C-Series, Coalescing Filters FAQ

FUNDAMENTALS: What Coalescing Filters Do

What is a coalescing filter for natural gas?

A coalescing filter, also called a coalescing filter separator or gas coalescer, is a filtration device that removes both liquid contaminants (water droplets, hydrocarbon condensate, compressor lubricating oil, liquid mists, and aerosols) and fine solid particulate from a natural gas stream. Unlike a simple gas scrubber or knockout drum that captures only bulk liquid slugs, a coalescing filter captures submicron liquid droplets and aerosols that would pass straight through a scrubber. FilterFab C-Series coalescing filters achieve 99.9% removal of all liquid droplets and solid particulate down to 0.3 microns, the finest filtration available in this product category.

How does coalescence actually work to remove liquid from gas?

Coalescence is a physical phenomenon in which tiny liquid droplets (too small and light to fall by gravity) are forced to combine into progressively larger droplets as the gas stream passes through a fibrous filter medium. As the gas flows through the fine pores of FilterFab's borosilicate fiberglass media layers, the microscopic droplets collide with the fibers and with each other, merging (coalescing) into larger, heavier drops. Once the drops become large enough that gravity overcomes the gas drag force, they drip downward off the outside of the cartridge and collect in the sump at the bottom of the filter housing, where they are periodically drained away. No liquid is carried further downstream.

What contaminants does a coalescing filter remove from natural gas?

FilterFab C-Series coalescing filters remove: (1) free liquid water and water slugs, (2) water vapor that has condensed into liquid droplets, (3) liquid hydrocarbon condensates (heavier hydrocarbons that condense out of the gas at operating temperature and pressure), (4) compressor lubricating oil mist and aerosols, (5) glycol carryover from dehydration systems, (6) amine solution carryover from gas treating systems, and (7) solid particulate down to 0.3 microns including pipe scale, dust, and fine debris. This dual capability (liquid AND particulate removal in a single vessel) is what distinguishes a coalescing filter from a simple dry particulate filter.

Where should coalescing filters be installed in a natural gas system?

Coalescing filters are typically installed at locations where liquids are known to be present or are likely to accumulate: (1) at wellheads and gathering system inlet points, where produced water and condensate accompany the gas, (2) upstream of gas compressors, to prevent liquid slugs from damaging compressor valves and internals, (3) downstream of compressor aftercoolers, where hot compressed gas is cooled and hydrocarbon and water condensation occurs, (4) at compressor fuel gas connections, to protect engine fuel systems, (5) downstream of glycol dehydrators where glycol carryover is possible, and (6) at any point where phase changes are occurring due to pressure or temperature changes.

What is the micron rating of the FilterFab C-Series coalescing filter?

FilterFab C-Series coalescing filters achieve 99.9% removal of liquid droplets and solid particulate down to 0.3 microns. This is among the finest filtration ratings available in the natural gas filtration industry. This sub-micron capability is made possible by FilterFab's proprietary reverse-flow, radially-finned borosilicate fiberglass coalescing cartridge, which combines multiple layers of borosilicate microfiber glass and felted polyester media. By comparison, many competitors' coalescing elements only achieve 1-micron or 3-micron ratings, making FilterFab's 0.3-micron performance a significant technical differentiator.

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SIZING & SELECTION: Choosing the Right Coalescing Filter

How do I size a coalescing filter for my natural gas application?

Coalescing filter sizing follows the same two-step process as dry filter sizing: (1) identify your line size and operating pressure to determine the appropriate C-Series model number from the part number chart, then (2) verify the model's rated SCFH capacity at your operating pressure exceeds your system's actual flow demand using the C-Series flowrate table. Coalescing filter flowrates are based on 1.5 PSI pressure drop for pressures up to 500 PSI, and 3 PSI pressure drop for higher line pressures. C-Series filters handle flows from 17,900 SCFH (C2 at very low pressure) up to 17,000,000 SCFH for a 12-inch model at 1,400 PSI. Contact FilterFab at 724-643-4000 if you need sizing assistance.

