As the business environment continued to improve in 2016, one of the main challenges faced at the Chattanooga facility was to improve finish mill reliability. Maintenance department personnel volunteered to review issues with the finish mill and its auxiliary equipment. The recurring issue, which stood out to team members, was with the MODCO cement conveying system, which conveys finished product to our cement silos.  Over a two-year period from 2015 through 2016, the MODCO system accounted for 14 percent of the finish mill downtime. Whilst examining the downtime in detail, the performance statistics revealed that over this period of time 93 mill stoppages or 166 hours were attributed to the MODCO, or 37 percent of the unscheduled mill downtime. Finish mill reliability performance was approximately 90 percent over this period. Improving the MODCO reliability was paramount.  

The MODCO system, commissioned in 2000 as part of the plant expansion, is a dual pod pneumatic system used to transport cement from the finish mill to the cement storage silos. It uses an arrangement of valves to pressurize, aerate, vent, fill and discharge the pods.

Cement is conveyed to the pod, a “radish” shaped vessel, through a flow control valve on top of the pod. When the pod is full, the outlet valve on the bottom opens and the pod pressurizes with sufficient air to ensure an even flow of material from into the transition chamber, where blending with conveying air occurs. The transition chamber is designed to produce laminar flow of the air-material mixture as it enters the pipeline, enhancing the energy efficiency of the system. A PLC manages the cycling operation of the pods.  

The particular issues experienced with the MODCO was reliability performance of the vent and butterfly type aeration valves. Premature failures due to high velocity airflow coupled with cement abrasion were the root cause. The failure of this equipment occurred in the valve body, the sealing surface and valve stem packing.

Vent valves used a single disk composed of cast stellite material, with a carbon steel body, which lasted approximately three months.  Upgraded valves were purchased and trialed. This new application incorporated ceramic internals to combat the abrasive media.  The gate seat of these new valves is spring loaded to ensure a tight seal to prevent cement particles from cutting the disk and body. The ceramic lined gate seats were the key to improved performance.  

Butterfly valves required replacement due to wear issues of the seat and failures of the actuators. The experiment with a new high performance butterfly valves commenced in 2016. There were four valves installed, two on each pod’s air supply lines. The high performance butterfly valve is unique due to its protected seat design. The seal material composition is polytetrafluoroethylene. The stainless steel seat protector’s purpose is to protect the seal from the abrasive environment. These valves have now realized close to two years of service life, versus the previous version, which lasted approximately four months.

The improvement in valve life has been significant due to the efforts of the maintenance project team.  

This maintenance improvement project has allowed us to achieve a finish mill reliability of 96 percent and helped us to reduce our maintenance costs over the last two years.