Retrofitting a Packed Tower with an Out-of-Round Shell

An existing packed tower is rarely as geometrically perfect after years of operation as it appeared on the original fabrication drawing. Thermal cycling, foundation movement, fabrication tolerances, mechanical loads, or previous repairs can leave the shell slightly oval rather than perfectly circular.

For random packing, a small degree of shell ovality may cause few obvious problems. For close-fitting structured packing, liquid distributors, support grids, collectors, and mist eliminators, however, the difference between nominal and actual diameter can determine whether a replacement internal can be installed at all.

A brownfield retrofit should therefore treat shell ovality as an engineering input rather than an inconvenient field deviation.

Why Nominal Tower Diameter Is Not Enough

A tower described as 2400 mm inside diameter does not necessarily measure 2400 mm at every orientation or elevation.

One direction may measure slightly larger while the perpendicular direction is smaller. The deviation may also change with elevation.

This matters because replacement equipment is normally fabricated before the shutdown. If the manufacturer receives only the nominal diameter, a circular internal can arrive on site and interfere with the shell in one direction while leaving excessive clearance in another.

Field cutting then becomes the only solution, increasing shutdown time and damaging fabrication quality.

Measure the Diameter at the Installation Elevation

Shell measurements should be taken where the new internal will actually be installed.

A diameter measured near the bottom manway may not represent conditions ten meters higher.

For critical internals, measure several directions across the shell rather than a single diameter.

The survey should record both the maximum and minimum clear internal dimensions.

Where lining is present, measurements should be taken from the finished lining surface rather than from the steel shell.

Structured Packing Is Particularly Sensitive

Structured packing is normally fabricated as blocks or segments intended to fill the column cross-section with controlled wall clearance.

Excessive clearance can encourage gas or liquid bypass.

Insufficient clearance can prevent installation or crush the packing edges.

A retrofit design may therefore require:

• adjusted segment geometry;
• flexible wall sealing;
• controlled field clearance;
• smaller individual blocks.

The objective is not to force the tower back into an ideal circle. It is to make the new packing work within the geometry that actually exists.

Distributor Fit Must Also Be Checked

Large trough, pan, or pipe distributors frequently span most of the tower diameter.

If the vessel is oval, distributor supports and peripheral clearances can become uneven.

This may create two different problems.

First, the distributor may physically interfere with the shell.

Second, a distorted support arrangement may make the distributor difficult to level.

Because gravity distributors depend on liquid head, mechanical fit and hydraulic performance are connected.

Check Support Rings Separately

The shell may be out of round while an existing support ring has its own distortion.

Do not assume that measuring the shell automatically defines the usable support diameter.

Record:

• support-ring inside opening;
• seating width;
• high and low points;
• local distortion;
• weld projections.

The new internal must interface with the support that actually carries it.

Avoid Excessive Field Clearance

A common reaction to uncertain dimensions is to make replacement internals much smaller than the vessel.

That improves installation tolerance but can create hydraulic bypass.

Large gaps around:

• structured packing;
• mist eliminators;
• collectors;
• distributors

can allow gas or liquid to avoid the intended flow path.

Retrofit tolerance should therefore be controlled, not simply maximized.

Use Segmentation Strategically

Sectional construction helps accommodate non-ideal vessel geometry.

Smaller panels can be positioned independently and are easier to adjust during installation.

However, excessive segmentation introduces more:

• joints;
• hardware;
• assembly time;
• potential leakage or bypass points.

The ideal arrangement uses enough segmentation to accommodate the actual shell without creating unnecessary installation complexity.

Consider Thermal Condition During Measurement

Tower dimensions can change slightly with temperature.

Field measurements are commonly taken during shutdown at ambient conditions, while the equipment operates hot.

For very large towers or installations with temperature-sensitive clearances, thermal expansion should be considered.

This does not mean the internal must match the hot geometry exactly. It means the designer should ensure that operating expansion will not create binding or damaging contact.

Check Whether Ovality Indicates a Larger Mechanical Problem

A small fabrication tolerance is different from significant structural deformation.

If shell distortion is substantial or appears to have developed during service, qualified vessel engineering may be required.

Tower-internal suppliers should not treat serious shell deformation merely as an internal-fitting issue.

The pressure vessel itself must remain mechanically acceptable.

Build the Actual Geometry into the Retrofit Drawings

The final fabrication package should clearly distinguish:

• nominal vessel dimensions;
• measured dimensions;
• design clearances;
• field-adjustable interfaces.

This prevents manufacturing personnel from “correcting” the design back to nominal dimensions.

Why This Matters for Retrofit Reliability

The best-performing tower internal is useless if it cannot be installed without uncontrolled site modification.

Conversely, an internal made excessively small simply to guarantee installation may create years of hydraulic bypass.

Successful brownfield engineering finds the middle ground: enough tolerance for the real vessel and enough fit to preserve process performance.