2.0 HOW TO CREATE A PROCESS AND INSTRUMENTATION DIAGRAM
When creating a P&ID, design engineers should follow these steps (see Figure 4.1 for an example of a typical P&ID):
1. Start with a PFD and expand into multiple sheets by breaking up the PFD into appropriate logical segments, if needed, to fully show all process, instrument, panel, and equipment details needed for a P&ID;
2. Create a legend sheet specific to the project, defining all symbols, abbreviations, numbering scheme, and other conventions to be used;
3. Develop an instrumentation numbering system based on the International Society of Automation (ISA) Standard ISA-5.1, and customize it to adapt to specific project or end user requirements. Some utilities have their own symbols that they use in their facilities. For consistency, any facility-specific symbol should be defined for all new utility projects. A well-constructed numbering or tagging system can be used in preparing design, operations and maintenance (O&M), and the asset management database;
FIGURE 4.1 Example of a P&ID.
4. Add the instrument bubbles for all process measurements;
5. Add bubbles as defined in ISA Standard 5.1 for process displays, including those for all panel-mounted and process-mounted field instruments;
6. Add all signal lines showing interconnections between all field sensors, transmitters, panel-mounted instruments, and control room equipment, including any basic interlocks using the appropriate ISA symbols;
7. Show all inputs/outputs (I/Os) to and from each programmable logic controller (PLC) and distributed control system (DCS);
8. Show power requirements for all instruments and panels. It is important to indicate where uninterruptible power supply (UPS) power is needed;
9. Add any special notes needed to supplement the information shown; and
10. Complete the drawing by ensuring that all devices are tagged according to the convention established in the legend sheet and that any connections to other drawings are correctly shown.
Process and instrumentation diagrams are typically supplemented by the I&C design documents described in the following subsections (see Chapter 3 for supplemental information on these drawings).
2.1. Process Flow Diagrams
Process flow diagrams that engineers create in the initial design phase establish significant equipment that will be used during each treatment process (Figure 4.2). These documents also include basic materials-balance information for expected operating conditions (e.g., normal dry-weather flow and maximum flow).
FIGURE 4.2 Example of a PFD.
Once design engineers have completed the PFDs and incorporated all owner comments, process, mechanical, or I&C engineers (depending on the organization) use them to develop P&IDs. These documents provide more details such as equipment, piping, instruments, signals, valves, and control panels needed for wastewater screening, pumping, primary treatment, aeration, and so on.
2.1.1 Symbols
Symbols and identification codes used to identify instruments in water and wastewater facilities are typically based on ISA standards, with modifications for the unique nature of water and WRRFs (Meier and Meier, 2011). Standard S5.1 (ISA, 1992) (Figure 4.1), which defines symbols used in P&IDs, was created to help promote uniformity in the instrumentation industry. The symbols in this voluntary, consensus-based standard are adaptable and can be used in many applications. Through regular updates of the standard, ISA has incorporated other symbol standards endorsed by the American National Standards Institute (ANSI) (http://www.ansi.org) and the Institute of Electrical and Electronics Engineers (http://www.ieee.org). Another standard used to develop P&IDs is ISA-5.3-1983, Graphic Symbols for Distributed Control/Shared Display Instrumentation, Logic and Computer Systems (ISA, 1983). The ISA also provides its library of standard symbols in an electronic format that can be easily imported into any computer-aided design (CAD) program.
The ISA’s standards only provide a framework for developing P&IDs; as such, documents prepared by different designers may vary significantly. Therefore, those responsible for developing P&IDs should include a legend defining all lines, symbols, abbreviations, instruments, and equipment-tagging number conventions used in the documents (see Figure 4.3 for a typical legend and symbol drawing).
2.1.2 Computer-Aided Design Software
Today, most engineers use a CAD program to prepare P&IDs. Advantages of CAD software include
• A standard library of symbols for all project documents;
• The ability to reuse relevant P&IDs from previous jobs;
• The ability to reuse a template for a system-specific P&ID;
• A well-documented record of changes (in accordance with good information-technology management practices);
• Multiple options for archiving documents on removable media (e.g., universal serial bus flash drive, recordable digital versatile disc, or tape backup) or long-term storage (e.g., network server or personal computer hard drive); and
FIGURE 4.3 Process and instrumentation diagram legend and symbol drawing.
• Several options for rapidly transmitting documents (e.g., e-mail, removable media, cloud storage sites, or intranet sites).
Commercial CAD software can produce two- or three-dimensional drawings for P&IDs and other engineering designs. The most popular programs use the Microsoft Windows operating system. An inexpensive option for preparing simple P&IDs is Microsoft’s Visio program (http://www.microsoft.com), which is available as a stand-alone product or part of a suite. Other vendors offer similar programs such as SmartDraw (http://www.smartdraw.com) and The Engineering Toolbox (http://www.engineeringtoolbox.com). A quick Internet search for P&ID software will yield several other vendors. Some manufacturers offer addon software for existing CAD programs that can thoroughly