unit procedure

A unit procedure is a structured part of a batch or manufacturing procedure that is executed on a specific piece of equipment, known as a unit. In ISA‑88 terminology, a unit procedure sits below the overall procedure in the recipe hierarchy and above operations and phases.

Core meaning

Within the ISA‑88 batch control model, a unit procedure:

• Represents the portion of the recipe carried out on a single unit (for example, a reactor, mixer, or filling line)
• Is composed of one or more operations, which are further broken down into phases
• Defines the ordered set of actions, parameters, and logic needed to complete a major step on that unit, such as “Charge and react” or “Filter and wash”

A unit procedure is typically implemented in a batch control system, DCS, PLC, or MES batch engine, and is referenced by both control recipes and equipment recipes. It is not just a narrative description or a standard operating procedure, although it is usually aligned with those documents.

Where it is used in manufacturing systems

In regulated and batch-oriented manufacturing environments, unit procedures commonly appear in:

• Recipe management systems, where unit procedures are defined as reusable building blocks for different products
• MES and batch execution systems, where they coordinate equipment phases, collect process data, and manage interlocks on a specific unit
• Automation systems, where control logic for each unit procedure is implemented in phases and linked to recipe parameters

Operationally, a batch procedure may call several unit procedures in sequence or in parallel, each tied to a different unit, to complete the full batch.

What a unit procedure is not

• It is not the entire batch procedure or master recipe.
• It is not a generic SOP document, even if it aligns with SOP content.
• It is not a single phase, step, or equipment command. Those are lower-level elements within operations and phases.

Common confusion

Unit procedure vs procedure: In ISA‑88, the procedure is the top-level ordered set of actions that defines how the batch is made. A unit procedure is a subset of that procedure tied to a specific unit. Multiple unit procedures can exist within one procedure.

Unit procedure vs operation: An operation is the level below a unit procedure. A unit procedure may contain several operations, and each operation may contain several phases.

Unit procedure vs equipment module or phase: Equipment modules and phases are elements of the equipment model and control logic. The unit procedure is a recipe element that orchestrates those lower-level elements for a particular unit.

Relation to the ISA-88 model

Within the ISA‑88 procedural model, the typical hierarchy is:

• Procedure
• Unit procedure
• Operation
• Phase

Within this structure, the unit procedure is the level where process intent for a specific unit is expressed in a form that can be executed and version-controlled in MES and control systems, and traced during batch review.