How do FilterFab coalescing filters use only 1–2 cartridges when competitors use 10 or more?

This is FilterFab's most significant technical differentiator. Our proprietary reverse-flow, radially-finned coalescing cartridge has an exceptionally high surface area due to its radial fin geometry, much greater than a straight cylindrical element of the same length. This larger active filtration area means each cartridge can process a much higher gas flow rate before reaching its pressure drop limit. Competing designs using simple cylindrical elements may require 10 or more elements to achieve the same flow capacity and removal efficiency that FilterFab achieves with just 1 or 2. Fewer cartridges means dramatically lower replacement cost, faster changeouts, and less maintenance downtime.

What pressure ratings are available for FilterFab C-Series coalescing filters?

C-Series coalescing filter housings are available in three standard pressure ratings: 285 PSI, 740 PSI, and 1,480 PSI — matching the full F-Series dry filter lineup. The 285-PSI models use ANSI 150# raised-face or flat-face flanged connections, 740-PSI models use ANSI 300# flanges, and 1,480-PSI models use ANSI 600# flanges. All ratings are available in line sizes from 2 inches through 12 inches. Choose the pressure rating that exceeds your system's maximum allowable working pressure with adequate safety margin as required by your applicable piping code.

Should I install a dry filter upstream of my coalescing filter?

Yes, in most applications it is considered best practice. A dry particulate filter upstream of a coalescing filter extends coalescing element life significantly by removing bulk solids before they can reach and foul the coalescing media. Coalescing cartridges are more expensive than dry particulate cartridges, so protecting them from premature solid loading makes economic sense. However, FilterFab C-Series coalescing filters DO remove both solids AND liquids, so a standalone coalescing filter will protect your system in the absence of a pre-filter. Discuss your specific contamination profile with FilterFab's engineering team to determine the most cost-effective configuration.

Is a coalescing filter the same thing as a gas scrubber or moisture separator?

No, these are distinct technologies with different capabilities. A gas scrubber or knockout drum uses gravity and flow velocity reduction to drop out bulk liquid slugs and large droplets, but cannot capture fine liquid mists, aerosols, or sub-100-micron droplets. A moisture separator uses inertial impaction or vane-type elements for intermediate droplet removal but still cannot capture submicron aerosols. A coalescing filter, by contrast, captures liquid in ALL forms (slugs, droplets, mists, and submicron aerosols) plus solid particulate down to 0.3 microns. For complete liquid protection upstream of a compressor or meter, a coalescing filter provides a level of performance that a scrubber or separator simply cannot match.

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TECHNICAL: Engineering & Construction Details

What is borosilicate fiberglass media, and why is it used in coalescing cartridges?

Borosilicate fiberglass is a glass fiber produced from borosilicate glass (the same base material as laboratory glassware), drawn into very fine microfibers — typically 0.5 to 3 microns in diameter. These ultra-fine fibers create an extremely dense, tortuous flow path that forces gas-borne droplets into contact with the fiber surfaces thousands of times, promoting coalescence. Borosilicate glass is chemically inert, resistant to hydrocarbons and water, dimensionally stable across a wide temperature range, and does not shed fibers that could contaminate downstream gas. FilterFab C-Series cartridges use multiple layers of borosilicate microfiber glass sandwiched with felted polyester media to achieve 99.9% liquid removal to 0.3 microns.

What is reverse-flow filtration in a coalescing filter, and why is it better?

In a reverse-flow coalescing design, the gas enters the filter element through the center core and flows outward through the coalescing media. As the gas travels radially outward through the coalescing media layers, liquid droplets coalesce and migrate radially outward with the gas flow. Gravity draws the droplets down the exterior of the element where they drip into the sump while the gas flows upwards toward the vessel’s outlet nozzle. This separation of the liquid drainage path from the gas flow path prevents re-entrainment, the condition in which already-coalesced liquid is picked back up by the high-velocity gas stream. Competing outside-in designs can re-entrain liquid at high flow rates. FilterFab's reverse-flow design ensures that coalesced liquid consistently drains away from the gas path.

