Use filtration in stages: a coarse filter or probe filter near the tapping for bulk dust, a finer filter after the cooler, a coalescing filter to remove liquid mist and oil aerosol, and a membrane filter as a final guard. ACCURRA-S includes an internal fine filter and a rotameter, and expects dust below 3 micron. A blocking filter shows as falling flow or pressure drop.
- Name the filter types and what each removes
- Order filters correctly along the sample line
- Recognise a blocking filter before it ruins a reading
- Plan filter replacement
Filter types
| Filter | Removes | Typical position |
|---|---|---|
| Probe filter, sintered | Coarse dust at the tapping | In the probe, with blowback if dusty |
| Particulate filter | Fine dust | After the cooler, before the analyser |
| Coalescing filter | Liquid droplets, oil aerosols and fine mist | Before the analyser on compressed or oily gas |
| PTFE membrane filter | Fine particles and aerosols; repels water | Last guard before the analyser |
Headline Filters, UK, whose range Sarvesh represents in India, supplies particulate, coalescing and PTFE membrane filters for analyser protection: headline-filters.
Order matters
- Remove the bulk first, with the coarsest filter nearest the tapping.
- Cool the gas, remove condensate, then fine-filter, because droplets wet and blind a fine filter.
- Finish with a membrane or fine filter immediately before the analyser.
- Never put a fine filter where liquid water can reach it without a coalescer or drain.
What the ACCURRA-S expects
The catalogue sample specification calls for a non-corrosive, non-toxic, non-flammable, non-condensing and dry sample, free of entrained oil, with dust below 3 micron. Every ACCURRA-S has an internal fine filter and a rotameter, but that filter is the last line of defence, not the only one. Where the sample falls short of the specification, add conditioning in front: How to design a gas sample handling system.
Sizing and replacement
- Size by flow and by dust load. A small filter in a dusty duct clogs fast.
- Fit a differential pressure gauge or watch the flow; a falling rotameter reading is the first sign of blocking.
- Replace elements on a schedule set by experience at the site, not only when the reading fails.
- Keep spare elements; see Maintenance schedule and spares for gas analysers.
Cleaning a blocked filter with compressed air and refitting it. Particles can be driven deeper and the element rarely recovers. Replace it.
Filters that remove the very thing you measure
Filters can adsorb or react with some gases, especially water and ammonia. Choose materials suited to the target gas and keep surfaces to a minimum for trace moisture.
- Filter in stages: coarse near the tapping, fine after the cooler, membrane last.
- Remove liquid before the fine filter.
- Falling flow or rising pressure drop signals blocking.
- Replace elements on a schedule and keep spares.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
Why filter coarse first and fine last?
Fine filters would clog with bulk dust; the coarse filter protects them.
What does a coalescing filter remove?
Liquid droplets and oil aerosol.
What is the earliest sign of a blocking filter?
Sample flow dropping on the rotameter, or pressure drop rising.
Key terms
Frequently asked questions
How often should analyser filters be changed?
By experience at the site; start with a regular interval and shorten it if the pressure drop or flow shows blocking.
Can I clean and reuse a filter element?
Not reliably. Replace it.
What dust size can ACCURRA-S accept?
The catalogue specifies dust below 3 micron.
Who supplies the filters?
Sarvesh represents Headline Filters, UK, in India.