NAMUR
5.1 Structure for Batch Control
There is a three-level hierarchy for the concept of control. The levels are Coordination, Procedural, and Basic control. See Figure7-1, which is not in 88.01. This hierarchy is necessary because each level requires a different way of thinking about a problem. In 2005, each level also requires different control equipment capabilities. This is not so much because different computers are required as it is that each level is worth more than the one below it, but that’s nothing that a free market can’t sort out.
Figure 7-1 Three-Level Hierarchy of Control
5.1.1 Basic Control
At the lowest, most basic level we find Basic control. There is no simpler control. It could also be called Basis control because the higher levels can not work without it. It is Basic control that can change and hold the state of the process material (e.g., pres- sure, temperature, agitation, composition of reactor headspace gas). It is Basic control that can change and hold the state of the equipment configuration (e.g., transfer header, heat exchange media).
Basic control includes the six types of control discussed in Chapter 3—regulatory, dis- crete, sequential, alarm, override, and interlock. Simple examples are the functions of a PID or an on/off device controller. 88.01 adds monitoring and exception handling.
Monitoring may just involve taking the measurement for someone else to monitor or it may mean accumulating trend or event data that is reported on demand. Exception handling is very basic because it is limited to one control module. Normally, exception handling takes place at higher levels that have a broader scope.
Basic control usually is able to respond to process conditions that represent deviation from the desired state and correct the deviation. If a device does not respond then either it is a command input like a switch or it is a monitoring device, devoid of feed- back control. 88.01 does not distinguish between open-loop and closed-loop control.
Basic control must be able to accept commands to change its setpoint, if it has an actuator. The source of commands may be a human or a machine. Basic control may be able to report the status of its efforts. For example, a light switch has a visible posi- tion, if not a visible effect, which serves to report its status.
Basic control for batch processes is similar to that for continuous processes. The batch control environment generates many more command-confirm dialogs to change modes and settings as the procedure progresses. Off is not one of the modes for a PID controller in a continuous process. The Off mode would command a PID to go into manual mode, disable low alarms, and close the valve, all with one command.
Basic control is carried out with physical equipment in a manner that is completely independent of the product being made. No part of Basic control resides in a recipe, although executing a recipe procedure may result in commands being sent to Basic control. The results of the command to Basic control may affect the execu- tion of the procedure.
5.1.2 Procedural Control
The middle level of the control hierarchy is responsible for animating Basic control.
The target states of individual Basic control schemes have to be changed to specified values in a specified sequence. These changes cause the controlled equipment to process a batch of material. The changes (commands) may originate from any process equipment level and propagate down to control modules.
Batch process control is not possible without Procedural control to sequence the activities of Basic control in such a way as to produce meaningful activity. Just as
physical equipment is static without Basic control, so Basic control targets are static without Procedural control.
Procedural control has a hierarchy consisting of procedural elements. See Figure 7-2, which is Figure 6 in 88.01.
Figure 7-2 Procedural Control Model
The top level is the Procedure. There is no higher level, so this is the entire procedure.
Since the Procedure can be quite complex, it may consist of Unit procedures. Each Unit procedure may consist of Operations. Each Operation may consist of Phases. There is nothing simpler than a Phase because a Phase orders the activities of Basic control.
5.1.2.1 Procedure
A Procedure orders a set of Unit Procedures. The Procedure should define the starting conditions for each Unit Procedure, even if there is only one Unit Procedure. There must be at least one Unit to contain the batch.
An example of a Procedure for a specialty chemical is an entity labeled “Terephthalic Acid Procedure.” The process for making Terephthalic acid (TPA) was used as an example in Chapter 2.
An example of a Procedure for a biological process could be “Miraculomycin Proce- dure.” The process grows cells that make a miraculous drug.
5.1.2.2 Unit Procedure
A Unit Procedure orders a set of Operations. The Unit Procedure should specify the starting conditions for each Operation. It is possible for a Unit Procedure to consist entirely of Phases, but this is not a good idea because there are no reusable subdivi- sions of the procedure.
An example of a Unit Procedure could be “TPA Reaction.” There are four nearly iden- tical units in the same process cell. They each empty into a common storage tank, so each unit does not make a different product. One Unit Procedure may be used for all Units. The Unit Procedure has to specify the Unit to be used.
A biological process uses one bioreactor to grow cells because there must be no chance that an unknown cell will also grow. After growth is complete, the cells are transferred to a second unit that will either filter the exhausted cells from a product in the growth medium (e.g., alcohol) or harvest the cells to extract a product from within the cells (e.g., insulin, interferon). Multiple unit procedures are required, as specified by the Procedure.
5.1.2.3 Operation
An Operation orders a set of Phases. The Operation should specify the starting conditions for each Phase. There must be at least one Phase to command an equip- ment entity.
An ordered list of operations for making TPA follows:
• preheat
• charge water
• charge DMT
• react
• cool
• dump
The operations are not complex, but product quality depends on the cooling opera- tion. There are separate charge operations because the 750 psi water charge is violent.
