Most people would classify multi-variable controlor advanced process control (APC)over the past 20 years as a slam dunk in terms of process control, automation and optimization success. But it can also be viewed, quite plausibly, as a major diversion that derailed the modern distributed control system (DCS) from achieving its natural place at the center of process control and automation progress. In the process, vast amounts of time, money and talent have been diverted from much more essential process control needs, in favor of APCs much over-estimated benefits.
As industry comes to terms with the idea that APC may be just another tool in the kit, rather than a game-changing technology, it will help the process control community to find renewed traction and move forward if we adopt a fresh paradigm based on the lessons we have learned, because a world of control and automation opportunities remain, but they are not going to come from APC.
Losing the pyramid.
The traditional pyramid paradigm, with APC prominently at its center, served well to initially focus industry on this promising technology. But the situation we find ourselves in today is an over-emphasis on APC on the one hand, even as it continues to exhibit intractable limitations on the other hand, such as:
Traditional control-loop design is the product of experience, not the lack of APC. Some handles are suitable for closed-loop control, but the majority are not, given the realities of industrial process and equipment operation. Most APCs are built on the opposite assumption that including all handles and interactions is optimal, and as a result, quickly devolve online, in hours or days, not months or years, to utilizing basically the same limited number of handles that were in use before APC ever came along. This is the well known clamped MV problem.
The APC algorithm is a good algorithm, but it is only one algorithm. DCSs have hundreds of algorithm function blocks that are readily combined in myriad ways to deliver smartly tailored solutions to almost any control or automation challenge. And again, the DCS function set is no accidentit derives from a long tradition of process control success.
APC is the enemy of agility. It is in vogue now, as it should be, to treat process plants as manufacturing plants, where agility is better understood to be essential. But the business and engineering model of APC, with its five-year life cycle, interim inflexibility and high cost of changes, is entirely at odds with the concept of agility.
A fresh paradigm will help the process control community get past APC and get on with the business of process control and automation. Fig. 1 shows one possibility. Its a working paradigm in that it serves not only as a point of departure for process control planning, but also to measure and guide progress at both management and engineering levels. It shows the potential contribution of multiple essential process control competencies and the portion that has actually been captured.
The first bar, something we are all familiar with (a modern DCS), provides a reference value for the others. For example, smart-field devices have the potential to bring as much additional improvement as the original DCS (another 100%). The percentages in Fig. 1 are overall industry estimates, but each corporation and plant site can put in their own numbers and appropriate competencies.
| Fig. 1. Proposed new process control |
paradigm emphasizes multiple essential
competencies and serves as a working
guide at both management and
Notably, huge opportunity remains untapped in the smart field, smart DCS control, and smart safety system areas. Far from being done, process control and automation are just getting started! HP
|The author |
Allan Kern has 30 years of process control experience and has authored numerous papers on advanced process control, decision support systems, inferentials, and distillation control, with emphasis on operation and practical process control effectiveness. Mr. Kern is a professional engineer, a senior member of ISA, and a graduate of the University of Wyoming.