As much as 50 percent of the material that comes from the ground during coal mining is waste – dirt, rocks and other impurities – which must be removed from the coal before it can be processed and burned. Coal is washed, crushed and sized at a coal preparation plant, where throughput can average 4,000 tons per hour. If process and wastewater streams are not well-managed, a coal prep plant can flood, becoming dangerous to work in or being forced to shut down.
In several locations at a coal prep plant, pumps are required to handle collected water:
- Holding ponds fed by runoff from raw and clean coal stockpiles
- Basement cleanup and conveyor belt-head sumps
- Truck washdown sumps
In most services, the water to be pumped is a muddy, abrasive slurry, laden with coal fines, briquettes, dirt, and even nuts and bolts from broken machinery.
The challenge at a coal prep plant in Kentucky
A recent shutdown at a coal preparation plant in eastern Kentucky was caused by the failure of a self-priming centrifugal pump in the main floor cleanup sump. The pump’s suction attracted solids at the intake, but because the pump’s hydraulics were inadequate to pull in heavy solids, the pump only dewatered the sump and left the settled coal and plant waste in the bottom of the sump. Eventually the self-priming pump’s hose was choked by the buildup of solids, and the pump starved and failed. The only way to remove the solids from the sump was to manually dig them out with a shovel.
The plant replaced the self-priming centrifugal pump with a 15-horsepower, high-head, durable, top-discharge pump with a mechanical agitator and wide clearances. Because of its straight path from intake to discharge, the pump chosen for the plant used the pumped liquid to cool the motor. Since it did not have an elbow joint like a side-discharge pump, it had fewer surfaces to wear and potentially fail. The top-discharge pump provided the plant with:
- Wear resistance
- All wetted parts constructed of abrasive-resistant chrome iron to extend wear life
- A replaceable, hardened wear plate that corrects problems from performance-damaging erosion
- Pump volutes cast from 200-Brinell-hardness ductile iron reduces abrasion possibilities at the point that slurry enters the discharge
The pump’s impeller can handle solid concentrations as high as 70 percent by weight, and the integration of an agitator allows for fluidization of the settled solids, transforming them into a slurry that is easier to pump and reduces clogging in the sump.
At the eastern Kentucky coal prep plant, the pump has operated problem-free for more than a year, with the cleanup sump consistently free of process debris. The pump’s agitator has kept particles suspended in the slurry, while large, 1 -inch holes in the strainer have allowed virtually all the solids to pass through. Because the pumping components – notably the agitator, impeller and wear plate – are constructed with abrasive-resistant, high-chrome iron, the pump has been successful and has completely dewatered the sump.
Although not maintenance-free, the pump continues to provide reliable operation in the coal prep plant’s harsh environment, successfully dealing with the high percentage of solids and eliminating the need to manually remove buildup.
Mike Bjorkman is vice president of BJM Corporation and has more than 30 years of experience in the pump industry. He serves as director of marketing and internet technology for BJM Pumps LLC and All Test Pro LLC, subsidiaries of BJM Corporation. He may be reached at 860-399-5937.
BJM Pumps, established in 1983 and headquartered in Old Saybrook, Connecticut, supplies electrical, submersible pumps to industrial and municipal markets throughout the United States, Canada and South America. For more information, visit http://bjmpumps.com.