(38c) A Simple Way to Calculate Control Loop Service Factor Taking in Account Plant Conditionals

Process safety considers several layers of protection to avoid that our process achieves an undesirable and dangerous situation. One of the most important of these protection layers is the basic process control system (BPCS). Nowadays, this protection layers is usually performed by distributed control systems (DCS) or programmable logic controllers (PLC), where each layer is configured as a control loop. Nevertheless, having these control loops configured is not enough. It is also necessary that the loop is closed and performing its job well. Although there are in the market several systems that provide means of analyzing control loops, they compete with the advanced process control (APC) and Process Information Management Systems (PIMS) in the acquisition of data from the BPCS, requiring a dedicated OPC server to get data in the required frequency. Secondly, many of the indexes calculated by them are of difficult interpretation by people outside the automation and process control environment. Thirdly, the information needed to calculate many of that index are already stored in PIMS systems. We have learned by experience that if a control loop is helping the operators to do their job, they will keep them closed on its correct operating mode (AUTOMATIC, CASCATE, REMOTE CASCATE, etc.). Based on this assumption, we decided that one of the simplest ways to track the performance of our control loops was to calculate the percent of time that control loops work on their target operating mode. This KPI is called by different name by different companies. We have chosen to call it service factor. All the information needed to calculate it is already stored in our process management information system (PIMS). To calculate this KPI is simple but there is a complication factor for continuous plants: conditionals. As soon as we started presenting this KPI for the plant engineers, we started receiving tons of justifications of why some loops were having a poor service factor. The problem was that the plant operation is not a steady state process. There are many situations when operators must break a loop (e.g.: during a reactor regeneration, unit start-up, etc…). In this paper we are going to present a simple approach using MS Excel and some PIMS store procedures to automatically take in account these conditionals and how the control loops are managed.