Why Edge Sealing Is Critical in Mist Eliminator Performance

A mist eliminator may have the correct mesh density, pad thickness, material, and hydraulic design, yet still show poor outlet performance because of one apparently minor installation detail:

the seal between the separator and the vessel wall.

Gas does not care how much engineering effort went into the active demister area. It follows the path of lowest resistance.

If a gap exists around the perimeter of the mist eliminator, part of the gas can bypass the mesh or vane pack entirely.

The result is simple:

the separator may perform correctly where gas passes through it, but the vessel still shows liquid carryover because some gas never entered the separator.

This makes edge sealing one of the most important—and most underestimated—parts of mist eliminator installation.

Why Gas Prefers an Open Gap

A wire mesh pad or vane pack creates intentional flow resistance.

Even when pressure drop is low, the gas still loses energy as it passes through wires or changes direction between blades.

An open gap around the edge offers much less resistance.

Gas therefore redistributes toward that opening.

This effect can be disproportionate.

A gap representing only a small percentage of the vessel cross-sectional area may carry more than the same percentage of the total gas flow because the hydraulic resistance is much lower.

That means a seemingly minor fitting problem can have a much larger effect on outlet carryover than its physical size suggests.

Nominal Vessel Diameter Is Not Always the Real Diameter

One common cause of edge gaps is manufacturing the demister from the nominal vessel diameter.

The actual clear internal diameter may be different because of:

• rubber lining;
• FRP lining;
• internal coating;
• corrosion;
• vessel ovality;
• weld beads;
• previous repairs;
• support-ring geometry.

For retrofit projects, this difference becomes especially important.

A pad manufactured from an old drawing may not match the real vessel after years of operation.

The actual installation diameter should therefore be verified whenever practical.

Vessel Ovality Creates Uneven Gaps

Large process vessels are rarely perfectly circular.

Fabrication tolerances, thermal cycling, mechanical loads, and long-term operation can create slight ovality.

A circular demister manufactured to one measured diameter may fit tightly in one direction and leave a gap in another.

This can produce localized bypass.

Because gas follows the lowest-resistance region, the gap may become a concentrated flow path.

The resulting carryover may appear to come from “poor demister efficiency” even though the actual problem is geometric fit.

Segment Joints and Edge Sealing Are Connected

Large mist eliminators are normally divided into sections for manway installation.

The outer segments must fit the vessel perimeter while the inner segment joints must fit each other.

If the segmentation is poorly designed, installers may create one of two problems:

• segments are compressed together and leave an outer gap;
• segments fit the perimeter but leave internal joint gaps.

Both create bypass paths.

The complete assembly should therefore be designed as one continuous separation surface.

The objective is not simply to make every individual section fit.

It is to make the assembled separator cover the full active area without creating low-resistance shortcuts.

Soft Mesh Can Deform Away From the Wall

Wire mesh pads are flexible.

During installation, transport, or operation, the perimeter can deform.

If the pad is not adequately restrained, a gap can appear between the mesh and vessel wall.

Gas force may enlarge the opening.

This is one reason the support and hold-down arrangement matters.

The separator must remain in the intended position during operation, not only during initial installation.

Edge Bypass Can Be Hard to Detect From Differential Pressure

One might expect bypass to produce an obvious pressure-drop change.

Sometimes it does.

A large bypass path can reduce overall differential pressure.

But small or localized gaps may not change the measured vessel pressure drop enough to make the problem obvious.

The pressure taps normally measure an overall condition.

They do not show exactly how much gas is passing through each part of the separator.

A vessel can therefore have apparently reasonable differential pressure while still suffering from localized bypass.

Visual inspection remains important.

Why High-Efficiency Mesh Cannot Compensate

Suppose a mesh is capable of very high removal efficiency for the target droplets.

That performance only applies to gas that actually passes through the mesh.

If 10% of the gas bypasses the separator, improving the mesh itself cannot remove droplets from that bypass stream.

The solution is mechanical, not hydraulic.

This distinction prevents a common mistake:

replacing a standard mesh with a denser or thicker pad when the actual problem is perimeter sealing.

A denser mesh may even increase the resistance difference between the active pad and the bypass gap, encouraging more gas to take the open path.

How Edge Sealing Is Achieved

The exact method depends on material and vessel design.

Possible approaches include:

• close dimensional fit;
• flexible mesh overlap;
• gasketed edges;
• retaining frames;
• wall clips;
• specially shaped peripheral sections.

The solution should not create excessive compression of the active mesh.

The objective is to eliminate significant bypass while preserving the intended separator structure.

Plastic Demisters Need Special Attention

PP, PVDF, and other plastic demister systems may behave differently from stainless-steel assemblies.

Plastic structures can experience:

• thermal expansion;
• creep;
• lower stiffness.

A fit that appears correct at ambient temperature may change during operation.

Material movement should therefore be considered when designing frames and supports for hot corrosive service.

How to Diagnose Possible Edge Bypass

Possible indicators include:

• persistent carryover even at moderate load;
• low or unexpectedly stable pressure drop;
• carryover appearing immediately after installation;
• visible wall gaps;
• uneven wetting near the perimeter;
• old vessel lining that changes internal diameter.

Inspection should examine the entire circumference.

One small region may be responsible for a significant portion of bypass flow.

What Should Be Checked Before Fabrication?

For new or replacement projects, confirm:

• actual clear vessel ID;
• lining thickness;
• vessel ovality if known;
• support-ring geometry;
• segmentation arrangement;
• edge restraint;
• operating temperature;
• demister material.

Photos and field dimensions are particularly valuable for retrofit work.

Final Engineering Perspective

Mist eliminator efficiency depends on forcing the gas through the active separation structure.

If gas can escape around the perimeter, the performance of the mesh or vane pack becomes irrelevant to that bypass flow.

Edge sealing is therefore not a minor installation detail.

It is part of the hydraulic design of the separator.

A reliable mist eliminator must provide both high separation performance through the active area and controlled gas flow around its perimeter.