Filtration quality is the single most important measure of hydraulic machine health. More hydraulic failures are a result of particle contamination than any other cause. Some inside information is required to effectively apply filtration, but it mostly boils down to size, location and quality.
Hydraulic filters are sized based on the flow passing through them at an acceptable pressure drop. It is important to consider pressure drop, which is the pressure developed by the friction of the fluid against and through the material of the filter assembly itself, because the majority of filters are installed with a bypass valve designed to open by the backpressure created by the clogged element. By sizing a filter intelligently, pressure drop is created only as the element becomes clogged with particles, preventing premature bypass, a condition when fluid passes through unfiltered.
To prevent excessive pressure drop, filter assemblies are often sized larger than required. An oversized assembly ensures little energy is lost to pressure drop, and as a corollary, larger filter elements are capable of holding more dirt before they become clogged. Because an adequately sized filter assembly provides both higher dirt-holding capacity and higher flow capacity, the choice to oversize a filter is limited only by real estate and price. With this in mind, choose filters as big as is reasonable. Never install a filter with borderline flow capacity, as it can go into bypass quickly and often, especially as it becomes clogged.
Additional considerations for sizing a filter are the fluid type and viscosity, as well as the quality and efficiency of the filter media. Highly viscous oil creates more backpressure than does thin oil, and you should also consider how oil viscosity will vary with extreme changes in ambient temperature, such as with mobile equipment. Obviously, cold oil is thicker, and a marginally sized filter will bypass until the fluid reaches operating temperature.
Filter media quality plays a part in sizing assemblies for two reasons: Finer filtration (i.e., lower micron rating), is more restrictive to flow because of the smaller gaps in the media, which are required to trap smaller particles. Also, the quality of the filter media plays a part because premium synthetic depth-media has a higher dirt holding capacity and will take longer to clog than cheap paper or cellulose media. If you are unsure of the variables when you are applying or purchasing a new filter assembly, it is advised to oversize the element and to use a high-quality medium.
Location determines type
By and large, hydraulic filters are used in three primary functional locations: return lines, pressure lines and off-lines. The venerable return-line filter is the most commonly used on a hydraulic machine, especially on mobile equipment. It is installed somewhere between the last valve or actuator of a hydraulic circuit and the reservoir so the oil is filtered as it enters the tank. This mounting location ensures oil in the reservoir is clean and limits the re-introduction of wear-related contamination generated in the hydraulic system itself. Oil should then be clean and once again ready for recirculation by the pump.
The spin-on filter is still the most popular choice for mobile machinery, but traditionally the quality of paper and cellulose media left much to be desired. However, the major players in the filtration game make higher quality synthetic fiber media for their spin-on elements, and other than the newer series of medium pressure filters of some manufacturers, the synthetic canisters can be installed onto existing low-pressure heads.
The low- to medium-pressure cartridge style filter assemblies are also popular for use in return lines. These units have removable bowls with which filter cartridges are inserted. When a filter element becomes clogged, the bowl is screwed off, the cartridge removed and replaced, and then the bowl is screwed back on again. This design is more ecologically friendly, as it requires no bulky steel outer shell, is simpler to manufacture, and once used, cartridge elements are easier to drain of residual waste oil.
Cartridge filter assemblies are often more expensive than their spin-on counterparts, but they’re available in a wider range of filter media, which is usually higher quality. In addition to synthetic elements, filter cartridges are also available in various media such as wire mesh, water-absorbing, paper, polyester, etc. It is important to consider the media, because so many are available, and not all are created equally. For example, some paper filter media absorb water, but others made from polyester do not. The synthetic media typically have higher dirt holding capacity, which should be factored into price considerations of your replacement elements; a synthetic element might cost twice as much as a paper element, but could hold four times the dirt.
The pressure filter assembly is also relatively common, and is useful for protecting the hydraulic system downstream of the pump. Sometimes contamination makes its way into the reservoir and is fed into the circuit via the pump. By placing a filter after the pump, anything introduced into the oil and not caught by the return filter will be trapped by the pressure filter, keeping the downstream components safe. Pumps sometimes wear rapidly or break apart internally, and pressure filters will catch the pieces before they damage anything else in the system. When used in this capacity, pressure filters often have no bypass valve, providing ultimate protection.
