The right solutions for bonding the bunds
Secondary containment is absolutely critical for securing storage tanks, says Matthew Lines, the technical services manager of ITW Irathane Futura. Here he explains the best approach
Secondary containment, also commonly referred to as 'bunds', is an area around a primary storage tank (the vessel that directly contains the material) which is designed to contain any loss whether from dripping valves or as a result of vessel rupture and so prevent the contents from escaping to the environment. It can either take the form of tank built in situ into which the primary vessel is placed or it can be integrated into a primary tank. The focus of ITW is on built-in-situ structures and more specifically those constructed from concrete, brick or blockwork.
Size and construction
Where a storage vessel requires bunding by law, (which depends on its size, use, location and media being stored), the bund must be able to contain at least 110% of the volume of the vessel. It should also be impermeable and leak proof to prevent any environmental contamination.
Bunds come in many shapes and sizes, with larger ones typically constructed from concrete, bricks or blockwork. In many cases these have been built specifically for the duty, although some have been rapidly constructed to meet an environmental need. What is often overlooked, even in the best thought-out bunds, is the it is made 100% leak-proof.
In the context of constructing a bund, the very thing that makes these rigid construction materials so good is one of their downfalls. However solid the ground, or however well constructed the bund may be, there is always an element of movement possible in the structure. Movement can occur for a number of reasons: loads will vary depending on whether the primary containment is full or empty; there will be expansion and contraction as the structure is affected by heat from the sun, and there may be movement from the substructure or subsidence. Whatever the source, movement or expansion can result in cracks - and a crack generally means a route for liquids to make their way out of the bund.
Some bunds remain unlined and for this reason, are not worth the time and money that was taken up in building them. Others I have seen have a less than satisfactory coating applied, often a simple epoxy, or worse still, a general masonry paint. These solutions may make a bund look good but do little to ensure total containment.
ITW has developed a range of solutions to the many problems encountered in the area of secondary containment and it is important to select the most suitable product. Where chemicals are relatively innocuous it is often possible to utilise one of the tried and trusted products from our Aqualine range.
These are a series of systems each finishing with a polyurethane top coat, the system as a whole being designed to accommodate existing expansion joints - where present - along with the ability to bridge cracks which arise from one of the unexpected causes mentioned earlier. For larger areas there is a range of polyurea based products to choose from.
Polyurea 2000 and Geothane 5020 offer speed of application and performance. In the case of Geothane 5020 this has been designed to enable the user to produce a bund where no concrete currently exists. For more aggressive chemicals, the next level of resistance is achieved through use of ITW's Chem C system. This is based on a vinyl ester blend and incorporates specialist fillers to produce a highly chemical resistant finish.
This system is rigid and hence is not able to accommodate concrete movement. But the incorporation of suitable matting in the system during application reinforces the finished coating such that in the event of any notable structural movement the lining may detach from the concrete shell, but remain intact and able to continue to provide full protection.
Where the most aggressive acids need to be bunded ITW can offer Epoxy Coat 7000 AR. This is an epoxy system able to withstand 98% sulphuric acid at ambient temperature. This highly specialised product is supplied as a two-component brush or roller applied coating that is typically applied in three coats to achieve an optimum coating thickness.
Where required, all these systems can be made anti-slip to facilitate walkways, with the addition of a suitably inert media, depending on the potential chemical exposure.
The newest of these systems is Geothane 5020, a fast set hybrid urethane/urea system specifically formulated to provide excellent diesel resistance.
This key performance attribute has led to widespread use by the MOD in the UK where old diesel storage facilities have been upgraded to meet current requirements with this unique seamless fast set system. In combination with primers, the Geothane 5020 can be applied to concrete, both old and new with the correct surface preparation.
However, there is another way to take advantage of this solution where no existing concrete bund structure exists. In combination with the proper ground preparation and a suitably selected geotextile, it is possible to produce a seamless bund of any size onto bare earth.