The need for by-pass filtration alongside factory-fitted full-flow filters

21 March, 2017

Most hydraulic machines are fitted with full-flow filters. These filters provide a ‘safety net’, protecting from catastrophic machine failure due to large particles lodging in moving parts. However, there lies a finer level of contamination that results in slow, irreparable damage to machinery, that cannot be combatted by full-flow filters.

Whilst oil may ‘look clean’, millions of particles smaller than 40 microns (the limit of visibility by the human eye) are contained within the oil. As these microparticles are pumped around the hydraulic system, they act like a fine sandpaper, slowly eroding the surface of parts. This soon results in reduced performance and efficiency. Over time these parts erode, before complete machine malfunction occurs, leading to costly repairs, part replacements and machine downtime.

Maintaining a constant level of clean oil via by-pass filtration can protect against this slow machine degradation and sluggish operating performance. Moreover, fitting by-pass filtration as a factory standard, will significantly prolong the lifespan of all machine components.

Figures 1 and 2 illustrate the difference between a system without effective filtration, and with CARDEV by-pass filtration. Figure 1: the grey area indicates the cumulative growth of dirt and water contamination in a hydraulic system with an ineffective (or non-existent) filtration system. Regular, expensive oil changes are required. Component wear due to contamination in the system leads to expensive and catastrophic machine malfunction. Figure 2: Factory-fitted by-pass filtration units and regular filter element changes maintain oil to a ‘cleaner than new’ standard. Oil wastage and component wear are significantly reduced; costs are kept at a minimum.

How clean does oil need to be?

Oil cleanliness standards are often set per application or industry. For example, aerospace industries demand higher levels of cleanliness than most others. There is, however, a greater consideration than the minimum requirements of cleanliness set by manufacturers; indeed, there is a significant benefit to maintaining oil cleanliness at levels cleaner than new – straight out of the barrel – oil.

Consider a machine that runs for 24 hours a day, operating with flow rates of 100L/min. The hydraulic oil is maintained at the cleanliness of the minimum requirement for new oil using full-flow filtration (ISO 4406 code 19/17/14). In this system, pumps and other machine components encounter up to 592.1kg dirt each year within the fast full flow. Degrading the ISO cleanliness by just one code for each micron rating to 20/18/15, increases this cumulative annual dirt mass to 1184.2kg; a staggering amount of sandpaper-like contamination to be pumped through expensive and up-time critical components.

Including by-pass filtration in the machine system can significantly reduce these figures. Circulating oil flow through a depth filter, such as a CARDEV filtration unit, can clean and maintain oil at ISO cleanliness of 13/11/8. In the same scenario as described above, this reduces the annual dirt mass to just 9.5kg, demonstrating the life-prolonging benefits of by-pass alongside full-flow filtration.

How does by-pass filtration work?

By-pass filtration takes a small proportion of oil flow from the main hydraulic circuit. This oil is pumped through the by-pass filter, removing up to 99.9% of contamination, before being deposited back into the main circuit. Running this by-pass loop throughout machine operation will compensate for any new ingresses of contamination, maintaining the oil cleanliness.

The cellulose-based depth CARDEV filters developed by Environmental Technologies Group Ltd. (ETL) focus on the effective filtration of both particles and water contaminants within oil. In the CARDEV by-pass units, oil is pumped slowly (2L/min) through a wound filter element. Long cellulose fibres within the element lead to the adsorption of particles and absorption of water, leaving clean oil entering back into the main hydraulic circuit.

How do we test this?

At ETL we have worked with the University of Leeds to develop a series of multi-pass tests specific to hydraulic by-pass filtration. Figures 3 and 4 show results from two testing procedures. All reported particle counts are taken upstream of the by-pass filter, monitoring the contamination levels of the tank (full flow) of the system; no additional full-flow filter is fitted. In figure 3, ISO Fine Test Dust in HV46 oil is injected into the main circulating tank at intervals, mimicking the small ingress of particulate contamination in a hydraulic system during operation. Top: Particle counts in the full-flow circuit decrease as the by-pass loop removes the contamination. Middle: Flow rate and pressure across the filter remain constant. Bottom: Efficiency of filtration for each micron rating.

Figure 4 describes a draw-down test where the by-pass filtration unit is used to decreased the contamination in the tank from ISO code 23/22/19 to 13/11/8. This scenario describes an initial system clean-up, or offline tank filtration, when large amounts of contamination need to be removed from a system during machine downtime. CARDEV offline units that provide this bulk clean-up service are also manufactured at ETL. Bespoke systems can also be designed in-house on request.

It is evident through the research and development done at ETL, that by-pass filtration should be a critical component in a hydraulic system. Well-developed units, such as the CARDEV by-pass filter tested here, save end users money, as well as protecting the environment from wasted oil. By-pass units such as these can be retro-fitted, as well as factory-fitted, and should be considered by manufacturers and end-users alike.

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