How to Build a Replacement Tower Packing BOM from GA Drawings and Site Measurements

A tower packing replacement purchase often begins with an old vessel drawing.

The drawing may show:

• tower diameter;
• several internal elevations;
• packed sections;
• distributors;
• support plates.

Purchasing still needs a practical Bill of Materials.

Turning a GA drawing into a replacement BOM requires more than calculating one cylinder volume.

Start With the Latest Reliable Drawing

Before calculation, confirm:

• drawing number;
• revision;
• equipment number;
• whether the drawing is as-built or original design.

An old construction drawing may no longer represent the actual tower.

Past revamps may have changed:

• bed height;
• internal elevation;
• distributor type;
• packing.

Identify Every Packed Section Separately

Do not combine all tower packing immediately.

Create individual entries:

Bed 1Bed 2Bed 3

Record for each:

• tower internal diameter;
• bottom elevation;
• top elevation;
• bed height;
• existing packing.

This prevents one arithmetic error from affecting the entire project.

Calculate Geometric Bed Volume

For a cylindrical packed section:

V=πD24HV = \frac{\pi D^2}{4}H

where:

V = packing volumeD = tower internal diameterH = packed height

Use the internal diameter applicable to that actual section.

Do not use vessel outside diameter.

Check for Diameter Changes

Some towers contain:

• different shell diameters;
• internal liners;
• transitions;
• local restrictions.

A single tower diameter may not apply to every bed.

Each section should be reviewed independently.

Distinguish Packing Height From Available Internal Space

The distance between two internals may not equal required packing height.

Space may be needed for:

• distributor clearance;
• gas disengagement;
• support structures;
• collectors;
• hold-down devices.

Use the actual specified bed elevation, not simply the distance between convenient drawing lines.

Add the Product Specification

Each BOM line should include:

• packing family;
• nominal size or model;
• material;
• required volume.

Example:

T-101 / Bed 150 mm SS316L Metal Pall Ring8.6 m³

For structured packing, include:

• model;
• material;
• total bed height;
• number of layers;
• segment drawing reference.

Do Not Convert Volume to Weight Too Early

The primary random-packing requirement is normally bed volume.

After the exact packing is confirmed:

Weight ≈ Volume × Bulk Density

If the replacement product changes, bulk density may also change.

Therefore the BOM should preserve the original m³ requirement rather than relying only on tonnes.

Verify Against Site Measurements

Before final purchase, compare drawing data with field information where possible.

Verify:

• internal diameter;
• support elevation;
• distributor elevation;
• actual bed height;
• manway size;
• existing packing.

This is particularly important for towers that have operated for many years.

Record Drawing-to-Site Differences

If site measurement conflicts with the drawing, do not silently choose one value.

Record:

Drawing: 2,400 mm IDSite measurement: approximately 2,385 mm

Then obtain engineering confirmation.

A small diameter difference can change:

• packing quantity;
• structured-packing segmentation;
• internal fit.

Include Spare Quantity as a Separate Line

Do not hide maintenance spare inside the calculated bed quantity.

Example:

Required bed quantity: 12.4 m³Installation/spare allowance: 0.6 m³Total purchase: 13.0 m³

This improves procurement clarity.

Internals May Need Their Own BOM

A packing replacement may also require:

• support grid;
• hold-down grid;
• distributor;
• redistributor;
• demister;
• fastening hardware.

Do not assume that replacing packing automatically means replacing all internals.

Inspect first and define scope.

Add Document References

A strong BOM can include:

• GA drawing number;
• packing datasheet;
• existing packing photo reference;
• material specification;
• project revision.

This makes the BOM auditable.

Example BOM Structure

For each line include:

Equipment TagBed NumberPacking TypeMaterialNominal Size / ModelTower IDBed HeightRequired VolumeEstimated WeightSpare QuantityDrawing Reference

The final commercial quotation can then follow the same structure.

Engineering Takeaway

A replacement BOM should be built from:

Latest Drawing + Actual Bed Geometry + Existing Packing + Site Verification + Approved Replacement Specification

The most dangerous shortcut is:

“The old invoice says 12 tonnes, so order 12 tonnes again.”

Replacement quantity should be rebuilt from the actual packed volume.