MATERIALS HANDLING
-FREEPUMPING PROBLEM
The low NPSHr value of two compact rotary lobe pumps ensures precise and reliable decantation of water from a vacuum tank, explains Roger Willis
in Australia. Since their high NPSHr value reduced the capacity of the plant, its management decided to replace both units with two Tornado T1 rotary lobe pumps, which were manufactured by Netzsch.
U Tornado pump unit
ntil August 2017, two centrifugal pumps were used to decant a fi ne coal screen vacuum tank in an open-pit coal mine of a company
T ese self-priming positive displacement pumps operate by two intermeshing rotors, transporting the conveyed medium continuously from the suction to the pressure side allowing accurate dosing. Since they operate with a very low NPSHr and are completely insensitive to the eff ects of cavitation, they are particularly well suited for the task in the Australian mine. T e two VSD-driven units have been successfully employed since August 2017. T ey ensure lower fl uid levels in the vacuum receiver. In an Australian open- pit mine, fi ne coal dust is extracted from a water- coal dust mixture with a temperature of 25°C. “For this purpose, the mixture is fi rst fed to horizontal belt fi lters,” says Steve Weir, business development manager at the Netzsch Australia subsidiary. T anks to gravity, water is collected in the transverse
groove on the fi lter belt and fed into a vacuum chamber. Liquid-ring pumps generate a vacuum in the chamber. T e vacuum sucks the fl uid from the belt fi lters into the vacuum tank. While the belt fi lters deliver water continuously, the vacuum tank allows only a specifi c amount of water. T e excess liquid must be removed from the tank and be pumped on to the next process step. Until August 2017, two centrifugal pumps were used for decanting this excess water. However, these pumps were not ideal. “Cavitation, or gas-fi lled bubbles, frequently develop in centrifugal pumps. T ey not only reduce the effi ciency of the system, but also make accurate dosing impossible,” says Weir. “T is, in turn, limited the throughput of the system.” T e management, therefore, decided to replace the pumps with other models, which were expected to discharge process water with more precision, reliability and effi ciency.
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