When a Filter is Not a Filter

Jim Fitch, Noria Corporation

Just like lubricants, filters have a life expectancy, and it’s not indefinite. Users should be aware that there are certain operating conditions that can abruptly alter a filter’s performance and shorten its service life. These events can silently turn what you think is a filter into a non-filter. The unpleasant consequences of most filter failures are particles that get a free ride into sensitive machine components and frictional surfaces, leading to damage and premature failure.

Sadly, most filters used on machines and vehicles have a limited ability or no ability to alert operators and technicians to faulty performance. Some may have pressure-differential (P-D) alarms or gauges, but even these devices can only respond to a fraction of filter-related problems that may occur. Of course, the most common condition to which these P-D indicators alert is a filter that is becoming plugged or is already in bypass. However, they can’t announce warnings for a host of other problems that often contaminate downstream fluids and components.

Good Filtration Run Amok
As much as we would like to believe that lube oil filters will operate reliably, many will fail or exhibit impaired performance at some point during use. Often these failures are due to no fault of the manufacturer or user. We can break down the problems into three categories:

  1. Dirt goes around the filter. Oil and particles will take the path of least resistance. This can occur when bypass valves get jammed or cocked, either in a full-open or a slightly cracked position. Seal failures (pinched, damaged or missing) can also send torrents of oil into bypass. Of course when filters become loaded, bypass valves will crack open, which enables fluid to escape filtration. Filters often plug prematurely due to a sudden ingress of particles, water contamination, sludge, biomass, etc. Under the worst conditions, filters have been known to plug after less than an hour of service. Additionally, high viscosity resulting from cold starts and oxidation can send fluid into bypass as well.

  2. Dirt goes through filter element gaps. Filter media can become stressed due to vibration, shock, thermal aging, chemical attack and mechanical fatigue (repeated stress reversals from cyclic flow or cyclic viscosity). Poor quality and defective filter media may have wide-ranging pore sizes. In such cases, more of the flow is first directed through the larger openings, reducing the overall capture efficiency of the filter. It is not unusual for filter elements to have leaky end-cap sealants and seam cracks, which are another source of bypass. Also, surge conditions can collapse filter center-tubes, sending a tsunami of fluid (and previously held particles) out of the discharge port and into your machine.

  3. Dirt migrates through media pores. Over time, particles begin to build up on the surface of filter media as well as the interstices (subsurface pores) of the media. After several months of accumulation, some of these particles make their way through and out by the drag forces of the fluid. This migration is normal but can become dangerously high as the filter ages or is exposed to cyclic conditions from cold starts, changing temperature, variable flow, surge flow, reverse flow, shock and vibration, and viscosity change. For some of these filters, the particle count downstream can exceed the concentration upstream.

Let Particles Talk
There is only one method to be absolutely sure that contaminant ingression is under control and that filters are working as required. This method is an online particle counter positioned upstream of the filter. We can say that if particle counts going into the filter are within targets, the filter, at minimum, is doing an adequate job. However, despite the fact that there are more than a dozen online particle counters on the market, there are very real economic and practical constraints to installing them on every machine of interest. Nevertheless, there are other ways to get similar information, albeit not in real time.

One method is to use a portable or bench-level particle counter. Particle counts can be scheduled periodically based on risk. These instruments are designed for the plant or field. They can provide a quick ISO code and particle count from bottle samples or from lines temporarily attached to sample ports or quick-connect fittings.

Another less-costly alternative is to use the patch-test method. Patch-testing involves drawing a sample through a membrane and then examining the particles on the surface of the membrane (patch) manually using a low-cost microscope. This will not only give you a rough indication of the particle count, but it also allows you to inspect the particles for composition, color, shape and texture. This inspection can help troubleshoot ingression problems.

Don’t Let Your Guard Down
Keeping tabs on the performance of filters is essential to machinery reliability. Yet too often, many people in the maintenance field seem to be oblivious to the importance and methods of doing so. The best strategy is the proactive strategy. You don’t have to detect and diagnose a problem that doesn’t exist.

The following are a few rock-solid ideas to avert problems with filters:

  • Invest in quality filters (housing, valves, change indicators and elements).

  • Over-size your filters to reduce fatigue caused by high-flow density, cold starts and surge-flow conditions.

  • Inspect new filters and store them in a clean and protected environment.

  • Learn how to install filters to avoid damage to filter media and seals.

  • Inspect filter housings carefully for stuck bypass valves, missing springs and sludge conditions.

Used filters are often a dead giveaway of filtration problems. Learn to read the warning signs. After all, the filter is a bone pile of past problems generated within the machine and transported by the fluid. Inspect used filters for:

  • Damage such as tears, buckled media, collapsed center-tubes, pinched seals, etc.

  • Unusual particles and soft, sludgy material on the media

  • Varnish on end-caps and/or center-tubes

  • Sludge and/or water in filter bowls/housings

  • Corrosion on filter housing or filter element surfaces

When problems are found, don’t forget to troubleshoot and remediate. Uncorrected root causes rob valuable resources and impair reliability. Learn from these lessons to better control contamination at the lowest possible cost.

Subscribe to Machinery Lubrication

About the Author

Jim Fitch, a founder and CEO of Noria Corporation, has a wealth of experience in lubrication, oil analysis, and machinery failure investigations. He has advise...