By Dan Hebert, Senior Technical Editor
Many batch operations have been standardized and improved through implementation of the ISA88 batch control standard. First published in 1995, the standard has become the common language to describe batch applications in the process industries. But ISA-88 doesn't have to be strictly limited to batch processes. Most processes defined as continuous are, in fact, a hybrid of batch and continuous. For example, an oil refinery operates continuously in normal situations—but converts to batch operation in abnormal conditions.
When an ISA88 philosophy is applied to the procedural operations of continuous processes, Yokogawa labels it Modular Procedural Automation (MPA). "We're using ISA88 principles to address procedural operations in industries that are outside the traditional batch realm," says Maurice Wilkins, vice president of Yokogawa's Global Strategic Marketing Center in Dallas, Texas.
A procedural operation is a set of tasks conducted the same way time after time to achieve a certain goal, such as starting or shutting down a unit. These operations can be manual, prompted or fully automated.
Batch processes are procedural, but typically involve sets of procedures running in parallel on varying process units, and almost always need to have built-in flexibility. ISA88 has defined models and terminology for procedural operations in batch processes, and one of the key aspects of the standard is the ability to run these procedural operations flexibly.
Procedural operations are also used in continuous processes for many tasks including grade changes, transition management, start-ups, shutdowns and emergency procedures. "In a recent study, the ARC Advisory Group indicated that continuous manufacturers now see effective and repeatable transition management along with the use of operator sequences as a competitive advantage," notes Wilkins.
"The procedural operations we're referring to would typically only apply to one process unit, and would in many cases need to do the same thing the same way every time under any circumstances. That is not to say that these procedures wouldn't be complicated. In fact some may even involve the use of advanced process control," explains Wilkins.
Automated procedural operations have been used in continuous processes, but they have typically been set up using strings of function blocks chained together or by straight line-coded applications. This is inefficient, hard to standardize and prone to error.
Non-standard procedural operations raise several problems. Plant operator knowledge and skill sets are leaving the workplace due to retirement. So are many of the engineers who designed these straight-line procedural applications, and modifying these spaghetti-coded applications can be difficult.
Safety is also a concern. "The cause of some recent industrial accidents has been partially due to the lack of good procedural-based emergency shutdown operations, or to an abnormal situation putting too much pressure on an operator in a crisis. This pressure can initiate out-of-sequence procedural operations with possibly disastrous consequences," notes Wilkins.
"MPA uses the ISA88 modules below the unit, such as phases, control and equipment modules. This modularity provides standardization and promotes repeatability and reproducibility across units and sites. It also improves safety, engineering, operator training and the retention of operator knowledge," he continues.
"The ISA88 standard has revolutionized the design, implementation and operation of batch processes. We think a new Procedural Automation Standard for Continuous Processes can provide the same revolution to the procedural operations of continuous industries," concludes Wilkins. He, along with other ISA88 veterans like Dave Emerson, Marcus Tennant and Control editor Walt Boyes have submitted a proposal to ISA to begin such a standards effort.