Control of Pollution (Oil Storage) (England) Regulations 2001

Stringent regulations should be considered when storing oil. Current legislation on the Control of Pollution in the context of oil storage provide an overview of such regulations.

This has implications for the design considerations of both storage tanks and secondary containment bunds.

This article is for process engineers, process safety engineers, civil engineers, plant designers, site operatives and maintenance personnel and those involved in the inspection and assessment of existing bunds in the context of oil storage.

Background

Containment systems centre on primary, secondary and tertiary forms. They are a critical part in the safe storage of hazardous chemicals (such as fuels) in bulk quantities.

Primary Containment

Primary containment is the first stage in the prevention of major accidents involving hazardous liquids. This type of containment is attained when equipment in direct contact with such material (e.g., storage vessels, pipes, valves, pumps and control/alarm devices) prevents it from leaving the controlled area.

Fixed tanks refer to storage tanks used to house and contain oil. According to regulatory requirements in regards to the Control of Pollution (Oil Storage)

3.—(1) Oil shall be stored in a container which is of sufficient strength and structural integrity to ensure that it is unlikely to burst or leak in its ordinary use.

4.—(3)b.(ii) must be adequately protected from physical damage;

4.—(3)c. if made of materials which are liable to corrosion, must be adequately protected against corrosion.

This means that added surface protection in most cases is a necessity; multiple chemicals have the capacity to corrode steel.

Corrosion-resistant systems centre on fixed resin-based coatings, such as vinyl ester-based fibreglass-based linings.

Although priorities should be focused on preventing breaches in primary containment, adequate secondary and tertiary containment options should be maximised. It was the failure of the secondary and tertiary containment systems at the Buncefield site which led to it being classed as a major accident to the environment (MATTE).

Secondary Containment

Secondary containment systems are designed to minimise the impact of any failure in the primary containment system. It is designed to prevent the uncontrolled spread of hazardous liquids in the event of a loss of integrity in storage tanks, pipes, and the overall primary containment system. It is also intended to offer a degree of storage capacity for firewater management. Secondary containment systems are housed outside of the primary containment and are usually comprised of concrete bunds lined with a chemically resistant coating that is impervious to the contained liquids for a sufficient period of time until corrective clean-up procedures can be followed.

Some of the most relevant points in terms of secondary containment design requirements centre on the following criteria:

3.—(2) The container must be situated within a secondary containment system which satisfies the following requirements—

(a) subject to paragraph (5), it must have a capacity of not less than 110% of the container’s storage capacity or, if there is more than one container within the system, of not less than 110% of the largest container’s storage capacity or 25% of their aggregate storage capacity, whichever is the greater;

(b) it must be positioned, or other steps must be taken, so as to minimise any risk of damage by impact so far as is reasonably practicable;

(c) its base and walls must be impermeable to water and oil;

(d) its base and walls must not be penetrated by any valve, pipe or other opening which is used for draining the system;

Bund design requirements necessitate the bunds to be ‘impermeable to water and oil’. Since bunds are usually made from concrete/mortar, they are not entirely impermeable. Concrete is highly porous and prone to cracks, which eventually lead to ingress.

Note: the HSE defines terminology used here as:

• “container” means a fixed tank, a drum or a mobile bowser or (even if not connected to fixed pipework) an intermediate bulk container;
• “drum” means an oil drum or similar container used for storing oil;
• “fixed tank” includes an intermediate bulk container which is connected to fixed pipework;
• “oil” means any kind of oil and includes petrol; and
• “secondary containment system” means a drip tray, an area surrounded by a bund or any other system for preventing oil that is no longer in its container from escaping from the place where it is stored.

Secondary containment design typically comprises a spill bund. Beyond the inclusion of the base structure of the bund – namely a base and wall, a bund lining is often included.

Functionally, according to CIRA, bunds should be able to withstand the static and hydrodynamic loads associated with:

• release of liquid from primary storage tanks
• release of water from hoses during fire fighting operations
• wind (50-year design life)
• potential impact by site vehicles (if not protected by barriers)

According to the BSTG report, “bund wall and floor construction and penetration joints should be leak-tight. Surfaces should be free from any cracks, discontinuities and joint failures that may allow relatively unhindered liquid transboundary migration.”

The BSTG report also cites that “ASTs containing substances that are flammable, highly flammable or extremely flammable shall be bunded to provide secondary containment of the dangerous substance as stated in policy statement references 6 and 7….”

In addition to bund structural design, many of the features of the bund itself can be enhanced by the installation of an effective lining system.

Types of Bund Linings for Fuel Storage

Any secondary containment lining should consider risk, cost and practicality. In spite of this, regulatory bund lining standards do not exist in a consolidated form.

Nevertheless, bund linings for fuel system should possess the following characteristics:

• Present a seamless surface that is defined by an extremely low porosity
• Remain chemically inert to extended periods of exposure to oil, gasoline and other substances including water
• Possess adequate strength and durability: impact resistance, abrasion resistance and a degree of flexibility to accommodate movement of the underlying structure
• Maintain integrity in response to the ignition of a flammable liquid. Note: fire protection can be incorporated into certain systems to meet BS 476: Fire tests on Building materials and structures.
• Low-to-medium cost for materials and installation
• UV-resistance
• Ease of repair and refurbishment

Vinyl ester linings – backed with fibreglass for added reinfrocement – are engineered to meet these requirements. In many sectors of chemical and fuel storage they are considered the “gold standard” for small-to-medium sized areas (up to 1,000 m2).

Note: the COMAH Containment policy stipulates that ‘bunds shall be impermeable’ and that ‘bunds shall have fire resistant structural integrity, joints and pipework penetrations’ – a requirement that covers the preparation of the tank base and foundation plus the selection of lining systems; concrete, earth or polymeric or polymeric and mineral composites.

The section below, adapted from the Safety and environmental standards for fuel storage sites report, presents some of the options available to planners considering what bund lining system to use. Key factors centre on advantages and disadvantages as well as ease of installation, cost and fore resistance. It is important to note that compatibility testing with the fuels being stored is also available if specific information is lacking. 

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Tertiary Containment

Tertiary containment – the third line of defence – minimises the impact of a failure of the primary and secondary containment systems. It is intended to offer an additional barrier to block the uncontrolled spread of hazardous liquids. Tertiary containment is attained via site drainage systems and sumps, diversion tanks and flexible booms. An example of usage would be when a bund overflows with hazardous liquid due to a loss of integrity in the main storage tank. It is intended to further prevent or slow the spread of such hazardous material.