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 is, every piece of equipment has a unique tag number, thereby avoiding uncertainty. Tag numbers should also be used in process descriptions.
2.2 Process and Instrumentation Diagrams
Process and instrumentation diagrams (P&IDs) are visual representations of process piping and equipment and the I&C components of the overall process control strategy. These diagrams are also the one place where every piece of equipment, including its tag number, can be found. The P&IDs can be included as an appendix of the PCN or referenced in the PCN.
2.3 System Components
Programmable logic controllers (PLCs), programmable automation controllers (PACs), distributed control systems (DCSs) and remote telemetry units (RTUs), and user interfaces such as operator interface terminals (OITs) and HMIs used in the monitoring of the process, site, or system are listed and their functions are described. This information can also be presented as a control system architecture drawing.
Control, data handling, and presentation systems such as supervisory control servers, historians (and associated trending and reporting packages), alarm escalation systems, and performance management systems should be included to establish the extent of process data management and how staff will interact with the process and receive data. Development of data integration and connections external to the control system (reporting, remote monitoring, and alarms escalation) should be implemented following the practices identified in Chapter 12.
2.4 Flow Charts
Flow charts visually display process logic that will be executed in field controllers or the supervisor control system. Flow charts represent the control algorithm or process using blocks to represent logic functions. A step or activity is represented as a rectangle, while an analysis or decision is represented as a diamond. Arrows link each block and display the flow of control logic. Flow charts allow complex “if, then, and, or” statements to be easily represented for implementation by the programmer and as a reference for the operator.
2.5 Control Hierarchy
Each level of the control system and monitoring and control functions are described in the following subsections.
2.5.1 Local Controls
Design engineers should describe any local equipment controls and detail their relationship and interactions with the PLC or supervisory control and data acquisition system. This information is especially important to ensure compatibility among all the engineering disciplines involved. Local and remote switches, e-Stops, hardware interlocks, relay logic, and single-loop controllers inform operations of the level of local control available and identify controls and functions that are done outside of the PLC for programmers. Programmers can use this information to suppress nuisance alarms from the PLC. The description of local controls should identify any safety or software permissives and interlocks that are bypassed in local control operations.
2.5.2 Motor Control Center Functions
Design engineers should detail all the functions performed by and at the motor control center. In particular, any work required by the electrical engineering group and the electrical subcontractor and supplier should be clearly identified. Power monitoring instrumentation and motor/line/transformer protection relays should be identified, with monitoring and protection functions included.
2.5.3 Controller Functions
Design engineers should detail the PLC’s (or other controller’s) role in both the specific process being described and the overall process control architecture. Control activities, software interlocks, and automation functions performed by the PLC/PAC/DCS/RTU will be described and operator inputs (setpoints) listed. Specific peer-to-peer communications used for controls or calculations will be listed.
2.5.4 Remote Manual Controls
Design engineers should describe the manual equipment controls available to operators from OITs or HMI stations. Devices that are manually adjusted to control the system are identified and their function described. To assist operators in performing complex manual operations that are not easily automated, equipment-operating tables should be used to identify the correct equipment state for repeatable manual operations. These tables will assist the operator in matching the equipment state to the required actions and will primarily identify pump status, speed, and valve positions. Table 5.1 presents a sample equipment-operating table.
2.5.5 Remote Automatic Controls
Design engineers should describe the automated equipment controls available to operators from OITs or HMI stations. This section describes controls that are implemented at the supervisory level and any calculations, summations, or logic used in the control functions.
TABLE 5.1 Sample equipment operating table.
2.6 Alarms
Design engineers should describe when and how local and remote alarms will be generated. The description should provide enough detail so programmers can create the appropriate software and operators will understand which responses are needed. This description will include alarm management in which specific alarms are suppressed for the same root cause. This includes a communications failure alarm suppressing all other alarms from a controller or a pump-off status suppressing a low-flow alarm.
Alarms that are escalated to a facility-wide speaker paging system or e-mail notifications should be identified for implementation in specific systems. Specific alarm limits should be included in the PCN for the alarms as a reference for the operator. These limits should be updated in the document and controller if the process or operational parameters are changed.
2.7 Setpoints
Design engineers should identify the setpoints for control loops or operations such as pressure setpoints for compressors, chemical dosing rates, liquid and gas flows, tank-fill levels, sump-level cycles, and high-level controls such as sludge retention times, dissolved oxygen setpoints, and bypass triggers. In addition to listing the setpoints, the narrative should describe how critical setpoints were assigned.
2.8 Performance Reporting Data
Design engineers should identify all
