Bunds Design & Maintenance: Ensuring Effective Bund

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Design requirements for bulk fluid storage necessitate an arrangement in a safe, easy to operate and maintain layout that is economical. These centre on: (1) safety, (2) ease of operation/maintenance for workmen and operators and (3) economical/cost-effective – such that real estate and plant area should be minimal whilst ensuring points (1) and (2) are addressed. If the area is scarce, the emphasis should be on finding an economically optimal layout.

“In all cases, the most important aims of the layout of these areas are to ensure that they are a safe distance from the process and public areas and to contain spillage from the vessels, such that the risks they pose to plant, people, and environment are controlled.”

When installing equipment such as pumps and control valves, it is recommended that consideration be given to ensuring that liquid effluent is not able to enter into these areas – or vice versa. This may require additional design precautions – particularly if the site is on a slope or if it is close proximity to external areas that are prone to flooding – and to mitigate such a risk

“Care should be taken in layout to avoid the flooding of—and ensure the containment of—sensitive areas such as pump pits and bunded areas. Liquid effluent must not be allowed to run onto adjacent property or vice versa. Extra precautions are necessary if the site slopes unfavorably or contains natural watercourses.”

22.23 Bunds

Bunds are an important form of secondary containment.

Accounts of bunding are given in The Design of Bunds (Barnes, 1990 SRD R500) and Bund Overtopping – The Consequences Following Catastrophic Failure of Large Volume Liquid Storage Vessels (Wilkinson, 1991 SRD R530).

Barnes discusses the bunding recommendations given in codes and standards; the philosophy of, and practice in, bunding; the design of bunds; the effectiveness and failure of bunds; and the use of high bunds and double integrity systems. He gives case histories of storage, and particularly bund, incidents and some statistics on storage failures. He draws particularly on the report Liquefied Energy Gases Safety (GAO, 1978) and on a study by Buckley and Weiner (1978).

22.23.2 Philosophy of Bunding

Bunds used to store liquids that are classed as (1) flammable, (2) toxic, (3) corrosive, and (4) reactive; and liquids which are stored at a temperature (1) above the boiling point and (2) below it; and liquids which have (1) a high vaporization rate and (2) a low vaporization rate.

For atmospheric storage tanks, containing such hazardous chemicals as acids and alkalis, it is again not usual to have a bund. If there is a danger of spillage onto roads, pathways, or working areas, or of hazardous interaction between spillages of two liquids, separation by distance is appropriate. Common practice is to provide a bund for refrigerated atmospheric storage of LPG and other LPGs and for ammonia, but not for pressure storage of LPG or ammonia.

Bund Design

There are 2 main elements of effective bund design. These centre on:

1. Bund capacity

The volumetric capacity of the bund at atmospheric conditions should be at least 110% of the capacity of storage containers over 205 litres capacity to accommodate foaming and surges AND/OR at least 25% of the capacity of storage containers up to 205 litres capacity.

Low bund walls

A high proportion of accidents relating to storage occur when operators are working inside a bundled area. In emergencies, the simplest exit route is via an open region of the bund wall, provided it is not too high; locating steps and ladders may not always be practical in an emergency. Nevertheless, there should be adequate fixed access over the wall, with a minimum of two methods of exit. Furthermore, the bounded area should have sufficient lighting inside and out.

The bund capacity is a function of the total volume (capacity) of the bund. It is a variable factor in bund design, on the basis that a low bund wall facilitates firefighting and ease of movement in the event of an emergency.

With a low bund system in particular, if the release of liquid is sufficiently large, it may flow as a wave that overtops the bund wall. Such overtopping has occurred in a number of cases. One of the most dramatic was in Qatar, where an LPG storage tank contained in a 50% capacity bund suffered a catastrophic failure. A ‘tidal wave’ of LPG overtopped the bund and caused massive destruction. Barnes quotes an estimate that, even if the capacity of the bund had been 100%, it would still have been overtopped by a wave some 5 m high.

Another major overtopping, from a bund that was 20% undersized, occurred at Moose Jaw, Saskatchewan, in 1980, following the catastrophic failure of an oil storage tank.

However, bund wall design should also consider the hazardous nature of the materials being stored. In the event of a tank failure, the magnitude of material release is dependent on the rate of chemical evaporation from the bund.

