What is Output Distribution? How Is It Helpful?

Demand Less to Produce More: Keep Final Control Elements Away From Their Extremes

Most of us know what it means to give 110%. Whether by a coach urging players to finish strong or a supervisor encouraging staff to hit a project deadline, many of us have been asked to go beyond some normal expectation of output. Even though we’re limited to giving 100%, operating at such a high level can result in great achievements. When that high octane performance level is required for only a brief period the impact on staff can be minimal. If demanded for an extended period, however, the impact can be long lasting and even debilitating. We’re only human, right?

It’s no surprise that process instrumentation has similar limits. Physical constraints limit final control elements to 100% of their operating range. Although valves and dampers can operate at their extremes they’re typically designed to function within a more moderate range. As with us, there are long-term consequences associated with operating equipment at an extreme for extended periods of time. What’s more, the negative impact can go beyond any single piece of production equipment and effect the process at large.  In that regard, demanding less of instrumentation can enable your facility to produce more.

Output Distribution is a key performance indicator (KPI) that sheds light on the performance and health of a facility’s process instrumentation. It calculates the distribution of a given valve or damper’s position during closed-loop operation. Equipped with this information production staff can proactively identify mechanical issues and/or challenges associated with control loop design. Additionally, staff can utilize it to proactively address instrumentation issues, thereby avoiding any negative impact and getting the most out of existing production resources.

• The Hot Seat

Practitioners often refer to operation of a final control element (FCE) close to its lower extreme as (near the seat). In the case of a valve or a damper this corresponds with the fully closed positions, and it is indicative of a FCE that is sized incorrectly. If the Output Distribution shows operation near the fully closed position, then the FCE is most likely too large and designed for higher capacity applications. Conversely, operation near the fully open position indicates that the FCE is undersized and incapable of delivering the necessary throughput.

• Constant Change

Production processes are highly dynamic and modest changes in their rate of flow can be expected. A sizable shift in output, however, can be indicative of a problem upstream. The Output Distribution metric can be used to detect such a change. A visible “step” in the data provides the time stamp and simplifies root-cause analysis. If the data spans the metric’s full range, then the FCE is probably poorly suited for controlling the process as it is showcasing oscillatory behavior.

• Nonlinear Shifts

Changes to the Output Distribution can also be associated with a shift in a control loop’s operating range. This is particularly true of nonlinear processes. Variation in the Output Distribution metric can be used to identify these control loops. Application of an adaptive tuning strategy may both improve control and enable the FCE to operate away from its extremes.

Output Distribution is one of several KPIs in the PlantESP Loop Performance Monitoring platform that enable process manufacturers to proactively identify and correct control loop performance issues. It alerts staff to operation that is potentially harmful to the associated instrumentation. Additionally, it enables staff to take appropriate steps to correct issues before other parts of the production facility are put at risk.