Manufacturer and distributor of Demag Cranes in South Africa, Terex Materials Handling & Port Solutions (MPHS) South Africa, earlier this year supplied a R33-million turnkey solution to State-owned water utility Rand Water, which comprised the design, manufacture and installation of two desludging bridge cranes.
Terex MHPS South Africa senior man-ager Richard Roughley notes that this com-prehensive offering included the commissioning and automation of the crane equipment, the long travel rails and the flocculation system for two sedimentation tanks.
“Demag’s machinery will play an integral part in Rand Water’s process of converting water from the Vaal dam and river into potable water. Flocculators mix incoming water with flocculent that attracts suspended solids. A resultant sludge then settles at the bottom of the tank,” he explains.
Roughley notes that the automated desludging bridge will scour the 10 000 m2 floor of the sedimentation tanks to remove this sludge layer. The sludge will then be treated and released back into the river. “In total, 200-million litres of water is expected to flow through this system of tanks, while 16-million litres of sludge will be pumped out by the bridges each day, thereby ensuring that the quality of the water is at the correct level for the next stage of cleaning.
The bridges travel 0.7 m/min during automatic operations and 20 m/min during transfer operations, while the pumping capacity of each bridge is eight-million litres a day. Each bridge spans 40 m and is fitted with six pantograph hoists that guide the pumps to sludge level, Roughley explains.
Meanwhile, he notes that designing the pantograph hoists took about six months. Research and development for the pantograph hoists comprised three-dimensional modelling of various configurations, which included finite element analysis – a computerised method for predicting how a product reacts to real-world forces – to determine the best solution, Roughley tells Engineering News.
Further, owing to the unique nature of this project, a number of challenges were encountered and effectively mitigated. “Our cranes have never been designed to handle water and we had a limited timeframe to learn the technical aspects of pumps and hydrodynamics,” he says, adding that the desludging process was a steep learning curve for Demag.
He admits that any system failure, owing to a lack of electricity or water, will result in filter blockages, which is a major concern. “To reduce this risk, the desludging bridges are fully automated and they communicate with the Rand Water control room regularly, thereby ensuring smooth and seamless operation at all times.”
The pantograph was also a new concept proposed by Demag, designed to be a more robust and reliable solution, compared with industry-standard sludge bridges. “This [specific project] required extra engineering to ensure first-time success. Logistics was another concern, as the cranes each weigh 60 t and are 40 m long and 8 m wide. However, Demag’s partner- ship with a trusted transport company solved this [problem]”
Roughley further highlights that Demag had an engineering competitive edge, owing to its 59 years of experience in developing advanced crane technologies and its 40 000 m2 under-roof manufacturing facility, where the company manufactures, assembles and tests machinery before transporting the completed machines in three parts, with the main structures and auxiliary equipment each transported in one piece.
“This is a major achievement, as we encoun-tered narrow bridges and challenging site conditions. In the past, all desludging bridges were delivered to Rand Water in parts, with final manufacturing on site,” he explains.
To facilitate this handover, Demag’s dedicated service team will train Rand Water personnel to operate and maintain the machinery. Roughley says the operational life span of the bridges is 20 years, but, with regular maintenance and servicing, they could last more than 30 years.
Although these bridges are prototypes, he is confident that this new type of concept will set a new standard in the local water industry, creating the potential for export to expanding international markets.
“This will be a proudly South African contrib-ution, as the local content of this project is 80%, which creates the potential for sustainable job creation and skills development in the foreseeable future,” concludes Roughley.