Fig. 7. Simplified representation of component arrangement for Multipass Test.

Estimating ingression rate

Assume that a new hydraulic system has been flushed properly before being put into service. If the system has a multipass filtration system with a given flow rate, the eventual stabilized level of contaminants will depend on the system's ingression rate and filter media removal efficiency. If filter efficiency is too low, the contaminant level will continue to increase due to the wear particles generated within the system and new particles entering from outside the system. (This is the scenario in most automobile lubrication systems, and why motor oil should be changed periodically.) If filter efficiency is high enough, the contaminant level will decrease and become stabilized, extending the service life of the hydraulic fluid. Because operating conditions vary, this is a kind of dynamic stability. The contaminant level varies within a range determined by these conditions.

Therefore, to select the appropriate filter media, it is necessary to have some idea of the ingression rate. Of course the ingression rate probably varies at different locations in the system, and depends on these factors:

• concentration of ambient airborne contaminants (which enter through worn filler/breathers, loose fittings, leaking seals, etc.)
• use or absence of an air-filter element in the reservoir breather
• number of components in the system or circuit branch
• types of components that make up the system, particularly if there are rotating components such as pumps and motors (some types wear faster than others)
• fluid velocity (because higher velocity often may accelerate wear - after flushing is completed)
• system pressure (because higher pressure also tends to increase wear rates)
• fluid temperature (excessive heat can cause fluid and additives to break down, creating contamination), and
• the filter media used (more-efficient media results in lower contaminant levels and reduced wear rates).

This parade of factors makes accurate estimation of ingression rates difficult. An estimate can be made by conducting particle counts on fluid samples taken from a system with known operating conditions and filtration efficiency. (In multibranch circulating systems, the reservoir frequently is picked as a convenient location from which to take samples.) Then, by using a simple filtration model based on the material-balance equation, an ingression rate can be inferred. Filtration specifiers can construct their own estimates using the same technique. Such an estimate may be more accurate than one of the averages published in a table. Computerized filtration models also are available that allow a large number of variables to be quickly manipulated in a kind of ingression-rate What if? analysis.

Cleanliness reference

In order to detect or correct problems, a contamination reference scale is used. Particle counting is the most common method to derive cleanliness level standards. Very sensitive optical instruments count the number of particles in various size ranges in a fluid sample. These counts are reported as the number of particles greater than a certain size found in a specified volume.

The ISO 4406 cleanliness level standard has gained wide acceptance in most industries today. A modified version of this standard references the number of particles greater than 2, 5, and 15 micrometers in a known volume - usually 1 milliliter or 100 milliliters. (The number of smaller-size particles helps predict silting problems. A high number of larger particles might indicate catastrophic component failure.)

Filter media

The filter media is that part of the element which actually contacts contaminant and captures it for subsequent removal. The nature of the particular filter media and the contaminant-loading process designed into the element explains why some elements last longer in service than others.

During manufacture, media usually starts out in sheet form, then is pleated to expose more surface area to the fluid flow. This reduces pressure differential across the element while increasing dirt-holding capacity. In some designs, the filter media may have multiple layers and mesh backing to achieve certain performance criteria. After being pleated and cut to the proper length, the two ends are fastened together using a special clip, adhesive, or other seaming arrangement to form a cylinder. The most common media include wire mesh, cellulose, and fiberglass composites, or other synthetic materials. Filter media is generally classified as either surface- or depth-type.

Surface media

For surface-type filter media, the fluid stream basically flows in a straight path through the element. Contaminant is captured on the surface of the element which faces the fluid flow. Surface-type elements are generally made from woven-wire cloth. Because the process used to manufacture the wire cloth can be controlled very accurately, and the wire is relatively stiff, surface-type media have a consistent pore size. This consistent pore size is the diameter of the largest hard spherical particle that will pass through the media under specified test conditions. However, during use, the build-up of contaminant on the element surface will reduce the pore size and allow the media to capture particles smaller than the original pore-size rating. Conversely, particles (such as fiber strands) that have smaller diameters but greater length than the pore size may pass downstream through surface media.

Depth media

For depth-type filter media, fluid is forced to take convoluted indirect paths through the element. Because of its construction, depth-type media has many pores of various sizes formed by the media fibers. This maze of multi-sized openings throughout the material traps contaminant particles. Depending on the distribution of pore sizes, the media can have a very high capture rate for very small particle sizes.

The two basic media that are used for depth-type filter elements are cellulose (or paper) and fiberglass. The pores in cellulose media tend to have a broad range of sizes and are very irregular in shape due to the irregular size and shape of the fibers. In contrast, fiberglass media consist of man-made fibers that are very uniform in size and shape. These fibers are generally thinner than cellulose fibers, with a consistently circular cross-section. The differences between these typical fibers account for the performance advantage of fiberglass media. Thinner fibers can provide more pores in a given area. Furthermore, thinner fibers can be arranged closer together to produce smaller pores for finer filtration. Dirt-holding capacity, as well as filtration efficiency, are improved as a result.

