Interesting facts about Advanced Process Control (APC)

Companies that have implemented successful Advanced Process Control (APC) projects suggest they’ve done just that and even better. Whether its new drivers, process controllers, or economic issues like higher yields, lower energy costs, and better financial performance, all these benefits could be enjoyed by the companies using this specific method.

How does Advanced Process Control (APC) operate, and how does it deliver on such lofty promises in the real world?
The common objective of Advanced Process Control (APC) is to find a way to manage complex interactions within a process better than primary process control. Primary process control works best when one variable and one actuator are related. However, often moving one actuator affects several variables at the same time. This is where primary process control methods lose some effectiveness and using advanced process control method is required.

Where does Advanced Process Control (APC) work?
When considering Advanced Process Control (APC) deployments, some are better candidates than others. Four typical elements can be used for this method are-

  • – The process that has high interaction
  • – The process that is highly constrained
  • – The process that has high energy costs
  • – Process with a high product value recovery

Where is the economic motivation of Advanced Process Control (APC)?
The word that comes up most frequently in discussions of Advanced Process Control (APC) is optimization. In this case, it means that the system can balance all the interacting elements in a way to achieve the most desirable outcomes laid out by plant management. Often the new strategy allows things that were formerly considered tradeoffs to advance in parallel beyond points that were considered possible with primary process control. Instead of looking at a problem in series, this system looks at all of the different information coming in as a single problem. Depending on the type of technology, and how it is configured, it will, in the best case, have a business or economic driver that will be something like:

  • – increase feed to the plant
  • – create higher-value products
  • – reduce energy consumption

How to install Advanced Process Control (APC)?
A large-scale of Advanced Process Control (APC) systems require a great deal of computing power since the system is constantly churning through extensive calculations. Consequently, early systems were typically installed separately from the installed distributed control system (DCS). Often this is still a logical choice since the system equipment can be selected from any vendor and does not need to be from the same DCS hardware provider. This has its own challenges in that DCS and APC systems have to be rationalized, but it avoids having to upgrade the DCS.

More current DCS platforms often have the capability to support embedded APC functions, which is particularly useful for smaller process units. This is certainly the simplest from an implementation standpoint and typically makes it the most transparent to the operators, but requires that you have a relatively DCS recent system to find such APC functionality. Additionally, the same DCS processors now have to do the APC related calculations along with normal process control duties, and this can compromise responsiveness depending on the complexity of the system.

Some companies choose to create their own Advanced Process Control (APC) strategies, beginning with designing their own process model. This may be forced on a company due to the extreme nature of the process involved that precludes the availability of an existing solution. Others believe that making is less costly than buying.

Suffering relapses
History showed us that all Advanced Process Control (APC) projects don’t succeed. There are various reasons, but most center around two main areas. Some project fails because the plant is not designed in maintaining equipment. APC won’t overcome valves that stick or unreliable sensor data. If field devices begin to behave erratically, the system attempts to tweak the operation, making the disturbances from minute to minute worse, while operators blame the main system for this behavior.

Other APC installations suffer from operators that simply cannot allow the system to do its job without human intervention. With APC ability to press limits and work against constraints, the system can push a process into areas where operators fear to tread. When operators give in to their fears, much of the system’s benefit is already lost. APC systems that aren’t used can get turned off permanently and the company loses its faith in the technology.

Companies that choose hold on to the Advanced Process Control can enjoy its benefits to the fullest. In order to make this happen, the plant has to maintain the top-notch performance of field equipment, combined with engagement from operators to let the system do its job. These very basic concepts make all the difference.

–Author: Peter Welander, Control Engineering

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