Oil Filter Element Examination

Abbas Vijlee, Schroeder Industries; Gus Schroeder, Schroeder Industries

Have you ever wondered why you couldn’t sufficiently clean a system even though you specified the best filter housing and media to do the job? Have you ever observed equipment running smoothly over a long period of time while oil samples taken from the same equipment continually show poor results? These scenarios are often confusing, leaving one bewildered and frustrated.

Filter Element

A good place to begin to unravel the mystery behind such dilemmas is to perform a thorough analysis on the used filter element. A careful visual examination of the element by experienced eyes can yield useful information. Tests and disassembling the spent element can also shed light on the condition of the system from which it was removed.

Although there is no textbook offering instruction on the art of examining used elements, a seasoned professional can interpret visual clues and test data into plausible explanations.

Visual Examination of Oil Filter Elements

Many problems can be identified with a simple visual examination. Significant aspects include the following element components, seals, pleat condition and media.

Element Components

Examine components such as the end caps, center core and outer wrap. If they are damaged, investigate whether damage could have caused the fluid to bypass the filter element. The fluid is not filtered any time fluid bypasses the element.

If the components show rust, it is possible that the fluid has water contamination. Water in oil could show up as high, abnormal particle counts. The water will also cause the element go into early bypass, which could result in dirty system oil.


There are various seals on an element. The obvious ones are the elastomeric seals on the outsides of the endcaps, an epoxy seal (or similar adhesive) between the endcap and the media and another seal along the longitudinal seam. Missing or improper seals provide a leakage path and may explain the increase in particle count.

A missing elastomeric seal is a big concern because if left in the housing, it could affect the sealing performance of the next and all future elements. One must examine the housing to make sure seals are not piling up. Take corrective action to prevent any leakage paths created by improper sealing.

Upon changing an element, the operator must make sure that the seals on replacement elements and spent elements match. If there are any missing parts, check the parts manual to ensure that the replacement part matches the drawing. After verifying that the replacement is correct, check the housing for the missing part.

Oil Filter

Pleat Condition

Most new elements have properly defined pleat shapes. Pleats should be longitudinal and evenly spaced. Compare the new and used element. Are the pleats straight? If they are wavy or bunched up, it may indicate that the element was subjected to high differential pressure or the flow was fluctuating frequently. In most hydraulic systems, fluctuation in flow is a normal phenomenon.

A properly sized filter should not be affected by flow fluctuation. Either condition (high differential pressure drop or flow fluctuation) could allow the housing bypass valve to open frequently and cause the oil to bypass the element, resulting in high particle counts.


Observe the media to identify the amount of dirt on its surface. If the entire surface is full of contaminant, the filter element was most likely in bypass and the fluid was not filtered.

Another sign of trouble can be revealed with an inspection of the media texture. If the old media is appreciably softer than the new media then it is possible that excessive heat or water is present in the system. As explained earlier, water may cause bypass conditions to occur and account for the higher particle counts. The water must be removed to improve the filter life and also to prevent future damage to system components.

If the element was subjected to high temperatures, the media may not show obvious signs of degradation but the oil properties may have changed. Oil may have oxidized and particle counts may have risen.

Perform Tests

In some cases, a fabrication integrity test (also known as bubble point test) may be appropriate. Its results indicate the degradation of the media. A rule of thumb is that the spent element should have an 80 percent bubble point of a new element. If the test were to show a lower bubble point, further examination would be necessary. A significantly lower bubble point would reflect damage to the media and even holes or tears in the media. This would cause bypass of the element.

One may also run pressure-drop tests on the used element. Compare the pressure drop of the used element to that of a new element. If the difference is high, it may indicate that the element was either in bypass or very near bypass.

Disassemble Filter Element

After thoroughly examining the element, disassemble the used element and spread the media to examine it for holes and/or tears by shining light up through the media.

Another condition to look for is contamination build-up in the valleys of the pleats. An experienced eye can detect how close the element was reaching its capacity. This would indicate that the filter could be bypassing. Check the filter element indicator for proper operation.

These suggestions may be used as guidelines for hands-on engineers or technicians. Unfortunately, there are no magic formulas. Because it takes time to accumulate experience and historical data, it is advantageous to have several people familiar with these practices actively involved in filter analysis and in documentation development. This will provide not only the necessary training annotations, but a historical record of a particular system as well.

What should one do when the unexplainable is encountered? If this is the first incidence, install another filter element. If it cleans up a system and stays clean, chances are good that the former element was a bad sample. If the trend persists, the alternative is to either refine the system or choose a different media or supplier. Be sure that you’ve chosen the correct housing and the best filtering media for the job. Work with your filter manufacturer and seek his assistance. Most filter manufacturers have similar programs and experienced people to help you.

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