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

Until August 2017, two centrifugal pumps were used to decant a fine coal screen vacuum tank in an open-pit coal mine of a company 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.

These 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 effects of cavitation, they are particularly well suited for the task in the Australian mine. The two VSD-driven units have been successfully employed since August 2017. They ensure lower fluid levels in the vacuum receiver.

In an Australian open-pit mine, fine coal dust is extracted from a water-coal dust mixture with a temperature of 25°C. "For this purpose, the mixture is first fed to horizontal belt filters," says Steve Weir, business development manager at the Netzsch Australia subsidiary. Thanks to gravity, water is collected in the transverse groove on the filter belt and fed into a vacuum chamber. Liquid-ring pumps generate a vacuum in the chamber. The vacuum sucks the fluid from the belt filters into the vacuum tank. While the belt filters deliver water continuously, the vacuum tank allows only a specific amount of water. The 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-filled bubbles, frequently develop in centrifugal pumps. They not only reduce the efficiency of the system, but also make accurate dosing impossible," says Weir. "This, in turn, limited the throughput of the system." The management, therefore, decided to replace the pumps with other models, which were expected to discharge process water with more precision, reliability and efficiency.

The essential requirement for the new units was their suitability for pumping media containing solid matter. They also needed to feature flow rate of 240m³/h, pressure of 3 bar, and NPSHr of 1.8m. The management selected two rotary lobe pumps from the Tornado T1 series. These pumps meet the requirements of the open-pit operator in terms of flow rate, pressure, delivery rate and suction; they also have exceptionally convenient compact dimensions. The key advantage over the previous technology is that these models are self-priming positive-displacement pumps, which guarantee continuous delivery. Inside the pumps, there are two synchronised intertwined rotors, which create negative pressure on the suction side drawing the medium to the rotary lobes without interruption. The lobes then convey the medium to the pressure side.

This conveying principle has a very positive effect when used in the relevant application. "The vacuum tank operates at a very low pressure, which in turn creates a very low inlet pressure for the water intake from the vacuum tank," says Weir. "Our pumps are well suited for this application, since they operate with a low NPSHr." They are, therefore, insensitive to the effects of cavitation, which may cause other types of pumps to fail quickly. Furthermore, the volume transported depends on speed. Thus, the desired quantity can be dosed exactly by setting the rotations per minute. When used in the mine in Australia, the speed is 218rpm, and the conveyed volume is 240m³/h. The particle sizes, which are in the range of 0.5mm, are causing no problem at all. The fluid viscosity is 5 mPas.

An essential feature of rotary lobe pumps is that their gear and pump chambers are strictly separate from one another. This special Gearbox Security System (GSS) ensures that the medium cannot enter the complex gearbox, even in the event of a shaft seal failure. This means high operational reliability. On the other hand, gear oil can never enter the pump chamber. The seal is lubricated and cooled by means of easily accessible flushing and sealing water connections.

Since August 2017, the two pumps have been employed to decant excess water from the vacuum tank, and to convey it to the clarifying or thickening tank. Water is recovered there, and then reused in the coal treatment plant. The two VSD-driven Tornado units ensure stable and lower liquid level in the vacuum receiver. The vacuum for draining coal fines is thus more reliable. "Higher vacuum and more uniform vacuum level lead to more efficient recovery of coal fines," explains Weir.

Roger Willis is with Netzsch