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

21 March, 2017

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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