ISA-88 structures a batch recipe into a tidy hierarchy of procedural elements so that complex production can be built, understood, and reused piece by piece. The unit procedure is the second tier of that hierarchy, and it is the one that ties a stretch of a batch to a single processing unit. Understanding what belongs at this level, versus the tiers above and below it, is essential for anyone building recipe structures. This guide explains what a unit procedure is, how it runs a batch through one unit, how it sequences operations, and how it fits between the procedure and the operation levels.
ISA-88 unit procedure in one line: An ISA-88 unit procedure is the second level of the procedural hierarchy, procedure, unit procedure, operation, phase, and it represents the ordered set of operations that a batch goes through within a single processing unit. It carries the batch through one unit's worth of processing, such as taking a reactor through charge, react, and discharge, by sequencing the operations that run in that unit, while the procedure above it strings unit procedures across several units to make the whole batch.
The defining trait of a unit procedure is that it is bound to a single processing unit. A unit in ISA-88 terms is a piece of equipment that carries out major processing activities, such as a reactor, a mixing vessel, or a dryer, and a unit procedure is the procedural element that runs a batch through that one unit from start to finish of its involvement. If a batch is made in a reactor, the reactor's unit procedure covers everything the batch does while it is in that reactor, and nothing that happens in other units. This one-to-one tie between a unit procedure and a unit is what gives the level its meaning.
Because it is scoped to a single unit, a unit procedure typically corresponds to a recognisable chunk of the process. A reactor unit procedure might take the batch through charging the raw materials, running the reaction, and discharging the product, which together are one unit's contribution to making the batch. That contribution is complete and self-contained: the unit procedure begins when the batch enters the unit's care and ends when the unit has done its part and the batch moves on. It is the natural unit of thinking about what one vessel does for a batch.
This scoping is also what makes unit procedures reusable and comprehensible. Because a unit procedure describes what happens in one unit, it can be understood on its own, tested against that unit, and reused whenever a batch needs that unit to do the same job. Engineers building recipes think in terms of what each unit must do, and each of those becomes a unit procedure, so the recipe is assembled from clear, unit-sized pieces rather than one undifferentiated block of logic. The hierarchy's clarity begins with this discipline of one procedure per unit.
A unit procedure does its work by sequencing operations, which are the level immediately below it in the hierarchy. An operation is a major processing activity within the unit that usually takes the material through a significant state change, such as heating, reacting, or cooling, and a unit procedure orders these operations into the sequence the unit must follow. The unit procedure is essentially the ordered plan of operations for one unit: do this operation, then that one, and so on, until the unit's part of the batch is complete. It holds the logic of what runs when inside that single unit.
The sequencing is not always a simple straight line. A unit procedure can include branches, choices, and loops, so that operations run in a fixed order in some cases and in a chosen or repeated order in others, depending on how the process needs to behave. The unit procedure is where that control-of-order lives for the unit: it decides which operation follows which, whether an operation repeats, and how the sequence responds to conditions. Below it, each operation contains its own finer sequence of phases, but the unit procedure governs the ordering of the operations themselves within the unit.
Keeping this responsibility at the right level is what makes the hierarchy coherent. The unit procedure sequences operations; it does not itself reach down to drive valves or set individual setpoints, which is the province of phases far below. Nor does it coordinate across units, which is the job of the procedure above it. By owning exactly the ordering of operations within one unit, the unit procedure occupies a clear middle ground: broad enough to represent a whole unit's contribution, specific enough to be a real sequence, and cleanly separated from both the cross-unit coordination above and the equipment action below.
Placing the unit procedure correctly against its neighbours prevents the most common confusion in recipe building. Above it is the procedure, which is the top-level strategy for making the whole product and which strings unit procedures together across all the units a batch passes through. If a batch is made by charging in one vessel, reacting in a reactor, and finishing in a third unit, the procedure is the overall plan that links those three units' procedures in order, whereas each unit procedure covers just one of those units. The procedure crosses units; the unit procedure stays within one.
Below the unit procedure are operations and, below them, phases. An operation is a major activity within the unit that the unit procedure sequences, and a phase is the smallest procedural step that actually causes process action. So the chain reads: the procedure coordinates the batch across units, each unit procedure sequences the operations within its unit, each operation sequences its phases, and each phase drives real equipment action. A recipe builder mapping out a batch decides at the unit-procedure level what each unit will do as a sequence of operations, leaving the finer detail to the levels below.
This structure carries naturally into how batch execution is monitored and coordinated by supervisory systems. As a batch runs, a SCADA or batch layer tracks which unit procedure is active on which unit, which operation within it is running, and how far the sequence has progressed, giving a picture of the batch's state that mirrors the ISA-88 hierarchy. For operations that span multiple sites or run many units, a cloud SCADA platform such as Merobix can present that unit-by-unit progress and the associated process data centrally, so a team can see which unit procedure each unit is executing and how the batch is advancing without standing at any single panel. The clean tiering that ISA-88 imposes on the recipe is exactly what makes such progress views clear, because the batch's state can be described in the same procedure, unit procedure, operation, and phase terms the recipe was built from.
A procedure is the top-level strategy for making the whole product, and it strings unit procedures together across all the units a batch passes through. A unit procedure sits one level down and covers just a single processing unit, sequencing the operations that run in that one unit. In short, the procedure coordinates the batch across units, while each unit procedure handles one unit's share of the batch.
A unit procedure contains an ordered sequence of operations that a batch goes through within one processing unit, and it holds the logic controlling their order, including any branching, choices, or loops. For example, a reactor unit procedure might sequence charge, react, and discharge operations. Each operation in turn contains its own phases, but the unit procedure governs the ordering of the operations themselves within that unit.
A unit procedure is bound one-to-one to a single processing unit, such as a reactor or mixing vessel, and it covers everything the batch does while it is in that unit's care, from when the batch enters until the unit has finished its part. This one-to-one tie makes each unit procedure self-contained, understandable, and reusable, which is why recipes are assembled from unit-sized pieces rather than one undifferentiated block of logic.
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