Continuous improvement of continuous processes

How ISA106 and procedural automation work to extend the benefits of batch standardization to continuous operations.

By Greg McMillan and Stan Weiner

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Greg: Batch processes have benefited enormously from the ANSI/ISA 88 Batch Control Standard. Here we look at what can be done to help provide the automation to increase the profitability, safety and reliability of continuous processes. I have always sought to find concepts that provide an understanding to see the commonality rather than getting lost in the details. It is kind of like the difference between an aerial view of a wilderness rather than being deep and lost in a forest. Some seem to enjoy thinking a process is so unique that only they can provide what is a totally special solution. What excites me are the lessons and principles developed from all processes that enable us all to make improvements. This is a key to the advancement of the profession. If every solution is totally unique, we don’t have a profession. Could it be that one of the reasons why there are not many universities in the world offering bachelor degrees in automation and process control is a lack of understanding and documentation of what is common in terms of practical proven solutions?

Stan: Dustin Beebe, president and CEO of ProSys and member of the committee for the Technical Report ISA-TR106, “Procedure Automation for Continuous Process Operations—Models and Terminology,” provides us with much needed insight, understanding and guidance to achieve an overall philosophy and approach that provides a framework for solutions to applications that may seem to be too complex and diverse for generalization.

Greg: In the engineering technology department where I spent most of my career, we made the most of the synergy of modeling and control. I tend not to see this today in specialty chemical and biological process applications. Dustin, what do you see?

Dustin: Increasingly recognized are the opportunities of procedural automation and state-based control, and the role of first-principle dynamic models in making the most of these opportunities.

We’re asking ourselves, how can we automate the actions by the operator, such as changing modes, outputs and setpoints for different states? If we can do this, we have repeatable actions that can be continuously improved to increase process safety, capacity and efficiency. The role of the operator is elevated to be more of a troubleshooter.

This automation can also deal with operator turnover. Not only is there a generation of operators retiring, but there is the risk that replacements may only stay in the job for a few years before moving on. The average amount of time Millennials stay in a job is about 18 months. We have larger issues than just training the replacements of retirees. Also, what was acceptable as a risk 20 years ago is far from permissible today.

The success of the ISA standard on batch control points to the need for a standard for continuous processes. We have the first step in terms of the ISA technical report on procedural automation to define the terminology and concepts and set the stage for a standard.

In batch processes, you automate every action. What we do today in continuous processes is not sufficiently defined or understood. We don’t need to automate all the actions for all of the operational states in the control system. We justify what needs to be automated in continuous processes differently than batch. We use state-based control as an effective style of procedural automation. For example, there may be operational states for each piece of equipment in terms of being idle, preparing for startup, and then filling, pressurization, heating up, going on temperature and composition control, running, making transitions in terms of product changes, and dealing with abnormal conditions such as failures, severe upsets and shutdown. The unit operation is always in a state.

An important aspect is preventing escalation of a problem to activation of the safety instrumented system (SIS) and possible shutdown. An automated, localized and coordinated response can be taken to operate in a degraded state that keeps a unit running. For example, consider the case of a loss of catalyst. There is five minutes of hell building up reactants. Instead of a shutdown, state-based control can manipulate a whole bunch of conditions to minimize reactant buildup.

Greg: Shutdown of a unit operation has far reaching consequences in terms of disruption of downstream and upstream operations, the suppliers of raw materials and users of the products. Shutdowns and startups stress equipment. The states during shutdown and startup are often the most hazardous and least efficient states. The loss in profitability goes way beyond the obvious loss in onstream time.

The November 2009 Control Talk column, Show me the Money – Part 1 mentions how Michel Ruel has eliminated manual operation and automated the best of operator actions.

When operators say a start or special mode is too difficult and situational to automate, I take this as a clue that there is an even greater upside including benefits by me taking the best of the operator actions and using knowledge from a working prototype to eliminate any operator actions. I have automated a lot of startups as seen in humorous retakes, such as “Wally and the Beave Automate Reactor Startups” in the Momentum Press book Process Control Case Histories: An Insightful and Humorous Perspective from the Control Room.

A particularly memorable experience was a centrifugal compressor that would shut down multiple times on each attempted startup. Operators claimed it was impossible to automate and that the startup was so tough, only they could do it. I found out from several operators their version of what they had to do. I thought about the first principles and what I knew about compressor control, and used a simulation to develop and test a strategy. The automation of the startup got the compressor running in a matter of minutes without any shutdowns. Previously, it might take one or more shifts to get to this point. The automation was able to preemptively deal with the very fast dynamics associated with compressor pressure and surge control. Operator actions are not fast enough, knowledgeable enough, multivariable enough, attentive enough or consistent enough. Also. operators lack the ability to anticipate the delays and trajectories of variables.

Trajectories of process variables into the future, as seen in MPC operator graphics and by means of simple computation for PID control (as noted in the 6/26/2016 Control Talk column “Future PV Values are the Future” can help mitigate the misinformation created by digital displays of current values and human nature. The trajectories help forgo human impatience and lack of anticipation. I can’t overemphasize how important these trajectories are for improving automation and human understanding.

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