Bunded Storage Tanks — Technical Selection Guide for Oil, Fuel and Chemicals

Why Single-Skin Tanks Are Now Non-Compliant for Most Applications

Single-skin fuel and oil storage tanks — those constructed with only one wall between the stored liquid and the environment — were standard practice for decades. However, following a series of high-profile pollution incidents, UK legislation now effectively prohibits single-skin tanks for commercial and industrial oil storage above 200 litres.

A single-skin tank offers zero secondary containment. A hairline crack, a corroded weld, or a failed fitting results in an immediate, uncontrolled release into the environment. The cost of remediating a hydrocarbon soil contamination event — which can require excavation, specialist disposal, and groundwater treatment — can easily exceed six figures. That is before any Environment Agency enforcement action or prosecution.

Bunded tanks solve this by providing a second containment layer. There are two primary configurations:

• Integral bunded tanks — the outer wall of the tank itself forms the bund. These are factory-manufactured, typically in polyethylene or steel, and arrive as a self-contained unit. There are no field joints to fail and installation is straightforward.
• Stand-alone bunds — a separately constructed containment structure (concrete, steel or GRP) surrounds one or more tanks. More common for large-volume or multi-tank installations. Requires careful design to ensure the bund base is impermeable and all penetrations are sealed.

Oil Storage Regulations 2001 (England): What They Require

The Control of Pollution (Oil Storage) (England) Regulations 2001 — commonly called the Oil Storage Regulations — set the legal minimum standard for oil storage at commercial, industrial, and institutional sites in England. Scotland, Wales, and Northern Ireland have equivalent legislation.

Key requirements include:

• Applies to any site storing more than 200 litres of oil for non-domestic use
• The bund must hold 110% of the largest single tank within it, or 25% of the total storage capacity — whichever is greater
• The bund base and walls must be impermeable to oil — no cracks, no porous materials
• No drain valve is permitted without a lock, and it must remain locked when not in use
• Fill points must be positioned inside the bund or have a drip tray capable of catching spillage
• Vent pipes must terminate safely away from ignition sources
• Fixed pipework should be above ground where possible for inspection

Oil Storage Regulations 2001 — Threshold Comparison

Site Type	Threshold	Bund Required?	Key Requirement
Domestic (home heating oil)	Any volume	Recommended, not mandatory	Good practice guidance only
Commercial / Industrial	>200 litres	Yes — mandatory	Full OSR 2001 compliance required
Agricultural (fuel oil)	>1,500 litres	Yes — SSAFO 2010	Secondary containment, locked drain
Any site (within 10m of watercourse)	Any volume	Yes — enhanced protection	Bunding plus additional precautions

Agricultural Fuel Oil — SSAFO 2010

The Water Resources (Control of Pollution) (Silage, Slurry and Agricultural Fuel Oil) (England) Regulations 2010 — known as SSAFO 2010 — cover agricultural fuel oil storage specifically. If an agricultural site stores more than 1,500 litres of fuel oil, bunding is mandatory. The regulations require:

• A bund designed to hold 110% of the tank capacity
• No filling point within 10 metres of a watercourse without additional protection
• All drainage from the bund to be controlled and not connected to any watercourse or soakaway
• At least 28 days written notice to the Environment Agency before constructing new facilities (for sites above 200m³)

Steel vs Polyethylene Bunded Tanks — Choosing the Right Material

Both steel and polyethylene (LDPE/HDPE) bunded tanks are widely used and both can be fully compliant. The choice depends on application, site conditions, and the stored product.

Steel Bunded Tanks

• Stronger — higher mechanical resistance, suited to sites with vehicle movement nearby
• Longer service life — properly maintained steel tanks can last 30+ years
• Fire resistant — steel does not melt or deform in heat the way plastic will
• Heavier — requires substantial base and lifting equipment for installation
• Corrosion risk — external and internal corrosion must be managed; coatings and cathodic protection are used on larger units
• Inspection — formal inspection to API 653 standard required every 5–10 years for large tanks

Polyethylene Bunded Tanks (LDPE/HDPE)

• Lighter — far easier to handle and install, no crane required for most sizes
• Corrosion-proof — will not rust, unaffected by the stored fuel
• UV stable — modern polyethylene tanks include UV inhibitors for outdoor use
• Lower temperature range — HDPE can become brittle at very low temperatures; check manufacturer ratings for exposed northern sites
• Not fire resistant — polyethylene will deform and burn; steel is preferred in fire-risk areas
• Lower cost — typically cheaper to procure for standard capacities

Capacity Sizing and Ullage

Correct capacity sizing is critical. Undersizing creates compliance and safety problems; oversizing wastes capital expenditure.

Ullage is the empty headspace left at the top of the tank. A minimum of 10% ullage must always be maintained — tanks should never be filled above 90% of their stated capacity. This is essential because:

• Fuels expand with temperature — filling to 100% on a cold day means overflow on a warm day
• Delivery drivers typically cannot accurately gauge fill levels from outside the tank
• An overfill prevention valve (OPV) at the fill point provides mechanical protection against overfilling

Size your tank so that your maximum anticipated delivery volume represents no more than 85–90% of total capacity. For example, a 5,000-litre delivery requires a minimum 5,500-litre tank (11% ullage at full delivery).

