A fishmeal plant is the hardest envelope in food processing
Of all the food processing buildings, the fishmeal plant is the one where a steel envelope is asked the most. It is hot, it is wet, and the air carries compounds that attack metal even when nothing is splashing.
The mistake almost everyone makes is to treat this as a coating problem. It is a vapour problem, and the vapour is inside.
The attack comes from the air, not from the floor
Cooking, pressing and drying release water vapour along with volatile compounds. That mixture rises, meets a cooler roof, and condenses.
The condensate is not clean water. It carries the same compounds the vapour carried, and it sits in the low point of every roof profile, on every purlin flange, and on every fixing. A roof that is wet on its underside for most of the year is a roof whose protection has to survive immersion, not just humidity.
Inside-out corrosion is invisible until it is structural
From outside, the roof looks fine. The painted face is on the outside, so the surface that is being consumed is the one nobody inspects.
This is why fishmeal plants surprise their owners: the failure is discovered when a purlin is found to be a fraction of its original thickness, usually during an unrelated repair. By then the fix is a roof, not a coating.
The practical answers, in order of value
First, keep the vapour off the steel. Where the process can be enclosed or extracted at source, that single decision removes most of the problem before any material is chosen.
Second, if the steel will be wet, design it to be wet and drainable: profiles that do not hold water, details without upward ledges, and no place where condensate can stand against a flange.
Third, choose the protection for the environment that actually exists — which at the roof underside is a wet, warm, chemically loaded interior, not a rural exterior.
The zone that is genuinely mild
Not every part of a fishmeal plant is aggressive. Offices, the bagging end, the workshop and the finished goods area can be ordinary industrial environments.
Applying the wet-area specification everywhere is a way of spending money that does not buy life; applying it nowhere is a way of buying a roof twice. The zone map is the useful document here, and it should be drawn on the same plan as the layout.
Fixing selection matters more than it looks
In an aggressive interior, the fasteners and the cut edges are the weak points. A coated sheet is only as good as the hole drilled through it.
That argues for details that minimise penetrations, for fixings specified for the environment rather than for the cheapest option, and for cut edges that are treated rather than left. It also argues for a maintenance route that lets somebody see the underside of the roof.
What I want before drawings
- The process flow, and where vapour is released.
- Whether the cooking and drying stages can be enclosed or extracted at source.
- The internal temperature and humidity range, in operation and when idle.
- The zone map: which parts of the building are wet and loaded, which are ordinary.
- How the underside of the roof will be inspected and cleaned.
- The wind speed for the site in km/h from the local code, not a round number — a coastal site brings its own airborne load to the outside faces as well.
A fishmeal plant will always be a hard environment. The choice is whether the building is designed for it, or replaced because of it.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If this decision is live on a project, send the layout and the constraint that worries you and I will tell you what it changes.
Related: corrosion class as a selection input · an oilseed press hall · a shrimp line is a floor problem · washdown in a poultry hall
