Packing Support Design for FRP Scrubber Towers

FRP scrubber towers are widely used for corrosive gases because the vessel shell can resist many chemical environments without the weight of a thick metallic structure.

However, an FRP shell does not behave like a steel shell. Packing support loads cannot be transferred through improvised clips, drilled holes or narrow contact points without checking the laminate.

The support grid, vessel ledge and shell reinforcement must be designed as one system.

Understand the FRP Load Path

The packed bed creates a vertical load from:

Packing weight

Operating liquid holdup

Fouling deposits

Temporary liquid accumulation

Installation loads

Possible maintenance loads

The support grid transfers this load into a ledge, clips or an internal frame. That component then transfers the reaction into the FRP shell.

FRP is directionally reinforced. Its strength depends on fiber orientation, laminate thickness, resin system and fabrication quality. A local bracket cannot be evaluated from shell thickness alone.

The vessel manufacturer should verify the laminate and attachment detail at every support elevation.

Prefer Distributed Support Where Practical

A full-circumference ledge distributes the packing load around the vessel and reduces concentrated reactions.

Even with a full ledge, local forces from primary beams must be considered. A beam end can place a significant reaction on a small area of the ledge.

The design may require:

Additional laminate beneath the ledge

Local gussets

Increased ledge width

Load-spreading pads

External shell reinforcement

Closely controlled beam locations

The support-grid supplier should provide beam reactions and required bearing dimensions. The FRP vessel manufacturer should confirm that the shell and ledge can accept them.

Do Not Drill the Shell Without Approval

Field installers may be tempted to drill holes for clamps, brackets or positioning bolts. An uncontrolled hole can cut reinforcing fibers and breach the corrosion barrier.

It may also allow process liquid to enter the laminate. Once liquid penetrates damaged FRP, repair can become difficult.

Any penetration should be part of an engineered detail with:

Defined hole location

Local reinforcement

Compatible sealing

Protected exposed laminate

Inspection requirements

Approved repair procedure

If the design does not require a penetration, mechanical arrangements should avoid creating one in the field.

Select Compatible Support Materials

FRP towers may contain support grids made from:

FRP

Thermoplastic materials

Dual-laminate construction

Corrosion-resistant metal

Ceramic components

Combinations of these materials

Selection depends on chemical exposure, temperature, bed load, span and fabrication capability.

A metal grid may provide high stiffness but requires confirmation of corrosion resistance. An FRP or thermoplastic grid may offer better chemical compatibility but can require deeper sections or additional beams.

The lightest or least expensive material is not always the lowest-risk choice.

Account for Long-Term Creep

FRP and thermoplastic supports can deform gradually under sustained load, particularly at elevated temperature.

A support that appears flat during installation may sag after months or years of operation if it was designed using only short-term material strength.

The calculation should use material properties appropriate for:

Operating temperature

Chemical environment

Expected service life

Continuous loading

Resin or polymer type

Manufacturing method

Reducing span with intermediate beams may be more effective than increasing panel thickness alone.

Control Bearing at Beam Ends

Narrow beam ends can damage an FRP ledge through high local contact pressure.

Bearing pads or wider end details may be required to distribute the reaction. Their surfaces should be smooth and should not create a hard edge against the laminate.

The design should also allow for dimensional tolerances. A beam that is too long can become wedged between opposite walls, applying horizontal force to the FRP shell.

Thermal expansion must be accommodated without permitting the beam to lose its required bearing length.

Protect the Vessel During Installation

FRP surfaces can be scratched, chipped or crushed by metal components moving through the manway.

The installation plan should include:

Segmented components sized for the manway

Defined lifting points

Edge protection

Temporary wall protection

Controlled lowering equipment

Match marks

Installation sequence

Tool and fastener control

Dragging a beam across the shell can damage the corrosion barrier even if the damage is not immediately visible.

Hot work inside an FRP tower should be avoided unless covered by a specifically approved procedure.

Verify the Finished Internal Diameter

FRP fabrication tolerances, joints and internal laminate buildup can change the usable diameter.

Field measurements should include:

Internal diameter in several directions

Diameter at the support elevation

Ledge width

Ledge elevation and levelness

Manway clear opening

Internal seams and protrusions

Existing nozzle intrusions

Locations of local reinforcement

Grid and beam sections should be designed from verified dimensions, especially for replacement projects.

Excessive field trimming can expose fibers or remove protective resin-rich surfaces.

Consider Chemical Exposure on Both Sides

The support grid may be exposed to liquid draining from the bed and gas rising from below. The ledge can remain continuously wet even when the main shell surface appears relatively dry.

Chemical concentration may also change during evaporation, shutdown or washing.

Material review should consider:

Normal gas and liquid composition

Cleaning chemicals

Startup and shutdown conditions

Maximum temperature

Condensation

Deposits

Possible solvent exposure

Fire-retardant requirements

Compatibility should cover resin, reinforcement, adhesives, fasteners and any elastomeric components.

Keep the Support Hydraulically Open

Mechanical strength must not create an unnecessary gas restriction beneath the bed.

The support should allow:

Uniform vapor entry

Free liquid drainage

Passage of expected solids

Access for cleaning where required

Minimal stagnant pockets

Closely spaced structural members may improve stiffness but reduce effective open area. Support layout should be reviewed together with the tower’s design gas and liquid rates.

Inspection Before Loading Packing

Before the grid is covered, confirm:

Ledge condition and dimensions

Complete panel seating

Beam-end bearing

Correct clamps and fasteners

Required expansion clearance

No damage to the shell laminate

No exposed fibers

Removal of temporary protection

Clean drainage and vapor passages

Correct panel match marks

Any FRP repair should be completed and cured according to the approved procedure before the packed bed is installed.