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Unraveling filter misunderstandings

02/28/2011 @ 9:22am

In the heat of maintenance, the tendency is to grab a filter, spin it on, and get back to the field. But what if the only replacement filter in your supplies is dented or cracked? Or the service manual calls for a fuel filter made with synthetic fiber, and the one you have on hand is made with cellulose?

When in doubt about these and other questions, ask your equipment dealer for guidance, advises the Filter Manufacturers Council. The Council also points out that dented engine oil filters should be avoided. Once the steel canister of a filter is dented, a concentration of stress in the canister material is created, making it more susceptible to fatigue.

That fatigue results from pressure pulses created by the pressure-regulating valve in the system. Oil filters are designed with a low-carbon steel to resist fatigue and are formed so the stress created by these pulses is equalized over the surface area of the canister.

A dent provides an area of stress concentration from pressure pulses and can greatly shorten the life of the canister since it results in stress cracks. As such, it's best to avoid using new filters that are dented and to replace existing filters that are damaged in use, the Council urges.

There is another issue that has occurred with the addition of new filtering media for lubrication, fuel, and coolant systems. Today, tried-and-true cellulose media have been joined by synthetic and meltblown technologies.

Some filters still use cellulose media made from wood pulp featuring large-diameter fibers.

Synthetics' Performance Advantage

Synthetic media, on the other hand, consist of very thin fibers created by polymer or glass fiber technology. Synthetic filters have a performance advantage over cellulose units in that they provide more area for fluid to flow over the filtering media to capture contaminants.

For example, contaminants that plug up 50% of the cellulose media surface area will only plug 10% of the synthetic media area. Besides having lower flow restrictions, synthetic media are also less sensitive to water that can cause cellulose filters to swell, thus restricting flow, the Council says.

Meltblown media filters have a specific use in fuel systems. These filters, consisting of polymer fibers made using a blown-process technology, offer good contaminant-retention capacity and water-separation performance.

The Council also warns that there can be confusion when it comes to the difference between a bypass vs. a full-flow lube filter. Oil that goes through the full-flow lube filter goes on to lubricate the engine. The bypass lube filter receives about 10% of the amount of oil that flows through the full-flow filters and filters that oil at a much higher efficiency.

The oil that flows through the bypass lube filter then returns to the sump. Due to the high efficiency of the bypass lube filter, it cannot handle the same volume of flow as the full-flow filter. A metering orifice is commonly used to meter the flow of oil through the bypass filter. Thus, you shouldn't use a full-flow filter to replace a bypass filter and vice versa.

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