During the past three years, the Pryor, OK plant has experienced recurring problems with the clinker cooler drag conveyor’s hold down roller on its Kiln 2 production line.  Having logged 36 kiln stoppages (265 hours of downtime) during that time, the plant made several attempts to address the causes of the failures.  In 2017, the plant abandoned attempts for modification and instead focused on completely redesigning the assembly.

The clinker conveying system consists of a single 300mm T-link drag that runs horizontally under the clinker cooler and then has a 15o incline from just past the clinker breaker to the discharge into the storage hall. The purpose of the hold down roller assembly is to maintain clinker conveying through transition to the incline.  Without the hold down roller, the drag chain would rise out of the material bed causing buildups that would lead to kiln stoppage.  The original design had a rigid mounted toothed sprocket roller at the transition to keep the chain against the bottom of the trough.  This design was problematic due to the inability to handle volume and size variations in the clinker.  When the clinker breaker hammers are new, the problem with clinker size variation is not as frequent.  However, as hammer wear progresses, it becomes increasingly difficult to keep the chain on track in the sprocket roller.  In addition, the rigid mounted roller did not tolerate clinker volume variation (from kiln pushes) and in a high volume events, acted as a dam in the conveying system. The buildup behind the roller, then caused excessive roller wear, further contributing to tracking issues and shut downs (e.g. the roller was replaced 17 times since 2015).  In an effort to improve reliability of the conveying system, the plant formed a reliability team consisting of key maintenance and production personnel.

One of the chief concerns for the team was to address both the tracking and damming issues in a single comprehensive design.  The team first opted to replace the toothed sprocket roller with a solid cast drum roller.  The prototype roller consisted of a 225mm cast roll with a cast in shaft.  Wear bars were installed on the cast roll to prevent wearing of the drum itself.  With a weight of 150kg, the team felt that this roll would deliver adequate downward pressure and resolve the problematic tracking constraint of the old design.  To resolve material damming issues, the team modified the design to allow the roller to lift slightly by utilizing a pivoting mounting assembly that would elevate over material buildup in the event of clinker surges.  The design and fabrication of the roller mounting assembly was completed inhouse in preparation for installation during the 2018 kiln shutdown.

The ideal weight of the hold down roller was estimated during the design phase and the plant had to add 30kg of additional weight during commissioning to improve the tension of the conveyor chain and achieve the optimal 25mm vertical sag recommended by the manufacturer.  Finally, the plant Automation Engineer added an amperage limit safeguard to minimize the clinker cooler material discharge in the event of a drag conveyor overload. Though it is still early in the 2018 campaign, no downtime has occurred due to hold down roller performance, and the Pryor plant is optimistic that the newly designed roller will continue to perform well and allow us to meet the kiln reliability target of 97 percent.