This article was printed in CONTROL's April 2009 edition.
By Dr. James Ford
Advanced Process Control (APC) is a fairly mature body of engineering technology, with Model-Predictive Control (MPC) being universally accepted as the preferred APC technology for most continuous refining, chemical and petrochemical processes. Almost all refineries and many chemical and petrochemical facilities employ one or more versions of MPC software, such as DMCPlus from AspenTech, RMPCT from Honeywell, Connoisseur from Foxboro Invensys, PredictPro from Emerson or others, to reduce plant disturbances and operate closer to constraints, and increase production, improve yields and quality and reduce energy consumption. Unfortunately, a large, bothersome portion of installed and commissioned MPC controllers have fallen into disuse and the failure list gets longer every day – in short, expensive APC software has turned into “shelfware.”
This article will discuss reasons why APC and, in particular, MPC have encountered difficulties and will give recommendations to improve the likelihood of successful APC implementation to help you avoid having your APC software turn into costly shelfware.
Early and Recent Developments
The first 20 years of APC focused on mitigating the destabilizing effects of process disturbances, both measured and unmeasured. Measured disturbances were handled by techniques such as feed-forward, decoupling and compensating, while unmeasured disturbances could only be dealt with via various feedback or “servo” control techniques (PID control and then later by model-based techniques, such as Smith predictors). These older APC technologies are referred to as Advanced Supervisory Control (ASC) and Advanced Regulatory Control (ARC). Figure 1 shows a typical pre-MPC control hierarchy diagram with examples of different APCs.
Figure 1. The hierarchical approach to APC design identifies the causes of disturbances in each part of the process and then layers solutions that deal with disturbances on top of the basic control system from the bottom up.
Unless implemented with some rather sophisticated decoupling control action, most ARC and ASC strategies tended to ignore multivariable control interactions, that is, the fact that one dependent control variable (CV) can be influenced by more than one independent, manipulated variable (MV). MPC was developed to deal with the multivariable nature of many control problems, and it has become the preferred technology for multivariable control problems and just about any control problem more complicated than simple cascades and ratios.
Prior to MPC, most successful APC engineers used a process engineering-based, hierarchical approach to developing APC solutions (Figure 1). The foundation of the control hierarchy is “basic” process control, and these single loops and simple cascades appear on P&IDs and provide the operator with the first level of regulatory control.
Most process units in refineries and chemical plants are very complex, highly interactive and subject to frequent disturbances. The basic control system is incapable of maintaining fully stable operation when challenged by these disturbances. Thus, the emergence of APC. The hierarchical approach to APC design identifies the causes of disturbances in each part of the process, and then layers solutions that deal with disturbances on top of the basic control system from the bottom up. Each layer adds complexity, and its design depends on the disturbances being dealt with. In reality, the ASC/ARC borderline in Figure 1 is rather fuzzy, and so this picture is somewhat oversimplified, but useful for discussion purposes. Some of the important advantages of this approach are
- Operators can understand the strategies. They appeal to human logic because they use a “systems” approach to problem solving–solving a big problem by breaking the big problem down into smaller problems and solving each of those–and they can be presented to the operator to look like normal cascades;
- The control structure is more suitable for solution at a lower level in the control system and often can be implemented without requiring added hardware and software;
- The controls “degrade gracefully.” When a problem prohibits a higher-level APC from being used, the lower-level controls can still be used and can capture much of the associated benefit.
Figure 2. MPC replaces the entire middle section of Figure 1, and the control hierarchy degrades to this diagram.
MPC replaces the entire middle section of Figure 1, and the control hierarchy degrades to Figure 2. The industry-accepted approach to MPC design is non-hierarchical, that is, the controller outputs directly to basic control set points or valves. There is, in fact, no hierarchy–just one large, flat MPC controller on top of the basic controllers moving all of them at the same time. Operators rarely understand them or what they are doing. In addition, they do not degrade gracefully. They are either “on” or “off.”