Demister Installation Types — Top, Bottom, Drawer and Segmented

Installation style is a vessel-access decision as much as a process decision. The correct process pad still fails the outage if it cannot pass the manway.

Choose from the maintenance path backwards

Before selecting a construction, trace the complete route from the shipping case to the final support ring. Check lifting limits, platform access, vessel nozzle or manway clear opening, internal obstructions, turning radius, support elevation and the safe working position for the installation crew. Nominal vessel diameter and nominal manway DN do not answer these questions.

The expected maintenance method matters too. A pad that is removed every turnaround may justify smaller, clearly numbered sections or a drawer arrangement. A rarely accessed clean-service pad may favor a simpler fixed support. The process engineer, vessel engineer and maintenance team should agree on the concept before the supplier fixes section geometry.

Top-installed

Pads lowered from above onto a support ring or beams. Common when the vessel top is open during turnaround. Needs a clear lifting path and correct support elevation.

Confirm whether a removable head, large top flange or construction-stage opening actually remains available for future replacement. A pad installed before final vessel closure is not automatically maintainable through the operating manway.

Bottom-installed

Pads installed from below against an upper retainer when top access is restricted. Retention hardware and wall seals matter — edge bypass destroys efficiency even if the mesh is correct.

The assembly must resist its own weight, vapor uplift and the agreed liquid or upset load. State who supplies studs, clips, grids and wall seals; otherwise a complete pad may arrive without the site-specific attachment parts.

Drawer / cassette style

Horizontal withdrawal of pad sections. Useful where frequent inspection is expected and axial space allows guide frames. Confirm frame material matches the process.

China fabrication reference: HG/T 21586-1998 (withdrawable wire mesh mist eliminator) is the companion reference to HG/T 21618 for this style. Its stated scope covers withdrawable elements for DN300–DN5000 equipment at atmospheric pressure, including service where elements require frequent cleaning, inspection or replacement. The standard’s background and selection appendix address sulfuric acid plants. China’s 2024 industry-standard review listed HG/T 21586-1998 for revision; until a replacement is formally published, the 1998 text remains the current controlled reference here. It is fabrication language, not a substitute for the process design basis, and it does not cover a pressurized-vessel design by implication.

Segmented pads for manway entry

Any pad that cannot pass the available access must be shipped in sections. Section width is set by the manway’s actual clear opening, the insertion path, lifting clearance and safe handling limits — not by a universal catalogue width or nominal DN. Sections assemble inside the vessel with grids and tie wires. State the actual clear opening on the RFQ, account for flanges and davit hardware, and confirm that each section can rotate through the full insertion path. A one-piece multi-metre pad is normally unsuitable unless the vessel provides a verified installation opening and lifting route.

Measurements that prevent a fit-up failure

Record dimensions from a common datum and show where each was taken:

• vessel inside diameter at the actual support elevation, including out-of-round readings;
• support-ring inside diameter, width, thickness, interruptions and flatness;
• distance from support to hold-down points and nearby nozzles, beams or distributors;
• manway clear width and height after accounting for the neck, flange, hinge, davit and insulation;
• narrowest part of the full internal transfer route and the space available to rotate a section;
• existing pad thickness, grid arrangement, flow direction and installed compression;
• required wall clearance and the proposed bypass seal.

Photographs should include a scale and orientation. A hand sketch with datums is more useful than many close-ups with no location. Where dimensional uncertainty remains, keep the drawing as a manufacturing proposal for approval rather than calling it an exact duplicate.

Verify on drawings

• Support ring elevation and open area
• Hold-down / anti-lift details
• Bypass seals at the wall
• Drainage path to avoid re-entrainment
• Section numbering for reassembly

Installation drawing and packing controls

The controlled drawing should show section outlines, match marks, top and bottom orientation, lifting or handling points where applicable, grid joints, tie-wire locations, wall seals and excluded site hardware. Section marks on the drawing, product and packing list should agree.

Thin grids and large sections can distort during road or sea transport. Define rigid support, moisture protection, stack limits and handling labels in the quoted scope. Packing does not correct an installation design, but it protects the verified geometry until the outage.

Incomplete drawings are normal on replacement jobs — vessel sketches and old pad photos are enough to start a controlled comparison, not enough to guarantee fit. The replacement-without-drawings guide gives the evidence sequence, and the inspection checklist provides a field capture list.

Standards and responsibility boundary

HG/T 21586-1998 provides controlled fabrication language for the withdrawable construction within its stated scope. It does not approve a vessel alteration, define a pressure-retaining attachment or replace the site’s lifting, confined-space and installation procedures. The vessel owner remains responsible for the safe access plan and any pressure-boundary work; the demister purchase should clearly separate supplied internals from site-installed supports and vessel modifications.