Organizing batch process control
By Bill Hawkins
The ISA-88 standard has had a major impact on increasing productivity in industries where batch manufacturing is used. Before 1990, the only standardization for batch process control came from the fixed offerings of vendors. Creative people did marvelous things with what was available, all differently. The computer was added to the available technologies, bringing many more ways to use them differently. People who operated batch processes began wishing for standardization. This became acute as acquisitive companies tried to sort out the different methods of process control in plants they had purchased.
Tom Fisher wrote and published "Batch Process Control" in 1990. While he was writing it, he started ISA88 in 1988 under the auspices of ISA. The committee discussed the ideas that Tom was putting into his book. Those ideas evolved until 1994, when balloting began on Part 1 of ISA-88.
The first ISA-88 standard formed the foundation for the standards that followed. We were careful to get it right and very nearly succeeded in 1995. It took 15 years for the next revision. By that time there was a large user base that would mightily resist major changes. That user base included vendors who developed systems as soon as possible to remain competitive and now had no resources to do another. Some of them missed the finer points of the standard, so there are different interpretations of the standard for sale. Once the vendor is chosen, the standard becomes what the system will do, not the words in the ISA standards. And it isn't perfect, but it's a lot better than it was before 1995.
The following is an abbreviated overview of the 2010 revised standard.
Physical models for batch processes
A generic batch process hierarchy starts with a process, which consists of process stages. These stages include physical and chemical changes to the raw materials, such as grinding ore or reacting chemicals to form new compounds. Below stages are process operations that do major activities, such as charging a reactor, reacting, or finishing the reaction. Below operations are process actions, such as add a material, transfer heat, and agitate.
The physical equipment used for batch processing begins with a unit, which is a vessel that contains part or all of one batch during a process stage. Above the unit may be a process cell, containing multiple units required for processing one or more batches. Below the unit may be equipment modules or control modules. Equipment modules roughly correspond to process operations. Control modules correspond to process actions.
Notice that the process hierarchy and the physical model both have four hierarchical layers. Later work on recipes revealed that physical layers were required for the enterprise (general recipes), site (site recipes), and areas. Areas were thrown in to make a seven-layer model, which was very popular at the time.
Batch process control functions
The following describes the means that a recipe would use to make a product. We settled on three kinds of control for batch processes: basic, procedural, and coordination.
Basic control is what holds the batch in a state given by one or more setpoints. Examples include discrete, regulatory, sequential, override, interlock, and alarm functions. Whatever it does, it does the same way regardless of the product being made. A control module may be as simple as a process measurement or a switch, or as complex as PID loops cascaded from an analyzer or sequential control that sets the configuration of a piping maze and washes it.
Procedural control determines the order in which basic control functions will be activated and establishes the required setpoints. It may be done by a person following a recipe, or it may be automated. If a control module follows a procedure, that procedure is built into the module as a fixed sequential control scheme. Control modules do not do procedural control in the sense used here.
There are four procedural elements in the procedural control model: procedure, unit procedure, operation, and phase. They are a means for splitting a complex problem into reusable modules with increasing amounts of detail.
A procedure is used to make a product. Execution of the procedure orders the start of unit procedures. A unit procedure is specific to one type of unit. Execution of a unit procedure orders the start of operations and may request the services of other units. An operation is a procedure that directs a group of phases to accomplish a major processing function. A phase directs the activities of control modules to directly affect the process.
Coordination control directs the activities of procedural control. It is required when a batch needs more than one unit for processing, or when a unit needs the services of shared equipment. It may require negotiation using prioritized requests from procedural control.
Tom Fisher drove the important concept that physical equipment alone was unable to do any processing. It needed control, so we had to model what we meant by equipment animated by control. The equipment described in the lower four levels of the physical model becomes controlled equipment in the standard. Procedural control elements combined with equipment entities can perform process functions that are necessary to make a product.
