Tank bund

Bunding, also called a bund wall, is a constructed retaining wall around storage "where potentially polluting substances are handled, processed or stored, for the purposes of containing any unintended escape of material from that area until such time as remedial action can be taken."

The term can also refer to dikes, but it is frequently used to describe liquid containment facilities that prevent leaks and spillage from tanks and pipes, though sometimes any barrier is referred to as bunding. Frequently, the liquids in these tanks and pipes are toxic, and bunding is used to prevent the liquid from causing damage (either by force or its chemistry). If a large tank has a catastrophic failure, the liquid alone can cause extensive damage.

If built properly, bunding is large enough and strong enough to contain the contents of an entire tank, though regulations may require it to be up to a third larger. When multiple tanks share a bund, the capacity is based on the largest tank. One of the most common designs for large tanks is a concrete or masonry wall around the tank with a concrete floor. The outside of the wall may be reinforced with an earth berm.

Concrete works very well for many liquids, but it is unsuitable for some applications like containing strong acids. Using earth berms for bunding is not recommended for most situations, though liners can be used to decrease permeability. Smaller tanks often use containers made of steel or plastic. The material used depends on cost, the chemical properties of the liquid and its density. Plastic tanks cannot hold very dense liquids at high wall levels. Large, exposed bunding will need a sump pump or some other system to remove precipitation, though it may also be used to transfer spilled liquid into another container. Rainwater must be treated if the liquid being stored is toxic because there may be small amounts of it surrounding the tank.

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Wikipedia