In many machines as much as 90% of all particles suspended in the oil can be iron or steel and can be particularly troublesome to operators. Frank Schollmeier of Valin Corporation looks at the advantages and benefits of magnetic filtration and how it can often outperform traditional methods.
As industrial filtration methods continue to evolve, sometimes choosing one can be a multi-faceted decision. There are a variety of factors that come into play when determining the most efficient method. In some circumstances, a combination of methods is truly the most optimal solution. One that seems to be an underutilized technology in the industry is magnetic filtration.
Most operations not currently using a form of magnetic filtration in its process can greatly benefit by implementing it. Equipment operators, maintenance technicians and reliability engineers all know and understand the importance of clean oil in achieving machine reliability. Additionally, most used oil analysts agree that in many machines, as much as 90% of all particles suspended in the oil can be iron or steel. These types of particles can be particularly troublesome.
Typically, one or both of any lubricated, sliding, or rolling surfaces will have iron or steel metallurgy. These can include any number of frictional surfaces utilized in gearing, rolling-element bearings, piston/cylinders, etc.
Magnetic strength By producing a magnetic field or loading zones that collect magnetic iron and steel particles, magnetic filters can often outperform traditional, mechanical filters. With magnetic filtration technology, magnets are geometrically arranged to form a magnetic field that has a non-uniform magnetic strength.
This technology is commonly used with rotating drums used for tool coolants with high levels of machining waste. Additionally, filter housings can be utilized with a complete set of magnets and no filter media, with an intent to only capture ferrous metals. Finally, there are flow through magnetic filters in use. These are filter housings where there is no filter element. Instead the magnets are in plates attached through a rod that acts like a conventional filter element.
But cannot mechanical filters remove particles that are roughly the same size? While it is true that conventional mechanical filters can remove particles in the same size range as magnetic filters, most of these filters are disposable and incur a cost for each gram of particles removed.
Additionally, using magnetic filters over particulate filtration will require a lower power consumption. This is due to flow restriction caused by the fine pore-size filter media. As pores become plugged with particles, the restriction increases proportionally, causing the power needed to operate the filter system efficiently to escalate.
Conditions The decision to use magnetic technology in a given application should always be influenced by various machine conditions and fluid cleanliness objectives. For example, what is the expected concentration of ferrous particles? What type of oil is used? What is the operating temperature, surge flow, shock and machine design?
As there are numerous commercial products, configurations and applications, certain advantages and disadvantages discussed in this article may not apply. However, understanding the various factors can serve as a great starting point for making the decision whether magnetic technology is a good choice in a given application.
Advantages First and foremost, magnetic filtration is considered a reusable technology. This, of course, means that it is a more cost-efficient technology than a traditional mechanical filter. The cost of removing a gram of particles from the oil with magnetic technology is a fraction of that incurred when only using disposable filters. So, from an apple to apple comparison, utilizing magnetic filtration as opposed to traditional, disposable technology will ultimately be friendlier to the bottom line.
There is also the issue of flow restriction as the filtration is in use. Unlike conventional filters, most magnetic filters do not show an increase in flow restriction as they load with particles. This can be extremely advantageous. While conventional filters can go into bypass when they become plugged with particles, magnetic filters continue to remove particles and allow oil flow.
Increased effectiveness One of the biggest advantages of magnetic filtration is that it does not always have to be an ‘either/or’ situation. In fact, when used in conjunction with conventional mechanical filters, magnetic filtration may lead to an increase in the mechanical filter’s effective service life. The impact can be so great that in certain cases, a mechanical filter’s life may be increased by two or three times as much. A common example of this is using a magnetic insert on a bag filter housing.
The next major benefit to utilizing magnetic filtration is how it protects the equipment that the liquid is running through. Specifically, servo and solenoid valves can be heavily damaged by particles that are magnetic (such as iron and steel). By continuously removing these particles with magnetic filters, the reliability of these valves can be substantially enhanced.
Magnetic filtration will also better protect against premature oil oxidation, which can lead to varnish, sludge and corrosion. Everything else being equal, the continuous removal of iron and steel particles by magnetic filters should have a positive impact on oil service life.
Magnetic filtration Both the variety of magnets used and ways in which magnetic filters and separators can be configured in a product’s design are both key factors that influence magnetic filtration. There is much more to their performance than simply the strength or gradient of the magnetic field. For example, the size and design of the flow chamber, total surface area of the magnetic loading zones, and the flow path and residence time of the oil are all important design factors. These factors influence the rate of separation, the size of particles being separated, and the total capacity of particles retained by the separator.
The magnetic force acting on a particle is proportional to the volume of the particle. However, it is also disproportional to the diameter of the particle. For instance, a two-micron particle is eight times more attracted to a magnetic field than to a one-micron particle. Because of this, large ferromagnetic particles are easier to separate from a fluid than smaller particles. The separating force is proportional to the magnetic field gradient and the particle magnetization (the degree to which the particle’s material composition is influenced by a magnetic field).
Particles made of iron and steel are the most strongly attracted materials. However, red iron oxide (rust) and high-alloy steel (for example, stainless steel) are only weakly attracted to magnetic fields. Conversely, some nonferrous compounds such as nickel, cobalt and certain ceramics are known to have strong magnetic attraction. Materials that cannot be picked up with a magnet, like aluminum are called paramagnetic substances.
Competing forces It is critical to keep in mind that there are competing forces which resist particle separation from the fluid. Oil velocity is one such example, which imparts inertia and viscous drag on the particle in the direction of the fluid flow. Depending on the design of the magnetic filter, the fluid velocity may send the particle on a trajectory toward or away from the magnetic field. This competing viscous force is proportional to both the particle’s diameter and the oil viscosity. If the particle’s diameter or the oil’s viscosity increases, the hydrodynamic frictional drag will increase proportionally. The magnetic attraction increases by a factor of eight when a particle’s diameter doubles, while the competing viscous drag sees only a 2X multiple. This is important to note and further demonstrates how large particles are more easily separated than small particles, even in an environment of considerable viscous drag.
Particle capture efficiency by magnetic technology can be categorized into three factors:
1. Particle characteristics The larger the particle, the easier it should be to separate. Additionally, if it consists of a highly magnetic material (for example, iron and low-alloy steel), it will be subject to a great capture efficiency.
2. Fluid characteristics The fluid conditions that best facilitate the separation of magnetic particles are low oil viscosity and low oil flow rate. If these conditions are met, extremely small, one-micron particles can be separated from the oil efficiently.
3. Magnet design Magnetic filters that employ high-flux magnets and are arranged in a way that develops high-gradient magnetic will be most effective.
Most filtration operations will have some use for magnetic filtration. If there are metal particles being filtered out in any way, it is generally a beneficial technology. Whether replacing traditional filters or enhancing their effectiveness, magnetic filtration can be very beneficial.