The Development of Structural Integrity Procedures for the Safe Working of Ceramics Industry Ball Mills with Cast Iron Ends

A ball mill at a ceramics materials company in North Staffordshire exploded in the spring of 2000. Luckily, nobody was injured as a result. Even so, the damage was so extensive that many in and around the ceramics industry rapidly concluded that the industry must introduce suitable operating practices so that the risk of a similar explosion was made acceptably small. As a result, a group of engineers, industry representatives and safety professionals has developed a set of Structural Integrity procedures for adoption in this industry. These mills and their operation are very different from the structures in those industries that normally draw upon Fracture Mechanics-based Structural Integrity principles. Hence, the development of these SI procedures for the ceramics industry is a good example of how their rapid acceptance could benefit traditional industries. Ceramics materials are ground wet, and so the mill must be sealed when running, to keep the resultant slurry inside. Since grinding produces a large amount of heat, the pressure in the mill rises as the water vapor and air inside attempt to expand. The mill therefore becomes a pressure vessel if it is left running long enough for the internal pressure to reach significant values. There are more than 250 ball mills in the UK industry. At least one half of those in the ceramics industry have cast iron ends, as did the mill that exploded. They are cylindrical, and rotate around a horizontal axis. Their construction typically involves a steel cylindrical wrapper, fastened, in different ways, to the two end plates. The end plates are therefore the critically sensitive components. The paper describes the work to assess the sensitivity of the cast iron end plates to the presence of cracks, and how this can be related to regular operating practice in the industry. The structure of the industry makes it heavily constrained against large capital expenditure, so effective SI procedures must be affordable by each company. The practicality of the procedures depends crucially upon non-critical flaws being observable by eye for a long period of mill running. As a result, the planned SI procedures can be implemented through a programme of education and training of all plant personnel and their supervisors and managers. This is an excellent example of how SI practice depends upon effective working of all relevant staff in an industry.