Make the right choices to achieve efficient bridge design

10th August 2017
Posted By : Joe Bush
Make the right choices to achieve efficient bridge design

Mark Wright, Managing Director of A&J Fabtech, looks at the choices that can determine the effectiveness of a scraper bridge installation. A scraper bridge starts life as a design drawing, setting out all the necessary components, materials and features required for a particular installation. The collective design and manufacture of the scraper must produce a durable structure that performs its task efficiently and reliably.

Settlement tanks, sedimentation tanks and clarifiers all perform a vital role in removing solids from a wastewater streams to improve the water quality in terms of clarity, oxygen demand and chemical contaminants. A vital part of the process is the continual removal of the solids as they either sink to the bottom of the tank or float to the surface. The scraper bridge, in all its forms, is designed to complete this process efficiently and reliably.

Meeting the standards
For applications within the UK that will be installed for one of the many water authorities, the design must abide by the stipulations laid out in the water industry mechanical and electrical specifications (WIMES). In addition to this, as a steel fabrication, which most scraper bridges are, the manufacture of the product must meet CE marking criteria, in accordance with BS EN 1090.

All manufacturers of steel structures destined for the water treatment industry will be well acquainted with these requirements and those involved with more complex equipment must conform with more specific regulations, such as those governing electrical installations.

Back to the drawing board
Every installation will present its own challenges, and almost every design will have some unique features that are required for any given application. To that end, it is essential to have a flexible approach to the design and manufacture of the scraper bridge.

Scale is the most fundamental difference and the size of the settlement tank will, in part, determine the style of the scraper bridge - the choice between a half, three-quarter and full width bridge is combined with the need for a rotating or a fixed bridge. Once this has been established it is important to consider the actual installation of the bridge assembly and how this will be accomplished.

To minimise the amount of time required to assemble and complete the construction of the scraper bridge, the use of a modular design can prove to be very advantageous. This approach allows the complete bridge to be assembled and tested by the manufacturer before being separated into its modules for transport to the customer. This enables the reassembly and commissioning of the new installation to be much faster, greatly reducing the time on-site.

Key choices to achieve efficient bridge design

Getting the basics right
Once the more basic requirements have been determined, then the choice of materials, drive systems, scum removal systems and diffusers can be addressed. Again, the position of the settlement tank under design within the treatment process will have a significant bearing on the selections. Primary settlement tanks will have a much heavier solids load compared to a tertiary tank and this will influence the design of the scrapers, their supports and the speed at which the scrapers will rotate.

Components that operate below the water will often be made from stainless steel in order to minimise corrosion and to promote durability. The construction of the bridge itself will often be galvanised steel and incorporate walkways and hand-railing to maintain the safety of operators and maintenance personnel.

Scrapers can be powered by a central drive system attached to a fixed bridge or as part of a rotating bridge that is equipped with a peripheral drive motor that locates on the wall of the settlement tank. Modern systems often use Variable Speed Drives (VSDs) to provide operators with the ability to alter the speed of the scrapers easily.

Selecting the best options
In order to create the most suitable design for a particular application, it is essential to select the correct options to meet the required specification. These can include automated lubrication systems that improve reliability as well as adjustable and reversible scraper blades that improve durability and minimise maintenance costs.

Another important area is the diffuser located at the centre of the scraper bridge, the design of which influences the flow of the influent and the settling conditions within the tank. Stamford baffles, McKinney baffles and Energy Dissipating Inlet baffles (EDIs) should be considered as part of the design proposal. In each case complete hydraulic design and flow modelling should be provided to demonstrate suitability for the project.

However, when the electrical system is considered, then the wider range of options can be really appreciated. From an operational standpoint, reliability and efficiency can be enhanced through the selection of various options. Warning signals from loss-of-motion detectors or torque overload protection, as well as drive carriage obstruction detection, can all minimise damage to the equipment and alert maintenance personnel to an issue.

The installation of sludge blanket detection equipment and its connection to the central process control system can provide significant improvements in water treatment efficiency. This information can be used to activate sludge pumps on an ‘as required’ basis rather than a timer, improving the energy efficiency of the whole process.

Creating the reality
The theory of creating an ideal settlement process can be achieved with relative ease, the more complicated part is delivering the reality in such a way that it is delivered on time, with minimal disruption to the existing process. It is at this point that the expertise and experience of the designers and manufacturers is really appreciated.

Working with a team that will deliver a turnkey project from initial design concept to final commissioning ensures a seamless conclusion to every installation.


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