Why Is There a Wall Around That Tank? To Catch the Whole Lot

A tank holding oil, chemicals or any liquid that must not reach the ground or a drain can fail in several ways, by splitting, by overflowing during filling, by a valve being left open or by being damaged. In every one of those cases a large volume arrives on the ground very quickly, and the only way to contain it is to have somewhere for it to go that is already built. A low wall surrounding the tank, with a sealed floor, creates a basin that holds the escape until it can be pumped out, and that is the whole function.

The required volume follows rules rather than judgement:

• •For a single tank, typically one hundred and ten per cent of its contents
• •The extra allowing for rainwater already standing in the basin
• •For several tanks, the greater of the largest tank or a share of the total
• •The volume of the tanks themselves deducted, since they occupy space
• •Walls kept low enough to fight a fire over and climb out of
• •Distance from the tank so a split does not jet over the wall

The basin fills with rainwater, which must be removed or it reduces the capacity available for an actual spill, and how that is done decides whether the containment works. A drain left permanently open makes the wall decorative, since a spill runs straight out through it, and that is the single commonest fault found on inspection. A valve that is normally shut and opened deliberately to release clean rainwater works, provided somebody checks the water first and closes the valve afterwards, which depends on procedure rather than on engineering. Automatic systems that detect hydrocarbons and close the valve remove the human step, and are standard on larger installations.

Not every containment is built in concrete, and the temporary equivalents are widely used and easily misapplied. Flexible trays and folding berms sit under a vehicle, a generator or a stack of drums and catch small leaks, and they work well provided somebody empties them after rain. Inflatable barriers can be deployed around a spill in progress. Drum pallets with a sealed sump underneath hold a proportion of what stands on them and are standard for chemical stores. Absorbent booms and mats deal with what escapes containment entirely. The common failure with all of them is treating a small tray as equivalent to a properly sized basin, which it is not.

Holding a volume of liquid is only part of the requirement and several other failures are common. The floor and walls must be resistant to whatever is stored, since a solvent can attack concrete, and a lining is frequently needed. The structure must withstand the pressure of a full basin, which is substantial and acts outward on walls that are frequently thin. It must be maintained, since cracks in the floor, holes where pipes pass through the wall and vegetation growing through joints all provide routes out. And it must be kept clear, because storing drums and equipment inside the basin uses the volume that the calculation reserved for a spill.

A sealed basin around a tank holds the contents if the tank splits, overflows or is left open, sized at typically a hundred and ten per cent of the largest tank with the space the tanks occupy deducted. Rainwater must be released through a valve that is normally shut, since a permanently open drain makes the wall decorative, and that is the commonest fault found. Storing drums inside uses up the reserved volume.