Demister Support and Hold-Down Requirements: What Must Be Defined Before Purchase?
A demister pad cannot function properly if it is not mechanically supported inside the vessel.
Yet RFQs often focus entirely on the mesh pad itself and leave the support arrangement undefined.
This can lead to one of the most common scope problems in demister procurement:
Who supplies the support and hold-down system?
Mesh Alone May Not Be a Complete Assembly
A wire mesh demister may require:
- lower support grid;
- upper restraining grid;
- tie wires;
- frame;
- clips;
- beams;
- vessel attachment points.
Depending on the equipment design, some of these may already exist.
Others may need to be included in the new supply.
The RFQ should say so.
Lower Support
The lower grid supports the weight of the mesh pad.
It should provide sufficient open area while preventing the mesh from sagging excessively.
Procurement should define:
- whether the support is new or existing;
- material;
- support elevation;
- interface dimensions;
- load requirements where relevant.
If the support is existing, the supplier may need drawings or measurements to ensure compatibility.
Upper Hold-Down
Gas flow can lift or disturb the mesh under certain conditions.
An upper hold-down grid or restraint system may therefore be required.
The buyer should not assume that it is automatically included with the demister pad.
Scope must be explicit.
Sectional Installation
For large vessels, both the mesh and support components may need to pass through a manway.
That means support grids may also require segmentation.
The RFQ should provide:
- manway size;
- maximum section dimensions;
- assembly method.
Otherwise, a full circular support could be fabricated but impossible to install.
Material Compatibility
The mesh, grid, bolts, clips, and vessel internals may use different materials.
This can create:
- corrosion concerns;
- galvanic compatibility issues;
- temperature limitations.
Procurement should therefore review the complete material combination rather than specifying only the mesh alloy.
Mechanical Load
The support system may experience loads from:
- mesh weight;
- liquid holdup;
- process upset;
- maintenance handling.
For large demisters, structural adequacy may require engineering review.
The buyer should identify whether the supplier is responsible only for fabrication or also for mechanical design.
Attachment Interface
A support system must connect to something.
The tower may have:
- support ring;
- brackets;
- beams;
- clips.
The RFQ should provide interface details.
Missing interface information is one of the main reasons shop drawings require repeated revision.
Drawing Approval
Before fabrication, the supplier drawing should show:
- support layout;
- hold-down arrangement;
- segment configuration;
- attachment points;
- material;
- dimensions.
This gives the buyer an opportunity to detect scope gaps early.
Inspection
Factory inspection can verify:
- dimensions;
- material;
- fabrication;
- fit-up;
- segment marking;
- fastening components.
Where possible, trial assembly can be useful for complex sectional systems.
Engineering Takeaway
A demister should be purchased as a defined mechanical system, not simply as a quantity of mesh.
Support, hold-down, attachment, segmentation, and scope boundaries should all be resolved before fabrication.