What are Bunds and Bunded Fuel Tanks
Bunds or Bund walls are generally used around storage tanks or drum storage areas where flammable chemicals or toxic liquids are held usually Fuel or Oil containment are the major usage. Alternative measures may be earth dikes (usually for very large tanks), sumps and interceptors. Bunds are also sometimes used within plant buildings for reactors and other process vessels. For materials that are normally gases at ambient conditions, bunds are used where flash fractions are sufficiently low to merit them. Therefore they are often used for refrigerated gases but not for the same gases stored under pressure. Bunded Fuel Tanks are one of the best ways to store these liquids
It is normal to limit the number of tanks in a single bund to 60,000 m3 total capacity. However, incompatible materials should have separate bunds. Tanks often have individual bunds.
Bunds should be sized to hold 110% of the maximum capacity of the largest tank or drum. This will allow some latitude for the addition of foam during response to the emergency. There are no set rules on the ratio between wall height and floor area and codes vary greatly with respect to recommendations of bund wall height. Low wall heights (1-1.5 m) are often used to facilitate firefighting but are poor defence against spigot flow (where a leak in the wall of a tank passes over the bund wall) or the tidal wave effect of a catastrophic tank failure. In some cases bunds up to height of the tank are used, but these are quite unusual. For high walled bunds, consideration will need to be given to the possibility of tanks floating as the bund fills, causing catastrophic failure.
Bunds are generally fabricated from brick/mortar or concrete but where liquids are being stored above their boiling point additional insulation, e.g. vermiculite mortar, may be added as cladding to reduce the evaporation rate. Such materials provide adequate chemical resistance to most liquids.
Maintenance of bunds is an important aspect, often overlooked, particularly in remote locations. A system of inspection should be in place to ensure the integrity of the bund. Also due consideration should be given to drainage to allow the removal of rainwater. This is another reason that the popularity of Bunded Tanks has increased due to maintenance and portability. This is normally achieved by incorporating a drain at a low point of a sloping floor with a manual valve, normally kept closed. Operating schedules should include daily opening of the valve to remove accumulated water, this will also assist in identifying minor leaks. However, with this system there is the problem that the valve may be left open or fail, thus reducing the effectiveness of the bund if a tank failure occurs. Also in winter conditions, ice may form blocking the drain. Failure to remove rainwater will reduce the capacity of the bund and may result in overtopping and if the substance to be contained is incompatible with water e.g. oleum, may result in an increased airborne release. Consideration of these scenarios should be included in the Safety Report.