How does the collected liquid drain from a coalescing filter housing?

After liquid coalesces and drips from the filter cartridge, it accumulates in the sump (the low point of the filter housing). The sump is equipped with a drain connection (typically a 1-inch FNPT coupling) for liquid removal. Drainage can be accomplished in two ways: (1) Manual drain - an operator opens the drain valve periodically to purge accumulated liquid; appropriate for applications with low liquid loading or frequent maintenance schedules. (2) Automatic drain - a float-operated or timer-controlled automatic drain valve opens when liquid reaches a set level and closes when the sump is emptied; appropriate for high liquid-loading applications or unmanned remote stations. FilterFab housings are furnished with a drain connection as standard.

Are FilterFab coalescing filter housings ASME code stamped?

FilterFab C-Series housings are designed and manufactured to ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 specifications, with all welds performed by Section IX certified welders. Standard housings are not U-Code stamped; however, ASME U-Stamp certification is available at additional cost for applications where a stamp is required by code, specification, or regulatory requirement. If your project requires a U-Stamp (common in refinery, petrochemical, or highly regulated utility applications) specify this at the time of quotation so inspection and documentation can be arranged through our authorized inspection agency.

What is the typical pressure drop across a new coalescing filter element?

C-Series flowrates are engineered around a design pressure drop of 1.5 PSID for operating pressures up to 500 PSI, and pressure drop of 3 PSID for higher line pressures. These are the "clean element" design basis values. As the coalescing element collects solids, pressure drop increases over time. Most operators plan element changeouts when differential pressure rises to 9–10 PSID, though actual practice varies by application. Installing a differential pressure gauge across the coalescing filter housing is strongly recommended to monitor element condition and plan maintenance proactively.

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APPLICATIONS: Real-World Use Cases

Do I need a coalescing filter upstream of my natural gas compressor?

Absolutely, this is one of the most critical filtration points in any gas system. Liquid carryover into a compressor causes multiple failure modes: liquid slugs can destroy compressor valves and piston rods in reciprocating compressors ("liquid slug" is the equivalent of hydrolocking an engine), compressor oil is contaminated, suction valves fail from particulate impingement, and unplanned shutdowns result. Coalescing filter installation upstream of every compressor, both at the suction inlet and on the fuel gas connection for engine-driven units, is standard industry practice and typically required by compressor OEM warranty terms. FilterFab C-Series filters sized for your compressor's suction flow rate will protect your equipment investment.

Why do I need filtration downstream of a compressor aftercooler?

When natural gas is compressed, its temperature rises substantially (sometimes several hundred degrees Fahrenheit). The aftercooler cools this hot, compressed gas back toward ambient temperature. As the gas cools, heavier hydrocarbons and water vapor that were in gaseous form at high temperature condense into fine liquid droplets and aerosols, a process called retrograde condensation. These liquid droplets are too fine to be captured by a simple separator and will carry forward downstream. A coalescing filter installed in the cool, high-pressure stream immediately downstream of the aftercooler captures these condensed liquids before they reach instrumentation, meters, or fuel gas systems.

Can a coalescing filter handle liquid slugs, or only mists and aerosols?

FilterFab C-Series coalescing filters are designed to handle liquid slugs, bulk free liquids, fine droplets, mists, and submicron aerosols. The housing sump has adequate volume to accommodate liquid slug events without flooding the element. However, for systems with extremely high or frequent liquid slug conditions, such as wellhead gas gathering lines during upset conditions or right after a pipeline pig passes, a C-Series Two-Stage design is recommended. This coalescing filter will have an open chamber first stage to collect and buffer the heaviest liquid loads, protecting the coalescing element from being overwhelmed and bursting open.

My turbine meter is giving erratic readings, could liquid carryover be the cause?

