2.11 Control System Architecture Diagram
Control system architecture diagrams are simple schematics of the control network topology and interconnections among PLCs, DCSs, I/Os, MCCs, VFDs, intelligent valve networks, and other control components (Figure 4.4). They also illustrate any necessary network equipment (e.g., Ethernet hubs, switches, fiber transceivers, and routers).
FIGURE 4.4 Example of a control system architecture diagram.
3.0 PROCESS AND INSTRUMENTATION DIAGRAMS AS COMMUNICATION TOOLS
3.1 Contract Bid Documents
Contract bid documents are key documents used by system integrators in developing a bid for the I&C portion of a contract. Water and wastewater utilities use P&IDs when obtaining bids from contractors. Basically, “contract documents” consist of contract specifications, P&IDs, and their supporting documents. Other contract documents needed also include equipment installation details.
3.2 End User and Owner Feedback
Owners and end users use P&IDs to review design documents to ensure that the design engineer has addressed their needs. Owners also use P&IDs for training and O&M.
During design reviews, control system users and owners tend to base their written or oral feedback on the project’s P&IDs. (It is important to make sure that they understand that other documents [e.g., written scope, design report, and standards] are used to implement the P&IDs.) Users and owners also use P&IDs to verify that the concepts presented in the design report, flow diagrams, written scope, and standards are actually implemented in the new or upgraded system.
3.3 Operator Training
Because P&IDs show process flow and all associated instruments, controls, panels, and significant equipment, they typically are used when training facility operators. Process flow diagrams are also used because they can be easier to understand than P&IDs. In addition, PCNs are used to supplement these documents during operator training.
3.4 Operator-Interface Graphics Development
Many WRRFs include graphics in their process control or SCADA system. These graphics are typically displayed on operators’ workstation screens. Depending on the organization’s preferences and practices, these graphics can be developed by the consulting engineer, contractor’s instrumentation system integrator, or user or owner.
Process and instrumentation diagrams and PFDs are used to develop process graphic displays. The graphics are designed to represent the process as closely as possible, as depicted in P&IDs and PFDs.
3.5 As-Built Documentation
At the end of a project, users or owners typically require contractors to provide a set of “record” or as-built P&IDs, that is, drawings that reflect changes that occurred during construction and startup. They not only serve as an accurate record of the new or upgraded facility’s actual equipment and system configurations, but also can be used as a training tool. As-built P&IDs also provide a basis for future renovations or upgrades.
3.6 Coordination with Other Design Disciplines
Process and instrumentation diagrams are the foundation of I&C engineering design. They are an essential element of the I&C design package. Process and instrumentation diagrams are multidiscipline drawings that present information from mechanical, process, I&C, and electrical groups. Mechanical and process groups use P&IDs to show various piping connections between process equipment, including main process equipment found in WRRFs (i.e., process tanks, wet wells, channels, pumps, bar screens, grinders, and specialized equipment), manual and automated valves, pipe reducers, in-line instruments, special piping requirements (double-wall pipes for chemicals lines, etc.), vendor-supplied packages, and interconnections to the process. Electrical groups use P&IDs supplemented by other documents (e.g., motor lists and equipment lists) to determine power requirements for instruments and motorized equipment, signal wiring requirements, networking requirements for the control system, interfaces to MCCs and variable-speed drives, and so on.
Mechanical specifications, or “specialty equipment specifications”, often have control specifications embedded in their sections. The mechanical or process engineer should be cognizant of the need to modify these control specifications to match the P&IDs; in some instances, P&IDs may need to be adapted to these controls. Control functions (e.g., interlocks, signal conditioners, timers, proportioning/integral/derivative controllers, etc.) are also an important part of the mechanical specifications and must be clearly identified using the ISA S5.1 symbols and guidelines.
4.0 INTELLIGENT PROCESS AND INSTRUMENTATION DIAGRAMS
Current P&ID trends include intelligent P&IDs using software. Today’s CAD software packages allow design engineers to create intelligent P&IDs with minimal data. Users simply choose appropriate symbols and tags from pull-down menus and place them in the drawings with the help of toolboxes containing symbol-placement and -modification capabilities. The software converts the drawing into an intelligent object model. Adding intelligence involves creating objects that include properties, methods, and relationships for each component (element) in the drawing. Thus, components that were symbols in a schematic drawing then become objects in an object model. Intelligent objects derive information from data entered with the object. Intelligent objects automatically inherit appropriate attributes from other objects in a process line (e.g., reducer size is based on the size of components on either side). Intelligent objects provide intelligent checking for data validity and consistent engineering practices. Software inherent to intelligent objects automatically
• Allows access to all pertinent component information by simply selecting the component on the P&ID. Software displays a list of piping, valves, instruments, instrument loops, and equipment in the P&ID;
• Reports conflicting flow, unconnected lines, and other inconsistencies;
• Checks for valid data input and inconsistencies between P&ID sheets;
• Provides integration with existing automation tools (design, engineering, construction, startup, and O&M);
• Provides links to project schedule, procurement databases and spreadsheets, and other information through a standard open database connectivity (ODBC) interface; and
• Handles symbol orientation and line breaks, reducing the number of steps necessary to place all in-line P&ID components. It also automatically manages all element symbols according to project standards, as defined by the user. In addition, it tracks all links on each drawing so, for instance, each arrow representing a link directing viewers to another drawing can actually send them to the correct drawing.
Computer-aided design software also allows design engineers to integrate various databases and other data repositories (spreadsheets, text files, other drawings, etc.) into one I&C database linking P&IDs with other supporting documents (Kelm, 2002; Knapp, 1999). This database contains all the data for every instrument, device, process line, and valve. Every change made to the database is instantly reflected in the P&ID. All documents are linked together via equipment tag numbers. The decision to use this software must be made early on in a project because of the related learning curve.
4.1