NAMUR
5.2 Equipment Entities
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.
5.2.1 Relationship Model
The relationship model is shown in Figure 7 of 88.01 and also in Figure 7-3. In general, a Procedural Element (Figure 6 or 7-2) directs the control of an Equipment Entity (Figure 2 or 6-2) to perform the functions of a Process Element (Figure 1 or 6-1).
Figure 7-3 Procedural Control/Equipment Mapping to Achieve Functionality Figure 7-3 is not an exact representation of Figure7 in 88.01. The physical model column says “Controlled Equipment” and not just “Equipment” because procedural control can’t be combined with plain old equipment. The first two columns were made plural with “(s)” at some point after “combined with a” had been written, per- haps because a transfer requires two of everything. Many procedures can run in many process cells, but it only takes one procedure running in one process cell to carry out one batch process specified by a recipe. Perhaps a recipe could include the processing in several cells, but 88 does not provide a way for the recipe to be distributed to the cells. There is no process control activity in an Area, just business functions.
The Process Functionalities on the right are an abstract list of a batch process and its subdivisions. Each process function has to be realized by one or more equipment entities, or the batch will also be an abstraction. The Procedural Elements on the left are an abstract list of a procedure and its subdivisions. When realized in a product recipe they provide the direction to animate the Equipment Entities so that the pro- cessing necessary to make a product will occur. To put it another way, you can’t produce a specific product unless you can perform the process functions. You can’t do any processing unless you have the right controlled equipment. The controlled equip- ment requires direction from procedural elements in order to do the correct
processing. We will soon see that the recipe provides the correct procedure.
The concepts of ownership and acquisition will be discussed later in this chapter. An equipment entity can be owned by another equipment entity. Ownership implies that only the owner may give direction to the owned equipment entity. Ownership may be established by wiring or configuration. Acquisition is the process of temporarily obtaining ownership of common equipment entities (not otherwise owned) by using a simple negotiation. A release process is used after the process function is complete.
The model of the relationship of procedural control to controlled equipment to process functionality is fundamental to the 88.01 standard. If you don’t understand this model, read on because things may clear up as you see how the model is used. If you can’t make your batch process fit this model, please write to ISA Standards and Practices with an explanation of the problem. Perhaps ANSI/ISA-88.01 will have to be revised.
5.2.2 Control in Equipment Entities
The control that is associated with an equipment entity may be one or more of the three subdivisions of batch control: Basic, Procedural, and Coordination, as shown in Figure 7-1. The recipe has to contain that part of procedural control that is specific to making a product. The equipment may have associated procedural control that does not change with different products. For example, a heat exchanger has to establish the correct flow of the correct medium no matter what the product may be. A premixer may have a product-independent procedure, requiring only material sources and amounts from the recipe. Reactors tend to have associated basic control that receives direction from a recipe procedure.
5.2.2.1 Process Cell
A process cell consists of units and common equipment modules. If there is only one unit then there are no common equipment modules. At a minimum, the cell must be able to receive a schedule of products to be made and retrieve the required recipes. The cell must be able to start the scheduled recipe in the chosen unit at the scheduled time.
Cells may be more complicated than that, which requires coordination of units and common equipment. The cell may be required to allocate equipment ahead of each batch, or the units may be able to manage equipment among them. Also, the cell may manage auxiliary activities like cleaning and sterilization, or the units may be respon- sible for making themselves ready. A cell may manage its material inventory based on its knowledge of scheduled batches, or material management may be a business function.
Complexity is relative, but it can be estimated from the number of units and common equipment in the cell. If any unit can transfer to any other unit then the complexity for that equipment count is at a maximum. This is reflected in the number of valves in the transfer piping. If there are no transfer selection valves then the paths through the units never vary, which is not particularly complex no matter how many units there may be.
A cell may have to manage a few equipment and control modules, but units own most of the modules.
Coordination Control:
The main purpose of the process cell is to coordinate activities within the cell. It should come as no surprise that there is far more coordination control in a cell than there is procedural or basic control. Coordination begins with the selection of units to run unit procedures so that each batch will be able to finish in a timely manner without waiting for a procedure in another batch. If there are transfers between units, the cell may coordinate these for best use of available resources. If batches have pri- ority levels (adding complexity to batch coordination) then the cell may determine which batch gets held or shut down so that a higher priority batch can run.
It may be necessary for cells to coordinate as peers because only the business levels are higher. The following conversation between process cells might occur:
PC1: “Chauncy, old peer, I seem to be running low on Ingredient B. Have you got some to spare?”
