Structured Packing in FRP and Lined Columns: Actual ID, Wall Fit & Support Design
Structured packing can be installed successfully in FRP, rubber-lined, plastic-lined, and other corrosion-resistant columns, but the packing should be designed around the finished internal surface of the tower, not simply the nominal vessel diameter shown on an early mechanical drawing.
That distinction matters.
A steel shell may be fabricated to one internal diameter and later receive a corrosion-resistant lining. An FRP tower may have its own manufacturing tolerances. Brick or other thick lining systems can further change the usable cross-section.
The structured packing eventually sees none of the original shell dimensions.
It sees the surface that actually exists after fabrication and lining are complete.
If that finished diameter is ignored, the packing may arrive:
- too large to install,
- too small and leave excessive wall gaps,
- or unable to fit around local lining irregularities.
For lined columns, dimensional coordination between the vessel supplier and the internals supplier should happen before packing production begins.
The vessel drawing may not show the diameter the packing actually needs
Consider a steel absorber that is specified as 2,000 mm internal diameter before corrosion lining.
If lining is later added, the free diameter available for the packing becomes smaller.
The reduction may sound modest when viewed as lining thickness on one wall.
But the packing sits between two opposite walls, so the effect applies around the entire circumference.
There can also be local variations around:
- lining joints,
- nozzle penetrations,
- repair areas,
- support attachments.
This is why an RFQ stating only:
Tower ID: 2000 mm
can be risky.
The useful dimension is:
finished clear internal diameter at the actual packing elevation.
For a new tower, that value should come from the finalized vessel/lining design.
For an existing tower, field measurement is often safer.
Round on the drawing does not always mean perfectly round in the vessel
Structured packing elements are manufactured to fit a circular cross-section.
Real industrial vessels can be slightly out of round.
FRP fabrication, lining work, field repair, long-term service, or shell deformation can all create dimensional variation.
A tower might measure differently:
- north–south,
- east–west,
- at another angle.
This matters particularly for structured packing because a large uncontrolled peripheral gap creates a preferred path around the bed.
At the opposite extreme, manufacturing packing to the largest measured diameter can make installation impossible at the narrowest point.
For a replacement project, a single diameter measurement is therefore weak information.
Measure at several directions and, for a tall bed, at several elevations where practical.
The supplier then has a realistic picture of the usable envelope rather than one idealized circle.
Wall clearance is a hydraulic issue, not just an installation tolerance
Some clearance is necessary.
Packing that has to be hammered or heavily compressed into the vessel is not correctly sized.
But too much clearance creates another problem.
Gas and liquid prefer easier flow paths.
If there is a continuous open annular space between the structured packing and tower wall, part of the process stream can bypass the corrugated packing passages.
The bed may then contain the specified volume of structured packing while using less than its intended effective area.
The problem becomes more noticeable when the tower is small or when the clearance is unusually large relative to diameter.
Lined towers make this more complicated because the wall may not have the smooth dimensional consistency of a precision metal bore.
The goal is therefore not zero clearance.
It is controlled fit without creating a significant bypass path.
Do not force the packing against a corrosion lining
An oversized metal packing segment can damage more than itself.
If workers force it into a lined tower, sharp or rigid packing edges may scratch, cut, chip, or locally stress the protective surface.
That defeats the purpose of the lining.
The risk depends on the lining type, but the general installation principle is simple:
structured packing should enter the vessel without needing the lining to act as a mechanical forming tool.
This is especially important during replacement jobs.
Crews working under shutdown pressure may be tempted to:
- bend packing,
- hammer sections,
- force oversized modules past a tight region.
Even if the packing eventually reaches its final position, hidden lining damage can create a corrosion problem later.
Correct dimensions are much cheaper than repairing a damaged corrosion barrier after startup.
The support system is often more difficult than the packing itself
A packing support in a conventional metal tower can often be attached to welded support rings or other metallic structures.
A lined or FRP column requires more thought.
The mechanical design has to carry:
- dry packing weight,
- operating liquid holdup,
- possible deposit load,
while maintaining enough open area for gas flow.
At the same time, the support arrangement must respect the corrosion-protection system.
A support attachment that penetrates, cracks, or bypasses the lining can become the weakest point in the tower.
For FRP equipment, the vessel manufacturer may need to incorporate the required support structure during fabrication.
For lined steel vessels, support rings and their lining details should be coordinated before the corrosion lining is completed.
Trying to invent a heavy packing-support arrangement only after the lining is finished can make the project unnecessarily difficult.
