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Chapter 5

      Process Control Narratives

      Jeff Coulson, P. Eng, PMP

      Jack Geisenhoff, P.E., PMP

       1.0 PURPOSE OF PROCESS CONTROL NARRATIVES

       2.0 TYPICAL PROCESS CONTROL NARRATIVE COMPONENTS

       2.1 Equipment and Tag Numbers

       2.2 Process and Instrumentation Diagrams

       2.3 System Components

       2.4 Flow Charts

       2.5 Control Hierarchy

       2.5.1 Local Controls

       2.5.2 Motor Control Center Functions

       2.5.3 Controller Functions

       2.5.4 Remote Manual Controls

       2.5.5 Remote Automatic Controls

       2.6 Alarms

       2.7 Setpoints

       2.8 Performance Reporting Data

       3.0 OTHER METHODS OF CONVEYING PROCESS DESCRIPTIONS

       3.1 Animations

       3.2 Static Process Models

       3.3 Interactive Process Models

       4.0 DEVELOPMENT, UPDATES, AND PRESENTATION

       4.1 Document Development and Updates

       4.2 Document Storage and Presentation

       5.0 REFERENCE

1.0 PURPOSE OF PROCESS CONTROL NARRATIVES

      Process control narratives (PCNs) are written for a number of reasons. They reveal the design engineer’s overall control philosophy and objectives. Process control narratives simply and clearly state how automation systems, including field controllers, human–machine interfaces (HMIs), and reporting and performance management systems, are expected to function. This ensures that suppliers, contractors, and programmers can implement the automation system to meet the objectives of the process. The PCN also serves as a reference and training document for operators and a facility.

      A coherent PCN helps coordinate the various disciplines involved in design. It enables process, electrical, and instrumentation and controls (I&C) engineers to work together effectively. Periodically reviewing PCNs with each discipline will significantly improve design coordination.

      Process control narratives help the construction team understand the intent of contract documents. They should make the requirements clear to the client, general contractor, I&C subcontractor, electrical subcontractor, and related suppliers. To encourage these parties to read descriptions during the bidding process, the engineer should refer to them as related documents in Part 1 (“general”) of each appropriate Construction Specifications Institute specification.

      Process control narratives also provide an operational framework for operations and management staff before, during, and after construction and once the automation system is in service. The narratives should detail how the processes are supposed to work. They can also be used as an operator training and maintenance tool as part of operations and maintenance manuals.

      Finally, PCNs are a source of institutional knowledge, which can be particularly important at water resource recovery facilities (WRRFs) where staff members familiar with design and operations are no longer employed at the facility. Indeed, a set of well-prepared PCNs at these facilities can be invaluable. Process control narratives represent the one place where all the interactions among various facility components are documented and explained.

2.0 TYPICAL PROCESS CONTROL NARRATIVE COMPONENTS

      This section outlines significant components of a PCN. Although some of these items may be unnecessary for simple I&C systems, they should all be included in the design of a relatively complex process or facility.

2.1 Equipment and Tag Numbers

      It is important to list critical equipment tag numbers and equipment descriptions in a table to provide information and to avoid process errors that can result if incorrect equipment and/or valves are operated. Although the International Society of Automation (ISA) has developed a widely used method for assigning tag numbers (ANSI and ISA, 2009), some utilities prefer to use site-specific tagging methods. However, the method or standard used is less important than the result; that