Typical process/procedure/equipment mapping to achieve process functionality
There are four types of recipe, as shown in "Recipe types model." Each type contains information required for identification and version tracking, a formula section with data for materials and processing conditions, equipment requirements for processing, the recipe procedure for making the product, and other information, which may be links to material safety data sheets and other things that are specific to the product made by the recipe.
The general recipe contains enough information to make the product anywhere, once transformed to use the processing functions available in a particular facility. It contains equipment requirements, but no references to specific equipment unless absolutely necessary. General recipes are developed and maintained at the highest level of an enterprise, which may be a single plant.
The site recipe is a general recipe that has been translated for the capabilities found at a specific site, presented in the local language. A site can be a plant or a geographic region. ISA-88 does not define plants.
The master recipe is a transformation of a general or site recipe into a recipe which uses the capabilities of a specific process cell. Master recipes are developed and maintained in a plant. The product scheduler for a plant must specify a process cell and its master recipe, and may specify an equipment configuration within that cell.
The control recipe is a copy of a master recipe that has been made specific to the equipment that will be used. The copy may be changed during execution as unplanned disturbances occur. At the end of the recipe, it represents the "as-built" record of batch processing.
Recipe types model
Separation of recipe and equipment control
Hierarchical modular design requires increasing amounts of detail going down the hierarchy. This is clearly shown in the recipe types. Recipes, once validated, are difficult to change, so the minimum detail about the equipment to be used is put in the recipe. The details of specific equipment are handled by an equipment procedural control hierarchy with four levels, from equipment phases to equipment procedures.
Recipe procedural elements are designed by recipe authors consulting with the control engineers who design equipment procedures. Recipe procedural elements are chosen to make products using process functions. Equipment control procedures are designed to provide process functions using specific controlled equipment, without any knowledge of the product being made. Recipes specify what is to be done in the equipment, but they don't tell the equipment how to do it.
ISA-88 does not specify how to link a recipe procedural element to an equipment procedural element, because it is not an implementation standard.
Information flow from general recipe to equipment entity
There are other batch control considerations (see Section 7 of the revised standard).
There are seven areas of control activity defined by ISA-88 as a way of modularizing the design of a batch control system. The top three activities-recipe management, production planning and scheduling, and production information management-interact with ISA-95 in ways that cannot be described in 1,500 words (see section 8 of the revised standard).
Process cell management
This activity converts a scheduling request to a control recipe, selects the cell resources to be dedicated to the batch, initiates the execution of the control recipe, maintains a state machine for each active batch, and cleans up the resource inventory when the batch is finished. It also produces the final batch report.
Control activity model
Each unit must manage its own resources and acquire and release other equipment itself or through cell management. When given a unit recipe, it executes that recipe with the aid of a unit state machine, sending start commands to the process control activity and receiving status messages indicating success or failure. It produces a batch report for the unit.
The commands produced by executing the unit recipe in unit supervision are translated to commands for specific equipment entities. If equipment procedures are involved (not basic control), they are executed by this activity. Otherwise, the commands are sent directly to the basic control points in control modules. Status is returned and, in turn, passed upwards as required. Data on other aspects of basic control are also passed upwards.
Personnel and environmental protection
This activity is outside the scope of ISA-88, so there are no details. A safety shutdown system would be part of this activity.
Completeness, compliance, and conformance
Section 9 of the revised standard contains statements that allow users to evaluate claims of compliance made by vendors.
Other parts of ISA-88
That is a summary of Part One. There are four more parts in the specification.
The complete ISA-88 standards are available at www.isa.org/standards, and ISA members can view the standards online at no cost.
Batch Control Systems - Design, Application, and Implementation, Second Edition
ABOUT THE AUTHOR
Bill Hawkins retired from 20 years at Hercules and 20 years at Rosemount in 1999. He continues odd jobs in process control, including four years of work on Foundation Fieldbus. He wrote Batch Control Systems, Second Edition for ISA and edited four volumes of WBF papers for Momentum Press. He served on ISA50 and ISA88 standards committees and contributed to ISA-18 and ISA-95 standards.