Get the right cartridge or bag filter

An inaccurately sized or improperly specified filter can undermine an entire high-performance liquid filtration system. There are a dozen key parameters to assess to determine the proper specification.

By Tim Mills, Kaydon Custom Filtration Corp.

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To accurately size a filtration system, the supplier will take into account the type of pump (such as positive displacement or centrifugal), existing relief valves and desired flow rate. The supplier also may recommend the installation of a differential pressure gauge on each filter, a pressure gauge on the discharge side of each pump and a vacuum gauge on the suction side. These gauges will greatly enhance the troubleshooting of system problems.

  • Existing pipe line size. It sounds simple, but it’s often overlooked: the filter vessel should properly fit the existing pipe. Generally, the filter should be sized to match the pipe to which it will be installed. Unless there’s some limiting factor — such as available installation space, low flow rate or an operating pressure exceeding 3,000 psig — the filter shouldn’t be installed with connections that are smaller than the existing pipe. If the pipe is 2 in., install a filter with connections that are at least 2 in.. This simple practice will eliminate excessive pressure drop.

  • Fluid physical characteristics. A Material Safety Data Sheet (MSDS) provides important precautions for the handling and use of the fluid, but a Fluid Product Data Sheet (FPDS) offers the most useful information for specifying filtration systems. So, be sure to have this information available when contacting a filtration supplier.

    The FPDS gives greater detail about the fluid’s physical characteristics, including viscosity, specific gravity, materials compatibility and recommended applications. The fluid manufacturer may be able to supplement the FPDS with specific recommendations about filtration.

  • Temperature. Provide the filter supplier with a realistic estimate of the fluid’s operating temperature. Consider the operating temperature range that you expect 99% of the time, adding ±10°F for safety. Unrealistically broad ranges will undoubtedly drive up system costs and may compromise performance.

    Temperature can affect many design decisions, including the specifications for seal material, adhesives and filtration media. Outdoor applications can present additional challenges. In cold climates insulation, heat tracing and circulation heaters can help to mitigate the effects of lower temperatures. Tropical and high humidity environments can take a toll on system components such as motors and instrumentation. Ensure that the motors are rated for high temperature conditions. Controls can be cooled and protected with a simple fan inside the control panel.

  • Space for installation. This is one of the most commonly overlooked aspects of a specification. Give the filtration supplier dimensional data detailing space limitations at the installation location. Consider the headspace above the filter necessary to allow easy removal.

    In some cases, it may make sense to opt for a horizontal filter instead of a vertical one or to put the filter at another location in the process line that provides more space. In addition, consider filter inspection and changing responsibilities. The operators must have an adequate work staging area as well as a place, such as a 55-gal. drum, for used filter disposal. An adequately sized work area will encourage good work practices, including frequent and safely performed filter changes.

  • Filter-element filtration rating. Such ratings can be confusing because the system for rating a process filter differs from that for an oil filter. An “absolute” rating from a process filter supplier may differ from an efficiency rating supplied by a hydraulic or lube oil supplier.

    It’s common to confuse a micron rating with an efficiency rating when they are two distinct specifications. For example, a filter may have the capability to block particulates of 10 microns or larger but may effectively block only 50% of those particulates. Therefore, the filter should be specified to match the desired purity of the stream.

    In critical applications, use a filter that can provide at least a 95% efficiency rating, preferably 99%. In general applications, a 50% rating suffices. The minimum efficiency target for turbine oil or hydraulic oil filtration should be 99.5%.

    In oil applications, choose an element that has been tested per ISO 16889 (Multi-Pass Filter Element Test). This test evaluates filter element efficiency and dirt-holding capacity and determines the micron size rating of the element at a given efficiency. To have a practical meaning, efficiency should always be combined with the micron rating. Any reputable filter manufacturer can easily give this information, so ask if the filter element is rated and tested per ISO 16889. If it isn’t, then the micron rating of the element may be questionable.

    Dirt-holding-capacity ratings aren’t particularly valuable. The contaminant particles used in the multi-pass test are lightweight, consistent in size and, therefore, don’t represent field conditions. The ratings can give a sense of relative service life among elements with the same micron ratings but actual field conditions may produce different results because of the random size and weights of the trapped particles.

    The best practice is to use rated elements that have been tested with a widely accepted method. Avoid elements that aren’t rated to any test or standard.

  • Choice of cartridge or bag filter. There’s no definitive basis on whether it’s better to specify a cartridge filter or a bag filter. Cartridge filters typically cost two-to-four times more than bag filters but their larger surface area yields a longer service life.

    A pleated cartridge filter is strongly recommended if the majority of the particulate contamination in the fluid is smaller than 10 micron. Pleated cartridge filters also offer higher efficiencies and are available with the micron ratings required to maintain a high degree of fluid purity.

    Not surprisingly, flow rate and viscosity also can affect your selection decision. Cartridge filters, especially high porosity ones that use micro-fiberglass or polyester material, better suit fluids with higher viscosities.

    Bag filters are good choices for applications with low viscosity fluids like water, low filtration requirements and larger particulates (40 micron and larger). Bag filters also may be easier to specify for materials compatibility. Bag filters require a larger housing to match the clean (initial) differential pressure of a pleated cartridge.

  • Filter vessel sizing. A common error is to specify a filter vessel with too few filter elements. Such undersizing often stems from a desire to keep initial costs down. However, it’s actually advantageous to specify a filter vessel that can contain a few elements more than the minimum. Oversizing will add some initial cost but the long-term savings will pay for the upfront investment.

    A filter is essentially a restriction in the flow line. The flow line is less likely to experience interruption or pressure drop when its restrictive qualities are minimized; a larger filter vessel reduces the strain on the system. In addition, it will offer better filtration performance because the fluid’s velocity as it travels through the system is reduced, allowing the filter to capture more particles — even if the filters are specified to a lower micron rating. Therefore, a larger filter vessel with additional elements can improve performance and reduce long-term maintenance costs.

  • Tim Mills is an applications engineer for Kaydon Custom Filtration Corp., LaGrange, Ga. E-mail him at

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