The typical hydraulic system is cleaned by a single filter in the circuit. Often this filter is located on the inlet line in front of the pump, to protect the pump and downstream components; so used, it is occasionally called a "suction filter" or "suction strainer." Many pump manufacturers object to this filter location, claiming that it "starves" the pump inlet, so the filter may be located at other points in the circuit, such as on the return line, in a pressure line, or on a bypass line.

Filtration for pneumatic systems is handled quite differently. In most industrial pneumatics, compressed air is supplied from a single compressor to a large number of operating systems, as a plant resource, much like light or electricity. Individual filters are used on the separate systems -- sometimes more than one filter per system. Often the filters are found in conjunction with regulators and sometimes lubricators comprising a filter-regulator-lubricator (frl) for the system.

Filters are rated on the ability to retain contaminants of certain size levels.

ISO 4406:
ISO 4406 is a hydraulic cleanliness rating system based on the number of particles larger than 5 and 15 ∝m in a 1-ml fluid sample. A standardized chart is then referenced to convert the particle counts into the ISO 4406 rating format. For example, a 1-ml sample containing 140 5-∝m particles (ISO 4406 range number = 14) and 28 15-∝m particles (ISO 4406 range number = 12) has an ISO 4406 cleanliness rating of 14/12.

ISO 4406 is an internationally recognized standard for expressing the level of particulate contamination in hydraulic fluid, and for specifying required cleanliness levels for hydraulic components and systems. The widely accepted system provides a consistent and meaningful vehicle for dialog between manufacturers and users.

Pall cleanliness code: Some users and manufacturers of hydraulic systems have complained about a potential problem with interpreting portions of ISO 4406. According to Pall Industrial Hydraulics Corp., fluid samples with a high silt content (0 to 3 ∝m) can be reported as clean according to ISO 4406. To differentiate between usable fluids with a clean rating and silt-loaded fluids with the same rating, Pall has proposed a cleanliness code with a three-number format. Rather than invent a new rating system, Pall suggests to simply expand ISO 4406 to include a third range number that considers the contamination level of particles larger than 2 ∝m.

Absolute rating: Absolute filtration rating specifies the diameter of the largest hard, spherical particle that passes through a filter under controlled conditions. The rating is also an indication of the largest opening through a filter.

To determine absolute rating, the filter is installed in a test system. Test fluid is first circulated through a clean-up filter until fluid sampling indicates no more than 0.0004 g of contamination per 100 ml. A specific amount of artificial contaminant is mixed with the clean test fluid, thoroughly agitated, and then a measured quantity of the mixture is passed through the test filter and collected in a container. The fluid is then passed through a very fine membrane filter with a typical pore size of 0.45 ∝m. The membrane filter is then examined under high magnification to determine the diameter of the largest particle captured. The diameter of the particle, expressed in microns, is the absolute filtration rating of the tested filter element.

Nominal rating: Nominal filtration rating is an arbitrary value determined by the filter manufacturer. The rating system refers more to the types and sizes of holes in the filter medium than actual filter performance.

Nominal filter ratings have many limitations. First, they do not present a clear indication of the largest-size particle that can pass through a filter. Second, it is a nonstandard system that lacks consistency from one manufacturer to another. As can be expected, nominal filtration ratings can lead to problems in the field. In fact, perhaps the most common filter-related application error is selecting an element based on nominal rating. This typically leads to contaminated systems and accelerated component failure. Due to these factors, nominal ratings have lost favor to the more sophisticated Beta filtration rating system.

The life of a hydraulic or pneumatic component depends on the type, amount, and size of contaminant particles passing through it. Since each component has a different resistance to contamination, the filtration level must be matched to the system's most sensitive component.

To ensure maximum reliability, ratio could be specified to maintain contaminant levels far below those actually required. But this approach increases both initial and maintenance costs; therefore, filter rating and size must be carefully matched to system needs to produce the most economical system.

Another important factor controlling system reliability is filter location. Placement of the filter in the suction, pressure, return, or bypass line could require different filter specifications because system parameters such as pump flow rate, reservoir size, reservoir contaminant level, contaminant ingestion rate, and flow rate can change with filter location. All these parameters combine to control the required filter ratio.

A suction-line filter removes contaminants before they enter the pump. Since the reservoir collects all generated and ingested contaminants, it can be considered as the contaminant ingestion point for the circuit.

A pressure-line filter removes contaminant either between the pump and the other components or between any of the components. The ingestion point is either between the pump and filter or at the reservoir. Similarly, with a return-line filter, the ingestion point can be either between the work components and filter or at the reservoir.

A bypass circuit allows a large portion of the total pump flow to bypass the filter. In such a circuit, the filter handles only the amount of flow necessary to maintain the contamination level required by the system components. This lower flow specification allows the use of a smaller, less-costly filter, but can still provide maximum component life. In the bypass circuit, the ingestion point can be after the work components but before the bypass line or at the reservoir.