Vent Pipes — Specification and Positioning

All bunded tanks must have a vent pipe to allow displacement of air during filling and venting of fuel vapour. The vent pipe must:

• Point downward at its open end — this prevents rain ingress which causes water contamination in the fuel
• Be fitted with a stainless steel mesh or flame arrester at the open end — this prevents an external flame or spark from tracking back into the tank
• Terminate at a point away from ignition sources — minimum distances from windows, doors, electrical equipment, and open flames apply (consult current ATEX guidance)
• Be at least 50mm internal diameter for tanks up to 10,000 litres
• Have no valves or restrictions that could cause pressure build-up

Fill Points and Overfill Prevention

The fill point is the highest-risk location for spillage during tank operations. Requirements under the Oil Storage Regulations 2001 are clear:

• Fill point should be located within the bund wherever possible, so any drips or splashes are contained
• Where the fill point is outside the bund, a dedicated drip tray must be installed beneath it — sized to catch a full hose-end spillage
• An Overfill Prevention Valve (OPV) must be fitted — this automatically closes when the tank reaches a set level, physically stopping the delivery
• The OPV float and mechanism should be inspected annually to ensure free movement
• Fill points must be clearly labelled with the fuel type and tank capacity

Draw-Off Valves and Level Gauges

How fuel is extracted from the tank has direct implications for spill risk:

• Bottom outlet draw-off is common but carries higher spill risk — any failure of the valve or connected pipework results in the entire tank contents draining. A bottom outlet valve should always have a secondary lockable valve.
• Top suction with a submersible pump is generally preferred as any pipe failure above the liquid level does not result in siphoning
• A sight glass or electronic level gauge allows the user to check tank contents without opening any fittings. Sight glasses must be robust (polysulfone or toughened glass) and protected from impact
• Gauging tapes should only be used where no other option is available, as they can introduce contamination

Tank Inspection Schedule

Regular inspection is a legal and practical necessity. The following schedule applies to most commercial and industrial installations:

• Annual visual inspection (owner/operator) — check for leaks, corrosion, structural damage to tank and bund, condition of fittings, operation of OPV, cleanliness of bund interior, security of access
• 5-year formal inspection (competent engineer) — detailed inspection of all components, testing of valves and gauges, assessment of remaining service life
• API 653 standard applies to large above-ground steel tanks — covers inspection intervals, minimum remaining shell thickness, and criteria for continued service
• Any visual sign of leakage, distortion, or structural damage should be treated as an emergency requiring immediate investigation

Heating Fuel, Gas Oil and DERV — Regulatory Differences

Not all fuels are subject to identical regulations. Understanding the distinctions avoids both over-compliance and gaps in protection:

• Heating fuel (kerosene / Class C2 fuel) — used for boilers and heating systems. Covered by the Oil Storage Regulations 2001. Low vapour pressure means lower explosion risk but it remains a significant water pollutant.
• Gas oil (red diesel / Class D fuel) — used for off-road vehicles, generators, and heating. Subject to OSR 2001. Red dye is added for HMRC duty purposes. HMRC may inspect tanks to confirm correct use.
• DERV (white diesel / road diesel) — standard road vehicle fuel. OSR 2001 applies to storage. Subject to stricter duty compliance checks. Some sites store DERV and red diesel separately, requiring separate bunded units.

Regardless of fuel type, contamination of a controlled water (river, stream, groundwater) is an offence under the Water Resources Act 1991 and the Environmental Permitting Regulations 2010. The Environment Agency can prosecute and issue enforcement notices for any spill, however small.

Browse Our Bunded Tank Range

Use the links below to find the right bunded tank for your application:

• Bunded Fuel Tanks — diesel, petrol, and gas oil storage
• Bunded Oil Tanks — heating oil, lubricating oil, and transformer oil
• Bunded Waste Oil Tanks — used oil collection and temporary storage

Frequently Asked Questions

Do I legally need a bunded tank in the UK?

Yes, for most commercial and industrial applications storing more than 200 litres of oil or fuel, the Oil Storage Regulations 2001 (England) require bunded storage. Agricultural sites storing more than 1,500 litres of fuel oil must comply with SSAFO 2010.

How much does a bund need to hold?

Under the Oil Storage Regulations 2001, a bund must hold either 110% of the capacity of the largest tank within it, or 25% of the total storage capacity of all tanks within the bund — whichever is greater.

What is the difference between an integral bund and a stand-alone bund?

An integral bund is built into the tank itself — the outer skin forms the containment. A stand-alone bund is a separate secondary containment structure (concrete, steel or plastic) that surrounds one or more tanks. Both are compliant, but integral bunded tanks are simpler to install and have no joints that can fail.

Can I have a drain valve on my bund?

The Oil Storage Regulations 2001 require that any drain valve on a bund must be locked shut when not in use. This prevents uncontrolled discharge of spilled oil. The key must be kept secure and accessible only to authorised persons.

How often should a bunded tank be inspected?

Owners should carry out an annual visual inspection covering the tank body, bund integrity, fittings and vent pipes. A formal inspection by a competent engineer is recommended every 5 years. Steel tanks should be inspected to API 653 standard.

What is ullage and why does it matter?

Ullage is the void space left at the top of a tank. A minimum of 10% ullage must be maintained — never fill a tank above 90% capacity. This allows for thermal expansion of the fuel and prevents overfill scenarios that could overwhelm secondary containment.