Why Perimeter Seal Gaps Cause Vapor or Liquid Bypass Around Tower Internals

Some tower internals are designed to collect, contain or redirect an entire process stream. For these devices, a gap between the internal and the tower wall is not merely a fabrication imperfection. It becomes an unintended hydraulic path.

Perimeter leakage can allow liquid to miss the distributor below or let vapor bypass the intended risers. The result may be poor separation even when the packing and the visible center area of the internal appear undamaged.

Which Internals Require Perimeter Sealing?

Perimeter sealing is commonly important for:

• Deck liquid distributors that use the tower shell to retain liquid
• Total liquid collectors
• Chimney trays
• Collector-redistributor assemblies
• Certain draw trays
• Cartridge-type contacting devices
• Internals designed to force vapor through defined risers

Not every tower internal requires a gas-tight or liquid-tight perimeter. Open trough distributors and packing support grids normally retain large open areas by design.

The required seal depends on the function. The drawing and specification must state whether the boundary is intended to be liquid-tight, vapor-restricting or simply positioned close to the shell.

How Perimeter Gaps Develop

Typical causes include:

• Tower-shell ovality
• Local distortion near nozzles and weld seams
• Incorrect measured tower diameter
• An unlevel or distorted support ring
• Missing or damaged gasket sections
• Insufficient gasket overlap
• Incorrect clamp spacing
• Loose perimeter hardware
• Panel installed in the wrong position
• Bolt holes left unsealed
• Thermal or mechanical movement
• Chemical degradation of the sealing material

A rigid circular internal cannot automatically seal against a shell that is not perfectly circular. The sealing arrangement must accommodate the specified diameter tolerance and local wall irregularity.

Liquid Bypass

On a deck distributor, a perimeter leak allows liquid to run directly down the tower wall instead of leaving through the designed outlet pattern.

On a total collector, leaking liquid may avoid:

• Mixing with the collected liquid
• A side-draw measurement
• A pump-around circuit
• The redistributor below
• The intended residence or disengagement zone

Even a moderate leak can be important at low liquid load because it represents a larger percentage of the total flow. The downstream packing may develop a heavily wetted wall region and a poorly wetted central region.

Vapor Bypass

A chimney tray or sealed collector normally directs vapor through engineered risers with defined free area.

If vapor can pass through a perimeter gap, the effective vapor-flow arrangement changes. Local velocity near the wall can rise, and vapor may interact with leaking liquid outside the intended contacting region.

Possible results include:

• Local entrainment
• Uneven pressure beneath the internal
• Disturbed liquid collection
• Reduced mixing effectiveness
• Poor downstream vapor distribution
• Lower apparent separation efficiency

A bypass can sometimes reduce measured pressure drop, so a “better” pressure-drop reading does not prove that the internal is operating correctly.

Seal the Complete Boundary

Inspection should cover more than the visible gasket between the internal edge and the shell. The complete leakage boundary includes:

• Perimeter gasket or seal strip
• Panel-to-panel joints
• Support-ring interface
• Clamp locations
• Bolt holes
• Corners and segment intersections
• Penetrations for pipes or instruments

The seal material must be compatible with process chemicals, temperature, cleaning method and expected compression. Too little compression permits leakage; excessive compression can damage the gasket or distort thin panels.

Testing and Acceptance

Where appropriate, a controlled leak or water-hold test can verify the assembled internal. The procedure must define the test liquid level, allowable leakage, observation points and drainage method.

The temporary test head must not exceed the structural capacity of the internal or support ring. Test water compatibility and complete post-test drainage must also be confirmed.

For an existing tower, shell measurements should be taken before manufacturing replacement internals. Assuming the original nominal diameter can produce an internal that is geometrically correct on paper but impossible to seal in the actual vessel.