Demister Inspection Checklist for Shutdowns and Replacement

A shutdown inspection should answer three questions before anyone reorders the same demister: did the pad fail because it was old, because the operating duty changed, or because the original selection was wrong?

This checklist is written for maintenance engineers, vessel inspectors, buyers and suppliers. It is not a substitute for a site safety procedure, confined-space permit or pressure-vessel code inspection.

Plan the evidence before the outage

Assign who will record process history, dimensions, material condition, photographs and samples. Prepare a drawing or simple location grid so every observation can be tied to a section and face. Agree which parts may be cleaned, cut or sampled and who owns the final disposition.

Collect operating evidence before shutdown: gas-flow range, pressure and temperature, differential pressure, downstream carryover symptoms, wash history, level or foaming events, run length and process changes since the last inspection. Physical evidence is much more useful when it can be compared with the event timeline.

1. Before entering the vessel

Confirm isolation, cleaning, gas test and confined-space permit according to site rules. Collect the vessel tag, service name, last operating pressure and temperature, and the reason for inspection: carryover, pressure drop, corrosion, routine turnaround or replacement without drawings.

Prepare site-approved measuring tools, marker, sample containers, camera, lighting and PPE suitable for the residue. Any electrical or battery-powered device must be approved for the site’s classified location where applicable; an ordinary phone, camera or flashlight must not be assumed safe. If the old pad will be copied, photograph it before removal.

Do not enter, move internals or collect samples until the owner has released the task under its applicable safety procedure. Residues may be toxic, corrosive, pyrophoric or biologically hazardous even after the vessel is open.

2. Photograph evidence

Take photos of the demister from below and above, the wall support ring, hold-down grid, tie wires, section joints, manway path and any labels. Include a scale or tape in the photo whenever possible.

For troubleshooting, photograph the failure pattern: edge bypass, center sagging, plugging layer, washed-clean channels, corrosion at grid contact points, missing clips or cracked plastic vanes.

Use one overall image, then medium and close views with the same location label. Keep original files and filenames. A close-up without orientation, scale or section identity is weak evidence for a replacement drawing.

3. Dimensional checks

Record vessel inside diameter, support ring inside diameter, pad thickness, number of sections, section width, manway clear opening, grid diameter, grid bar spacing and distance from liquid level or inlet device if visible.

Large pads are normally segmented for manway access. Do not assume a catalog diameter is enough: section size and lifting clearance often decide whether the replacement can be installed.

Take more than one vessel and support-ring diameter where out-of-roundness is possible. Record the datum, instrument and practical measurement uncertainty. Measure the narrowest verified access route, not only the manway face. Do not infer an unmeasured vessel modification from an old pad that may already be distorted.

4. Mechanical condition

Check for sagging, compression loss, torn mesh, broken grids, loose tie wires, displaced sections and bypass gaps at the vessel wall. A mesh pad that looks clean can still fail if vapor bypasses around the edge.

For vane packs, check vane deformation, blocked pockets, missing hooks, broken welds and spacing changes. For plastic internals, look for thermal distortion or chemical embrittlement.

Record whether damage is upstream, downstream, central, peripheral or associated with a support. Deformation in the flow direction, a localized impact and uniform sagging imply different load histories. Do not straighten a critical part before its original condition is documented.

5. Fouling and corrosion

Describe the residue: salt, polymer, oil, gypsum, catalyst fines, rust, scale or biological slime. If possible, keep a small dry sample for the process team.

Corrosion should be separated by location. Mesh corrosion, grid corrosion and support-ring corrosion can have different causes. Mixed materials can create galvanic couples when liquid films are conductive.

Label samples with vessel, location, face, date and collector. A deposit result should be interpreted with liquor chemistry and operating conditions; finding an element in a sample does not by itself establish the corrosion or fouling mechanism.

6. Decision rule

Reorder the same design only when the old pad gave acceptable life, the process duty has not changed and the failure is ordinary wear. If there is repeated flooding, plugging or corrosion, review velocity, K factor, mesh density, material, wash system and alternative internals.

Classify the outcome explicitly:

• return to service — construction is sound and any cleaning or minor correction is approved;
• replace in kind — duty and design remain suitable, with controlled dimensions and material;
• replace with engineering change — media, material, support, washing or segmentation changes;
• process action before replacement — upstream, chemistry, level, distribution or operating cause remains open.

An engineering change needs a controlled drawing and approval. Do not let a purchase order silently turn an inspector’s observation into an unreviewed redesign.

RFQ evidence to send

Send photos, dimensions, service chemistry, operating temperature and pressure, gas flow, liquid carryover concern, destination and document requirements. If data is incomplete, state what is missing; a transparent incomplete RFQ is easier to review than a guessed specification.

Include the inspection map, operating timeline, deposit or material results, old section plan, support and access survey, required outage date and the selected route above. Use the replacement-without-drawings guide when no controlled drawing exists and the corrosion guide when material failure is involved.

Safety source and jurisdiction limit

For U.S. workplaces, OSHA’s permit-required confined-space requirements illustrate why hazard evaluation, isolation, atmospheric control, trained roles and rescue arrangements precede inspection. Other jurisdictions and sites have their own controlling law and procedures. This guide deliberately does not prescribe entry limits or authorize vessel entry.