The lowest level of procedural element in the procedural control model.

The smallest element of procedural control that can accomplish a process-oriented task is a phase. A phase may be subdivided into smaller parts. The steps and transitions as described in IEC 848: 1988 document one method of defining subdivisions of a phase. 

A phase can issue one or more commands or cause one or more actions, such as

• Enabling and disabling regulating and state-oriented types of basic control and specifying their set points and initial output values
• Setting, clearing, and changing alarm and other limits
• Setting and changing controller constants, controller modes, and types of algorithms
• Reading process variables, such as the gas density, gas temperature, and volumetric flow rate from a flow meter, and calculating the mass flow rate through the flow meter
• Conducting operator authorization checks.

The execution of a phase may result in

• commands to basic control;
• commands to other phases (either in the same or another equipment entity); and/or
• the collection of data.

The intent of the phase is to cause or define a process oriented action, while the logic or set of steps that make up a phase are equipment specific. Examples of phases include the following:

• Add VCM.
• Add catalyst.
• Heat.

Specification notes

You can use  SFC diagrams for phases.

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