An approved lining is not automatically the right lining. What NSF 61, BS 6920 and KTW test, where they stop, and how the application decides the outcome.
A tender for a fire water or potable storage tank arrives and one line decides the shortlist: the coating must hold a potable water approval. Three suppliers send certificates, all valid, all issued by recognised bodies, and all for different products with different chemistry and different film thickness. The approval does not rank them. It only says that each of them, applied as tested, does not leach anything above the permitted limits.
What the certificates actually certify
The European and American schemes differ in method but share a logic: a cured sample is immersed in water, and the water is analysed for compounds that migrate out of the coating.
- NSF/ANSI 61 covers drinking water system components in North America. It evaluates the health effects of what leaches into the water and assigns a maximum contribution to a reference dose. It is product-specific and formulation-specific.
- BS 6920 assesses non-metallic products against taste, odour, colour, growth of aquatic micro-organisms and extraction of substances. It is a UK scheme widely accepted elsewhere in Europe.
- KTW-BWGL is the German assessment basis, and it is the most demanding of the three in requiring the full formulation to be evaluated, including a check that all ingredients appear on the positive list.
- A certificate covers one product, one colour, one thickness, one cure condition. Change the hardener, add a pigment, apply it 100 microns thicker than tested, and the approval no longer applies to what is on the wall.
The chemistry that usually gets specified
- Solvent-free epoxy, two-pack, applied at 300 to 500 microns per coat. The workhorse for steel potable tanks. High solids means low solvent to be retained and released, which matters when the first tankful is drinking water.
- Glass-flake epoxy where the water is aggressive or the tank cycles between wet and dry. The flakes extend the diffusion path and improve barrier performance, but the film is harder to inspect for pinholes.
- Aromatic polyurethane or polyaspartic where a fast return to service is required. Faster cure means shorter flushing, at the cost of tighter humidity control during application.
- What does not belong here: zinc-rich primers. Zinc is not acceptable in a potable water lining. A tank interior is a non-zinc system by definition, which is also why the surface preparation standard has to be met without the fallback of a sacrificial primer.
Where the approval stops and engineering starts
A certificate says the product is safe in contact with water. It says nothing about whether it will still be intact in fifteen years.
- Adhesion on the actual substrate. A welded tank has weld seams, heat-affected zones, fillets and corners. An approved product applied at 60 percent coverage on a fillet weld will fail there first.
- Pinholes. A lining is an electrical and physical barrier. One pinhole through 400 microns puts water in contact with steel, and the corrosion that follows is concentrated and fast.
- Cure before filling. Solvent retention and incomplete crosslinking both release compounds into the first volume of water. The standard answer is a defined flush, but flushing does not fix an under-cured film.
- Edge and nozzle detailing. Manholes, nozzles, brackets and the roof-wall junction need stripe coats. They are also the places an inspector cannot reach with a spark tester without scaffolding.
Application, in the order it matters
- Surface preparation to Sa 2½ with a defined profile, and salt testing before the first coat. Inside a tank the blast abrasive is recovered and recycled, which means contamination accumulates in the abrasive itself.
- Stripe coats on every weld, edge and nozzle before the full coat, in a contrasting colour so the inspection can see it.
- Airless spray, plural component, heated. The viscosity of a solvent-free material at 15 °C is not the viscosity it had in the laboratory at 23 °C. Heat the material and confirm the pump ratio.
- Holiday testing at the specified voltage. A wet sponge is not adequate for thick solvent-free films; use a high-voltage detector and record every repair.
- Thickness survey on a grid, not at convenient spots. Low thickness over a weld toe is the standard defect that passes a casual inspection.
- Cure verification before filling. Solvent rub plus hardness, at the tank temperature, not the laboratory temperature.
The first fill is part of the specification
Most potable water specifications include a flushing and sampling regime: fill, hold, drain, refill, sample. Two practical points are usually missing from the document.
- Water quality affects the result. Chlorinated water extracts differently from raw water. The sample has to be taken under the conditions in the approval, or the result is not comparable.
- Temperature accelerates everything, including the release of residual compounds. A tank that will sit in the sun at 40 °C should be cured and flushed as if it were a hot service, not a cold one.
Repair and maintenance years later
The lining will be repaired at some point, and the repair has to be as approved as the original. Keep the product data sheet, the batch numbers and the application records. A lining that cannot be matched to its original specification is a lining that has to be replaced rather than patched, and that decision is made years after the paperwork was thrown away.
The certificate is the entry ticket. What determines whether the tank passes its first sample and its tenth-year inspection is the film thickness at the weld toe, the holiday test that was actually performed, and whether anyone measured the cure before the valve was opened.