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.

PowerFlow Fluid Systems supports industrial sustainability with innovative and environmentally responsible solutions for optimizing performance of virtually all fluid systems throughout the plant.  https://www.pwrfs.com/

DEPTH VERSUS SURFACE FILTERS – THE CONTAMINANT IS THE KEY

Introduction

Disposable liquid filter cartridges are typically designed for either “Depth” or “Surface” filtration.  These terms relate to the construction of the filter media and the mechanisms used to trap and retain particles.  Each is suited for contaminant characteristics.  These characteristics should be defined and considered in the selection of the filter media.

Categories of Solid Particles

Rigid Solids:

A rigid particle is a solid material that does not deform or readily breakdown into smaller particles.  During filtration a rigid particle maintains its size and shape.  Examples include sand, mol sieve dust and activated carbon fines.

Deformable Solids:

A deformable particle is a semi-solid material that can change its shape when a force such as pressure is applied to it.  During filtration deformable solids have the potential to change shape.  Examples include gels, waxes, and biofilms.

Shear-Sensitive Solids:

A shear-sensitive particle is a solid material that readily breaks apart into smaller particles when a shearing force is applied.  During filtration shear-sensitive particles have the potential to change shape and size, as well as generate additional smaller particles.  Examples include agglomerated solids, iron sulfide and asphaltenes.

Three Filtration Mechanisms

There are three key mechanisms used for filtration of particles.  Filter media is constructed to exploit one or more of these.  They are direct interception, inertial impaction, and diffusional interception.

Direct Interception:

A sieve is a structure with openings, voids, pores or holes of a specified size and quantity.  Sieves stop particles by direct interception.  This refers intercepting particles on the surface of the structure because they are too large to fit through the openings.  A pump suction strainer is a good example of a sieve.

Inertial Impaction:

Inertial impaction exploits Newton’s First Law, which states that a body in motion at a constant velocity will remain in motion in a straight line unless acted upon by an outside force.  If a contaminant particle is suspended in a fluid stream and is denser than the fluid, it will have higher inertia than the surrounding fluid.  Inertia is related to a body’s resistance to changes in direction of its movement.  When a fluid enters a fiber matrix such as a filter media it encounters a tortuous path created by the fiber matrix.  As the flowing liquid weaves around fibers on its way through the media, the contaminant particles with greater inertia will depart from the flow path of the fluid and continue a straight path, colliding with the fibers.  After collision, the particles may adhere to the fiber or reenter the fluid stream and continue these collisions until coming to rest or exiting the filter.  When properly applied, this mechanism allows particles that are much smaller than the openings in a filter media to be captured.  A good example is a fiber mat-type HVAC panel filter.  Although the open area is so large that you can see through the filter, it still captures small particles that have impacted the fibers.

Diffusional Interception:

Diffusional interception refers to the capture of particles as they diffuse according the principle of Brownian Motion.  Particles diffusing through a filter media come to rest after randomly impacting one or more fibers.  This mechanism works on particles that are small enough that they move randomly through a fluid by bouncing around between molecules.  This is applicable to submicronic particle filtration and primarily considered in the design of gas filters.  However, diffusional interception is at work to some degree in all filters.  Media can be engineered to increase its contribution to the overall filtration performance.  It is referred to as engineered depth media.  The engineering challenge is to increase the probability of fiber impacts within the media as much as possible with minimal performance degradation pertaining to other metrics like pressure drop and service life.

Depth Filtration

Depth filters trap and retain particles within the cross sectional area of the media and exploit all three filtration mechanisms.  Increasing this area provides more filtration capacity; deeper (thicker) filters have the potential to trap and retain more particles.  Well-designed depth filters are constructed with graded porosity.  This means the pores are more open on the upstream surface layers and become tighter toward the downstream layers.  This allows contaminant particles to penetrate deep into the media and be trapped throughout it with smaller particles accumulating within the inner layers and progressively larger particles building toward the outer layers.  By design this maximizes the dirt holding capacity, while leaving space for fluid to traverse the media without prematurely building up pressure drop.  Examples include fiber mats, sintered metal filters, melt blown and string wound cartridges.

Depth filters are ideal for applications where the contaminant particles have a broad range of sizes, allowing all layers of the media to be fully utilized.  Deformable and shear-sensitive particles are also filtered more efficiently with depth filters.

Surface Filtration

As its name suggests, a surface filter traps and accumulates contaminant particles on its surface.  The media is generally a thin layer of woven or non-woven fibers creating a mesh.  The area between the fibers creates pores that sieve out particles from the flowing fluid like a screen.  While some of these filters have very thin layers of graded porosity that enhance performance, direct interception is the primary filtration mechanism at work.  In addition to the direct interception occurring at the surface, contaminant that cannot penetrate accumulates and forms a filter cake.  As the pores plug and the cake builds, the filter becomes more efficient and is eventually completely obstructed.  Plainly, the service life is directly related to the surface area.  So many surface filters are designed with pleated media and other creative ways to pack as much useful media surface area into the filter geometry as possible.  Examples of surface filters include strainers, screens, and panels or cartridges with very thin synthetic and paper media.

Surface filters are well suited for applications containing particles with relatively predictable profiles, such as rigid particles that form a porous cake on the filter media surface.  They are also useful where very specific sizes or types of contaminants are targeted and work well for applications where a high flux rate or a cleanable filter is advantageous.  They should not be expected perform well when the contaminants are deformable or shear-sensitive.

Summary

In summary there two fundamental categories of disposable cartridges.  These are surface filters and depth filters.  Each exploits fundamental filtration mechanisms in different ways to target contaminants.  Surface filters use direct interception to capture rigid solid particles on their surfaces and build a filter cake that helps improve filtration over time.  Depth filters use all three filtration mechanisms to trap and retain rigid and semi-solids particles when there is a broad particle size distribution that maximizes utilization of its many layers.  Understanding this basic concept and the contaminants to be targeted prior to selecting a filter cartridge will improve the odds that the filter will produce the desired results efficiently.  Know filters and let the contaminant dictate the type you use.