What Must Be Checked Before Replacing Plastic Packing with Metal Packing?
Changing from plastic packing to metal packing may be considered when higher temperature resistance, improved mechanical strength, or different hydraulic performance is required.
But the conversion introduces one major difference that is sometimes underestimated:
metal packing can place substantially greater load on the existing tower internals.
A proper retrofit evaluation must therefore consider both process performance and structural compatibility.
Check the Increase in Packed Bed Weight
Bulk density varies widely between packing materials.
A metal packing bed may weigh several times more than a similar volume of plastic packing.
This additional load is transferred to:
- packing support plate;
- support beams;
- attachment points;
- tower shell connections.
The existing support structure should therefore be reviewed before the new packing is ordered.
Evaluate the Support Plate
Inspect the support for:
- corrosion;
- distortion;
- cracked welds;
- beam deflection;
- remaining wall thickness;
- allowable load.
An old support that safely carried lightweight plastic packing may not automatically be suitable for a much heavier metal bed.
Check the Tower Shell and Attachments
In many towers, support beams transfer packing weight into shell-mounted clips, rings, or brackets.
These connections also require evaluation.
The issue is particularly important in older columns where corrosion may have reduced structural margin.
Review Corrosion Resistance
One reason plastic packing is used is strong corrosion resistance in many chemical environments.
Replacing it with metal requires a fresh materials review.
Possible choices include:
- carbon steel;
- stainless steel grades;
- specialty alloys.
The correct material depends on process chemistry, temperature, chloride content, oxidation conditions, and contaminants.
Using stainless steel does not automatically eliminate corrosion risk.
Recalculate Hydraulic Performance
Metal packing can have thinner walls and high open area, but actual performance depends on geometry.
Do not assume a metal packing with the same nominal size will produce identical:
- pressure drop;
- flooding capacity;
- efficiency.
Hydraulic recalculation is required using the proposed packing characteristics.
Check Distributor Compatibility
The existing liquid distributor may have been designed around the wetting behavior and hydraulic characteristics of plastic packing.
The new metal packing may change:
- liquid spreading behavior;
- irrigation requirements;
- effective wetting.
Distributor condition and uniformity should therefore be checked.
Consider Process Temperature
One reason for changing to metal may be an increase in operating temperature.
If the new process operates hotter than before, check not only the packing but also:
- gaskets;
- distributor materials;
- support materials;
- mist eliminator;
- plastic piping inside the tower.
Replacing only the packing may leave other temperature-sensitive components exposed.
Check Galvanic and Mixed-Material Issues
Introducing a new metal can create compatibility issues with existing metallic internals.
Where dissimilar metals are exposed to conductive liquid, galvanic corrosion may become relevant.
The entire internal materials system should therefore be reviewed rather than considering the packing alone.
Reconsider Bed Restraint
Metal packing is heavier than plastic packing and is generally less susceptible to lifting.
However, a retaining grid may still be required to control bed movement and protect the packing during operating upsets.
The existing top grid should be inspected for corrosion and adequate open area.
Evaluate Installation Logistics
The heavier material affects shutdown handling.
Consider:
- lifting;
- bag size;
- manual handling;
- manway access;
- loading method;
- support loading during installation.
For large towers, the total packing weight can be substantial.
When Plastic-to-Metal Retrofit Makes Sense
The conversion may be useful when:
- operating temperature has increased;
- higher mechanical strength is needed;
- plastic ageing has become a problem;
- the process chemistry is compatible with the selected alloy;
- hydraulic performance supports the change.
Why a Structural Review Is Essential
The major hidden risk of this retrofit is assuming that the old support system can carry the new bed.
The packing may be chemically and hydraulically suitable while the existing support structure is not.
For that reason, packing bulk density and total bed weight should be calculated early in the retrofit study.