Micron (μ) is used interchangeably with micrometre (μm). One micron is one millionth of a metre: 1μ = 1μm = 0.000001m.
The smallest lengthscale that the human eye can see is 40μm. So even if your oil ‘looks clean’, particulate contamination can still be a huge issue.
A filter’s beta rating describes the efficiency at which particles are retained within the filter. Larger beta ratings indicate a more efficient particle retention rate.
Beta ratings are denoted with a subscript number for the micron size being considered: β4μ means the beta rating for the filter at 4 microns, or how efficiently 4 micron particles are removed by the filter.
Beta ratings describe the relationship between the number of particles upstream and downstream of the filter.
e.g. For 4μ particles with 1000 particles per ml upstream of the filter, and just 10 particles downstream of the filter:
This relates to efficiency of the filter as follows:
Oil cleanliness can be measured in terms of ISO code 4406. Each level in the code represents the number of particles within a ml of oil that are equal to or larger than 4, 6, or 14 microns in diameter. The counts are cumulative (since a particle that is larger than 6μ is also larger than 4μ, for example), each ISO code represents a range of particle counts and the threshold of particle counts for each ISO code is double that of the one before.
e.g. ISO code 19/17/14 means that particles of diameter ≥4μ have ISO code 19, diameters of ≥6μ have ISO code 17 and ≥14μ have ISO code 14. This translates into particle counts as:
Between 2500 and 5000 particles of diameter ≥4μ per ml oil, between 640 and 1300 particles of diameter ≥6μ per ml oil, and between 80 and 160 particles of diameter ≥14μ per ml oil.
The full range of particles counts for ISO code 4406 can be found here*hyperlink to page with full ISO code table – change to be per ml?*.
The ISO code cleanliness of any oil can be measured using an in-line or offline particle counter. Particle counters work by passing a known volume of oil through the device flow aperture (set by factory calibration) and shining a laser beam from one side of the aperture to the other. Particle size can then be calculated by: the voltage drop due to the formation of shadows from any particles on the opposite side of the tubing to the laser beam (the voltage drop is proportional to particle size); or via light scattering techniques.
A full-flow filter is usually fitted to all industrial hydraulic systems. All oil pumped through the machine passes through this type of filter at the same large flow rate. These types of filter protect machine components from large particles (e.g. 50μ) that may cause catastrophic failure to moving parts. In this respect, full-flow filters provide a safety net for immediate failure. They do not, however, provide protection for long-term damage caused by smaller particles that contaminate the oil.
By-pass filters deal with much lower flow rates: they sit alongside the full-flow system and take about 10% of the flow. These filters remove much smaller levels of contamination (<1μ), maintaining the oil to a low level of contamination as the machine is used. These filters are often considered to provide oil polishing.
Fitting a by-pass filter along with a full-flow filter on any hydraulic machinery will significantly increase the time between oil changes.
Low-flow filters (such as those used in by-pass filtration) can be used to clean offline, stationary tanks of contaminated oil. In this scenario, flow is circulated through the filter directly from the offline tank, and back again, until the oil has reached the desired level of cleanliness.
There are several variables that affect the length of time it takes to clean oil:
- Initial and desired cleanliness levels
- Oil viscosity and temperature
- Shape of the tank
- Presence of turbulent flow or mixing within the tank
An ideal scenario for these variables will lead to efficient and effective filtration of the offline tank of oil. Even without this, it is possible to design a system for effective filtration of any offline tank (e.g. adding a heater to lower oil viscosity; introducing more flow to create turbulence or mixing).
The human eye can only see particles larger than 40μm. Millions of smaller particles contaminate oil even though they cannot be seen. These smaller particles slowly wear-away at components, which in turn introduces further contamination through abraded machinery. This leads to increased machine downtime. Furthermore, contaminated oil will reduce the efficiency of the hydraulics when in use.
Cleaning oil to as low as ISO class 13/11/8 with by-pass filtration will prolong oil life, reduce downtime, and increase efficiency.
The minimum cleanliness requirement for new oil has an ISO level of 19/17/14. At this level, particles in the oil are pumped around the hydraulic system at full flow, all operating hours.
For example, a system with a 100L/min flow, operating 24 hours a day, 365 days a year will be pushing up to 592.1kg of dirt through its pumps and other machine components, when maintained at ISO 19/17/14 (min. cleanliness new oil).
If the same system was maintained to ISO 22/20/16 (similar to that achieved by a 25 micron nominal full-flow filter), the pumps and machine components would see 4737kg dirt every year!
Reducing this cleanliness level to 13/11/8 reduces this level of dirt to just 9.5kg. That’s a reduction of 98.4% of particle weight from new oil at ISO 19/17/14, and 99.8% for ISO 22/20/16.
Unsurprisingly, this reduction in dirt seen by the hydraulic components will prolong their life.
Water contamination causes excessive corrosion to parts and oxidation of the oil. This in turn causes degradation of the base stock, promotes the formation of acids and can hydrolyse additives present in lubricating oil. Each of these significantly reduce the life of the oil.
Cellulose depth filters can remove free and dissolved water contamination from oil. As the oil passes through the filter, water is adsorbed by the long cellulose fibres within the filter media, leaving dry oil.
Hydraulic oil becomes cloudy when water is present above the saturation point of the oil. At this point, water is suspended in the oil (forming an emulsion) or separates out as free water, settling at the bottom of a tank.
The saturation point of hydraulic oil can be as low as 200-300ppm, or 0.02% water in oil.
It is known that just 1% water in oil can reduce the life of lubricated parts by 90%1
Once the filter approaches its full water-holding capacity, filtration efficiency will decrease. Pressure drop across the filter element is increased due to moisture-induced swelling of the cellulose fibres within the element, and as more oil is pumped through the filter, its structural integrity is challenged. If left unchanged, the filter element will not only dramatically lose efficiency in filtration of particulate and any further water contamination in the oil, but will risk filter degradation and media migration into the oil, causing further contamination.
No. Whilst it may take a long time to reach the maximum water-holding capacity of the filter, it would be advisable to replace the filter in advance of this to ensure filtration efficiency is not compromised.
Even if you start off with a large amount of free water in your oil, after the first clean and filter change, regular by-pass filtration will ensure the water contamination never reaches this point again. Any further low-level water ingress into the oil is less problematic and future filter changes will be much less regular.
The frequency of filter changes will depend on how quickly water contaminates the system: if water continues to ingress to reach 780ml each day, then the filters will need changing each day. But if 780ml is in fact built up over a week or a month (due to a lack of filtration present in the system to compensate for it) then the time between filter changes will also increase proportionally.
Yes. However, only when the water is introduced into the system slowly, relative to the oil tank volume. Introducing 780ml at once as free water will cause the filter to become saturated, requiring a filter change before all 780ml have been absorbed out of the oil.