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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Installing a high-performance liquid filtration system can be a sound investment for your process or lubrication streams but you can easily undermine the entire system’s performance by installing an inaccurately sized or improperly specified filter.

Because it must serve as a barrier to contaminants, the filter acts as a restriction or obstacle to an otherwise free-flowing stream. The filter specification should lead to a filter that keeps the process or lubrication stream flowing at the desired capacity while achieving the required fluid purity standards.

It’s common to strive for short-term cost control by specifying a filter to support the minimal performance threshold. However, specifying without a thorough understanding of guidelines can result in an undersized filter. An undersized filter creates an unwanted restriction, a pressure drop on the line, and thus a strain on the entire system. In addition, the filter itself will suffer strain and, as such, yield a brief service life.

Conversely, a filter that’s properly fitted — or even conservatively oversized — can produce long-term savings. A larger filter can support a system with a less expensive pump and motor. In addition, its superior cleaning capabilities will result in less frequent fluid and filter change-outs.

The essentials

Accurate filter specification requires a review of 12 critical parameters. Several of these are interrelated. Let’s look at each of them.

  1. Flow rate and regularity of flow. A filtration system obviously must be able to accommodate the stream’s flow rate. Indeed, flow rate is the most critical parameter in determining the appropriate filter size; so, flow rate data will be among the supplier’s first requests. Flow rate analysis also will consider the regularity of the stream’s flow — whether it’s steady, intermittent, variable, or potentially subject to sudden increases.

    If flow rate information isn’t readily available, check the system’s original mechanical drawing for details. If you still lack data, the supplier will request additional information to estimate flow rate and will probably recommend a slightly upsized filter, to err on the side of caution.

  2. Reservoir size and usage rate. These data can be used to support calculations of overall system flow rate. In addition, they’re particularly important in applications that recirculate oil through a side-stream (kidney loop) filtration system because the filter must be able to adequately clean the fluid within the required reservoir turnover rate and time.

    If the filtration system is undersized, the contaminant either will enter the system more quickly than the filter can process it or will linger and settle in the reservoir to create a lasting contamination issue. By specifying a filter that cleans the contents of the reservoir more quickly and more frequently, you can avoid system damage from residual or acute contamination. This attribute is particularly important for systems with smaller, modern reservoirs with higher usage rates.

    The traditional rule-of-thumb for reservoir turnover is to remove and clean 10% to 20% of the fluid per hour. Today, filtration systems can be specified for an even higher turnover rate. To accurately assess the performance of a filtration system fitted to a re-circulating system, test the purity of the fluid as it enters the reservoir and exits the filtration system.

  3. Viscosity. Analysis of the process or lubrication stream’s viscosity also is essential to proper filter fitting because viscosity can affect flow rate. While water and other low-viscosity fluids experience little variation in viscosity, the viscosity of other fluids such as oils can change significantly with the operating temperature (or temperature variations throughout the system). So, a filtration system that processes higher viscosity fluids (Figure 1) requires special attention for a sound specification fit. Viscosity can be an important factor, for instance, in turbine and gear oil applications.

    Figure 1. This unit provides particulate removal to 3 micron and water removal to less than 100 ppm while handling flows from 50 to 800 gal/min.
    Figure 1. This unit provides particulate removal to 3 micron and water removal to less than 100 ppm while handling flows from 50 to 800 gal/min.

    Ideally, the filter size specification, as well as pump and motor specifications, will be based on the measured or predicted viscosity of the liquid at the time it flows through the filtration system. Equipped with this information, it’s easier to produce a reliable prediction of the fluid’s flow rate and the performance demands on the system.

  4. Material compatibility. The filter vessel and element obviously must suit the fluid. Otherwise, filtration system parts could deteriorate and enter the fluid stream — meaning that today’s O-ring could become tomorrow’s contaminant. This parameter is particularly important in applications using synthetic and ester-based lubricants, although compatibility also should be considered with systems using mineral-based fluids.

    Consider all aspects of the filter, including the vessel, adhesives (if present), seals, filter media, and instrumentation (gauges, flow sights). A compatibility check can be done quickly and easily by comparing filtration system materials with those currently used in the process. If the system has been successfully operating, then a filter fabricated of the same materials should seamlessly integrate with the existing system. The fluid’s supplier, of course, can provide information on compatibility and hazards.

  5. Pressure and differential pressure. The system’s operating pressure (pump pressure) determines filter vessel sizing. It’s a good practice to size the filter vessel for a pressure at least 33% higher than the maximum operating pressure to ensure that the vessel is well within the limitations of its pressure rating. Filter element sizing is based on the differential pressure across the filter.

    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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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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