Failure investigation of a cement grinding mill

The cement grinding mill in question was a side-driven drum type with a shell size of 3050 mm (internal diameter) × 13800 mm (length). It was designed to take a load of about 220 tons. Grinding action was achieved by steel balls inside the drum, which crushed the cement as the drum rotated. The discharge end head was of carbon steel casting made to Japanese material specification SC46. On site investigations revealed that the discharge end head had a through-thickness crack around the whole circumference in the 240 mm thick section. Samples of material were removed for microstructural examination and mechanical testing. Techniques used include examination of fracture surfaces, metallographic examination of microstructures and tensile testing. The results of microstructural examination and tensile testing were supplemented by finite element stress analysis calculations. The evidence suggested that the discharge end head had been subjected to stresses higher than that envisaged in design. The probable cause of failure was overloading or improper alignment, or a combination of both.