Storage tanks — the simplest equipment with the largest consequences

A storage tank is the simplest-looking equipment on a site — a large welded steel cylinder holding liquid at atmospheric pressure. It is also responsible for several of the largest incidents the industry has had.

The reason is scale. Everything else on a plant holds a few cubic metres in a pipe or a vessel. A tank farm holds the inventory.

The vapour space is the whole question

Roof	How it works	Used for	Watch out for
Fixed roof, cone or dome	A permanent roof with a vapour space beneath it	Low volatility products — water, diesel, heavy oils	Breathing losses, and a vapour space that may be flammable
Fixed roof with blanketing	The vapour space is held under nitrogen	Products needing protection from air, or kept out of the flammable range	Blanketing gas supply becomes a safety system
External floating roof	The roof floats on the liquid, so there is no vapour space	Volatile products — crude, gasoline, naphtha	Rim seals degrade; rainwater must drain off the roof
Internal floating roof	A floating deck inside a fixed roof	Volatile products where weather or dust must be kept out	The space above the deck needs ventilation and gas detection
Dome roof over floating deck	A fixed geodesic dome over an internal floater	Retrofits, and sites with heavy rain or snow	Access for inspection is more restricted
Pressure storage, sphere or bullet	A pressure vessel, not a tank — contents kept liquid under pressure	LPG, butane, propane	A different code, different relief, and a very different hazard

The whole question is the vapour space. A fixed roof leaves a volume of air and vapour above the liquid that breathes in and out with temperature and level — losing product, and sometimes sitting in the flammable range. Floating roofs remove that space entirely.

A fixed roof leaves a volume of air and vapour above the liquid. That space causes two problems:

• It breathes. As the tank warms during the day, or fills, vapour is pushed out. As it cools, or empties, air is drawn in. That is product lost and emissions released — and it brings fresh oxygen into the tank.
• It may be flammable. Depending on the product and the temperature, the mixture in that space can sit in the explosive range.

A floating roof removes the vapour space entirely by sitting on the liquid surface. That is why volatile products — crude, gasoline, naphtha — are stored under floating roofs, and why heavier products that do not evaporate much are not.

Breathing, and the two ways it goes wrong

A fixed roof tank must be able to let vapour out and air in. That is what the pressure-vacuum (PV) valve on the roof does.

On the pressure side, emergency venting is sized for the fire case, exactly as relief devices are in vessels and relief. And the roof-to-shell joint is often deliberately frangible — designed to fail before the shell does, so an overpressure lifts the roof rather than splitting the tank and releasing the whole inventory.

Overfill

Hazard	What happens	Protection	Note
Overfill	Product overflows the roof and forms a large vapour cloud at ground level	Independent high level alarm and trip, separate from the gauging system	The cause of several of the industry's largest vapour cloud explosions
Vacuum collapse	The tank is sucked inward and buckles as it empties or cools	Vacuum relief on the pressure-vacuum valve, kept clear	A tank is far weaker against vacuum than against pressure
Overpressure	Roof lifts or the shell splits as vapour is generated faster than it can vent	PV valve and emergency venting, sized for a fire case	The roof-to-shell joint is usually designed to fail first, deliberately
Floor corrosion	The underside of the floor corrodes where nobody can see it	Cathodic protection, a liner, and out-of-service inspection	Leaks go straight into the ground and may run undetected for years
Differential settlement	The tank tilts or the shell distorts as the foundation consolidates	Levelling surveys against fixed datum points	Floating roofs jam if the shell goes out of round
Rim seal fire	Vapour at the floating roof seal ignites, often from a lightning strike	Seal maintenance, bonding, and rim seal fire protection	The commonest tank fire, and usually survivable if caught early
Sunken floating roof	The roof takes on rainwater or product and sinks	Roof drains kept clear, pontoon inspection	Creates a large exposed liquid surface with no vapour control at all
Static ignition during filling	Charge accumulates on the liquid surface and discharges	Bonding, low initial fill velocity, bottom filling	Worst at the start of a fill, when splashing generates most charge

Tanks look like the simplest equipment on a site and have caused some of the largest incidents in the industry. Almost none of the failures below are exotic — they are ordinary things happening to a very large inventory.

Overfill deserves its own treatment, because it has caused several of the industry’s largest vapour cloud explosions.

The sequence is always similar. A tank is filled beyond capacity. Product cascades over the roof or out of vents. It falls as a spray, evaporates readily, and forms a large, dense vapour cloud at ground level that spreads across the site until it finds an ignition source.

What sits underneath

Two failure modes come from below the tank, where nobody can see.

Floor corrosion. The underside of the floor sits on the foundation, in contact with whatever moisture is there. It corrodes from the outside in, invisibly. A leak goes straight into the ground and can run for years before anyone notices. Defences are cathodic protection, an impermeable liner, a leak detection layer, and out-of-service inspection under API 653.

Settlement. A full tank is an enormous distributed load, which is why tanks sit on compacted pads with ring beams rather than slabs, and why they settle measurably when first filled — the point made in foundations.

Uniform settlement is tolerable. Differential settlement tilts the tank and distorts the shell, and a floating roof in an out-of-round shell will jam. That is why tanks are levelled against fixed datum points on a routine cycle.

Floating roof specifics

Rim seals close the gap between the roof and the shell. They degrade, and vapour escapes through them — which makes a rim seal fire the commonest tank fire there is, frequently started by a lightning strike. It is also usually survivable if caught early, which is why rim seal fire protection exists.

Roof drainage takes rainwater off the roof through a flexible drain to the shell nozzle. Block that drain and water accumulates until the roof sinks — leaving a large exposed liquid surface with no vapour control at all.

Static during filling is a real ignition source, particularly at the start of a fill when splashing generates most charge. Bonding, low initial velocity and bottom filling are the controls, as covered in earthing and bonding.

What to take away

• The vapour space is the design question. Floating roofs remove it; fixed roofs breathe.
• A tank is far weaker against vacuum than pressure. The vacuum side of the PV valve is the one that folds it flat.
• The roof-to-shell joint is often deliberately the weak point, so overpressure lifts the roof rather than splitting the shell.
• Overfill protection must be independent of the gauge, or one failure removes both the warning and the defence.
• Floor corrosion and settlement happen where nobody can see.
• Keep roof drains clear and rim seals maintained.