Demister Fouling and Clogging — Causes and Fixes

Clogging is one of the most common demister failure modes in wet scrubbers and flue gas desulfurization (FGD) towers. A plugged pad raises pressure drop, distorts flow, and can worsen downstream duct corrosion when mist breakthrough increases.

Fouling is a mechanism, not one material

Deposits can be hard crystalline scale, soft slurry solids, sticky polymer or hydrocarbon, corrosion product, biological material or a mixture. They can arrive in the gas, precipitate on the media, form by reaction or remain after incomplete washing. The useful fix depends on which mechanism is active.

Map the first deposit location before cleaning. An upstream face loaded uniformly, a hard band at a temperature transition, edge buildup and isolated clean channels each lead to different questions. Appearance is evidence, not a complete diagnosis; retain a representative sample where site rules permit.

Typical root causes

• Gypsum, ash or slurry solids bridging the mesh
• Incomplete reaction chemistry leaving sticky intermediates
• Velocity too low for the required inertial capture, or too high for stable drainage, causing flooding, re-entrainment and characteristic deposit patterns
• Wrong style: knitted mesh used where a vane pack was needed
• No wash / spray provision where solids are expected

Read pressure drop with other evidence

Instrument impulse lines and zero should be checked before the pad is condemned. A differential-pressure trend across a larger vessel section may include packing, trays or duct losses in addition to the demister.

What not to do

Ordering a much denser mesh “to catch more mist” often accelerates plugging. Density is not a substitute for solids handling design.

Do not clean aggressively before the failure is documented. High-pressure water, steam or chemicals can damage fine wire, deform polymer internals or erase deposit evidence. The owner should approve a cleaning method compatible with the media, residue and confined-space controls.

Practical responses

1. Confirm whether the duty is clean mist or solids-laden.
2. For solids-laden FGD/scrubber service, evaluate vane packs, hooked channels, or combined mesh + vane — and PP where metal corrodes.
3. Review wash sprays, drain pots and offline cleaning access.
4. If replacing in kind, document pressure-drop history and deposit composition before copying the old density.

Decide between cleaning, replacement and redesign

• Clean and return to service when media and supports remain mechanically sound, the deposit is removable and the operating cause has been corrected.
• Replace in kind when the previous run length was acceptable, duty is unchanged and the failure is ordinary wear or accidental damage.
• Change construction when recurring plugging is tied to media passage size, drainage, washability or material.
• Correct the process first when precipitation, reaction chemistry, upstream solids removal, foaming or liquid distribution is the controlling cause.

Changing to vane internals may improve open area and washability but can change droplet removal, height, supports and orientation. It is an engineering change, not a purchasing substitution.

Evidence package for the RFQ

Send photos of the fouled pad and a short process note (SO₂ scrubber, quench, evaporator, etc.). Include deposit analysis if available, location map, clean and end-of-run pressure drop, run length, gas and liquid cases, solids and chemistry, wash-system layout and history, old media construction, support survey and downstream symptoms. Style and material recommendations should follow the failure evidence, not a generic catalogue page.

Freeze the selected geometry, material, section plan, wash interface and inspection scope in the PI. A dimensional and material QA package can verify the replacement construction; it cannot guarantee that unchanged process chemistry will stop deposits.

Use the wet scrubber guide to review washing and the maintenance checklist to preserve operating and shutdown evidence.

Authoritative operating evidence

The U.S. EPA’s FGD inspection manual distinguishes scaling and soft solids deposition and links mist-eliminator inspection to pressure drop, slurry conditions and washing. Koch-Glitsch’s severe-service guidance explains the use of open vane paths and irrigation where wire mesh is at plugging risk. These sources support mechanism-based diagnosis, not a universal replacement interval or cleaning setting.