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Considerations for the Design of an ISA-S88 Equipment Model: Part B

A preferred approach to the design of an equipment model is based upon a sequence of seven steps. 

  1. Determine the boundaries of the process.
  2. Determine the units and containers that are involved.
  3. Follow the flow of materials.
  4. What is done to the materials in a unit?
  5. Identify unit tags.
  6. Determine equipment arbitration.
  7. Determine the Parameter and Report values.

The paper provides details of these seven steps and how they can be used in the development of an ISA-S88 equipment model. 

  1. The boundaries of the process:

The upstream boundary covers incoming materials whereas the downstream boundary refers to all finished product storage activities, usually storage of the products of the process.  The vertical boundary indicates integration with the ERP to receive orders and to report materials produced and consumed.  The fourth boundary covers reports to be generated, particularly any reports that are specifically requested by customers as well as the programming standards to be used.

  1. Determination of units and containers:

This is a very important activity.  The FactoryTalkBatch program is priced per unit.  Consequently, poor definition of the number of units in the model can result in high costs rendering the project not viable.

Containers are typically tanks, drums and pallets, supper sacks, etc. i.e., the pieces of equipment in which materials are stored.

The traditional definition of a unit is a piece of equipment which combines and/or transforms ingredients to add value to an interim product or to the final product.  We believe that this can be misinterpreted and therefore recommend thinking about the Unit the following way:

“any location, whether equipment related, or non-equipment related in which a sequence or procedure is performed”

Based on this definition, an empty room or area in which manual pre-weighing of materials is conducted is a unit, even though no automated equipment is involved or required.  Clean-in-Place equipment that requires a sequence or recipe to make up the cleaning solutions will also be defined as a unity.  This approach extends to those parts of the CIP equipment that requires cleaning independently of a tank or vessel, e.g. sections of a transfer line through which materials move between tanks.

What is NOT a unit?  Typically heat exchangers are not units unless an independent procedure is performed in it.  An example is the heat exchanger that is used only to control the temperature of the water and is not counted as a unit.

The Equipment Editor (a component of FactoryTalkBatch) is used in the development of the equipment model and is an excellent place to document units and containers.

  1. Follow the flow of materials:

This step requires the identification of the phases that (a) bring materials into the process system, (b) move materials within the system, and (c) transfer materials (usually the products) out of the system and into storage.  A straightforward approach is to begin at the top and center of the units in P&ID and follow the perimeter of the diagram identifying what actions are taken at each piece of equipment interacting with the unit. For example, the action of adding water, oil; transferring material to another tank; unloading the tank.

  1. What is done to the materials:

This is effectively a continuation of the previous step, following the P&ID to identify the actions that occur.  For example, an action may be to agitate the mixture or possibly to heat/cool to a specific temperature.  Some phases could be used to control pressure or to prompt an operator to perform manual tasks, such as submit a sample to the laboratory or prompt closure of a lid of a vessel.

It should be noted that not all phases to be identified in the equipment model are reflected in the P&ID, these will indicate what the equipment can do.  The model will include tasks that are performed by operators, these tasks also being defined as phases.

  1. Identify unit tags:

Unit Tags can be values of weight, temperature, level, or pH in some cases, that are displayed at each unit in the equipment model and are available to the recipe author to allow decisions to be made in the recipe based on process conditions.  Example: “start adding a second material after the tank has reached a desired weight” or “add material after the temperature drops below or exceeds a target temperature.”

The Equipment Editor can again be used to document the phases for each unit.

  1. Determine equipment arbitration.

Example: the need for arbitration applied only to recirculation at tanks 301 and 302.  The actions will occur one at a time using the shared resource.  If the units were programmed in different controllers it will be necessary to create two phases, one for 301 and one for 302, and to provide a recirculation loop as a resource to allow arbitration.

  1. Parameters and Reports:

This is where the bulk of the project work is defined.  Keep in mind that the engineers building the project may not be the user creating recipes nor operating the batch system.  It is important, therefore, to use meaningful names for parameter and report names, be consistent with the naming conventions.  For example, the representation of a parameter might be:


and the corresponding report should be


Set points for given parameters may be changed by an operator and it is very important that ALL changes be documented in the phase reports.

It is convenient to use parameters to prompt operators.  If equipment has faulted or is in manual mode, then the action cannot be completed.  To counteract this a phase parameter can be created to prompt the operator to check the equipment.  This is shown as


The OO indicating “operator origin,” specifying how the phase is to be configured.  Note that since the recipe author does not need to know how equipment is configured a parameter should relate to the basic function at hand and allow the logic to adapt for differences in the equipment.

Parameters originate at all levels.  The control module may have a parameter that addresses the question “how long should a valve take to open before it is considered faulty?”  On the procedural side, parameters may originate at all levels, i.e. the procedure, unit procedure, operation, and recipe phase.  Most of the parameters will be at the recipe phase and equipment phase levels.  On the procedural side, some parameters at the recipe phase level may be deferred to the procedural level to create an interface with the ERP. Similarly, parameters at the equipment phase may be deferred to lower levels.  “Not all parameters required to perform a task need be in the recipe.”

Batching activities are recorded in the event journal.  Report values need to be useful in determining the product quality as well as providing an understanding of what occurred during the execution of the recipe.

Posted In: White Papers