State-based control isn’t new but chemical companies now are showing increasing interest in the approach, according to vendors such as ABB, PAS, ProSys and Siemens.
This has much to do with the 2010 launch of the ISA106 committee on Procedure Automation for Continuous Process Operations by the International Society for Automation, Research Triangle Park, N.C. Prior to this, only a small number of chemical companies, notably Dow, had extended the concepts of batch operations into their continuous manufacturing environments. The new standard brings an opportunity to formalize this activity.
“Some companies had automated portions of startup procedures, equipment switching procedures, etc., but few did things on a grand scale. And part of the drawback was also the horsepower and performance factors on the process controllers to support running large-scale procedures for long periods of time,” notes Dave Huffman, OGP business development, chemicals, ABB, Wickliffe, Ohio.
ISA106 defines a process state as “a definable operating condition of process equipment as it progresses from shutdown to operating and back to shutdown. Each process state represents a unique operating regime that supports the process equipment’s objectives of processing an input into a desired output.”
“To more clearly state this in terms of the state-based control objective, it is a definable operating state in which the control system needs to take action to achieve the desired objective for that operating state,” says Dustin Beebe, president/CEO, ProSys, Baton Rouge, La.
So, a state is any well-defined phase in which a system can exist. The phase can be either steady state (such as distillation) or transitional (such as heating up), where specific process values are being controlled to a desired set point.
“Each phase must include definition of the specific inputs that allow entry into the phase, as well as those required to exit to a subsequent state. Unlike a typical sequence of a batch system, which is executed and ultimately completes, a state-based control system is always in one — and only one — of its defined states. Examples of states may include: stopped, ready to start, starting, production, stopping. fault, etc.,” explains Paul Morgan, senior consultant in process automation, Siemens USA, Spring House, Pa.
But why choose state-based control in the first place? “There are many reasons why state-based control is the way to go. Process plants have become increasingly more complex, leading to greater potential for accidents due to human error. The challenge is to optimize production rate and increase uptime without compromising process safety. State-based control presents a proven and effective strategy for achieving safe and profitable production. It utilizes historical production information… and the knowledge of the best operators to automate complex processes,” says Eddie Habibi, CEO, PAS, Houston.
Morgan points to four key benefits: better safety (through risk reduction), improved alarm management, reduced operator workload, and better maintenance planning with asset management.
“Improved safety is one of the key benefits to utilizing state-based control. The startup and shutdown operations of the continuous process are arguably the states most likely to encounter a dangerous demand to trip, yet these states are frequently manually monitored and controlled. Couple this with the high demands on the operators to achieve full operation, and mistakes are more likely to occur,” he says.
Such situations also can complicate alarm management, which he describes as a critical issue at the corporate level. With state-based control, alarms not appropriate during certain phases of the plant can be concealed from the operator using smart alarm hiding. As a result, the operator doesn’t get distracted by nuisance alarms and can retain focus. However, these alarms still are recorded in the archive system for completeness.
Alongside the lack of distractions, the automated nature of state-based control also helps reduce operator workload by making a far simpler and less stressful task of both startups and shutdowns.
Lastly, with state-based control, maintenance of the plant’s assets can be better predicted. Starting and stopping devices typically is harder on them than continuous operation. “Having visibility into these demands allows scheduled maintenance, reducing unplanned plant downtime due to failures,” he adds.
ABB’s Huffman notes that any company interested in continuous improvement in its manufacturing discipline will have an interest in procedural control. After all, he points out, the company already has written procedures that generally describe, in a lot of words, how the process should be started up or shut down, how to prepare the process for maintenance, how to make product grade changes, etc.
But, he asks, are these procedures followed to the letter every time? And should they be followed exactly as they are written? “In many facilities, the answer to both of these questions is ‘no.’ Frequently, the written form of the procedure is out of date because minor changes to the process over a period of time have not been captured in the formal procedures. Handwritten notes may be scribbled into sections of a ‘working copy’ that are not part of the actual procedure document file, or information is simply missing that some or all of the operators just seem to know but is not written down at all,” he notes.