When Does a Packing Support Grid Need a Retaining Screen?
A support grid must hold the packing while allowing gas and liquid to pass freely. When grid openings are larger than the smallest stable dimension of the packing, an additional retaining screen may be considered.
However, adding a fine screen can reduce hydraulic capacity and create a fouling point. It should not be installed automatically.
Why Packing Can Fall Through a Support
Packing loss may occur because of:
- Grid openings larger than the packing
- Packing entering openings in an unfavorable orientation
- Deformed plastic packing
- Broken ceramic pieces
- Thin metal packing collapsing
- Vibration
- Incorrect loading
- Large gaps between support segments
- Excessive peripheral clearance
Nominal packing diameter alone may not represent the smallest dimension capable of passing through the grid.
What Is a Retaining Screen?
A retaining screen is installed above the primary support to reduce the effective opening size.
Possible constructions include:
- Heavy wire mesh
- Expanded metal
- Perforated plate
- Bar screen
- Secondary grid
Unless specifically designed otherwise, the screen should not be treated as the main structural support.
Hydraulic Penalty
A retaining screen can:
- Reduce free area
- Increase local gas velocity
- Trap broken packing
- Collect fibers and solids
- Increase pressure drop
- Restrict liquid drainage
- Promote flooding
The effective open area after overlap and support contact is more relevant than the screen’s catalog open area.
Fouling Risk
A fine screen is especially risky in service containing:
- Fibers
- Scale
- Crystals
- Polymer
- Biological growth
- Corrosion products
- Ceramic fragments
- Dust-laden scrubber liquid
Broken packing can form a layer on the screen and progressively block the bed base.
Better Alternatives to Evaluate
Depending on the project, alternatives may include:
- Smaller primary grid openings
- Different support geometry
- A coarse secondary grid
- Larger or stronger packing
- Improved segment-joint design
- Controlled packing installation
- Replacement of heavily broken packing
Inspection Questions
Confirm:
- Smallest packing dimension
- Packing deformation under load
- Expected breakage
- Screen material
- Wire or plate thickness
- Finished open area
- Screen support span
- Attachment method
- Cleaning access
- Segment overlap
- Manway fit