5.2 Units

Units contain all of the physical equipment control required to perform the basic processing actions on the batch. The definition of a unit is one of the most critical but often contentious steps in applying the S88 Design Pattern. Several simple rules, however, can be applied:

• A unit operates on only one batch at a time.

  Note	The ISA 88 standard actual says that "Units are presumed to operate on only one batch at a time," but for normal S88 use it is better to follow the rule of one batch in a unit at a time.

  Note	This rule is modified by the NS88 Design Pattern that can be applied to discrete and continuous production.

  This does not mean that a batch is only in one unit at a time; in fact, during material transfers a batch is in two units at the same time.

  This does not mean that a unit must be a single vessel. A unit could be no more than a section of pipe, a spot on a floor (used to perform manual actions), or a collection of related vessels which must simultaneously operate on the batch. For example, a reaction vessel with a recycle vessel may be considered a single unit because both vessels simultaneously operate on the batch. An area in a warehouse, where casks of material are aged under temperature control, could also be defined as a unit.

• Units operate relatively independently of one another, except when transferring material.

  Note