The pros and cons of suction filters
 Suction filters are an important element in ensuring trouble-free operation in various machines such as mobile concrete pumps, field stackers, hydrostatic closed-circuit drive systems or drawing frames.  Ideally, both suction and full-flow return filters should be installed in the oil tank. The best results in terms of flow quality can be obtained by mounting the filter horizontally below the oil surface.  Due to diminishing oil volumes in mobile applications, the full-flow return filter and suction filter for the closed circuit have been integrated in a single housing.  Ensures closure of the inlet valve before the filter cover is opened. |
The topic of suction filtration is rather a hot potato, dividing hydraulic engineers into one camp and filter manufacturers into the other. While machine designers and filter specialists promote the use of a suction filter, hydraulic engineers advise against them, claiming bad experiences with such devices. Who are we to believe? Is there really any point considering suction filtration, and if so what aspects need to be taken into account? To gain a better understanding of this problem area, we should take a closer look at hydraulic processes and potential faults in the intake section of a pump. Enlarging the volume inside the pump (piston, gear wheels, etc.) creates a vacuum. The air pressure at the surface of the fluid in the tank is usually atmospheric (or slightly higher due to pre-tension). This higher pressure causes the fluid to be sucked into the intake section of the pump. This process may be hindered by two possible faults which are equally harmful to system components. The first of these is that of the additional, undissolved air sucked in with the fluid. A sharply falling oil level in the tank, a poorly designed tank and oil return pipe, a leaky suction pipe or inadequate bleeding during start-up result in extreme vibrations when the poles of a concrete pump are extended, the familiar stick-slip effect or ultimately to worn cylinder gaskets. Secondly, air bubbles change from a dissolved to an undissolved state on reaching vapour pressure (from a negative pressure of approx. –0.3 bar), resulting in the production of oil vapour. This mixture of gas and oil vapour implodes under the excess pressure to which it is exposed at the discharge end of the pump. This is the notorious cavitation effect, which may also be compounded by the micro-diesel effect caused by the combustion of the vapour. Consequently, the solid contamination of the system is increased by cavitation erosion and rust. The increased wear also reduces the lifetime of such important components as the pump. Reasons for this include excessively high starting viscosity, insufficient ventilation of the tank and increased resistance in the suction pipe. These faults should not be underestimated, particularly since the actual cause is often not detected until it is too late for some components. The so-called filter experts recommend what they call a “super” suction filter and the cheapest possible air breather and bleeding filter for the tank. This is a sure way to encourage regular cavitation, for besides ensuring inadequate ventilation, the suction filter also generates additional resistance in the suction pipe. Suction filters are, however, an important guarantee of fault-free operation in many machines such as mobile concrete pumps, field stackers, hydrostatic traction drive with closed-circuit or drawing frames – particularly where the supporting system (chassis) doubles as a tank structure. As part of a plant-specific filter strategy, suitable suction filters prevent contamination with “coarse” solid particles and hence sudden pump failures. What’s more, they fulfil important functions in the fluid circuit, for whatever the tank design and type of plant type, the suctioned oil is constantly contaminated by abraded matter and the original installation dirt. Without suction filtration, these particles are churned up, aggravating wear and tear on the pump and downstream valves and increasing the number of fine particles due to disintegration. RT filters offer a special advantage in this respect: besides individually designed fleece and wire mesh combinations, they also have a specially developed, highly effective magnetic prefilter. Since such special conditions do not permit the use of microfilters, particles smaller than 20 μm pose a tricky problem for suction filters. At the same time, suction pipes and filters are supposed to be designed for low flow rates. These are two good reasons for using a magnetic core, which is positioned inside the filter element. Tests have shown that a permanent magnet installed downstream of a microfilter w = 3 μm (without a magnetic stage) still traps a significant quantity of micro-fine solid particles up to a size of 0.1 μm. Magnetic cores have a very long-lasting effect and thus not only increase the lifetime of the filter, but also have a positive effect on the filtration result without impairing the hydraulic system. RT suction filters are available for all mounting positions, with different versions for mounting inside or on top of the tank, or in a horizontal or vertical position. Line filters can also be used as suction filters, however, provided the supply line to the filter can be sealed while the filter element is being changed. This option is of course not without risks for the operating procedure, which is why such solutions are only recommended in special cases (e.g. for test bench construction). The ideal solution is to install both suction and full-flow return filters in the oil tank. The best results in terms of flow quality can be obtained by installing the suction filter horizontally below the oil surface. If it is also possible to mount the pump below the suction filter, then an oil column pressure of h x γÖI is exerted on the pump, which does not therefore need to generate a vacuum. This also guarantees a longer lifetime for the filter. For safety reasons, the suction filter must not be equipped with a bypass valve, otherwise a constant bypass flow will be generated during operation and optimal maintenance will be unable to be guaranteed due to the impossibility of detecting the degree of element contamination. Instead, the user is urgently recommended to use a vacuum switch to monitor the suction filters. If the suction filter is mounted in the ideal horizontal position (as in the case of RT filters), easy maintenance is also guaranteed. For this purpose, the filter must have an inlet valve, which automatically prevents oil from escaping when the element is changed, the suction filter being arranged below the oil surface. In the improved version of this solution, the inlet valve is guaranteed to close before the filter cover is opened and the seal broken. The volume of oil thus enclosed can be cleanly removed via specially provided drainage connections before the cover is opened and the filter element changed. According to the experience of RT, this effective combination of mechanical and magnetic filtration provides the best protection for the overall system. In the case of systems incorporating control valves, proportional valves, pilot stages or similarly sensitive components, additional line filters, also known as “police filters“, are used. Depending on the type of pump protection, the working hydraulic fluid may require separate filtration. In such applications, the full-flow return filter has proved highly successful as a classic filter system. These downstream system filters must always be finer than the suction filter. It should be noted in this connection that the suction filters in such combinations are an important means of relieving and hence prolonging the lifetime of expensive system filters. |
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