Yes, liquid carryover is a well-documented cause of turbine meter malfunction. When liquid droplets or slugs pass through a turbine meter, they can cause the rotor to spin irregularly (creating false high readings), can erode the rotor and bearings (leading to eventual under-reading), or can cause momentary rotor stalls and restarts (producing custody transfer errors). If you are experiencing erratic meter readings and your gas supply has any potential for liquid content (even low-humidity distribution gas can carry condensed water) installing a coalescing filter upstream of the meter is the appropriate corrective action. Contact FilterFab at 724-643-4000 for a filter sized to match your meter's line size and operating pressure.

What is "compressor fuel gas" filtration, and do I need a coalescing filter?

Many natural gas compressor stations use engine-driven compressors (large gas engines similar to automotive engines but much larger) that burn pipeline gas as fuel. The fuel gas supply to these engines must be extremely clean (free of liquids, compressor oil carryover, and fine particulate) to prevent engine valve fouling and combustion problems. Coalescing filters on the compressor fuel gas supply line protect the engine fuel system from liquid and particulate contamination. This is a separate and additional filtration point from the main compressor suction filter, and it is typically recommended by the engine OEM as a maintenance requirement.

Are coalescing filters suitable for RNG, biogas, and hydrogen blend applications?

FilterFab C-Series coalescing filters can be applied in RNG (renewable natural gas) and biogas applications, where elevated moisture, hydrogen sulfide, siloxanes, and biological particulates are common contaminants. Filtration is especially critical in RNG systems to protect sensitive downstream equipment from the more aggressive contamination profile of non-pipeline-quality gas. For hydrogen blend applications, where natural gas contains an admixed percentage of hydrogen, contact FilterFab's engineering team to discuss media compatibility and appropriate housing materials, as hydrogen's smaller molecular size and potential for embrittlement in certain alloys may influence material selection. Our engineering team is available to support emerging clean energy applications.

What happens if a coalescing filter is not drained regularly?

If liquid accumulates in the coalescing filter sump and is not drained, several problems result: (1) The sump fills until liquid contacts the bottom of the filter element, causing the element to become flooded. A flooded element loses its coalescing efficiency and restricts gas flow substantially. (2) Liquid carryover resumes as the flooded element can no longer function. (3) Pressure drop across the housing rises rapidly as the element becomes liquid-saturated. (4) In severe cases, liquid slugging can occur downstream of the filter. Proper maintenance includes regular sump inspection and draining, or installation of an automatic drain valve on high-liquid-loading applications to prevent sump overflow. FilterFab can supply this automatic drain system.

How do I know if I need a dry filter, a coalescing filter, or both?

The answer depends on what contaminants are present in your gas stream. If your gas contains only dry solid particulate (pipe scale, rust, dirt), a dry F-Series filter is sufficient. If your gas contains liquids (water, condensate, compressor oil) with or without particulate, a coalescing C-Series filter handles both. In many pipeline applications, particularly compressor stations, wellheads, and custody transfer metering, both types are used in series: a dry F-Series filter upstream to capture bulk solids and extend coalescing element life, followed by a C-Series coalescing filter to remove all remaining liquids and fine particulate. FilterFab's engineering team can help you determine the right configuration based on your specific gas quality and system requirements.

Can coalescing filters be installed outdoors in cold weather environments?

Yes, with appropriate precautions. The primary cold-weather concern for coalescing filters is that accumulated water in the sump may freeze, blocking the drain connection and potentially causing housing damage. For outdoor installations in freezing climates, consider: (1) insulating and heat-tracing the sump and drain line and (2) installing an automatic drain valve set to drain frequently before liquid can accumulate and freeze.

Are FilterFab C-Series coalescing filters made in the USA?

Yes, FilterFab Manufacturing Corporation has produced natural gas filtration equipment in Pittsburgh, Pennsylvania since 1986. All C-Series coalescing filter housings are fabricated in our Western Pennsylvania facility by ASME Section IX certified welders. The proprietary reverse-flow, radially-finned borosilicate fiberglass coalescing cartridges are also domestically produced. For projects with Buy American Act requirements, federal infrastructure funding compliance, or domestic content documentation needs, FilterFab's Made in USA manufacturing can support your procurement requirements. Contact us at 724-643-4000 or sales@filterfabmfg.com for documentation.