Tanks, pipework and ventilation
Everything that holds water, fuel or air and cannot be bought ready-made to size. There is almost no choice of shape here: it is set by the space the thing has to fit into and by what will flow through it.
How a tank is built
A rectangular tank is more interesting than a round one precisely because it does not hold itself. A cylinder is pushed out evenly and its wall works in tension; a flat wall under the same head of liquid bows outwards. So bands of stiffening run across it every 400–600 mm — on the outside, so that there is nothing inside for sediment to catch on.
The second thing people forget is the fall of the floor. One or two per cent towards the drain and the tank empties completely. A level floor looks tidier, but a centimetre of liquid and sediment always stays in it, and in winter that remainder splits the welded corner from the inside.
What jobs on vessels look like
A rectangular tank is made to fit: for a cellar or boiler room where a round vessel will not go through the door. A horizontal vessel on saddles and a vertical tank are factory geometry, but the manways, nozzles and pipework for a particular boiler room are still welded to order.
A word on pressure. Anything working under internal pressure is a different job: a qualified procedure, a qualified welder and documentation for the product. A tank for water, fuel or rainwater needs none of that.
Pipework joints
Six ways of joining two pipes differ not in strength — all of them have enough — but in what happens in ten years' time when the joint has to be opened. A welded joint opens with a grinder; a flange or a union opens with a spanner.
The sliding support is what gets left out most often. A steel pipe twenty metres long grows by a centimetre and a half when it warms by 60 °C. If it is clamped rigidly at two points, that growth pulls the fixings out or bends the pipe itself; a clamp that slides costs pennies and removes the question.
Ventilation
Here a fabrication shop does less than you would think. Ductwork is a catalogue product: spiral pipe and rectangular duct with flanges are cheaper bought than welded. Three things get welded: the hood to fit, cut-ins into finished duct, and transitions nobody sells.
A hood over a range or a wash-up is welded from stainless rather than galvanised sheet, and not for looks: over grease and steam galvanising lasts a couple of years. A cowl at the outlet is needed wherever wind can kill the draught — which is almost any roof lower than the building next door.
Where to go next
Stainless for kitchens and food plants, tables and sinks are in stainless equipment. If the vessel already exists and has started leaking, see repairs and rebuilding. Price and lead time are in how ordering works.
Frequently asked questions
Can you weld a pressure vessel?
That is a different job. A pressure vessel needs a qualified welding procedure, a qualified welder and documentation for the product. An ordinary tank for water, fuel or rainwater, working without internal pressure, is made without any of that — and costs a quarter as much.
Why is a rectangular tank dearer than a round one of the same volume?
Ribs. A cylinder holds its own shape: the pressure pushes it out evenly and the wall works in tension. A flat wall under the same head bows outwards, and it has to be stiffened with bands every 400–600 mm. A rectangular tank is not chosen to save money but because a round one will not go through the door.
How much fall should the floor have?
One to two per cent towards the drain — that is enough for the tank to empty completely. A level floor looks tidier, but a centimetre of liquid and sediment always stays in it, and in winter that remainder splits the welded corner.
Is it cheaper to weld extract ductwork or to buy it?
Ductwork is bought: spiral pipe and rectangular duct with flanges cost less than any welding. Three things get welded — the hood to fit, cut-ins into finished duct, and transitions nobody sells. The rule is simple: standard is bought, non-standard is welded.
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