Piping and Instrumentation Diagrams (P&IDs) use specific symbols to show the connectivity of equipment, sensors, and valves in a control system. The following sections will outline general information about P&IDs that is necessary to to know before trying to draw one.
1.1 P&ID vs. PFD
P&IDs may often be confused with PFDs, or process flow diagram. P&IDs and PFDs generally utilize the same notation for equipment. However, they serve different purposes and provide different information. The purpose of a PFD is to show exactly what a process does during operation, and a P&ID shows all controllers, valve types and the materials that are used in construction. A PFD shows the connectivity and relationships between the major equipment and materials in a process. However, it also includes tabulated design values such as normal, minimum, and maximum operating conditions that a P&ID does not contain. A PFD does not include minor piping systems or other minor components that a P&ID typically includes. The difference between P&IDs and PFDs is that P&IDs typically include more information regarding piping and safety relief valves than process flow diagrams. P&IDs do not contain operating specifications that PFDs contain, such as stream flows and compositions. It is important to note that differences between PFDs and P&IDs will vary between institutions. Most corporations maintain designated standards to create and modify the documents. Both PFDs and P&IDs are controlled documents and need to be maintained with a document control procedure. To see an industrial example of PFD vs P&ID refer to, PFD/PID industry example Wiki Page.
1.2 Information Incorporated in P&IDs
The following information is given on a P&ID that is not explicit on a PFD:
- ALL valves and valve types
- Controllers present
- Controller architectures
- Pipe diameters, materials of construction, and insulation properties (including minor piping systems)
- Equipment materials of construction
1.3 Uses of P&IDs
- Develop operational methodology
- Develop safety philosophy and safeguards
- Develop control philosophy
- Serve as a basis for control programming
- Serve as a communication document for how the process works
- Serve as a basis for equipment design, pipe design, estimating cost, purchasing
- Use for evaluation of construction process
- Train employees
- Serve as a conceptual layout of a chemical plant
- Provide a common language for discussing plant operations
1.4 Characteristics of P&IDs
- Grouped by specific section of plant
- Show the connections between all the sensors and actuators
- A general schematic - NOT a layout and NOT to scale. It should be noted that P&IDs do not specifically imply the following: same elevation of equipment, relative sizes, where valves are located on the equipment, how close equipment is to each other, and impeller types/location. They are also not the same as control or incidence diagrams. This type of information can be seen in either a plant layout drawing (can be either satellite view, showing distance between units, or a slice of building, showing height of units) or construction drawings, such as plant blueprints.
- Must be clear and uncluttered
- Must be systematic and uniform. P&IDs are used extensively in industry to document plant information. These documents need to be easily read by anyone working within the company, and easily explained to anyone else. OSHA audits can occur anytime and it is imperative that operational information can be provided to the auditor when requested. Without standard notation, it would be very difficult to go from plant to plant within your company and understand the P&IDs.
- Are generally highly confidential and have restricted access, as they can be used to replicate a process
1.5 What A P&ID Is Not
- Not an architectural diagram of a process (should show the flow of material across the plant floor between sensors and actuators, not necessarily corresponding to a 3D location)
- Does not need to be drawn perfectly to scale
- Does not imply any relative elevations
- Do not need manual switches
- No actual temperature, pressure, or flow data
- Leave out any extensive explanations
1.6 What A P&ID should include
- Instrumentation and designations
- Mechanical equipment with names and numbers, and their specifications such as material, insulation, maximum flow rate, working pressure and temperature, maximum power etc.
- All valves and their identifications
- Process piping, sizes and identification
- Miscellaneous - vents, drains, special fittings, sampling lines, reducers, increasers and swagers
- Permanent start-up and flush lines
- Flow directions
- Interconnections references
- Control inputs and outputs
- Interfaces for class changes
- Vendor and contractor interfaces
- Identification of components and subsystems delivered by others
- Intended physical sequence of the equipment
1.7 P&ID Revisions
- Revisions should be clearly identified
- Regularly issued to all related employees at each significant change to the process, as well as at benchmark points
- If small changes are made that don't warrant a completely new revision, "red pencil" additions are generally accepted between issues
- 15-20 revisions are typical during process design
- All revisions need to be communicated to EVERYONE so that only the latest revision is used. This is critical in order to avoid serious (not to mention expensive) construction mistakes. (Typically outdated P&ID is discarded to avoid confusion)
1.8 How do you generate a P&ID?
P&IDs can be created by hand or computer. Common programs, for both PC and Mac, that create P&IDs include Microsoft Visio (PC) and OmniGraffle (Mac). For creating a P&ID by hand or on a computer, please refer to the P&ID Standard Notation Wiki Page for standard equipment notation as well as computer templates.