Material compatibility has to continue below the packing bed
A project may correctly select PP or PVDF structured packing because the gas and liquid are corrosive.
Then it specifies an ordinary metallic packing support because the support is mechanically convenient.
That deserves review.
The support sees essentially the same process environment and may experience even greater liquid contact because everything draining from the bed eventually reaches it.
The same applies to:
- bed limiters,
- distributor components,
- fasteners,
- collectors.
Not every internal has to use exactly the same material.
Mechanical requirements can be different.
But each material needs its own compatibility justification.
The corrosion strategy should not stop at the packing surface.
FRP towers create a useful lightweight combination with plastic packing
Plastic structured packing can be particularly attractive in FRP scrubbers because the two materials often serve the same broad class of corrosive, moderate-temperature gas-cleaning applications.
Low packing weight reduces the mechanical load transferred into the FRP vessel and support structure.
Installation is also easier because individual modules can be handled without heavy lifting equipment.
This does not eliminate structural design.
A large plastic bed still carries its own weight plus liquid and deposits.
And because polymer stiffness decreases as temperature rises, the support still needs to provide stable bearing.
Lightweight should be treated as an advantage—not as permission to ignore mechanics.
Thick or irregular lining can affect more than the packing diameter
The lining also changes how other internals fit.
A liquid distributor designed from the bare shell drawing may no longer match:
- finished diameter,
- support points,
- nozzle projection,
- available clearance above the packing.
The same issue can appear around feed pipes and manways.
For this reason, the complete internal layout should preferably use one dimensional basis.
If the packing supplier designs from the finished ID while the distributor supplier designs from the unlined shell ID, the components can arrive at site with incompatible dimensions even though both suppliers followed their own drawings correctly.
A coordinated tower-internals drawing can prevent this.
The drawing should reflect the vessel as the internals will actually see it.
Replacement projects need special caution around old linings
An old lined tower may no longer match its original drawing.
The lining may have been:
- repaired,
- patched,
- locally replaced,
- built up around damaged areas.
Some regions may therefore project farther into the vessel than others.
If the old packing is removed during a turnaround, this is an excellent time to inspect and measure the actual surface before manufacturing replacement elements.
There is also another question:
Why was the old packing difficult to remove?
If elements were wedged tightly against repaired lining or visibly distorted around the circumference, copying the original packing diameter may repeat the same installation problem.
For replacement work, the old packing is useful evidence, but the present vessel condition should take priority over a decades-old drawing.
Segmentation should follow the real installation route
A packing section may fit the tower diameter but still fail to pass through the access opening.
Lined columns can have additional restrictions around:
- manway necks,
- internal flange surfaces,
- protruding nozzles,
- lining build-up.
Segment dimensions should therefore be checked against the smallest usable opening along the entire installation route, not only the manway nominal size.
This is also where material choice matters.
A flexible plastic element may tolerate handling differently from a ceramic element or rigid metallic module.
The installation plan should reflect the actual product being supplied.
A module that can only enter the vessel after severe bending is not well segmented.
A lined column retrofit should start with measurements, not packing manufacture
For an existing corrosion-resistant tower, useful RFQ information includes:
- finished internal diameter at several directions,
- diameter at several relevant elevations,
- tower lining type and approximate thickness,
- packing bed height,
- packing material,
- process chemistry and temperature,
- support arrangement,
- manway clear opening,
- nozzle or lining protrusions near the bed,
- current distributor and collector arrangement,
- photographs after old packing removal,
- known lining repairs or irregularities.
If the tower is not yet open, original drawings can be used for preliminary quotation.
Final manufacturing dimensions can then be confirmed during the shutdown measurement.
That is often safer than producing all replacement packing from nominal drawings before anyone has seen the actual internal surface.
The finished tower is the dimensional authority
The corrosion lining exists to protect the vessel.
Structured packing exists to provide gas-liquid contact.
Neither should compromise the other.
The packing needs enough dimensional control to:
- install without damaging the lining,
- avoid excessive wall bypass,
- sit correctly on its support,
- remain stable through operation.
That is why lined and FRP columns require a slightly different procurement mindset.
The steel-shell drawing may define the vessel.
It does not necessarily define the space available to the packing.
For structured packing manufacture, the finished internal geometry is the dimension that matters.
Once the vessel supplier, lining contractor, and internals supplier work from that same geometry, most of the difficult fit problems become preventable rather than something the installation crew has to solve inside the tower.