So whilst the total bund capacity may need to reach 110 % of the tank capacity, the ratio of the wall height to floor surface area may need to be adjusted to account for the hazardous nature of the stored chemicals. More specifically, the extent of evaporation will increase with a larger surface area and so a lower floor surface area and higher bund wall height may be preferred.

A high bund gives a much smaller total area for vaporization. Another approach applicable with a low bund system is the use of a ditch within the bund, which for all but the largest releases, reduces the effective area for vaporization. There are a number of bund floor materials, such as insulating concrete, which give an appreciable reduction in the rate of vaporization. Another approach to the reduction of vaporization is to blanket the liquid surface, either with foam or plastic spheres.

There is an increasing tendency to install high bund walls, for which there are two main designs. In the first, the bund is some one-half to two-thirds the height of the tank wall and is located about 7–8 m from it. In the second, the bund is the full height of the tank and separated from it by a distance of 3 m or less. There may be a weather shield between the tank and the bund so that the existence of a separate bund is not obvious. In both designs, the bund is structurally independent of the tank.

A limitation of a high bund wall would be the potential fallout should the bund topple over. Such instances have been reported extensively and have led to a failed bund.

2. Wall design

Bund walls are typically constructed from concrete.

However, the porous nature of concrete and its tendency to degrade over time due to impact, freeze-thaw cycles and abrasion can cause premature failure via a breach of the bund. As such, measures to protect the bund with a protective concrete lining are a necessity.

Bund construction materials

In the selection of materials for construction, factors that have to be considered are the mechanical strength, the vaporization rate, and for low-temperature liquids, the resistance to thermal shock. The materials used both for bunded areas and for bund walls are mainly earth and concrete. It is also made of insulating concrete.

Recognized bund lining systems centre on fibreglass (GRP) – a polyester or vinyl ester-based resin combined with a glass fibre reinforcement as well as multi-layer epoxy and polyurethane resins. Whilst the latter two are often painted on spray-on lining systems composed almost entirely of resins, the fibreglass system often incorporates a variety of different reinforcement materials. Usually, E-glass chopped strand matting is used to provide reinforcement, but a more specialist C-glass is selected for more aggressive chemicals.

Bund wall and floor linings

Fire-resistant bund linings

Fire-resistant linings and coatings are popular in many areas of construction. One area would be for fibreglass flat roofs, which incorporate a flame retardant ingredient to ensure this. In the context of bunds, fire retardant bund lining systems have been patented and similar designs are available via Strandek.

The presence of a fire inside a bund would be classed as a significant event. A lining that is not flame retardant would be at an increased risk of developing a leak, enabling contained liquid to leave the bund after the fire has ceased. As such, flame retardant linings can be engineered to provide an enhanced degree of resistance. Fibreglass linings, for instance, can incorporate flame retardant ingredients, making them regulatory compliant.

Chemically resistant bund linings

Chemically resistant bund linings are tailored toward the chemicals being stored in associated tanks.

Whilst there is no perfect solution, many linings can be engineered to be highly inert to the chemicals that they are designed to house. Furthermore, bund linings would only be exposed to very little quantities of stored chemicals or exposed to them for limited periods of time (prior to disposal). Therefore, long-term chemical resistance is not as important as, say, the inner lining of the storage vessel, particularly when regular inspections are performed.

Indeed, chemical resistance required is a multiple of exposure time, the temperature of exposure and the extent of exposure.

At Strandek, we utilise a broad range of resin systems with broad chemical resistance. In many cases, laboratory-based testing has been performed by third parties to demonstrate the performance of such systems.

Controllable factors that can be mitigated should be accounted for. Assuming the bund is correctly designed and the components are within specification, the overall condition of the bund itself should be considered.

Probably one of the most controllable factors – but often one of the most overlooked ones -which would dictate the capacity for the bund to house chemicals in a safe and compliant manner is its condition. In terms of the integrity of the bund wall, several simple questions should be answered:

(1) Is it structurally sound?

(2) Does it have a protective concrete lining?

(3) What is the condition of the lining?

These can be identified via a bund inspection and survey – something Strandek can offer. Such secondary containment bund surveys are able to identify faults in the lining and where relevant Strandek can provide and perform recommended actions to rectify the faults.