How to Design Tower Internals for Seismic and Horizontal Inertial Loads

Tower internals are often designed carefully for vertical operating weight and differential pressure but treated as insignificant during a seismic review. That assumption can be unsafe. Packing beds, liquid-filled collectors, support beams and segmented trays all have mass. When the vessel accelerates laterally, that mass generates horizontal reactions, local bearing forces and connection loads that must reach the shell without allowing panels to slide, impact or disengage.

Start with the Project Seismic Basis

The internal supplier should not invent a generic acceleration value. Obtain the project-defined horizontal and vertical seismic demands, applicable load combinations, installation elevation, vessel orientation and required analysis method from the vessel or structural engineer. The acceleration experienced by an internal may differ from the ground acceleration because the vessel amplifies motion along its height.

Clarify the condition to be analyzed: empty vessel, normal operation, shutdown with retained liquid, fouled condition, hydrotest or another project-defined state. Combining maximum liquid inventory, maximum deposits and an unrelated seismic extreme may be unnecessarily conservative, while ignoring credible retained liquid can be unconservative. Each included mass needs a documented basis.

Build a Complete Seismic Mass Inventory

Include the self-weight of trays, beams, collectors, distributors, supports, bed limiters and fasteners. Add packing mass, normal or credible maximum liquid holdup, deposits, insulation attached to internal components and any permanent piping supported from the internal.

Random packing behaves as a granular bed rather than a single rigid block. Structured packing is assembled from blocks or layers with joints and wall clearance. The design must define how lateral force transfers from the packing to wall wipers, retaining rings, support grids and beams. It is not sufficient to multiply total bed mass by acceleration if the resulting reaction has no credible physical path.

Liquid on trays and in collectors can move relative to the metalwork. A simplified equivalent mass may be adequate for some projects, while large liquid inventories or unusual geometries may require a more detailed sloshing or dynamic assessment. The vessel engineer should define the level of analysis expected.

Trace Horizontal Loads to the Shell

A typical load path may run from packing into a perimeter restraint, through a support grid into beams, and from beam seats into shell rings or clips. At every interface, check sliding, local bearing, bolt shear, weld load and the possibility that clearance permits impact before restraint engages.

Support beams designed only for vertical bending may lack lateral stability. Thin tray panels may rack if their clamps resist uplift but not in-plane movement. Removable manway panels and access doors are frequent weak points because their connection pattern differs from surrounding panels.

Shell attachments require reactions for both principal horizontal directions. An asymmetric beam layout, off-center downpipe or partial collector can generate torsion as well as direct shear. The internal vendor should return reactions in a form that the vessel designer can use, rather than simply marking the assembly “seismic suitable.”

Control Movement Without Preventing Thermal Expansion

Seismic restraint and thermal movement can conflict. An internal may need to slide during heat-up but remain captured during a lateral event. Use defined fixed points, guided supports, keeper plates, slotted connections or clearances that provide both functions.

Do not eliminate every gap without checking thermal growth. Conversely, do not use an expansion slot so long that a beam seat can walk out of engagement. Keeper geometry should remain effective at the maximum cold and hot positions. Nonmetallic internals need special attention because lower stiffness, creep and larger thermal expansion can change restraint clearance over time.

Check Vertical Acceleration and Load Reversal

Vertical seismic acceleration can increase or reduce gravity reactions. When combined with pressure transients or buoyancy, it may create uplift at a connection normally loaded downward. Bed limiters, tray clamps, beam keepers and ceramic support contacts should be checked for load reversal where required by the project basis.

Brittle ceramic components should remain in compression and should not be used to bridge an unintended lateral gap. Plastic or FRP panels require broad bearing areas so impact does not create local cracking or permanent deformation.

Avoid Common Design Shortcuts

Several shortcuts cause recurring problems:

checking only the largest support beam and ignoring small removable panels;

treating packing as attached to the shell without showing restraints;

applying seismic force but omitting operating liquid or deposits;

providing shell reactions for only one direction;

using friction as the sole restraint without an approved coefficient and normal force;

adding rigid stops that block thermal expansion;

assuming vessel-code compliance automatically covers vendor internals.

The analysis should match the actual installed configuration, including field splices, segment joints, access openings and clearances.

Fabrication and Installation Checks

Drawings should identify fixed points, sliding points, lateral keepers, engagement dimensions and orientation. Shop inspection should verify keeper gaps, beam-seat lengths, bolt grades and weld sizes. Field inspection must confirm that shims, gaskets or lining thickness have not reduced required engagement.

Match-mark asymmetric components and record final clearances. If field trimming or relocation changes a restraint, obtain an engineering disposition rather than accepting the change as a fit-up adjustment. After a significant seismic event, inspect accessible joints, displaced packing, cracked nonmetallic parts, opened wall gaps and permanent beam movement before relying only on overall tower pressure drop.

Procurement Data and Deliverables

An RFQ should state the seismic design basis, load combinations, operating masses, fouling allowance, allowable shell reactions, material temperature and required calculation format. Vendor deliverables should include the mass table, load paths, connection checks, support reactions, movement clearances and inspection dimensions.

Engineering Takeaway

Seismic design of tower internals is a load-path problem. Every horizontal and reversed vertical force must pass through identifiable restraints and attachments while preserving the movement required for operation.