How to Reverse-Engineer Existing Tower Internals When Drawings Are Missing

Older process towers often remain in service long after their original fabrication drawings, supplier records, or internal datasheets have disappeared.

During a shutdown, the plant may discover that it needs to replace:

• a liquid distributor;
• packing support grid;
• collector;
• redistributor;
• demister frame;
• hold-down grid;

but nobody can locate an approved drawing.

The maintenance team may have only:

• photographs;
• rough dimensions;
• an old equipment tag;
• the physical internal inside the tower.

At that point, purchasing a “similar” replacement is risky.

The safer approach is to reverse-engineer the existing internal systematically.

The goal is not merely to copy what the old component looks like.

It is to establish enough verified information to manufacture a replacement that:

fits the vessel, interfaces with existing supports, preserves the intended process function, and can actually be installed through the available access.

1. Begin With the Function, Not the Shape

Before measuring anything, identify what the internal actually does.

Ask:

• Is it a liquid distributor?
• Collector?
• Redistributor?
• Packing support?
• Bed limiter?
• Demister support?
• Feed device?

Two internals can look similar but have different functional requirements.

For example, a perforated plate may be:

• a support;
• distributor;
• gas/liquid collector component.

Copying geometry without understanding function can reproduce an old problem.

2. Establish the Tower Reference System

Every measurement needs a reference.

Record:

• tower tag;
• vessel north or 0° reference;
• relevant manway;
• installation elevation;
• shell internal diameter.

Without a common reference system, dimensions taken by different people may not fit together into one usable drawing.

3. Record the Installation Elevation

Determine where the internal sits relative to a reliable vessel datum.

Possible references include:

• tangent line;
• known nozzle centerline;
• support ring elevation;
• approved vessel benchmark.

Do not simply write:

“approximately 1.5 m below the manway.”

That may be useful for locating the part, but not for fabrication.

4. Measure the Real Tower Diameter

An old drawing may call the vessel:

2400 mm ID.

The actual shell may not be exactly round.

Measure several directions where possible.

Record:

• minimum ID;
• maximum ID;
• lining thickness;
• local obstructions.

The replacement must fit the real installation space.

5. Measure the Support Interface

For a support grid or collector, the surrounding shell diameter is only part of the problem.

Measure:

• support ring width;
• ring inside diameter;
• ring elevation;
• support-beam layout;
• beam width;
• clip location;
• bracket dimensions.

A replacement internal must transfer load into the same support system unless the support system is also being redesigned.

6. Measure the Complete Overall Geometry

Depending on the internal, record:

• overall diameter;
• length;
• width;
• height;
• panel thickness;
• trough depth;
• frame depth.

Avoid measuring only one convenient segment and assuming every other segment is identical.

Older internals may contain:

• edge pieces;
• special nozzle cutouts;
• asymmetric panels.

7. Map Every Segment

For segmented internals, create a plan view.

Assign temporary identifiers:

ABCD

or:

1234.

For each segment record:

• dimensions;
• orientation;
• connection to adjacent segment;
• support location;
• bolt pattern.

Photographs should include the identifier in the image where practical.

8. Measure Bolt Holes and Joint Interfaces

For bolted assemblies, record:

• bolt diameter;
• hole diameter;
• hole spacing;
• edge distance;
• overlap;
• gasket arrangement;
• washer configuration.

Do not only measure the finished assembly diameter.

If the replacement segments cannot bolt together, correct overall diameter will not save the project.

9. Map Nozzle and Vessel Interference

Record nearby:

• feed nozzles;
• drains;
• instrument penetrations;
• thermowells;
• beams;
• clips;
• manways.

Measure both:

• elevation;
• angular position.

This is particularly important for distributors and collectors.

10. Determine the Material

Visual appearance alone may not prove material grade.

Where material is important, investigate using appropriate:

• historical documentation;
• PMI;
• laboratory analysis;
• project records.

Do not assume every old stainless-looking component is SS316L.

11. Measure Sheet and Plate Thickness Carefully

Old equipment may have lost thickness through:

• corrosion;
• erosion;
• chemical attack.

A measurement from a corroded area may not represent the original design thickness.

Take several measurements where practical and distinguish:

existing remaining thickness

from

probable original nominal thickness.

The replacement does not necessarily need to copy corrosion loss.

12. Distributor Reverse Engineering Needs Hydraulic Data

For a liquid distributor, geometry alone is not enough.

Record:

• number of outlets;
• outlet diameter;
• outlet type;
• outlet elevation;
• distribution-point pattern;
• feed arrangement;
• trough dimensions.

Then compare those measurements with actual operating flow.

If the old distributor caused poor distribution, simply copying every detail may reproduce the same limitation.

13. Collector Reverse Engineering Needs Gas-Passage Information

For collectors and redistributors, record:

• deck area;
• gas risers;
• riser size;
• riser quantity;
• liquid collection path;
• draw-off point.

The internal must handle both:

• gas passage;
• liquid collection.

14. Support Grids Require Load Information

When recreating a support grid, determine:

• packing material;
• packing bulk density;
• bed height;
• tower diameter;
• expected liquid and fouling loads.

An old support can be measured, but the replacement should still be checked against the current service.

15. Demister Replacement Requires More Than Frame Size

For a mesh or vane demister, record:

• diameter;
• thickness;
• segment arrangement;
• frame;
• support;
• hold-down;
• flow direction.

If the process duty has changed, the separation element itself may require re-evaluation.

16. Manway Dimensions Must Be Included

Before finalizing segmentation, measure the actual clear opening:

• width;
• height;
• neck depth.

Also note:

• platforms;
• piping;
• internal obstructions.

A replacement piece that matches the installed diameter but cannot enter the tower is unusable.

17. Photograph With Scale References

Photographs become far more useful when they contain:

• measuring tape;
• ruler;
• segment tag;
• orientation arrow.

Take:

• overall views;
• joint details;
• supports;
• nozzle interfaces;
• damaged areas.

Do not rely on one wide-angle photo.

18. Separate “Copy Exactly” From “Improve the Design”

At the end of the survey, decide whether the objective is:

Like-for-Like Replacement

Preserve the existing configuration as closely as practical.

Fit-Compatible Upgrade

Maintain interfaces but improve selected design features.

Full Revamp

Reassess both mechanical and hydraulic design.

This distinction should be agreed before the supplier starts drawing.

19. Convert Field Notes Into a Controlled Drawing

Do not manufacture directly from scattered photographs and WhatsApp measurements.

Create a controlled drawing showing:

• tower ID;
• orientation;
• dimensions;
• elevation;
• segmentation;
• material;
• interfaces;
• revision.

Then obtain engineering confirmation.

20. Mark Uncertain Dimensions

If a critical dimension cannot be measured accurately, do not invent it.

Mark it:

TBC — To Be Confirmed

or

Field Verification Required.

One clearly identified unknown is safer than one false precise number.

Reverse-Engineering Workflow

Identify Function↓Establish Tower Datum↓Measure Shell and Support Interfaces↓Map Segments↓Measure Connections↓Map Nozzles and Obstructions↓Verify Material and Thickness↓Record Process-Critical Features↓Check Manway Access↓Create Controlled Drawing↓Engineering Review↓Fabrication

Engineering Takeaway

Reverse engineering a tower internal is not simply:

measure the diameter and copy the shape.

A reliable replacement requires:

Function + Geometry + Elevation + Orientation + Support Interface + Material + Segmentation + Access + Process Duty

When original drawings are unavailable, the physical tower becomes the primary source of truth—but the field data still need to be converted into a controlled engineering basis before manufacturing begins.