The piping and instrumentation diagram is the single most important document in a process plant. It’s not a scale drawing — it’s a logical representation of every piece of equipment, every pipe, every valve, every instrument, and every control loop in the facility. When I review a P&ID set, I’m not just checking for errors. I’m reading the design intent of the engineer who drew it.
Here’s what I’ve learned about creating P&IDs that actually communicate what matters — and avoiding the mistakes that cause problems during construction and operation.
What a P&ID Is (and What It Isn’t)
A P&ID is not a 3D model. It’s not a scale drawing. It’s not a wiring diagram. It’s not a bill of materials (though it drives the BOM). A P&ID shows:
– All process equipment (tanks, pumps, heat exchangers, vessels, etc.)
– All process piping (line size, material, service, insulation)
– All valves (type, size, actuation)
– All instrumentation (sensor type, location, control function)
– All control loops (how instruments connect to final control elements)
– All safety devices (relief valves, rupture disks, safety instrumented systems)
If you have a question about what’s in the plant or how it’s controlled, the P&ID should answer it. If it doesn’t, the P&ID is incomplete.
The Line Numbering System: The Backbone of Traceability
Every line on a P&ID has a unique line number. A typical format: 6″-P-1001-CS-H. This means 6-inch diameter, Process service, line number 1001, Carbon Steel, Hot insulated. The specific format varies by company, but the principle is universal: every line number is unique, and no two lines share a number.
The line numbering system does several jobs:
– Links the P&ID to the line list (a separate document with detailed info on every line)
– Links the P&ID to the isometric drawings used for fabrication
– Provides a unique identifier for every pipe during construction, testing, and maintenance
If your line numbering system is inconsistent or has gaps, everything downstream — construction, testing, commissioning — gets more difficult.
Instrument Tagging: ISA Nomenclature That Actually Communicates
The ISA-5.1 standard defines instrument tag format: first letter = measured variable, second/third letters = function. For example:
– FIT = Flow Indicator Transmitter (measures flow, indicates locally, transmits to control system)
– LIC = Level Indicating Controller (measures level, indicates, controls a final element)
– PSHH = Pressure Switch High-High (discrete switch, trips at high-high pressure condition)
– TAL = Temperature Alarm Low (alarm function, triggers at low temperature)
The most common mistake I see: inconsistent alarm and trip designations. PSH versus PAH — both mean “pressure high,” but PSH implies a switch (discrete) and PAH implies an alarm from an analog transmitter. Different devices, different wiring, different logic in the control system. Be precise.
Valve Specification: More Than Just a Symbol
A gate valve symbol and a globe valve symbol look similar on a P&ID. But they have completely different functions — gate valves are for on/off isolation, globe valves for throttling. Using the wrong symbol on the P&ID means the wrong valve gets ordered and installed.
Every valve on a P&ID should have, at minimum:
– Valve type (gate, globe, ball, butterfly, check, etc.)
– Size (which may differ from the line size for control valves)
– Actuation (manual, pneumatic, electric, hydraulic)
– Fail position for actuated valves (FO = fail open, FC = fail closed, FL = fail last position)
The fail position designation — FO, FC, FL — is critical for process safety. A valve that fails closed when it should fail open can cause overpressure. A valve that fails open when it should fail closed can drain a tank or flood an area. These designations should be reviewed during the HAZOP, not assigned arbitrarily.
The Common Mistakes That Haunt Projects
Missing vents and drains. Every section of pipe that can be isolated needs a way to drain liquid and vent gas. This includes pipe between block valves, pump suction and discharge, and dead legs. If you can valve it off, you need to be able to drain and vent it. I’ve seen projects where the piping was installed without drains — the operators had to break flanges to drain water for maintenance, which is slow, messy, and not how you want to run a plant.
Pressure relief that only exists on paper. Every vessel and every section of liquid-filled pipe that can be blocked in needs overpressure protection. The P&ID must show where relief valves discharge — not just a symbol pointing to nowhere. Atmospheric discharge, flare header, closed drain system — the destination matters for safety and environmental compliance.
Instrument ranges that don’t match the process. A level transmitter on a tank should have a range that covers the entire operating range plus some margin. But I’ve seen P&IDs where the instrument range on the data sheet doesn’t match the nozzle elevations on the vessel drawing. The result: the level transmitter can’t actually measure the full range it claims to. This gets caught during commissioning if you’re lucky, or during the first upset condition if you’re not.
A good P&ID set is worth the time it takes to produce. It prevents construction errors, simplifies commissioning, and provides the documentation operators need to run the plant safely. The key is consistency — consistent symbology, consistent line numbering, consistent instrument tagging, consistent level of detail. When I look at a P&ID set across a whole project, I should be able to tell it was drawn by one team with one standard, not five teams with five interpretations of what “good enough” means.