Why Corrosion Allowance Alone Cannot Protect Liquid Distributor Performance
Adding corrosion allowance to a liquid distributor may preserve its mechanical thickness, but it does not guarantee stable hydraulic performance throughout its service life.
Distributor performance depends on small holes, notches, level surfaces and liquid-retaining joints. Local corrosion can change these features long before the main plate loses structural strength.
Structural Life and Hydraulic Life Are Different
A distributor may remain mechanically intact while suffering from:
- Enlarged outlet holes
- Unequal hole geometry
- Perforated trough walls
- Leaking seams
- Damaged fasteners
- Distorted thin plates
- Corroded support points
- Rough surfaces that collect deposits
The distributor has therefore lost hydraulic accuracy even though it has not collapsed.
Corrosion Can Enlarge Outlets
For a gravity distributor, outlet flow is strongly related to opening area.
If corrosion enlarges some holes more than others:
- Those outlets discharge more liquid
- Distributor head changes
- Other zones receive less flow
- Original turndown performance is lost
Localized attack near one feed zone can create concentrated maldistribution.
Deposits Can Produce the Opposite Effect
Corrosion products may also accumulate inside outlets and reduce their effective diameter.
The same distributor can therefore contain:
- Enlarged holes caused by metal loss
- Restricted holes caused by deposits
- Completely blocked holes
- Leaking joints
This creates a highly irregular irrigation pattern.
High-Risk Locations
Inspect:
- Weld heat-affected zones
- Crevices
- Bolted overlaps
- Trough bottoms
- Stagnant corners
- Feed-impingement zones
- Liquid-vapor interfaces
- Support contacts
- Drain connections
- Areas beneath deposits
A general corrosion-rate value may not represent these localized conditions.
Corrosion Allowance Does Not Correct Material Selection
Extra thickness cannot compensate reliably for:
- Pitting
- Crevice corrosion
- Stress-corrosion cracking
- Galvanic attack
- Incorrect alloy
- Poor weld treatment
- Chemical concentration changes
Material compatibility remains the first decision.
Define the Required Service Life
The project should specify:
- Design life
- Inspection interval
- Expected corrosion mechanism
- Normal and upset chemistry
- Cleaning chemicals
- Temperature range
- Required hydraulic tolerance
- Replaceable or permanent components
A replaceable distributor may follow a different economic strategy from a welded internal that is difficult to remove.
Inspection During Shutdown
Measure more than remaining plate thickness.
Also inspect:
- Outlet diameter distribution
- Notch condition
- Joint leakage
- Trough levelness
- Fasteners
- Support integrity
- Deposit pattern
- Drainability
- Feed-impingement damage