What is new in the field of magnetic filtration?
	Assessment of a highly sensitive technology in view of the revised filter standards
	Hand in hand with the consistency of the filtration effect, the high dirt retention capacity of the magnetic core brings about an increase in the lifetime of the downstream filter element. As experience shows, permanent magnets not only attract ferromagnetic abraded matter, but also filter out large quantities of fibres, core sand residues, non-ferrous metal abrasions and similar non-magnetic matter. In the RT magnetic filter design, the magnetic system with the sharp-edged field poles is given a cylindrical shape and assembled inside a thin-walled, non-magnetic tube with a smooth surface. In order to maximise the effectiveness of magnetic filters, it is essential to make the liquid pass through a hydrodynamic flow straightener. The catchword of the year 1999 in the filter industry is undoubtedly "ISO MTD". Now that the long since discontinued production of ACFTD - also known as Arizona dust - has finally been replaced by the ISO MTD test dust, the relevant standards for evaluating filter performance have now also been drafted. These are known as ISO 4406:xxxx, whereby "xxxx" stands for the anticipated introduction date. This is a further sign that the industry is currently in a process of transition. As already reported, the use of the new test dust means that exactly the same filters will now receive a higher or lower rating than before. At the same time, the evaluation of the filtration result, i.e. the oil purity class, will also be changed in the ISO standard FDIS 4406.2 so that the standard based on three particle size categories - >2, >5 and >15 mm - which has not yet become generally valid will undergo a value shift as the calibration sizes are changed to >4 mm(c), >6mm(c) and >14 mm(c). As a result, the first code number will be increased by a value from 0 to 2 - 1 being the rule of thumb - while the second and third code numbers remain unchanged. What with all these changes, value shifts and re-evaluations, it is hardly surprising that users are feeling confused and are no longer sure which criteria to use when assessing the performance of the filters on offer. One thing hasn´t changed, however: the test medium still bears no relation to the actual material composition of the impurities in a hydraulic circuit. And this is precisely where the magnetic pre-filtration stage comes in, which is being improved and applied on a "non-standardised" basis by RT-Filtertechnik GmbH (formerly Regeltechnik Friedrichshafen GmbH). If we look at the wear on the components of hydraulic systems, we find that it depends largely on the nature and degree of the impurities found in pressure fluids. It is assumed that approx. 70% of the resultant failures are due to solid impurities. In many cases, these consist of ferritic abraded matter generated by the hydraulic components themselves. By effective magnetic pre-filtration with a downstream mechanical filter system, it is possible to guarantee optimal cleaning of the process medium. Tests have shown that a permanent magnet topped with a micro-filter "w = 3 mm" (without a magnetic stage) still attracts a significant quantity of super-fine solid particles measuring up to 0.1 mm. As experience shows, permanent magnets not only attract ferromagnetic abraded matter, which is generated continuously within systems, but also filter out large quantities of fibres, core sand residues, non-ferrous metal abrasions and similar non-magnetic matter. The quality of these particles can amount to up to 30% of the total dirt holding! It should also be noted that minute ferromagnetic particles stick together as they pass through a magnetic field. This happens when one of these particles becomes a dipole in the magnetic field. In other words, it too becomes a magnet, which in turn attracts other particles and merges into an agglomeration, in which the individual particles are only held together by the relatively weak forces of their remanent magnetism. If such an agglomeration finds its way into a narrow gap, it still won´t cause any damage, as the generated forces it will cause it to separate immediately into its original small components, and, depending on the flow rate of the medium, be flushed through the system, given a particle size range of 1...5 mm. An important factor here is an effective magnetic filter design. For this, it is necessary to generate the strongest possible magnetic fields within the filter element, through which the process medium is sent for cleaning. This can be achieved by fracturing the field poles, for example, or making them sharp-edged or pointed. On closer observation of the separation process, we can see that impurities have to travel at right-angles to the fluid stream in order to stick to the field poles. This movement is considerably obstructed by turbulences and eddy currents in the flow of fluid within the separating chamber. Such eddy currents can occur at any time, particularly during the entry of the fluid stream into the filter housing. In order to design a highly effective magnetic filter, it is therefore essential to send the fluid stream through a hydrodynamic flow straightener - a concept familiar from fluid mechanics - in order to stabilise it. In the RT magnetic filter design, this problem is solved by giving the magnetic system with the sharp-edged field poles a cylindrical shape and assembling it inside a thin-walled, non-magnetic tube with a smooth surface. Only then is it possible to achieve a smooth flow with the sharp-edged and fractured field poles necessary for generating an inhomogenous field. In some cases, it is also a good idea to equip the magnetic core with longitudinal wings, in order to prevent the liquid from rotating around the magnetic system. That way the liquid flows mainly along the surface of the magnetic system and the impurities move towards the field poles and stick to them. It is extremely important that the fluid stream should be as free from eddy currents as possible in the area affected by the magnetic force. In such a stabilised fluid stream (ideal flow rate = 0.25 m/s), the magnets can pick up and retain even the tiniest particles very efficiently without them being carried off again by eddy currents. A further advantage of magnetic pre-filtration is its extreme durability. Effective magnetic filters - if suitably positioned - can increase the lifetime of hydraulic components as well as prolonging the service of hydraulic fluids. Advances in magnetic technology over the past decades have allowed magnetic adhesion forces to be increased considerably. The permanent magnets used include the brand groups OXIT (barium-ferrite), OERSTIT (aluminium-nickel-cobalt) and also SECOLIT (rare earth-cobalt). Even long periods of use should not lead to any deterioration in the performance of the magnetic components. Hand in hand with the consistency of the filtration effect, the high dirt retention capacity of the magnetic core brings about a valuable increase in the lifetime of the downstream filter element. The strong influence of magnetic filtration on a flow medium is illustrated by a further example which can be found in the relevant specialist literature (Schüler-Brinkmann) and which brings home the principle very clearly, even though it refers to an area of application outside the classical hydraulic sector:Transformer insulating oil is circulated by feed pumps. In the process, ferritic particles generated by abrasion inside the pump get into the oil circuit. Operation of the system without a magnetic filter over a period of 500 h showed a constant, uniform deterioration of the dielectric (non-conducting, insulating) loss factor. On activation of a magnetic filter, the loss factor fell again over roughly the same period to the initial value. In conclusion, the use of magnetic filters - provided they are correctly built according to physical principles - can be said to offer major advantages. By contrast, filter structures which only provide magnetic pre-filtration in the form of a magnetic plug or annular magnets positioned in the oil stream directly inside the filter head can only be described as technically inadequate. Apart from their low dirt retention capacity and difficulty of maintenance, such structures are hardly able to prevent the Fe particles from being washed away, especially considering the relatively high turbulence in this area. A further problem with these solutions is that, when the system is under load, the strong pulsing of the volumetric flow and the corresponding pressure fluctuations can cause particles to break off from the essentially brittle magnetic material and be washed into the system.