Gary Little discusses some of the issues surrounding tank selection and installation
The circular storage tank industry has over the years experienced a substantial increase in technical and system innovation. Traditionally the water industry depended on concrete and welded tanks built using well-established methods, which were highly labour intensive and required extended durations of site activity.
As is often the case, agricultural demands for storage drove innovation, with pre-cast concrete and bolted tanks coming to the fore. These new systems progressed to meet increased industrial demands. Specifiers in the water industry have, over recent years, faced the demanding criteria of keeping abreast of new tank systems. The traditional standards for concrete tanks always form the basis of new concrete tank systems; bolted steel tanks have not been so easy to standardise due to the various types of material and coating used.
Significantly, the tank industry is often judged by the basic products and not by the project management and instillation skills of the tank contractor. Installation skills must be taken into account when selecting a tank supplier.
It is also important to understand that the tank industry should be understood to include roofs, access steelwork and pipework. The key interfaces with the civil contractor for basework, mixing and process requirements, and the programmed planning make consultation with the tank specialist contractors critical. However, main contractors do not give the priority that these areas demand.
The industry operates on asset life and tank selections are often driven by this method. Concrete tanks have traditionally been viewed as having a life expectancy of 40 to 60 years with steel being around 20 years. Recent pre-cast concrete systems have tended to bring down the asset life governed by BS8007 (the standard for concrete water storage structures) but are still considered to have longer life than steel tanks.
Steel tanks, while often having excellent factory-applied coatings such as glass fused to steel and epoxy, are totally dependent on skilled installation methods. For the purpose of comparison, the traditionally built, poured in-situ concrete tanks are not covered in this article. The system of formwork built on site, pouring part rings of wall with numerous vertical and horizontal joints are increasingly out of favour. They are very labour intensive, costly and require extended time on site.
More importantly, the lack of skills and workforce in the construction industry has driven the proprietary tank systems forward. Welded tanks are supported by detailed industry standards and are do not lend themselves readily to innovation.
The water industry has increasingly sought to improve the skills of its specialists. Recent developments such as the Achilles UVDB Verify system approval provides the utilities and main contractors with an industry standard of installation and general capability of specialists.
The high levels of health and safety, and quality assurance and installation skills demanded by UVDB Verify provide a sound and measurable level to judge tank suppliers. It should always be borne in mind that a tank can only be a valuable asset if installed correctly.
Large contractors are, in the main, financially driven. In our competitive market, this is understood by all. However, it has long been considered that tank selection, be it material or contractor driven, is always driven into a Dutch Action with important issues such as installation and industry controls such as UVDB taking a complete back seat. Both main contractors and tank suppliers are equally to blame for the downgrading of the importance of the tank specialist.
The water industry is beginning to recognise that tank specialist contractors have a key role to play in planning and engineering innovation. There are significant financial advantages to be gained by the water plc and framework contractors if specialists are consulted early enough. Any visit to a treatment works will reinforce the view that tanks are key to any project performance. Why then have specialists been driven to the bottom of the pecking order of importance?
Tank selection is key to initial civil works with piling and foundations often being the first work on any site. Therefore early involvement of the tank supplier is very important but involved consultation at this early stage is often not carried out.
Key interfaces with pipework, odour control equipment, access steelwork and mixing all involve the tank supplier. Unfortunately, consultation is often inadequate.
Tank selection should be considered as an early opportunity to involve a specialist who, if selected on the correct basis, will provide solutions and suggestions for an improved project.
A classic example of when a tank specialist should be consulted is with projects involving digesters and gas holders. Here, all elements of tank detail become critical and technical consultation with a specialist brings added value. This opportunity is commonly overlooked often resulting in loss of savings in time and money and increasing chance of technical complications.
So, as the various tank systems are covered, it must be recognised that project management and installation methods cannot be divorced from any selection process. To do so eliminates many of the benefits that a system can deliver. So yes, select a tank on its life expectancy, quality of coating, speed of installation – but do not omit the crucial element of project and installation controls all being
carried our under the appropriate health and safety considerations.
Pre-cast concrete tanks provide factory-controlled manufactured panels with a high-quality steel-mould finish on one side, with various types of finish on the other. Here, specification of the acceptable finish is critical and can often lead to dispute.
Standard panels are designed to BS8007 but the design depends on post-tensioned cables. This often makes design interpretation difficult to all but the specialist. The larger the pre-cast panel, the more vulnerable it becomes when being handled in the factory – and more importantly, on site. The panels are sealed with gaskets or grout, making the installation work highly important. The use of pre-cast tanks is well established, they have demonstrated longevity both in above-ground installations and partially buried structures.
Pipe penetration, manways, launders and roofs all require special consideration with pre-cast concrete tanks, which are made up of straight panels placed to make a circle. The post-tension cables must not be damaged, operation and manoeuvre controls should be carefully followed.
Pre-cast tanks are very quickly installed and provide commercial advantages when compared with in-situ concrete-poured structures. Early involvement in planning is important to ensure sufficient mould production space meet the programme on site.
Clear access on site is crucial as an intensive few days of panel delivery and crane work must be carried out. Large diameter structures are available with panel heights of 10-12m being available. Installation must be carried out by specialists.
Poured in-situ concrete tanks
There are numerous formwork systems available in for circular concrete tanks. The full benefit of using formwork systems can only be enjoyed when a design-and-build approach is taken.
Thinner concrete walls are achievable with design innovation. There are tie-less systems available that depend on ring tension externally and support systems internally. Formwork systems often allow for jointless tanks to be installed with all the obvious advantages this brings. However, when using formwork systems, strict control of tolerances can be difficult to achieve and is dependent on the size of the circle involved. Formwork specialists can deliver programme savings and material savings when compared with less traditional systems. Clearly all the pros and cons of ready mix concrete apply with formwork systems suppliers.
Bolted steel tanks
Factory applied coatings to pre-punched steel have been available for many years. The system depends on high-strength steel sheets being designed to accommodate the hydraulic loading.
The thickness of the sheets decrease as the height of the tank increases. Often the top ring of sheets will be 2.0mm in thickness. Very large structures can be installed and can accommodate all the typical tank fittings. The tank structure itself is dependent on bolts and sealant/gaskets. While the factory applied coatings are generally of the highest standard, the tank system itself depends on the bolts and sealing methods for its water tightness.
This makes the tank system highly dependent on the quality of the installation. Steel tanks remain the most competitive solution and stock manufacturing enables fast moving programme demands to be met. Steel tanks are susceptible to damage and periodic inspection is recommended. The correct choice of coating is clearly important with varying aggressiveness of liquids being required to be stored.
In conclusion, pre-cast concrete, in-situ concrete and bolted steel tank systems all offer varying advantages and disadvantages. Specifiers need to take into account asset life, programme needs and cost. The quality of installation and project controls can have a huge influence on the quality of the final installed structure.
The tank industry works to recognised standards, and full understanding of them will improve the selection of it products. The water plcs and main contractors have an important role to play. They can improve the performance of any project by applying factors other than simply the cost. Clearly what can seem to be a bargain can often cost more later. Interface consideration of tank covers, access steelwork and civil work is crucial and your tank specialist should be involved at an early stage to allow for full benefits to be enjoyed.