Bunding

Bunding

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Encyclopedia
Bunding, also called a bund wall, is the area within a structure designed to prevent inundation or breaches of various types.

Liquid containment


The term can also refer to dikes, but it is frequently used to describe liquid containment facilities that prevent leaks and spillage from tank
Tank
A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

s and pipe
Pipe (material)
A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases , slurries, powders, masses of small solids...

s, though sometimes any barrier is referred to as bunding. Frequently, the liquids in tanks and pipes are toxic, but bunding is used to prevent the liquid from causing damage (either by force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 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
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 or masonry
Masonry
Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

 wall around the tank with a concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 floor. The outside of the wall may be reinforced with an earth berm
Berm
A berm is a level space, shelf, or raised barrier separating two areas. Berm originates in the Middle Dutch and German berme and came into usage in English via French.- History :...

.

Concrete works very well for many liquids, but it is unsuitable for some applications like containing strong acid
Strong acid
A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...

s. 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
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 or plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

. The material used depends on cost, the chemical properties of the liquid and its density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

. Plastic tanks cannot hold very dense liquids at high wall levels. Large, exposed bunding will need a sump pump
Sump pump
A sump pump is a pump used to remove water that has accumulated in a water collecting sump basin, commonly found in the basement of homes. The water may enter via the perimeter drains of a basement waterproofing system, funneling into the basin or because of rain or natural ground water, if the...

 or some other system to remove precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of 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.

The bund may have a roof to prevent precipitation from getting in, but steps must be taken to provide adequate ventilation
Ventilation (architecture)
Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...

 when storing flammable liquids. If the wall is over a meter high, it may require a ladder or steps to allow people to escape quickly. Another design uses a channel that drains the liquid to a secondary container.

When the risk of tank failure is not as likely or when it would not cause extensive damage, the bunding may be designed to merely contain small leaks from hose
Hose (tubing)
A hose is a hollow tube designed to carry fluids from one location to another. Hoses are also sometimes called pipes , or more generally tubing...

s and valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...

s. This bunding may not be able to contain the entire volume of the tank. Plastic and steel are used, but another common method is making a hump or lip around the perimeter of a concrete floor. Some bunding is temporary, such as short-term chemical storage in the field. A hump or slope type bunding is helpful when vehicles need access to the area. There is also a type of bunding that compress
Compress
Compress is a UNIX compression program based on the LZC compression method, which is an LZW implementation using variable size pointers as in LZ78.- Description of program :Files compressed by compress are typically given the extension .Z...

es when a vehicle passes over and expands once it has passed.

Regulations


Bunding is a legal requirement in many countries particularly around tanks, storage vessels and other plant that contain liquids which may be dangerous or hazardous to the environment.

Particular examples which receive specific attention in the UK, the rest of Europe and the USA are oil and fuel storage tanks and transformers at electricity sub-stations which are filled with oil for cooling and insulation purposes.

It is reasonably easy to construct a "water-tight" bund around the base of a tank or vessel. A concrete base and a sealed wall of masonry, brickwork, concrete or even prefabricated steel provides the holding capacity.

Bunded oil tanks are generally a requirement from most insurance companies as opposed to single skinned oil storage tanks. Due to the safer oil storage solutions brought about by bunded oil storage tanks, some places in the UK can see the Environmental Agency ask you to install bunded oil tanks.

Holding capacity


Almost all regulations require a holding capacity of 110% of the capacity of the maximum capacity of the biggest tank within the bund or 25% of the total capacity of all the tanks within the bund whichever is the greatest. In addition further guidelines in some countries (eg. the UK) recommend additional measures such as providing sufficient "freeboard" or height of wall above the maximum holding capacity to accommodate dynamic factors such as surge in situations of major tank failure or storm driven waves in larger bunds. As a rule (and unless specific local laws prevail) most operators work to the 110% capacity guide.

Unwanted water build-up and removal


As noted above, electricity sub-station transformers contain significant amounts of oil. A 110kVA transformer may have up to 40,000 litres of cooling/insulating oil contained within the body of the transformer and its associated coiling radiators and storage tanks. Unlike ordinary fuel storage tanks, these are complex structures, with a higher propensity for leakage of the oil. Using the UK as an example, as electricity industry privatisation took place in the 1980s and 1990s the new electricity companies were made aware of their environmental responsibilities. Most area and national companies realised that they had several thousand transformers, many of which had been leaking into the ground below them for many years. The companies embarked on an upgrading programme involving the construction of "water-tight" bunds to retain any oil leakage and to prevent further pollution and contamination.