DMT and water don’t mix well without the aid of a powerful agitator, but the agitator can’t run during the water charge.
An ordered list of operations for growing cells might include:
• clean
• sterilize
• charge growth medium
• inoculate
• growth region A
Again, the operations are not complex. Quality depends on all of them being done precisely according to a validated plan. Cleaning is necessary because life is messy.
Sterilization heats and holds the heat long enough to kill the kind of cells that can live in human ambient conditions (i.e., not those on deep-sea volcanic vents). The growth medium may have been made in a unit procedure preceding growth, or it may be made in a common resource. It is brought to the proper temperature, pH, and dis- solved oxygen levels during the operation. Inoculation adds cells grown from a much smaller process that will make the specific drug desired. The more cells, the faster growth goes to completion, but also the more chances for contamination. During the growth regions, control models are applied to adjust growing conditions as the cell mass grows to the limit of sustainability.
5.1.2.4 Phase
The Phase is the lowest element in the procedural model. The phase should specify the commands that are to be sent to controlled equipment to change settings and perhaps modes. The phase would then wait for a response from the controlled equip- ment and then take one of the conditional actions specified in the phase. This may cause the next phase or phases to start, or it may start an exception-handling activity.
A phase can also send a command to an operator through a human interface device and then wait for a response.
The comment in 88.01 about subdividing refers to the steps or instructions required to accomplish the process-oriented task assigned to the phase. The fact that the lowest element is not basic but can be broken up suggests that the procedural model is incomplete. This will be cleared up in Section 5.3.3, where the separation of recipe and equipment procedures is described.
When batch control is not automated, an operator receives a recipe from a supervisor, probably as several sheets on a clipboard. In those sheets is a page for the procedure, showing the unit procedures to be run. Depending on the complexity of the process, each unit procedure may have separate pages showing the list of operations for each.
More sheets describe the phases in each operation and the sequence for running them.
Each phase has a list of instructions telling the operator how to operate and observe various pieces of controlled equipment. This may result in some equipment actions running in parallel, up to the limit of what the operator can handle. Some phases may be used more than once, but only one page is needed to describe the actions to be taken to implement that phase. The operator refers to this page each time that the procedure mentions the phase.
Automation consists of replicating what the operator does to basic control devices.
That requires the ability to do several steps in parallel such as open valves A, B, and C, then monitor temperature and level to determine when to close A at temperature, B at level, and C when the mix tank runs dry. Other steps may cause the operator to modify alarm limits and tuning constants or make calculations based on current operating conditions. The operator may be prompted to do something for which there is no actuator or to gather electronic signatures as required.
Macros
The concept of a procedural element that is a collection of any number and type of pro- cedural elements did not make it into 88.01. Macrois a programmer’s term for a named set of code that is used more than once in exactly the same form. When a programmer writes the name of a macro in a program then the compiler sees the entire set of code that was grouped into the name and inserts it into the code. The subject of macros as procedure elements came up when people began using Operations as macros, with no clear relationship to a process function. SP88 decided that macros were not procedural elements, partly because they messed up the symmetry of the models.
Any use of macros is dependent on the system implementation, which is not specified in 88.01. If a batch process is properly structured with meaningful operations and phases then there is no need for macros. The things that are repeated are process operations and actions, not arbitrary segments of a procedure.
5.1.3 Coordination Control
Procedural control represents partial automation of the operator’s function in batch control. Coordination control, by directing the activities of Procedural control, repre- sents partial automation of the supervisor’s function. This level of control is required when a process cell can run more than one recipe at the same time. Timely action is required, so this level is still considered real-time control.
Communication may consist more of transactions than it does of command and response. The following is a polite translation of the conversation between Coordina- tion Control (CC) and one of the units in the cell:
CC: “Unit 9, are you available to run a new recipe?” Unit 9: “Yes. I am yours to command.”
CC: “Unit 9, here is your next recipe. Start when ready.” Unit 9: “Recipe received, integrity good. Starting now.”
Unit 9: “CC, will need a filter in 32 minutes. Please select one.” CC: “Filter B is reserved, available in 35 minutes.”
Unit 9: “Acknowledge Filter B in 35 minutes.”
CC: “All units, we will run out of steam at this rate. Do not start anything that requires steam.” All units reply.
CC: “Unit 7, please go to Hold and remain there until advised.” Unit 7: “I hear and obey.”
When Unit 7 goes to hold and closes its steam valve, the steam pressure returns to normal. This ends the example. The translation was derived from binary data on the communication link, but binary could be natural language in ten years or so.
The specific functions of coordination control will be covered later, in the discussion of Control Activities.
The extent of coordination may exceed the borders of the process cell when there are utilities, transportation, or storage that are common to several cells and whose use must be managed. Communication at this level is among peers unless an area’s busi- ness function coordinates the activities of its process cells.