What to Do When the Actual Tower Diameter Does Not Match the Original Drawing
Tower internals are normally fabricated from approved vessel drawings.
However, retrofit projects frequently reveal a difficult reality:
the actual tower dimensions do not exactly match the old drawing.
The difference may be only a few millimeters, or it may be large enough to prevent an internal from fitting.
This becomes especially important for:
- liquid distributors;
- support grids;
- mist eliminators;
- structured packing;
- hold-down grids.
A supplier should never assume that an old drawing automatically represents the current vessel.
Why Drawing and Field Dimensions Can Differ
Several reasons are common.
Fabrication tolerance
Large vessels are not perfectly cylindrical.
The internal diameter may vary slightly around the circumference.
Corrosion or lining
An old carbon-steel tower may have:
- corrosion allowance loss;
- rubber lining;
- FRP lining;
- coating;
- internal repair layers.
These can change the usable internal diameter.
Historical modification
The tower may have been repaired or modified during previous shutdowns without updating the original drawing.
Measurement location
The tower diameter near one elevation may differ from another because of:
- deformation;
- weld seams;
- internal rings;
- local reinforcement.
Therefore, “tower diameter” should not always be treated as one perfect number.
Which Dimension Should Control Fabrication?
For retrofit internals, the actual usable field dimension is usually more important than the historical nominal diameter.
However, one measurement is not enough.
For large internals, the supplier should ideally receive measurements at several directions.
For example:
- 0°–180°;
- 90°–270°;
- additional diagonal directions if possible.
This helps determine whether the tower is slightly oval.
Why Small Differences Matter
Random packing itself may tolerate dimensional variation.
A fabricated internal may not.
For example, if a distributor is designed too close to the nominal shell diameter, it may:
- fail to enter the final position;
- contact the wall;
- distort during installation;
- lose leveling capability.
If the internal is too small, excessive wall clearance may create:
- liquid bypass;
- gas bypass;
- poor sealing.
The design therefore needs controlled installation clearance.
Structured Packing Requires Special Attention
Large structured packing layers are divided into segments, but the assembled layer still needs to fit the real cross-section.
If the vessel is oval, segments fabricated from an ideal circular drawing may require field modification.
That creates unnecessary installation risk.
For replacement projects, a dimensional survey is therefore highly valuable before production.
Mist Eliminators Need Wall Sealing
A mesh or vane-type demister normally needs effective sealing around the tower wall.
If the actual diameter is larger than expected, gas may bypass the demister at the perimeter.
If the assembly is too large, installation may become impossible.
The supplier should therefore confirm both:
- final assembled diameter;
- wall-sealing method.
What If the Difference Is Discovered After Fabrication?
The available options depend on the design.
Possible field solutions may include:
- adjustable perimeter seals;
- modified edge plates;
- trimming selected segments;
- replacing certain frame sections.
However, field modification should be the exception, not the standard plan.
It is usually cheaper to verify dimensions before manufacturing.
How Much Clearance Should Be Used?
There is no universal clearance value for every tower internal.
The correct allowance depends on:
- vessel diameter;
- internal type;
- material;
- thermal expansion;
- sealing requirement;
- manufacturing tolerance.
For example, a plastic internal may require different thermal-expansion allowance from a stainless-steel internal.
Therefore, suppliers should avoid using one fixed rule for all projects.
A Useful Retrofit Measurement Sheet
Before fabrication, record:
- Actual tower ID at installation elevation.
- ID in multiple directions.
- Support-ring ID.
- Support-ring elevation.
- Manway dimensions.
- Nearby nozzles.
- Internal obstructions.
- Existing lining thickness.
- Photographs.
- Measurement reference points.
This information can prevent a large amount of rework.
Final Engineering Principle
For new construction, approved fabrication drawings may be sufficient.
For older retrofit towers, field dimensions should verify the drawing before custom internals are manufactured.
The objective is not merely to know the nominal tower diameter.
It is to understand the actual installation envelope.