PC3: “Sorry, Throckmorton, I don’t. Have you tried Lancelot? Keep up the good work and my best to you and your units.”
(The humor in this, such as it is, will be lost on you if you have no knowledge of British peerage.)
Procedural Control:
Another purpose of the process cell is to acquire and initiate recipes. Procedural con- trol is required to examine the recipe procedure for unit recipe procedures and to pass them on to the proper units in a timely manner. Any other procedural control done by the cell is not related to a product recipe. Perhaps the cell does cleaning procedures for transfer piping.
Basic Control:
The interlock propagation example given in 88.01 seems to be for coordination rather than basic control. A cell may give direction to a control module that it owns, but it makes no direct use of basic control. Interlocks initiate changes in procedural control, possibly at the cell level.
5.2.2.2 Unit
The main purpose of the control that is associated with a unit is to execute that part of the recipe procedure that has been assigned to the unit. This requires that procedural commands be issued to the equipment and control modules that are owned by the unit. No coordination is needed if the modules are owned. If the process cell is com- plex, the unit may require coordination control in order to deal with other units and common equipment modules. The unit relies on its equipment and control modules to do basic control.
Coordination Control:
If the unit has to use equipment entities that it does not own then it must negotiate with the other entities to acquire their services and release them from service when done.
Alternatively, the unit may have to make a request to the process cell to coordinate the connection. The following conversation between a unit and a common equipment module provides an example, where U is unit and EM is equipment module:
U4: “EM3, can you supply 300 liters of hot water now?”
EM3: “No, Fred, but you can get it in 15 minutes. Shall I put you down for that time?”
U4: “I’ll take it, EM3.”
EM3: “OK, Fred. Your confirmation number is 377.”
U4: “Roger 377, EM3. Thanks.”
This conversation is equivalent to a person making a reservation at a hotel. Talking to the process cell is like talking to a travel agent, perhaps asking for any available hotel.
Procedural Control:
The main purpose of the unit is to translate the recipe procedure into commands that can be given to the equipment entities that it owns or has acquired. This may require access to documents containing product-independent procedure elements such as phases and operations. For example, the recipe procedure may contain an operation named “Bulk Charge” that specifies a standard material and amount but contains no procedure. The unit controller would get a copy of the standard procedure for Bulk Charge and then execute that procedure, which may refer to standard phase proce- dures. The documents would be paper for an operator or disk files for a machine.
Basic Control:
The unit always uses its equipment and control modules to do basic control. That’s why 88.01 has such modules.
5.2.2.3 Equipment Module
The main purpose of an equipment module is to provide a standard interface to a control function. The module encapsulates the control function and hides the details from the unit and the recipe procedure. A common function like heat exchange may be done in several different ways, but all of them require a temperature setpoint and perhaps a few other parameters. The equipment module conceals the details of the valve switching that are required to select the heat exchange medium. The operator or automator just sees a temperature setpoint.
If the module is common to other units and not owned by only one unit then it will need coordination control. If the module can execute a phase, so that the phase pro- cedure is not in the recipe or in the standard procedural element documents, then it
needs procedural control. An equipment module must do all of its basic control through control modules that it owns or modules that it can acquire.
Equipment modules can be recursive. The equipment module that contains all others provides the interface to the unit, so that all of the others are hidden. The underlying modules may be exposed when the mode of the top equipment module is manual.
Coordination Control:
The module must coordinate its component parts. Most of this is done by the connec- tions of the parts and some combinatorial logic, which is normally a configuration activity. Configured or procedural exception handling logic also affects the interaction of the parts. Logic may be configured to propagate mode changes. All of this seems like basic or procedural control. No negotiation is required.
If a unit or cell does not own an equipment module then the module must have coor- dination control in order to manage changes of ownership. An equipment module that is serving as common equipment must be able to be the server end of the acquire or release dialog.
Procedural Control:
An equipment module is required to have procedural control. The only procedural element that it can run is the phase. Operations and unit procedures only run in units, regardless of how capable the equipment module controller may be. If an equipment module could run an operation, it would be a unit. Further, an equipment module can only run one phase at a time, although the phase procedure may cause simulta- neous things to happen.
Basic Control:
An equipment module sends commands to the control modules that it owns, which perform the necessary basic control.
5.2.2.4 Control Module
A control module has nothing below it in the hierarchy of equipment entities. A con- trol module owns instruments that are in contact with the process material or auxiliary process streams like steam or chilled water. A control module has exclusive use of an actuator, if it has one, but it may share a measurement with other entities.
