P And Id Symbols

P And Id Symbols

Piping and Instrumentation Diagrams, or P&IDs, rely on a standardized set of symbols to communicate how a plant is designed and instrumented. The symbols are mostly defined by ISA-5.1, with some overlap from ISO 14617 and ANSI Y32. The convention is simple enough on paper but the real world has some messiness built in. The most complete reference is the ISA S5.1 standard document. You can also pull free symbol libraries from several places. AutoCAD users often grab the EzyCAD P&ID block set. Revit has its own library in the Family content folders. For Plant 3D, the symbol blocks come with the install under the default libraries folder. If you need something quick and editable, the ISA symbol PDF at isa.org works fine as a reference sheet. A P&ID symbol breaks down into a few consistent parts. There is the graphical shape that identifies the equipment type, the tag that identifies the instrument function, and the line styles that show what the connection carries. Instrument loops use a circle with a tag inside. Control valves have a distinct body shape with an actuator on top. Equipment like heat exchangers, pumps, and vessels each get a standard outline. Line types matter more than people give them credit for. A solid line is a process fluid line. A dashed line means an instrument air or electric signal. A phantom line with alternating long and short dashes marks steam tracing or mechanical linkages.

Tags follow the ISA format. The first letter is the measured variable, like P for pressure or T for temperature. Subsequent letters describe the function, so FC means flow controller. The loop number is just a project identifier, usually sequential. When you see a tag like PV-101, that is a pressure valve at loop 101. If the tag has two letters before the dash, the second one modifies the variable. PCV-101 would be a pressure control valve. The way I learned to read these fast was not by memorizing shapes. I spent a week mapping out common instrument callouts on a single drawing and noting where every tag referenced back to its corresponding legend. Once I stopped trying to recall symbols from memory and started using a quick reference, reading time dropped from about 45 minutes per drawing to roughly eight minutes for the same work.

Common pitfalls with symbol application

One thing beginners consistently mess up is the distinction between a valve symbol and an instrument symbol. A control valve has its own shape separate from any instrument circle. Drawing the instrument tag directly on the valve body instead of in a separate circle causes confusion during review. Another frequent issue is mixing line styles. Some draftspeople draw all lines as solid process lines even when they are carrying instrument tubing. The reader then has to guess whether a dashed line means a signal or just a mistake. I once caught this on a live installation where the piping spec called for instrument air lines but the drawing showed them as process lines. The field techs routed pneumatic signal lines through the same trays as high-pressure steam condensate. We caught it during a design review, but it should have been obvious from the symbol set alone. A more subtle problem is the placement of annotation text. ISA allows tags to sit inside or outside the circle. Most companies pick one convention and stick with it for the whole project. When you have multiple engineering firms contributing to a single P&ID, the text placement becomes inconsistent and harder to scan. I recommend enforcing a project-specific symbol standard early, before the first drawing is issued for review.

Practical method for building or editing a P&ID

Start with the instrument schedule. It is easier to place tags after you know what instruments exist than to reverse-engineer tags from the drawing. Next, lay down the major equipment outlines using the standard shapes. Do not fuss over decorative detail. A vessel is a rectangle with rounded ends. A pump is a circle with a triangular discharge. A heat exchanger is two concentric circles or a shell-and-tube outline with channel heads on both ends. Then add the process lines using the correct line type. After that, insert the instrument circles and connect the signal lines with dashed lines. Finally, apply the tags and any additional notes. If you are working in a CAD environment, create a symbol palette with the most common items. Group them by category: process equipment, control valves, measurement instruments, safety devices. A well-organized palette saves more time than any keyboard shortcut combination. I keep mine in a pinned sidebar so it stays visible while I work. Dragging a symbol from the palette is faster than navigating a tree menu or searching through a library folder. When it comes to symbols for P And Id Symbols that deviate from ISA, keep a deviation log. Some plants use custom shapes for proprietary equipment or specialty valves. Document every non-standard symbol in a legend section at the front of the drawing. Readers should never have to guess why a symbol looks different from the standard set.

Software options and file formats

Most engineering teams use one of three platforms. AutoCAD with a P&ID add-on is the most common in North America. Plant 3D includes the symbol libraries out of the box. SolidWorks PDM occasionally gets used for smaller projects. For lighter work, Visio with a symbol stencils file is acceptable but lacks the tag management features of dedicated tools. All of these export to PDF for distribution and DWG or DGN for archival. Tag data is sometimes embedded as smart objects or attached databases. If your project requires cross-referencing tags across multiple disciplines, use a platform that supports tag linking rather than flat drawing exports. I ran into a case last year where a client imported a Plant 3D drawing into AutoCAD and lost all the tag hyperlinks. The geometry looked identical, but the embedded instrument schedule data disappeared. We had to manually re-tag every instrument and re-associate the circles. It took about three hours for a twenty-drawing package. If you are sharing P&IDs with external reviewers, confirm the software version and file format before issuing anything.

What the symbols do not cover

ISA symbols describe the function and interconnection, not the physical dimensions or material specs. A pump symbol tells you the pump exists and its tag number. It does not tell you the seal type, the motor horsepower, or the pipe schedule. That information lives in the piping isometric, the equipment datasheet, and the instrument index. Treating a P&ID as a replacement for those documents will cause problems. I have seen contractors order the wrong valve trim because someone assumed the P&ID symbol implied a specific body material. The symbol only shows the function. The material spec is in a separate note or datasheet. Another blind spot is the representation of software-based logic. Modern control systems implement interlocks and sequences in code. The P&ID shows the hardwired safety chain, like a rupture disk or a redundant pressure switch, but it rarely shows the programmable logic that triggers a shutdown sequence. If you need to understand the full safety story, you must also pull the cause-and-effect matrix or the PLC program documentation. The P&ID alone will not give you that picture.

When the symbol set breaks down

For highly complex processes with dozens of interlocking control loops, the standard symbol density becomes hard to read. The drawing fills with overlapping circles and tag labels until you cannot distinguish one instrument from another. In those cases, splitting the P&ID into functional sections is the practical solution. Draw the primary process stream on one sheet, the utility streams on another, and the safety instrumented system on a third. Keep the main P&ID at a high level and reference the detail sheets by tag or by area. This keeps the symbols readable and reduces errors during field changes. Another scenario where standard symbols fall short is when dealing with non-conventional instrumentation, such as coriolis mass flow meters, neutron density gauges, or online gas chromatographs. These devices have specialized symbols that not every library includes. You either adapt the closest ISA equivalent and add a note, or you create a custom symbol and log it in the deviation register. I prefer the latter approach because it keeps the drawing self-contained. A note that says "see deviation log for symbol definition" is useless if someone does not know where the log is stored.

Final notes on usage

P&ID symbols are a communication tool, not a decoration. The goal is clarity for anyone who reads the drawing years after it was drawn. Consistency beats elegance every time. Pick a symbol standard, apply it uniformly, document any exceptions, and keep the reference materials accessible. If you do that, the drawings will serve you correctly during design, construction, and operations.