Fig. 4. Baffle in reservoir slows fluid velocity so large contaminant particles can settle to bottom. Diffuser prevents churning action which might entrain air in fluid.

Fig. 5. Suggested steps in hydraulic-filter selection process.

Fig. 6. Complex relationship among operating variables, components sensitivity to contaminants, and hydraulic system performance involves many factors.

Microbial growth

Over time, water contamination can lead to the growth of microbes - minute life forms such bacteria, algae, yeasts, and fungi - in the hydraulic system. And the presence of air exacerbates the problem. Microbes range in size from approximately 0.2 to 2.0 µm for single cells, and up to 200 µm for cell colonies. Left unchecked, microbes can destroy hydraulic systems just as they destroy living organisms. Under favorable conditions, bacteria can reproduce (doubling themselves) as rapidly as every 20 minutes. Such exponential growth can form an interwoven mat-like structure that requires significant shear force to break up. This resistance quickly renders a fluid system inoperable. Besides their mass volume, bacteria produce acids and other waste products which attack most metals. When this happens, fluid system performance is degraded, and components fail more rapidly.

Evidence of microbial contamination

The first indication of microbial contamination may be the foul odor that comes from waste and decomposition products of the microbes. Fluid viscosity may increase due to the mass of material produced by these organisms. At the same time, the fluid may have a brown mayonnaise-like appearance, or slimy green look.

Unfortunately, by the time these symptoms appear, system components and the fluid itself may be severely damaged. This could require a major overhaul or replacement of the system.

Properly selected filters will remove microbes. But without adding biocides (substances capable of destroying these living organisms) to the fluid, fast-growing microbes can place a heavy load on system filters. Combined with wear debris and chemical reaction products, microbial contamination can result in rapid plugging of filter elements, requiring frequent replacement.

Water and air are essential for microbe growth. Eliminating water and air from a fluid minimizes microbial problems. But some systems use water as the base fluid, and air is very difficult to exclude from fluids in operating hydraulic systems. With water and air present, microbes can usually find some fluid component to feed their growth. When water can not be controlled by exclusion or removal, biocides should be added to the fluid. A biocide combined with properly selected water-absorption filters can help minimize chemical-reaction byproducts and microbial contamination.

Exclusion practices

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.

Removal mechanisms

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?

Component sensitivity

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.

Additional information sources

Manufacturers' recommendations can be augmented by information that is available from other sources. For example, OEMs and research laboratories have carried out projects to analyze the sensitivity of pumps, valves, and other components to contaminants. As a result, guidelines and standards for hydraulic-fluid cleanliness have been published. These guidelines attempt to interrelate diverse factors such as:

• fluid lubricity (e.g., water-base fluids have lower lubricity than oil)
• abrasiveness of the contaminants commonly found in hydraulic systems
• system duty cycle and cycle rate (high pressure and high cycle rates, combined with contaminants, lead to earlier fatigue failures)
• component replacement cost
• design life objective in terms of mean time before failure (MTBF); a common goal today is 10,000 hours or more, and
• degree of risk associated with contaminant-related failures (high risk of personal injury or high cost of lost production dictates a need for cleaner fluid.)

Fluid variables and system variables both have an effect on a component's sensitivity to contamination. This sensitivity eventually is reflected in system performance, Figure 6.

The International Standards Organization (ISO) recommends cleanliness levels for various types of components, see the table below. The levels are stated in terms of industry standards that have been recognized for the past 20 years. Many fluid power designers apply these recommendations as rules of thumb. Many specifiers now accept and use ISO 4406 (see table on page A/99) as a means of designating the fluid cleanliness required for their systems.

Importance of records

Still, component manufacturers' and industry guidelines should be modified by experience. That requires gathering enough operating and maintenance data over sufficient time and from enough systems to provide confidence to make decisions. The data gathered should include the results of regular fluid analysis on systems. The categories of data collected might include:

Fluid variables - flow, pressure, temperature, and viscosity for circuit branches with the most sensitive or expensive components.

Fluid analysis - particle counts in various size ranges (e.g., >2, >5, >15, >25, >50, and >100 µm), spectrochemical analysis (e.g., most likely metals and other contaminants), and water content (% by volume).

Filtration information - model number and manufacturer for the filter(s) and element(s) protecting the circuit for which other data was gathered; element performance ratings in terms of beta ratios and dirt-holding capacity.

Maintenance data - date system placed in service; dates and descriptions of routine maintenance performed (including element replacements); reading of the filter element condition indicator (e.g., "needs replacement" or "in bypass"); dates and descriptions of component failures, including manufacturers' names and model numbers; failure mode analysis (e.g., fracture, corrosion, wear, etc.) also would be very helpful in determining if contamination was a factor in any failures.

PC data base and statistical analysis programs also can be used to correlate failures with fluid contamination levels. This will create a picture of the contamination tolerance of the most sensitive components. It also allows for the calculation of MTBFs for specific components, certain circuit branches, or the system as a whole.

Obviously, this is data the user must collect. Still, manufacturers can monitor warranty claims as an opportunity to capture some of this data, and create a clearer picture of component sensitivity. That may cover only the first year or two of service. A close relationship with customers and distributors can provide an opportunity to gather similar data over longer periods of time as replacement parts are ordered.

Contamination - dynamic, not static

Another reason for regular fluid analysis is that the contamination level changes with time, and varies by location in the system. At any point, the amount of contamination present in the fluid depends on three factors:

1. How contamination much was in the fluid when the system was started
2. How much was added to the fluid from all sources during operation (Ingression rate is the term used to describe the amount of contaminant entering the fluid per unit of time.)
3. How much contamination left the fluid due to all removal mechanisms (settling, and filtration or separation)

C_T = C_t + C_a - C_s

where:
C is contaminant
T is any point in time
t is time since start of process
C_a is amount added since t
C_s is amount removed since t

The term material balance is used because the equation calculates the net difference between the amount of material or contaminant entering and leaving the fluid, and adds this difference to what was already there. The calculation applies to a specific location in the system.

In a circulating system, contaminants not removed will appear at the filter inlet again, along with new contaminant added to the fluid. This is called a multipass system because the fluid and contaminant make multiple passes through the filter. As a result, the contaminant concentration in the system fluctuates continuously.

If we consider the initial start-up of a system, the contaminants already present are there as a result of manufacturing processes or have entered with new fluid. (Each milliliter of fluid out of the original barrel typically contains at least 2,500 particles that are 5 mm and larger in diameter.) A few minutes after start-up, the particulate level will be considerably higher due to flushing action of the fluid as it flows through new components and piping to pick up debris. Eventually, more particles enter the system through the reservoir breather and imperfect seals. Still more will be added over time due to internal wear.