Particle counting

Knowing the cleanliness level of the hydraulic fluid in a system is the basis for selecting contamination-control measures. Particle counting is the most common method of deriving cleanliness-level standards. Very sensitive optical instruments count the number of particles in various size ranges in a measured fluid sample. These counts are reported as the number of particles greater than a certain size found in a specified volume of fluid.

The ISO 4406 Cleanliness-Level Standard is accepted in most industries today. A widely used, modified version of this standard references the number of particles greater than 2, 5, and 15 µm in a known volume - usually 1 or 100 milliliters. The number of particles greater than 2 and 5 µm is a reference point for silt particles, those which can cause clogging problems. The 15-µm size range indicates the quantity of larger particles present, those which contribute greatly to possible catastrophic component failure.

To identify a cleanliness level, the number of particles in the sample for each of the three measured sizes is referred to the ISO 4406 chart, and given an appropriate range number. If a fluid sample contained between 1,300 and 2,500 2-µm and larger particles (range 18); between 320 and 640 5-µm and larger particles (range 16); and between 40 and 80 15-µm and larger particles (range 13); the sample would be classified as 18/16/13. Note that the numbers that make up the ISO cleanliness-code classification will almost never increase as the particle size increases.

Most manufacturers of hydraulic (and load-bearing) equipment conduct tests and then specify an optimum or target cleanliness level for their components. Exposing components to hydraulic fluid with higher than optimum contamination levels may shorten the component's service life. It always is best to consult with component manufacturers and obtain their written fluid-cleanliness-level recommendations. This information is needed in order to select the proper level of filtration. It also may prove useful for any subsequent warranty claims, as it may draw the line between normal operation and excessive or abusive operation.

The Multipass Test

The filtration industry uses the ISO 4572 Multipass Test Procedure (also recognized by ANSI and NFPA) to evaluate filter element performance. During the Multipass Test, Figure 7, fluid circulates through the test circuit under precisely controlled and monitored conditions. The differential pressure across the element being tested is continuously recorded, while a constant amount of contaminant is injected upstream of the element. On-line laser particle sensors measure the contaminant levels upstream and downstream from the test element. (Note that the results of this test are very dependent on flow rate, type of contaminant, and terminal pressure differential.)

The Multipass Test determines three important element-performance characteristics:

1. Dirt-holding capacity.
2. Pressure differential of the test filter element.
3. Separation or filtration efficiency, expressed as a beta ratio.

Beta ratio

The beta ratio (also known as the filtration ratio) is a measure of a filter element's particle-capture efficiency. Therefore, it is a performance rating. Here is how a Beta Ratio is derived from Multipass Test results. Assume that 50,000 particles, l0 µm and larger in size, were counted upstream from the test filter, and 10,000 particles in that same size range were counted downstream from the filter. Substituting in the equation:

β_x = (N_U)/(N_D)

where: x is a specific particle size,
N_U is the number of particles upstream, and
N_D is the number of particles downstream.

Therefore, β₁₀ = 50,000/10,000 = 5

This result would be read as "Beta ten equal to five." Now, a beta ratio number alone means very little. It is a preliminary step to finding a filter's particle-capture efficiency. This efficiency, expressed as a percent, can be found by a simple equation:

Efficiency_x = 100 (1 - 1/β)
Efficiency₁₀= 100 (1 - 1/5) = 80%

In this example, the particular filter element tested was 80% efficient at removing 10-µm and larger particles. For every five particles in this size range introduced to the filter, four were trapped in the filter media.

Contaminant loading

The term contaminant loading in a filter element refers to the process of filling and blocking the pores throughout the element. As contaminant particles load the element's pores, fewer open paths remain for fluid flow, and the pressure required to maintain flow through the media increases. Initially, the differential pressure across the element increases slowly because plenty of open pores remain for fluid to pass through. The gradual pore-blocking process has little effect on overall pressure loss.

Eventually, however, successive blocking of media pores significantly reduces the number of pores open for flow. The differential pressure across the element then rises exponentially as the element nears its maximum life. As the element continues to load with contaminant, the pressure differential across the filter will continue to increase. This goes on until the bypass valve (if installed) opens, the element (if without bypass protection) fails structurally, or the clogged element is replaced. The quantity, size, shape, and arrangement of the pores throughout the element accounts for why some elements last longer than others.

Consider two of the most common filter media: cellulose and fiberglass, For a given media thickness and filtration rating, there are fewer pores in cellulose media than fiberglass. Accordingly, the contaminant-loading process would block the pores of the cellulose media element more quickly than an identical fiberglass media element.

Multi-layer fiberglass media elements are relatively unaffected by contaminant loading for a longer time. The upstream media has relatively larger pores to capture larger particles; the downstream media layer with very small pores captures the greater quantity of small particles present in the fluid.