In this section you will learn
- The S88 physical model
- The importance of a Unit (and how to spot one)
We saw in the last section that recipes are segmented and organized by the procedural control model. Equipment is similarly segmented (or modularized) by using another S88 tool known as the Physical Model. The model has seven levels and is shown in Figure 10. Like the procedural control model, S88 uses an entity-relationship diagram to describe the physical model.
Figure 10: The Physical Model
Let’s start describing the physical model levels from the top.
Enterprise is really a fancy name for company. In very large companies, Enterprise might refer to a division or business unit. S88 states that corporate, divisional or business unit activities and decisions are made at the Enterprise level. This includes deciding which products will be made, where they will be made and when they will be made.
Likewise, Site is another name for plant. Probably the most common method for determining sites is based on geography, but that doesn’t mean two sites cannot physically be adjacent. Different sites can make different products, but they don’t have to. Similarly, different sites can have different manufacturing processes, but they may be the same.
Areas are sections of a site. Like sites, areas can be organized by various means, including physical layout or business function. Under S88, not every section of a plant need be part of an area, especially if that section has nothing to do with batch control.
While S88 defines all seven levels and considers each important, it’s the bottom four levels that really matter. Those four are shown in Figure 10 by a slightly darker color. The reason for this extra importance is that batches are typically made and contained within the level called process cell, and the remaining three levels are all subsequent components of a process cell.
A process cell contains all of the equipment required to make batches. Sometimes the term train is used to describe the equipment used to make a batch. A process cell may have more than one train, and the order of equipment used to make a particular batch is called a path.
In other words, a process cell typically provides the boundaries of the equipment necessary to make a batch. While not prohibited, paths do not usually cross process cell boundaries. A process cell may be processing more than one batch at any given time (as long as equipment is available).
The ALL-Important Unit
Batching activities are focused on something called a unit. In other words, batch processing occurs in units. A unit combines ingredients, performs a reaction, or otherwise adds value to your product or interim product. Units are commonly vessels like a mixing tank or reactor, but don’t narrow your definition of a unit to just a vessel.
A good way of spotting a unit is by determining if a piece of equipment must run a recipe in order to operate. If so, it is a unit. If not, it is used by a unit.
The table below shows some examples of what we consider to be units, and what we don’t:
|Example||A Unit||Not a Unit|
|Ingredient Storage Tank||√|
Pumps aren’t units by themselves, they just move things around, and you typically don’t need an entire recipe to operate a pump. (As we discussed above, a pump can be included as part of a unit.)
We don’t consider storage tanks to be units either. Remember that a unit performs a major processing activity, and the last time we checked, storing wasn’t one of those.
Washing machines and dishwashers are units since they more-or-less run a recipe. We don’t think refrigerators are. While some food, like pudding, may use a refrigerator to react, a fridge is really nothing more than a storage place.
Finally, here are some presumptions S88 makes about units:
- A unit frequently contains or operates on a complete batch of material. That is, the unit holds the entire batch quantity at a time. While common, this is far from a mandate.
- A unit may contain or operate on only a portion of a batch. It is not unusual for a batch to be split into two or more sections, where sections can be processed in parallel with multiple units or processed sequentially through the same unit.
- A unit does not operate on more than one batch at a time. From a record-keeping point of view, this made sense. It’s hard to track batches or lots, if you combine two or more of them into one unit.
Besides the main unit equipment (vessel or otherwise), a unit is made up of a collection of the last two subordinate physical model levels: equipment modules and control modules. These last two levels are so important to applying S88 that they warrant their own tutorial section. That’s what we’re discussing next.