No other equipment entity can be directly connected to the process, so all commands that cause process actions to take place end up in control modules.
The main purpose of a control module is basic control.
Coordination Control:
If an equipment module, unit or cell does not own a control module then the module must have coordination control in order to manage changes of ownership.
Procedural Control:
There’s no need to worry about procedural control in a control module because it is not supported.
Basic Control:
A control module may perform any or all six of the types of basic control. See Chapter 3 for a discussion of the six types of control.
5.2.3 Structuring Equipment Entities
This section of 88.01 discusses some of the basic ideas necessary to locate the bound- aries of process cells, units and equipment modules on a P&ID of a batch process. This would have been handy when discussing the physical model, but full knowledge of the relationships among controlled equipment entities was necessary. You now have that, so it is time to discuss structuring a process with equipment entities. Of course, an exact description of structuring methods would be a specification for implementation, so SP88 backed off from being specific in order to reach consensus.
As you might expect, the structure of the controlled equipment greatly influences the ease with which a recipe can be translated into process actions that will build a product. This suggests that several structures should be tried on paper before choosing to commit one to equipment and control hardware.
The third paragraph under Section 5.2.3 in 88.01 suggests that structuring might be trickier than you expect. If you do it wrong, you risk losing the benefits of the standard because the standard is built around the previously described controlled equipment entities. In practice, there has been more trouble defining units and equipment mod- ules than process cells and control modules.
Structuring requires a clear understanding of the purpose of each equipment entity in relation to its process function.
5.2.3.1 Process Cells
What follows is more about general rules for identifying equipment entities than it is specific to the process cell. A process cell is defined by the limits of its procedural and coordination control, except for coordination with other cells. The control limits are defined by the required process functionality. The general rules follow:
• The processing function of any equipment entity must be clear and precise.
Ambiguous language like “in some cases” should be replaced by definitions of the exception cases. “Does the job well” requires more explanation.
• The function performed by an equipment entity should correspond to a process function. The function should be independent of the product being made, except for the quantity and quality specified by the recipe.
• A subordinate equipment entity should execute its process task independently of any other equipment entity. This relieves the ordinate task from controlling the
details of the subordinate task. The master task simply wakes up the subordinate and tells it what to do, then does nothing further with the subordinate except to listen for a response, either “Done” or “Error.”
• An equipment entity should have minimum interaction with other equipment enti- ties, unless some cooperative activity like a material transfer is in progress. This allows the entities to be designed independently.
• Each equipment entity must have clear boundaries. Each equipment entity has exclusive control of the equipment entities that it owns. Clear boundaries are required to show the limits of ownership and to make sure that there is no chance that two equipment entities can command the same subordinate equipment entity or actuator.
• Equipment entities of the same type must have the same function, so that an operator cannot make a mistake by operating two instances of the type in the same manner.
• When two equipment entities must interact, the coordination of the interaction is done by the same equipment entities or by one entity at the next higher level. There is no higher level to coordinate process cells.
5.2.3.2 Units
A unit cannot be defined without first knowing what major processing activities are required and how they match the capabilities of the unit’s equipment. What follows is specific to a unit.
The unit may handle one or more major processing activities (process stages), such as reactions that cause a complete chemical change, physical mixing, separation, or cell growth.
A unit should be able to operate independently of any other unit, except when it is time to transfer contents between units. Extra coordination control will be required if the utilities are limited or common resources are used.
A unit operates on only one batch at a time. Otherwise, you have a discontinuous process, and the batch ID is meaningless.
5.2.3.3 Equipment and Control Modules
The definition of a unit’s process functions should reveal a set of process actions that are required in order to perform the unit’s tasks. Equipment modules can do procedural control that is related to one or more process actions. Control modules use basic control to accomplish a process action. Equipment modules may own control modules.
Summary
This chapter covered the structure for batch control as a hierarchy of coordination, procedural, and basic control. The hierarchy for procedural control is procedure, unit procedure, operation, and phase. Equipment entities were introduced as controlled equipment, relating each item in the control hierarchy to the equipment hierarchy of process cell, unit, equipment module, and control module. The relationship of proce- dural elements to equipment entities to process functions was described. Further definition of the equipment hierarchy was developed from rules for any equipment entity and rules specific to an equipment entity level in the hierarchy.
88 Batch Control Concepts, Part 2
This chapter covers the entire recipe part of 88.01 Section 5, Batch Control Concepts.