Questions for the Author

Q & A’s offer quick answers to common or not-so-common questions. The following questions originated after reading “The Future of Filtration,” an article published in the July - August 2001 issue of Practicing Oil Analysis magazine. Jim Fitch, the Publishing Editor asked Philip Johnson of Donaldson, the author of the article, the following questions.

Jim: What are the benefits, if any, of using a coarse filter media followed by a finer filter media in series?

Philip: The benefits of using two elements in a series are questionable, particularly if you are trying to control fine dirt particles. In some situations, the performance of a filter can be enhanced by larger particles because larger dirt particles are trapped on the surface of the media making the filter more efficient. A relatively coarse filter ahead of a finer secondary filter in a series means that the secondary filter would only receive small, stratified fine particles of dirt and would not have the benefit of the sieving effect caused by larger dirt particles lodging on the surface.

Jim: What benefits of filtration performance come from doubling the filter size by using a larger volume canister? Does this reduce flow density, face velocity by one-half or more pressure differential effect?

Philip: Pressure is force per unit area. Doubling the diameter of a filter increases the area by a factor of four, and at the same time, reduces the face velocity. The downside is that it makes both the filter housing and the element more expensive to make with the same structural strength to handle the same pressure. The more common way to increase the capacity of a filter is to either add more pleats, increasing the surface area of the media, or increase the length of the element.

Jim: Does doubling the filter size translate to doubled dirt-holding capacity?

Philip: Dirt-holding capacity is a function of many things, not just the size of the filter. If you were to double the size of a filter under the same flow conditions, you could find some unusual results. Logic suggests that by doubling the size of the filter, you would double the dirt-holding capacity. In most cases, this would be true. However, at low flow rates, you may find that the dirt-holding capacity significantly increases but that the efficiency actually reduces its ability to capture very fine dirt particles.

Jim: How does the cost of removing a gram of dirt by a filter installed on a full-flow pressure line differ from the cost of an
off-line low-pressure installation?

Philip: Low-pressure filters are usually less expensive than high-pressure filters and require less service parts. Each time you service a high-pressure filter, it is advisable to change the O-ring seal and backing ring (if there is one). The seals on low-pressure filters tend to last longer. There are several advantages to using an off-line filter over a full-flow pressure filter. For example, you can usually service an off-line filter without interrupting the hydraulic process. In addition, an off-line filter can be fitted to the size of the system, not to the maximum flow of the system. In large hydraulic systems, going to an off-line system is often the least expensive and most effective.

Jim: How does water contamination influence filter efficiency and dirt-holding capacity?

Philip: Water is one of the worst contaminants in modern hydraulic systems. With cellulose filters, water can cause the media to swell and bind. In many oils, if water is allowed to contaminate the oil, it will form sludge that will rapidly plug a filter element. Many filters are prematurely blocked due to problems associated with water contamination. If a hydraulic system has been running well, then suddenly experiences an increase in filter element replacement, check for signs of water contamination.

Jim: Does a filter’s capture efficiency typically improve or worsen over the life of the filter? How do surge-flow conditions affect this?

Philip: The two figures below show that efficiency changes significantly, both throughout the loading-up process and also through fluctuating flow conditions. One filter manufacturer may quote an equivalent or better efficiency (b-ratio) than another manufacturer, yet the two products may perform differently in loading-up under steady-state flow conditions and also under pulsating-flow conditions. The new ISO multi-pass test is a valid procedure for ranking filters, but it does not guarantee efficiency throughout the life of the element.

Jim: What special problems or failure conditions are associated with cold starts?

Philip: Cold starts usually create a higher differential pressure across a filter element because the oil is more viscous. This often leads to element bypass which, in extreme cases, can cause element collapse if the filter has been incorrectly sized. Many filter manufacturers test their filters using an oil with a viscosity of approximately 30 cSt at a temperature of 40°C. However at -20°C the viscosity is 75 times higher. Likewise, the viscosity of oil that is 68 cSt at 40°C increases by 150 times when the temperature is reduced to -20°C (both examples assume a typical viscosity index of 95). To size a filter properly, one must know the highest viscosity and highest flow rate, under the most extreme cold start conditions.

Jim: Does Donaldson have any experience with defoamant (methyl silicone) additives being removed by glass-fiber filters?

Philip: Donaldson tested the adsorption of silicon antifoaming additives in synthetic filter media elements. A collection process can occur in the element, but normal additive formulations used by oil manufacturers will usually be sufficient to overcome any adverse effects.

Jim: Does a filter have a shelf life?

Philip: Cellulose filters do have a shelf life because paper media oxidizes over time. We recommend that cellulose elements be used within three or four years from the date of manufacture.

Jim: What is the best strategy for timing a filter change?

Philip: If possible, service a filter in response to the differential restriction indicator. If scheduled maintenance is the accepted program, maximize the filter element and hydraulic oil life. The less a filter is changed, the less likely it is for contamination to enter a hydraulic system.

Jim: How does a filter’s Beta rating (capture efficiency) correlate with target cleanliness levels using the ISO Code (ISO 4406:99)?

Philip: Cleanliness control of a fluid system can be more a function of the system design than filtration efficiency. High-efficiency filters can clean up a system faster than a coarser filter. Coarse filters however, will slowly filter out fine dirt particles and are usually less expensive. Cleanliness control addresses how much contamination enters a system, how much is generated because of wear and how the filter controls ingression sources. Cleanliness control becomes significantly less expensive in terms of filtration when you start with clean oil and a well-sealed hydraulic system.


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