A bridge over troubled water

Vexamus Water has completed the installation of the water industry’s largest ever fixed bridge to be designed and built in the UK. Situated adjacent to Manchester’s popular Trafford Park shopping centre, the project at Davyhulme WwTW is part of a major United Utilities’ (UU) undertaking to control odours at the site. The imposing 50m Vexamus bridge had to meet many mechanical engineering design requirements in order to accommodate a geodesic dome connected to a fume scrubbing system.

Handling daily influent flows of 40 litres x 103/d, with 450m3/d sludge at 3% solids being processed, an existing system comprising two open tanks with half bridge scrapers has been replaced by the new bridge and dome. A fixed bridge was chosen because the top of the concrete wall used previously by the rotating bridge wheels was required for the dome ring beam.

Water Industry Mechanical and Electrical Specifications (WIMES) were used for sludge load (60kg/m2 of blade surface) and 3kN/m2 for bridge loading design. Consideration was also given to supporting the extremities of the rotating arms with a track supported on the inside of the outer wall. However, detailed structural and mechanical analysis indicated the rotating assemblies could be supported from a central tower in conjunction with a fixed bridge design. Generally, the structural elements were manufactured from grade 43A structural steel protected by shot blasting, aluminium metal spray and several coats of polyamide cured epoxy, bolted together with A4 stainless steel fasteners. Offering excellent resistance to corrosion, the A4 fixings are based on 316 stainless steel. An open-ended GRP tunnel was attached to the fixed bridge, which bisects the dome. Aluminium chequer plate flooring was selected on the main bridge access to provide a sealed tunnel because the dome has a slight negative pressure. The outer operations walkways, which are inside the dome area and run parallel to the main walkway, use non-slip GRP grating.

A key element of the design is the central tower that not only supports the bridge but also has to withstand torsion due to sludge load, as Vexamus’ technical director Alan Downham explains: “To protect the integrity of the tower, we have made an allowance of 2mm on structural members for corrosion. The two main bridge elements use friction joints to connect the sections and these halves are pivoted at the machine room and use sliding shoes at the other to permit any expansion or contraction.” He added: “We chose a 3m diameter slewing ring with external teeth, not on the basis of load capacity, but to provide sufficient space for the four vertical supports to connect the bridge halves to the tower and the machine room.” Twin electric planetary drives are used to drive the system. Planetary gearboxes, which have been pioneered by Vexamus, offer exceptional torque and are very compact. Protection of the drives is achieved using electronic load monitors – another first by Vexamus – offering superior performance to mechanical torque limiters.

The gear teeth and slewing ring bearing are automatically greased, while removable inspection covers allow inspection of the slewing ring and pinions. Proprietary lifting davits and sockets are incorporated into the design to facilitate installation and removal of the drive system.

To achieve motion, the upper flange of the slewing ring is attached to the underside of the machine room and lower flange to the top of the rotating cage. Two triangular frames are attached to the bottom of the rotating cage by pivot assemblies.

These arms are supported along its length by adjustable stainless steel ropes attached to a bracket at the top of the cage. This bracket is welded to the structural frame, which also supports the diffuser drum. A series of pivoted steel/rubber blade assemblies angled between 30/40° form a continuous helix that sweeps the sludge into the central hopper. Outer peripheral speed of the blades is approximately 2m/min. Four adjustable shoes on the lower end of the cage locate on a stainless steel ring attached to the tower to prevent excessive structural deflections due to the possibility of abnormal loads.

Utilising scoops, scum escapes the diffuser and is then collected by an angled GRP blade, secured by vertical supports from the lower triangular arms. An existing pivoted scum box, approximately 25mm above the water level, is located in each tank.

At a predetermined frequency the scum box is dipped below the water surface to collect the scum by the actuation of a rod and hydraulic cylinder. The control hydraulics are located outside the tank to permit accessibility for repairs and routine maintenance. The frequency of scum operation can be varied by a dial on the panel to give actuation between 0-10 revolutions of the tanks.

It is unlikely the slewing ring will ever need to be replaced because the loads are relatively light. Nevertheless, Vexamus has ensured provision for replacement, by incorporating a cantilevered frame into the design. After disconnection, the bearing is manoeuvred to the extremity of the frame and a removable opening in the dome permits cranage access for the bearing to be removed.