Filter cost generally increases with increasing size and increasing ratio, with size having more effect. Since suction and pressure-line filters must handle higher-pressure flows, these installations may require larger, more-expensive filters. Return-line and bypass filters, on the other hand, operate at lower pressure and, thus, are usually less expensive.

Pneumatic filters also require careful selection, but the process is not quite as involved. The reasons for this are simple: Because pneumatic devices operate at lower pressures than hydraulic components, tolerances in them are typically much larger. Accordingly, the opportunity for abrasive and silt-induced wear is considerably reduced. Also, air does not entrain and carry along particles at the same density as hydraulic fluid, so the task of filtering a pneumatic system is reduced.

The situation is much more critical if the air is to be used with fluid logic components that are sensitive to the presence of oil aerosols. For these jobs, most experts recommend various types of coalescing filters. A typical coalescing filter passes through four levels of filter media, including a foamlike core, a special filter which may be fiberglass, a perforated shroud of steel, and a foam cover. Such a filter is claimed to coalesce over 99% of all oil and water aerosols, in addition to removing solid particles of 0.01 ∝m or larger. In systems with significant quantities of oil, condensate, or scale, a roughing filter extends the life of the coalescing filter.

Most pneumatic filters of up to 2-in. pipe size are made with a removable reservoir bowl. This feature permits maintenance of the filter without the need to break pipe connections. When the filter is depressurized, the bowl can be removed and cleaned, and the filter element can be replaced or cleaned as required. Filter bowls are available in transparent plastic and in metal. Bowl guards to fit over transparent bowls are available from most manufacturers; metal bowls are considered safer in severe environments. To provide visibility, metal bowls may include a sight glass.

All compressed air filters must be drained. Manual drains on small filters are usually simple petcocks at the bottom of the bowl; large filters are drained manually through a valve. Automatic drains are available for all sizes of filters to simplify maintenance.

Normally, the choice of a specific element is pretty easy. If simple chip control is required of a filter, the element can usually be a straight wire mesh or screen with relatively coarse holes. Such screens are often rated by mesh size or by pore size.

For chip control, a so-called 100-mesh screen provides absolute filtration of about 220 ∝m. A 200-mesh screen filters to 105 ∝m, a sintered-woven wire mesh may filter to as fine as 25 ∝m absolute, and resin-impregnated paper (disposable) will filter to around 30 ∝m absolute.

If silt control is the required function, special filters that are capable of 1 to 5-∝m absolute filtration are necessary. Unlike the various metal chip-control filters, which are cleanable, the silt-control filters are nearly always made of a material that cannot be cleaned; they must be disposed of when they become filled.

Whichever filter element is used, its collapse pressure should be considerably higher than the pressure flow through the filter bypass valve at peak surge. A generous margin of safety here insures against filter collapse.

If the system requires that no dirt be allowed to pass through the bypass valve, experts recommend a filter without bypass; such a filter element must have a collapse pressure equal to the operating system pressure.

All materials used in the filter element must be compatible with the hydraulic fluid to be used over the entire temperature range expected in service.

Because most hydraulic systems fluctuate pressure and flow, filter elements used in them must be designed to withstand resultant cyclic stress. Filter elements designed for long life under fatigue conditions are available and required in most hydraulic systems.

Filter elements chosen must have adequate on-stream life. Element life is measured by manufacturers with a dirt-capacity test, in which the weight of an artificial contaminant added to the filter to attain a preselected differential pressure is measured. Normally, filter manufacturers can recommend a filter with adequate life for a particular application. Because service conditions may vary, this service life is seldom guaranteed; for critical applications, it may be necessary to "derate" the filter to ensure that the filters do not fill or break up toward the end of service lives.

Also, most manufacturers can furnish filter housings that will adequately support the filter element and restrain the fluid. Make sure that the housing selected allows easy servicing: An inconvenient filter simply isn't changed or serviced often enough, to the detriment of the hydraulic system. The housing should open quickly with a minimum of tools for element changes.

If the flow of filtered fluid cannot be interrupted, a duplex filter can be used. A duplex provides two filters separated by a three-way valve. Flow continues through one while the other can be serviced. Or if the system must be continued in operation, but can tolerate short periods of unfiltered flow, a conventional filter with a servicing bypass can perform the same functions as a duplex unit.

Bypass valves in filters must be capable of handling the maximum flow that can be expected through the filter assembly if it clogs up. With full flow through the bypass valve, the orifice should be large enough to avoid excessive differential pressure on the filter element. Bypass valves should be located so that collected dirt on the filter element is not swept downstream by the flow of oil passing through the valve.

Differential pressure indicators are options on nearly all filter housings; they are useful and should be considered. Typically, they inform operating personnel of accumulated contaminant buildup in the filter, and provide danger signals so that the elements can be serviced.

The devices include electrical switches, continuous-reading visual indicators, visual indicators with memory. The memory indicators show the highest differential pressure that the filter element has experienced, and are useful for jobs where an operator cannot watch the differential-pressure indicator continuously.