Protecting a process analyser from contamination is one of the most demanding filtration challenges in industrial gas handling. Sample systems must deliver a clean, representative gas stream to the analyser — free from particulates, liquid aerosols, and moisture — without altering the composition of the sample itself. Choosing the wrong filter type, or placing it incorrectly in the sample loop, can mean costly analyser downtime, drifting calibration, or outright instrument failure.
What Is a Sample System Filter?
A sample system filter sits between the process tap and the analyser inlet, conditioning the extracted gas stream so that it meets the analyser's inlet specification. In a fast-loop or bypass sample system, the filter is typically installed close to the process tap to protect downstream components — flow controllers, pressure regulators, and the analyser cell itself — from particulate damage and liquid slugs.
Three distinct filter types are used in sample conditioning, each addressing a different contamination mode:
- Particulate filters — remove solid particles (rust, scale, catalyst fines) down to 0.3 µm or finer.
- Coalescing filters — capture liquid aerosols (water, hydrocarbon mist) and drain them away from the gas stream.
- Membrane separators — provide an absolute liquid barrier using a hydrophobic PTFE membrane, allowing gas to pass while blocking any liquid phase entirely.
The correct selection depends on the process fluid, the expected contamination load, and the sensitivity of the downstream analyser. For a full overview of element types, see the R+F FilterElements element range.
Fast-Loop Sample Systems: Why Filtration Placement Matters
In a fast-loop design, a high-velocity bypass stream is drawn from the process line and returned downstream, with a small slip stream diverted to the analyser. The fast loop minimises sample transport lag — critical for real-time process control — but it also means the filter must handle the full process pressure and temperature, not just the reduced conditions at the analyser inlet.
For high-pressure sample taps (above 100 bar), the filter housing must be rated accordingly. The R+F instrumentation and high-pressure filter range includes housings rated to 400 bar, suitable for wellhead, pipeline, and reactor sample points. The RF-H-150 (100 bar) and RF-H-170 (400 bar) are the most commonly specified housings for process analyser protection in these services.
Selecting the Right Filter Element for Analyser Protection
Element selection for sample systems differs from standard compressed-air filtration in one critical respect: adsorption. A standard activated-carbon element (RF-AC) will strip trace hydrocarbons, sulphur compounds, or moisture from the sample — exactly the components the analyser may be measuring. In most sample conditioning applications, adsorption elements are therefore excluded from the sample path and used only on instrument air supplies or purge gas lines.
The preferred elements for sample system filtration are:
- RF-P (particulate) — borosilicate glass microfibre, 99.99% efficiency ≥ 0.3 µm, low pressure drop, chemically inert binder. Suitable for dry gas streams with solid contamination.
- RF-C (coalescing) — borosilicate glass microfibre, 99.99% efficiency ≥ 0.1 µm for liquid aerosols. Drains coalesced liquid to a sump or auto-drain. Essential where condensation or carry-over is possible.
- RF-GMS-170 membrane separator — PTFE hydrophobic membrane, absolute liquid barrier. Used as a final protection stage before the analyser inlet, particularly in wet gas or two-phase flow services.
For ultra-trace analysis (ppb-level measurements), SilcoNert-coated variants of the RF-P and RF-C elements are available, minimising surface adsorption of reactive species. Consult the process gas filtration range for housing and element compatibility data.
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Comparing Filter Configurations for Common Analyser Applications
The table below summarises recommended filter configurations for the most common process analyser applications. All configurations reference R+F membrane separators and element types available from R+F FilterElements.
| Application | Primary Filter | Secondary / Final Stage | Housing |
|---|---|---|---|
| Natural gas / pipeline | RF-C coalescing | RF-GMS-170 membrane | RF-H-150 (100 bar) |
| Refinery process gas | RF-P particulate | RF-C coalescing | RF-H-160 (250 bar) |
| Wellhead / high-pressure | RF-P particulate | RF-GMS-170 membrane | RF-H-170 (400 bar) |
| Instrument air / purge | RF-C coalescing | RF-AC adsorption | RF-H-150 or inline |
| Semiconductor / ultra-pure | RF-P (SilcoNert) | RF-GMS-170 membrane | RF-H-110 (SS, 100 bar) |
Use our free Engineering Tool to get a filtration recommendation for your specific application in under 2 minutes.
Inline Filters for Point-of-Use Analyser Protection
Where a full sample conditioning skid is not justified — for example, on a secondary analyser or a portable instrument — a disposable inline filter offers a compact, low-cost alternative. The RF-DIL disposable inline filter provides particulate protection at the analyser inlet without requiring a separate housing or element change-out procedure. The entire unit is replaced at the service interval, eliminating the risk of contamination during maintenance.
For applications requiring moisture removal at the point of use, the RF-DIA inline adsorber (molecular sieve or activated carbon fill) can be installed in series with the RF-DIL. This combination is widely used on portable gas analysers and temporary monitoring installations where a permanent sample conditioning system is not available.
Maintenance Considerations for Sample System Filters
Sample system filters operate in demanding conditions — often at elevated pressure and temperature, with process gases that may be corrosive, toxic, or flammable. Maintenance intervals must be based on actual differential pressure monitoring rather than fixed time schedules, as contamination load varies significantly between process taps and operating conditions.
Key maintenance points for sample system filters:
- Monitor differential pressure across each filter stage. A rise above 0.5 bar typically indicates element replacement is due.
- Check auto-drain function on coalescing filter sumps at each inspection. A blocked drain will cause liquid carry-over to the analyser.
- Replace membrane separators (RF-GMS-170) if liquid breakthrough is observed — a wetted PTFE membrane cannot be regenerated by drying.
- For sour gas (H₂S) services, specify K-type elements with FKM seals. Standard NBR seals are not compatible with H₂S above trace concentrations.
For guidance on element selection and replacement schedules, the coalescing vs particulate filter elements guide provides a detailed comparison of element types and their appropriate applications.
- A sample system filter sits between the process tap and the analyser inlet, conditioning the extracted gas stream so that it meets the analyser's inlet specification.
- In a fast-loop design, a high-velocity bypass stream is drawn from the process line and returned downstream, with a small slip stream diverted to the analyser.
- Element selection for sample systems differs from standard compressed-air filtration in one critical respect: adsorption.
- The table below summarises recommended filter configurations for the most common process analyser applications.
Regulatory and Standards Compliance
Sample system filters for process analysers are typically specified to meet ISO 8573-1 cleanliness classes for the instrument air supply, and to comply with ATEX or IECEx requirements where the sample gas is flammable. R+F FilterElements housings are available with ATEX-rated pressure relief and stainless steel construction suitable for Zone 1 and Zone 2 installations.
For oxygen service, EPDM-O₂ seals and oxygen-compatible element binders are mandatory. The oxygen filtration safety guide covers the specific requirements for filters in oxygen-enriched gas streams.
Related Reading
- Coalescing vs Particulate Filter Elements — Which Do You Need?
- ISO 8573-1 Compressed Air Quality Classes Explained
- Hydrogen Electrolysis Filtration — Protecting Electrolyser Stacks
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