What is in this article?:
The most common source of system impairment is fluid contamination. Without proper preventive measures and fluid conditioning, component failure can result.
The first defense against fluid contaminants is preventing their entry into a hydraulic system. After that, removing contaminants before system start-up prevents much damage that can occur early in a system's life. Thereafter, well-planned routine maintenance will maintain the fluid in peak condition. Here are some of the initial positive steps that can be taken:
- fit the reservoir with baffles and return-line diffusers, Figure 5, to prevent churning that whips air into the fluid
- equip the reservoir with a breather having an air-filter element with a rating of at least 99% efficiency at 2 µm
- make sure all fittings are properly tightened (besides causing leakage, loose fittings can allow airborne dust to be sucked into the system)
- flush the system thoroughly before it goes into service
- prefilter fluid before filling the reservoir (it should be as clean as your specification for the system fluid)
- inspect filter indicators to make sure they are working
- use boots and bellows to protect cylinder rods and seals
- replace filter elements before the filter bypass valve opens; otherwise, the system will operate with no filtration
- replace any worn seals and hoses promptly; if not done, the negative effects are the same as loose fittings
- practice good housekeeping whenever a system is opened for maintenance; protect replacement components from contamination, and
- analyze fluid regularly to detect problems such as overheating, leaking water, clogged heat exchangers, additive breakdown, etc.
Once contamination is in the fluid, it may be reduced and controlled by settling, outgassing (e.g. in aerated liquids), filtration/separation, and fluid replacement. The first two mechanisms - settling and outgassing - occur naturally, but their effect can be enhanced by controlling the fluid environment through system design. The latter two also require human involvement, again during system design or in maintenance activities after installation.
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 in the fluid. The flow rate of the fluid also helps determine how quickly a contaminant will settle. A contaminant transported by a fluid will stay in suspension if the flow velocity supplies enough lifting force to overcome gravity. If flow is turbulent, it is more likely that contaminants will stay in suspension.
As mentioned previously, the reservoir can be designed with baffles and return-line diffusers to reduce fluid velocity enough so that larger particles will settle. 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 moving parts. Therefore, system designers want a reasonable degree of turbulence in the hydraulic system so that smaller particles remain in suspension. This is as true for the reservoir as elsewhere in the system. A tapered reservoir bottom will help prevent the collection of smaller contaminant particles due to its reduced bottom surface area and tendency to extend the turbulence effect. As in many design projects, reservoir construction and piping configuration involves compromises.
Outgassing can be thought of as the inverse 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. Whether the air actually leaves the liquid or not depends on the relative surface tensions and partial pressures of the air and the liquid. The lower the turbulence in the reservoir, the more likely it is that a contaminant will leave the fluid by way of outgassing or settling.
Natural mechanisms, such as settling and outgassing, cannot by themselves reduce contamination to an acceptable level. In the absence of filtration and separation devices, the only alternative is to replace the fluid at periodic intervals. Even with adequate filtration, fluid replacement cannot be postponed forever. This certainly is true for automotive lubricants, and points out a fundamental fact of fluid life. There is an economic trade-off between the cost of buying, installing, and servicing filters and separators, and the cost of replacing the hydraulic fluid more often.
Fluid conditioning objectives
The objective of hydraulic fluid conditioning is to lower total operating costs. If the system can meet or exceed minimum standards for fluid cleanliness, one or more of these intermediate goals can be achieved:
- reduce maintenance requirements for the fluid system and components
- improve the performance of the system and its fluid
- assure the quality of the final product by improving machine operation, and
- enhance safety and/or reduce risk of injury to personnel (for example, by eliminating the need for maintenance on or around operating equipment).
Appropriate fluid conditioning increases the mean time between hydraulic component failures. Still, this benefit has to be properly balanced against the cost of purchasing the 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. One general process is illustrated in the filter-specification flow chart, Figure 5.
Many questions should be answered regarding contaminant removal:
- how clean does the fluid be have to?
- what size particles must be removed?
- how many particles within a given size range need to be removed?
- how efficient must the filter media be in terms of the percentage removal of a given size range - and in terms of dirt-holding capacity?
- will the fluid contamination stabilize at an acceptable level for a given combination of filters and media?
As the flow chart implies, specifiers need to have a feel for the sensitivity of hydraulic components to contaminants of various sizes and concentrations. Designers and users have observed that some components are more sensitive to contaminants than others. For example, they may have seen a certain pump quickly fail, while another type lasts for months in the same system. They also probably have noticed that higher pressures and flow rates tend to make all components wear out more quickly. Those who are particularly observant may have noticed that the higher the concentration of airborne contaminants around systems, the sooner they fail. These factors combine to influence the service life of components.
Another point is that filter media with small pore sizes frequently are more costly, and must be replaced more often than coarser media. For practical economic reasons, designers must find a compromise between costly ultrafine filtration and the cost of early component failures. This compromise is to have fluid only as clean as it needs to be, not as clean as possible.
Designers tend to rely on their own experience as well as information from component manufacturers to determine how clean hydraulic fluid needs to be. Some conservative manufacturers assume that worst-case conditions exist and specify a very low acceptable level of contamination for their components. Others take a middle-of-the-road approach, and specify cleanliness for more or less average conditions.