Pressure filters must be rated for the nominal pressure of the circuit, which can be 3000 or 5000 psi or higher. This higher-pressure capability dictates that the filter be manufactured from steel, which means they are quite heavy. Pressure filters use cartridge style elements, and in most circumstances, the elements have a high collapse rating to withstand the possibility of higher-pressure differential, especially when there is no bypass valve.
The third most common location for a filter is in what’s called a kidney loop, which is also referred to as an offline filter circuit. A kidney loop gets its term from the methods of blood cleaning outside the body, such as with a dialysis machine, and how it bypasses the online filters, the kidneys. A hydraulic kidney loop is a circuit with a dedicated pump whose only job is to circulate tank fluid to clean it (and usually cool it). Most high-end hydraulic machines use a combination of pressure, return and offline filtration, and typically the offline filter has the finest filter quality. Because the kidney loop isn’t exposed to the flow and pressure spikes of the system, it provides a reliable method of ultra-fine filtration quality that introduces no backpressure into the working circuit.
Although any filter can be used in a kidney loop, filter media quality is available much finer than even the best synthetic media found in pressure and return filters. Additionally, some offline filter media has extremely high dirt holding capacity, allowing them to remain running on the machine, in some cases, for years.
Examine filter ratings
Hydraulic filter quality varies as much as the construction and application of the units themselves. Filters are qualified by both the average size of particles they trap and the efficiency in which they’re trapped. Filter quality is expressed in the micron, which is a thousandth of a millimeter, and micron rating describes the theoretical maximum sized particle the medium will allow to pass. For example, a 10-μm filter should trap any 10-μm particle passing through it. In reality, this is not the case. Filter ratings are either nominal or absolute, and each describes the general capacity for the filter to remove particles of the specified size at a specified efficiency. A filter with a nominal rating is low efficiency, and a filter with an absolute rating is high efficiency. The problem is that any manufacturer can describe their filter with any micron rating as long as they publish the efficiency of the media, so you must be careful when selecting them.
Filter efficiency is often expressed by beta ratio, which is a ratio derived from a test measuring the number of particles upstream of the filter to downstream of the filter. Test dust is added to the oil of a sophisticated test rig that counts the number of particles before the subject and then counts the particles again after the fluid has passed through. Particle micron size must be specified when describing beta ratio, and conversely, no filter micron rating is worthwhile without knowing the beta ratio at which it was tested. Once filter beta ratio is calculated, we can further deduce the filter efficiency to decide if a filter is absolute or nominal.
If 100,000 5-μm particles are measured upstream of the filter and 100 particles are measured downstream of the filter, the filter
has a Beta Ratio of 1000, and is expressed as β5 1000.
• To now calculate efficiency, we subtract one from the beta ratio and then divide by the beta ratio.
For example: (1000-1)/1000 = 0.999 or 99.9%.
• Our theoretical test filter is 99.9% efficient at removing 5-micron particles.
Absolute filters must have βx ≥ 75 (x = specified micron size), according to ISO 16889: 1999, which is 98.7% efficiency. Anything below these numbers is considered a nominally rated filter, and such filters can drop as low as a beta ratio of 2, or 50% efficiency. Avoid nominally rated filters, if possible. When filter competence is low, the excess of particles turns the fluid into a liquid lapping compound, of which its abrasiveness further wears components, subsequently increasing the abrasiveness of the fluid itself. To be safe, use only absolute filters with >75 beta ratios.
A filter should be of minimum quality for the most sensitive hydraulic component in the system. Checking the datasheets of all components will help, as they will specify their required oil cleanliness level, and often the minimum micron filter rating. However, there’s no evidence it’s possible to have “too clean” of oil, and filter quality should be as fine as space, cost and pressure drop will allow.
High-pressure filter kits such as these are designed to protect against wear and malfunction, and are usually placed near control valves or other critical components.