Why distillery tanks are TIG welded with nitrogen purging

When we fabricated the fermenter, pre-fermenter and yeast settling tanks for the Greenative Petrochem plant at Latur — around 80 tonnes of stainless equipment — every process weld was TIG welded with nitrogen purging. Here is why that specification exists.

What happens without purging

Stainless steel resists corrosion because of a thin chromium oxide layer on its surface. When you weld it, the back of the weld reaches a temperature at which it will react with whatever gas is touching it. If that gas is ordinary air, the chromium burns and you get a rough, discoloured, oxidised layer on the inside face — heat tint, often called sugaring when it is bad.

That layer is not just cosmetic. It is chromium-depleted, so it corrodes preferentially. In a fermenter it also gives yeast and bacteria something to hold on to, which is exactly what a hygienic vessel is supposed to avoid.

What purging does

Purging replaces the air on the underside of the joint with nitrogen before and during welding, so there is no oxygen present to react. The result is a clean, bright root that keeps its corrosion resistance and can actually be cleaned in service.

Doing it properly means sealing the volume, letting the purge run until the oxygen level is genuinely low, and holding it until the weld has cooled past the reactive range. Rushing any of those three steps produces a weld that looks acceptable from outside and is compromised where it matters.

Why TIG rather than a faster process

TIG is slower than the alternatives, and on thin stainless that is the point. It gives fine control of heat input, which limits distortion on large thin shells, and it produces a root profile clean enough for hygienic duty without heavy grinding afterwards.

Fabricating on site rather than in a shop

Tanks of this size cannot be trucked complete. They are rolled and prepared as shell courses and assembled at the plant, which means the purging discipline has to hold in a yard rather than a controlled workshop. That is largely a matter of preparation: plan the seams, plan the purge dams, and do not let a shift end mid-joint.

The tanks at Latur were fabricated, erected and tested this way, alongside mild steel tank work, 1,500 running metres of handrailing and 2,300 square metres of gratings.