the pump is a sensitive component in a system (Figure 4). In most systems, the return filter is the last component fluid passes through before entering the reservoir. Therefore, it captures wear debris from system working components and particles entering through worn cylinder rod seals before such contamination can enter the reservoir and be circulated. Since this filter is located immediately upstream from the reservoir, its pressure rating and cost can be relatively low.
In some cases, cylinders with large diameter rods may result in flow multiplication. The increased return line flow rate may cause the filter bypass valve to open, allowing unfiltered flow to pass downstream. This may be an undesirable condition and care should be taken in sizing the filter.
Duplex assemblies are available for both pressure and return filters to provide continuous filtration (Figure 5). A duplex assembly is made with two or more filter chambers and includes the necessary valving to allow for continuous, uninterrupted filtration. When a filter element needs servicing, the duplex valve is shifted, diverting flow to the opposite filter chamber. The dirty element can then be changed while filtered flow continues to pass through the filter assembly. The duplex valve should be designed for neutral center to allow uninterrupted flow.
Even with the best filter system, contaminant ingression rates can easily exceed removal rates. Simply changing filters or replacing the fluid are expensive options, but these remedies will not eliminate the contamination overload problem. An off-line filtration loop (Figure 6) is totally independent of the equipment’s hydraulic or lubrication system, and is necessary to reduce system contamination to an acceptable level. For most systems, the optimum filtration design is a combination of suction, pressure, return-line and off-line units. Off-line filtration, also called recirculating, kidney loop or auxiliary filtration, consists of a pump, filter, electric motor and appropriate hardware connections. Fluid is continuously pumped out of the reservoir, through the filter and back to the reservoir.
With this polishing effect, off-line filtration is able to maintain a fluid at a constant contamination level. As with a return line filter, this type of system maintains overall cleanliness, but does not provide protection for specific components. An off-line filtration loop has the added advantage of being relatively easy to retrofit on an existing system that has inadequate filtration. Also, the filter can be serviced without shutting down the main system.
Most major manufacturers offer a number of off-line filtration options. For example, Parker Hannifin’s PVS purification system uses vacuum dehydration to remove dissolved and free air and water. It also incorporates a polishing filter that removes solid contaminants down to 1 micrometer. The system heats the fluid under vacuum to 150 F to convert water to vapor. A vacuum pump draws out the vapor and the water-free, gas-free fluid passes through a final particulate-removal filter. This process also removes atmospheric oxygen and nitrogen that may mix with hydrocarbons to form varnishes, sludge and carboxylic acids. The fine-particulate filter also removes existing varnishes and sludge. Vacuum dehydration is essential wherever large volumes of water are present.
Smaller, portable filter carts and hand-held pump/motor/filter units are ideal for prefiltering and transferring fluids into reservoirs or to clean up existing systems.
Fluid analysis-an essential part of any maintenance program-ensures that the fluid conforms to manufacturer specifications, verifies the composition of the fluid, and determines its overall contamination level. This is often achieved with a patch tests, through use of a portable particle counter or through laboratory analysis.
A patch test is a visual analysis of a fluid sample. It usually involves taking a fluid sample and passing it through a fine media patch. The patch is then analyzed under a microscope for both color and content, or the particles on the patch counted. Using this comparison, a user can get an estimate of a system’s cleanliness level. These techniques are time consuming and prone to error and have been supplanted by more sophisticated, on-site analysis methods.
On-site fluid analysis with a portable particle counter provides a number of immediate advantages including:
- certification of fluid cleanliness levels,
- accurate, immediate results,
- an early warning to help prevent catastrophic failure of critical systems,
- compliance with customer cleanliness requirements and specifications,
- compliance with equipment warranty requirements,
- determination of new oil cleanliness level, and
- fluid viscosity and temperature verification.
Portable counters let users see immediate results and download them via software for trend analysis in raw particle counts and ISO and NAS cleanliness codes. On-board counters, permanently connected to a system, provide continuous monitoring and can catch problems as soon as they occur. Preset triggers warn of significant changes in the fluid so timely corrective action can be taken. Counters and flow sensors mount on a machine and transmit data to a hand-held device containing firmware and software. The device can log all fluid-condition data or feed it directly into a data-acquisition system.
Of the several technologies for particle counting, laser-based systems provide the highest degree of accuracy, precision, and repeatability down to the 2 micrometer range. Such systems operate by detecting the shadow created by a particle as a fluid sample flows past an area of intense laser light in a sensor. A solid-state photo diode detects the momentary decrease in light and creates an electrical pulse that is proportional to the particle size. Particles are then individually sized and counted with the totals converted into ISO and NAS classifications.
Viscosity of the fluid can be calculated using a transducer to measure pressure differential across a fixed restriction at a known flow rate. A thermistor placed directly in the flow path provides accurate fluid temperature monitoring.
A complete laboratory analysis, performed on a small sample of fluid, can identify potential problems that cannot be detected by other techniques. The resulting report provides results of an individual sample or a trend of several different samples. Sampling technique is crucial to gathering reliable and repeatable data. Often, erroneous sample procedures will disguise the true nature of the system fluid. A National Fluid Power Association standard (NFPAT2.9.111-1972) and an American National Standards Institute standard (ANSI B93.13-1972) outline the recommended procedures for extracting samples from a fluid system.
David Brown is the Market Sales Manager for the Filtration Group at Parker Hannifin Corp. in Metamora, Ohio.