They immediately encountered the problem of water build-up from rain being retained by the now "water-tight" bund; the unwanted rain-water reduces the holding capacity of the bund. Once the water level reaches more than 10% of the holding capacity of the bund, it is no longer fit for purpose and the water must be removed. The water is also likely to be, at best, moderately contaminated with a small film of oil on the top of it or, at worst, substantially contaminated by a thick layer of oil. This is worse on older, leakier transformers. This also can apply to any oil storage tank

Oil floats on water and, if still clean enough to see through, has a different refractive index than the water below, making the oil/water interface difficult to judge. This makes manual pumping difficult and unsafe. Removing the entire contents for disposal as hazardous waste is expensive and environmentally unacceptable. However, in the UK at least, the latest regulations require some formal method to be put in place for the removal of the rainwater. One of the systems recommended is an automatic pump system which is able to discriminate between oil and water. A good system should work continuously and automatically and must fail to safety (e.g. not pumping). It should also provide alarms for conditions such as high water (indicative of pump or system failure) and high oil to warn that action to skim off the waste oil should now be done. These automatic pump systems are usually referred to as "BundGuards".

Bunding failures


In 1919, a 15-meter high molasses tank in Boston burst
Boston molasses disaster
The Boston Molasses Disaster, also known as the Great Molasses Flood and the Great Boston Molasses Tragedy, occurred on January 15, 1919, in the North End neighborhood of Boston, Massachusetts in the United States. A large molasses storage tank burst, and a wave of molasses rushed through the...

, killing 21 and injuring 150. A few other tanks have failed in a similar manner in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, but they have usually resulted in relatively few deaths. However, bunding's main purpose in unoccupied areas is to prevent environmental
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

 damage.
Bund wall failures have occurred in the UK, such as at Warrington
Warrington
Warrington is a town, borough and unitary authority area of Cheshire, England. It stands on the banks of the River Mersey, which is tidal to the west of the weir at Howley. It lies 16 miles east of Liverpool, 19 miles west of Manchester and 8 miles south of St Helens...

, Cheshire
Cheshire
Cheshire is a ceremonial county in North West England. Cheshire's county town is the city of Chester, although its largest town is Warrington. Other major towns include Widnes, Congleton, Crewe, Ellesmere Port, Runcorn, Macclesfield, Winsford, Northwich, and Wilmslow...

  in 1994. A polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...

 tank holding about 30 tonnes of 40% aqueous caustic soda sprang a leak about halfway down the main wall. The corrosive fluid jetted out over the bund wall, causing great damage to the surrounding factory and adjacent premises. The bunding at Buncefield also failed to contain petrol and other fluids.

Access containment


For access restriction, a 1m high bund (or berm) will obstruct most cars, vans and caravans, but further controls are needed to obstruct unregulated "motocross
Motocross
Motocross is a form of motorcycle sport or all-terrain vehicle racing held on enclosed off road circuits. It evolved from trials, and was called scrambles, and later motocross, combining the French moto with cross-country...

". An example of this can be seen at Delapré Abbey
Delapré Abbey
Delapré Abbey , or more properly, the Convent of St Mary De La Pré, was founded as a Cluniac nunnery about the year 1145, situated in the meadows of the River Nene to the south of Northampton ....

 in Northamptonshire
Northamptonshire
Northamptonshire is a landlocked county in the English East Midlands, with a population of 629,676 as at the 2001 census. It has boundaries with the ceremonial counties of Warwickshire to the west, Leicestershire and Rutland to the north, Cambridgeshire to the east, Bedfordshire to the south-east,...

, where bunding was added to the edge of the historical park land to keep out motorcyclists and travellers
English Travellers
The term English Travellers refers to certain groups of ethnic nomads, and may mean:* Romanichals, or English Gypsies of Romani origin* Irish Travellers, living in the United Kingdom* New Age travellers in Britain in the 1980s and 1990s...

.

Notable containment failures

  • Boston Molasses Disaster
    Boston molasses disaster
    The Boston Molasses Disaster, also known as the Great Molasses Flood and the Great Boston Molasses Tragedy, occurred on January 15, 1919, in the North End neighborhood of Boston, Massachusetts in the United States. A large molasses storage tank burst, and a wave of molasses rushed through the...

  • 2005 Hertfordshire Oil Storage Terminal fire
    2005 Hertfordshire Oil Storage Terminal fire
    The Buncefield fire was a major conflagration caused by a series of explosions on 11 December 2005 at the Hertfordshire Oil Storage Terminal, an oil storage facility located near the M1 motorway by Hemel Hempstead in Hertfordshire, England. The terminal was the fifth largest oil-products...


External links