Commissioning Control Systems – Closely Coupled Process

In a previous blog, I addressed some of the challenges of commissioning a multiple input/multiple output (MIMO) process that is characterized by process interaction. Also, in Chapter 12 of Control Loop Foundation – Batch and Continuous Processes  we address the special considerations associated with tuning an interactive process.  In such a process, each manipulated input may impact multiple controlled parameters. In general, this interaction may impact the control of two or more loops. A cycle may be established in which the loops fight each other. Similar loop interaction may also be observed in a closely coupled process where interaction and/or positive feedback is created by the process design.

At Emerson Exchange this fall, I will be a co-presenter in a workshop on “Commissioning Highly Interactive Process – An Approach for Tuning Control Loops”. In this workshop we will examine a skid mounted process for CO2 recovery that is characterized by a high degree of interaction. The size limitations of the skid allows for little liquid and vapor holdup between flash vessels, heaters and heat exchanger which contributes to this being a closely coupled process. A picture of the process is shown below.

In the workshop, details will be provided on the approach we took in tuning the control and the performance achieved. Commissioning the control loops associated with a highly interactive process may be complicated by the process interact. Automated tools for commissioning PID control loops are not designed to take into account loop interactions. The order in which loops are tuned was planned based on the process design. Following this commissioning order all loops were commissioned using DeltaV Tune. An improvement is the control response to process disturbances was noted after the loops were tuned to minimize the impact of process interaction. If you attend Emerson Exchange, then this workshop may be of interest if you face similar challenges in commissioning your control system.