Signs it is Time to Replace Filter Elements

Most industrial filter elements are disposable and must be replaced periodically.  It is important to monitor the performance of filters and understand the signs that it is time to replace the elements. Before examining this in more detail, consider the general construction and operating characteristics of cartridge-style filter elements.

For our purposes we will consider a cylindrical (candle type) filter cartridge.  Used in swimming pools to industrial hydraulics this is very common style.  These filter cartridges consist of a cylindrical structure that supports any number of materials that serve as the filter media.  As fluid flows through the cartridge, it passes through the media where solid contaminants are trapped.  As the contaminants accumulate and block the flow channels (or pores) in the media, flow is restricted and differential pressure across the media increases.  This pressure drop is the most important parameter to monitor.  

Gauges, transmitters, service indicators or other sensors are commonly used to monitor the status of filters.  Initially and for most of a filter’s life, pressure differential across it increases gradually in linear fashion.  However, near the end of its service life, the pressure drop accelerates sharply.  This indicates that it is time to prepare to replace the filter elements.  In a relatively short period of time the pressure drop will reach the manufacturer’s recommended maximum and the cartridge should be changed.  This is the normal course of a filter element service life.

As with everything, there are a few exceptions.   Let’s consider the signs that something has gone wrong even though the pressure drop may seem okay. Knowing these can prevent serious process problems that result when spent filter elements go unnoticed.

Signs of a problem:

Equipment failure – If a machine has failed due particulate contamination, this is the most obvious and urgent sign that a filter element needs to be replaced.  Unfortunately, purpose of the filter was defeated and the equipment it was supposed to protect failed.  This is almost certainly much more costly than replacement elements.  This shed light on the value of proper monitoring and training focused on critical filters.

Signs of equipment fouling – When a filter is in service too long there is significant risk that contaminant will start bypassing.  There are several possible causes for this, including damage to the filter cartridge, opening of filter bypass valves and filter unloading.  The latter refers to the dislodging and migration of contaminant through the filter due to excessive pressure and/or high fluid velocity within the media. If contaminant bypasses the filter there is a significant risk that it will accumulate in/on downstream equipment.  This fouling manifests in many ways. Smoky exhaust, vibration, sputtering, plugged guard filters, low oil pressure and sluggish controls/response are just a few of the possible symptoms.  However, this is greatly dependent upon the type of equipment.  The signs of equipment fouling should be researched, posted and used for training purposes.

Reduced flow of the filtered fluid – Without any pressure compensation, the flow of filtered fluid will decrease in proportion to amount of contaminant trapped in the filter.  This is a result of increasing pressure loss as trapped contaminant accumulates and blocks flow channels (pores) in the media.  Depending on the fluid system and the equipment design low flow may cause several problems.  Again, this should be researched, posted and used for training.  When signs of low fluid flow appear, one of top items on the troubleshooting list should be to check all upstream filters.  

Spike in transmitter data – Dialing in the time resolution and reviewing the minute by minute data logged by a filter’s pressure transmitters is an often overlooked and very useful diagnostic tool for filters.  Unlike lower resolution trending, zooming in and reviewing the log or even trending at higher resolution can uncover a spike that resulted in filter damage that, in turn, resulted in a drop to zero or a nominal differential pressure.  Fluid contamination often occurs in bouts related to process or environmental upsets.  When a bout occurs, it may overwhelm the filter causing the differential pressure to spike.  The sudden high pressure could rupture or collapse the filter element immediately.  This usually relieves the pressure and the gauges or transmitter readings drop to a unalarming level.  This could mislead operators and result in a perplexing equipment malfunction that could remain unresolved until further equipment damage, complete failure or a safety issue occurs.  When a filter seems to be lasting much longer than expected or there is equipment malfunction that could be related to a poorly performing filter, it is always wise to either visually inspect the filter elements or review the transmitter logs for a seconds to minutes-long spike in differential pressure.

Filter material found in equipment or reservoirs – Usually if this happens it means the filter either ruptured, collapsed or underwent another form of mechanical failure due to a pressure spike or a temperature or chemical incompatibility.  Unfortunately, if the filter is coming apart and making its way into downstream equipment, serious problems have likely already occurred.  Obviously, there needs to be an investigation into the cause of the failure and the filter elements need to be replaced with a suitable alternative.

One final piece of guidance with regards to monitoring and care of filter elements concerns process exposure time.  There are many possible mechanisms that are responsible for exposure-related filter media degradation, including thermal, chemical and pressure cycling and more. It is a good practice to get an estimation of the shelf life and application specific exposure limits for each type of filter.  This will not be the same for all filters in all applications.  Obviously, the process has a significant effect on the rate of degradation of various materials.  Exposure degradation, such as fiber oxidation, can result in poor performance and mechanical failure prior to a filter reaching its maximum rated differential pressure (contaminant capacity).  Applications with very light contaminant loads may require periodic filter replacement based on exposure time.

Similarly, filters behave differently when they are exposed to multiple fluid phases.  If a filter is operating in liquid phase and gas bubbles are present, this could result in higher differential pressure. It may be necessary to periodically bleed the gas from the filter housing to reduce pressure.  Likewise, if a filter is operating is dry gas phase and entrained liquid wets the filter media, this will result in more pressure drop and may wash contaminants out of the filter.  

Filters are a very important and often misunderstood component of many processes and equipment systems.  It is advisable to investigate and understand the application of each filter and ensure that operations and maintenance personnel are properly trained.  Simply monitoring filters and replacing the filter elements as required could avert serious operational issues and improve a systems overall performance.

The points above, are intended to help understand critical filter performance and operating characteristics and how these are related to filter element service life.  The objective is to provide guidance on how to monitor and identify when it’s time to replace filter elements.  When the time comes, searching the vast landscape of the internet will lead to myriad options for filter elements and suppliers.  Considering the importance of understanding filter characteristics and their applications, the advantages of working with a filtration specialist instead of a retail outlet should be obvious.  PowerFlow Fluid Systems offers hands-on experience with all types of filtration equipment and applications.  Contact us today for filtration products, services and free consultations.

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