Bunds provide a ‘second line of defence’ for controlling hazards events. They are a form of enclosure that surrounds drums or tanks, providing emergency containment in the event of a failure. Bunds can help to stop hazardous materials such as flammable or toxic liquids leaking onto the ground or into surface water, protecting the environment from chemical spills.
Typically bunds are constructed using concrete or masonry, which may be reinforced and lined depending on the volume and nature of the material being stored. They wrap around single tanks or a group of tanks and provide secondary containment should the primary containment fail.
In many countries bunds are a legal requirement for the storage of certain materials. In the UK, it is best practise to bund hazardous materials unless a risk assessment shows that this is not necessary.
Generally, bund capacity should be 110% of the largest tank or 25% of the total tank capacity, whichever is the greater. Space can be limited on many sites, so a small footprint can be desirable. The higher the bund walls the smaller the area required for the same bund capacity. However, there are limits as to how high the bund walls can be in order that they do not prevent firefighting or more general access and to allow natural ventilation. As a consequence, they are typically 1 to 1.5 metres high. However, in some circumstances they may be as high as the tanks themselves.
Ideally the bund should be protected from rain accumulation, if not rainwater will need to be removed at regular intervals, to ensure the bund has sufficient remaining capacity in the event of a tank failure. This can be achieved by a valved drain that is opened to remove accumulated water. However, regimes must be in place to ensure that the valve is generally closed, and to prevent drain blockage.
It is normal to limit the number of tanks in a single bund to 60,000 m3 total capacity. Incompatible materials should have separate bunds.
Bunds should be hydrostatically tested after construction is completed, before tanks are positioned within the bund walls. It is important that bunds are tested regularly after being constructed to ensure they continue to provide the protection that is required.
Alternatives to bunds include earth dikes, sumps and interceptors.
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