Piping & Instrumentation Diagrams (P&IDs)

Introduction to Piping & Instrumentation Diagrams (P&IDs)

In the world of automation, process control, and instrumentation, the Piping and Instrumentation Diagram (P&ID) is the primary roadmap of the plant. A P&ID illustrates the physical sequence of equipment and systems, as well as how these systems are instrumented and controlled. For a Control Systems Technician, the P&ID is arguably the most critical document for troubleshooting, installing, and maintaining process instruments. The industry relies heavily on the ANSI/ISA-5.1 Instrumentation Symbols and Identification standard to ensure that a P&ID drafted in one facility can be understood universally by technicians anywhere else in the world.

Functional Identification Letters

The cornerstone of the ISA-5.1 standard is the functional identification system, commonly known as the instrument tag number. An instrument tag is typically enclosed in a circle (or other symbol) and provides immediate context about what the instrument does. A typical tag consists of a functional identifier followed by a loop number (e.g., FIC-101).

The First Letter: Measured Variable

The first letter of an instrument tag always identifies the measured or initiating variable. This is the physical property of the process that is being monitored or controlled.

• P: Pressure or Vacuum
• T: Temperature
• F: Flow Rate
• L: Level
• A: Analysis (e.g., pH, oxygen concentration)
• V: Vibration or Machinery Analysis
• W: Weight or Force

Succeeding Letters: Function

The letters following the first letter describe the readout or passive function and the output function of the instrument.

• I: Indicator (readout function)
• R: Recorder (readout function)
• C: Controller (output function)
• T: Transmitter (output function)
• V: Valve, Damper, or Louver (output function)
• S: Switch (output function)
• A: Alarm (annunciator)

Example Breakdown

Let's analyze a few common instrument tags:

• PT-205: Pressure Transmitter for loop 205.
• FIC-301: Flow Indicating Controller for loop 301. This device measures flow, displays the value locally or on a screen, and acts as a controller to maintain a setpoint.
• LSH-404: Level Switch High for loop 404. This is a switch that activates when a liquid level reaches a predetermined high point.
• FV-102: Flow Valve (specifically, a control valve adjusting flow) for loop 102.

Instrument Bubble and Circle Symbols

The shape of the instrument symbol on a P&ID tells a technician what kind of system is processing the signal, while the horizontal lines inside the shape dictate where the instrument is physically located.

Shape Definitions

1. Single Circle: A discrete, standalone instrument. This means the device has its own dedicated hardware (like a field transmitter or a standalone panel meter).
2. Circle within a Square: A shared display or shared control device. This typically represents a function existing within a Distributed Control System (DCS).
3. Hexagon: A computer function. This is used when a supervisory computer system performs a specific logic or calculation task.
4. Diamond within a Square: A Programmable Logic Controller (PLC) function. It denotes that the control logic or interlock is handled by a PLC.

Location Lines

The lines running horizontally through the center of these shapes indicate the physical location and accessibility of the instrument to the operator.

• No Line: Field mounted. The instrument is physically located out in the plant process area, right next to the pipe or vessel. Examples include field pressure transmitters or local temperature gauges.
• Solid Line: Primary location, normally accessible to the operator. This usually means the instrument is mounted on the main control room board or is displayed on the primary DCS screens.
• Dashed Line: Auxiliary location, normally inaccessible to the operator. This instrument is usually mounted behind a panel, inside a local motor control center (MCC), or inside a field marshaling cabinet.
• Double Solid Line: Auxiliary location, accessible to the operator. Usually a secondary control panel in the field.

Piping and Signal Line Symbols

Instruments do not operate in isolation; they must interact with the process fluid and communicate with other control devices. P&IDs use different line types to differentiate between process pipes and various types of control signals.

• Process Pipe: A thick, solid continuous line. This represents the actual piping carrying the fluid (water, gas, oil) through the plant.
• Electrical Signal: A dashed line, or a solid line with hash marks (three diagonal slashes spaced out). This represents standard electrical communication, such as 4-20 mA analog signals, 24V DC discrete signals, or 120V AC power.
• Pneumatic Signal: A solid line with double diagonal slashes (//). This represents compressed air or gas used for instrumentation, commonly a 3-15 psi pneumatic signal driving a control valve.
• Hydraulic Signal: A solid line with the letter L intersecting it, indicating high-pressure hydraulic fluid.
• Capillary Tube: A solid line with small X marks. This represents a fluid-filled capillary tube, typically used for remote diaphragm seals on differential pressure level transmitters to isolate the sensor from corrosive process fluids.
• Software or Data Link: A solid line with open circles. This indicates a digital communication link between systems, such as an Ethernet connection between a PLC and a DCS, or a Modbus/Profibus fieldbus network.

Practical Scenario: Tracing a Control Loop

Imagine walking down a P&ID in the field. You trace a thick solid line representing a water pipe. On this pipe, you see an orifice plate symbol connected by solid lines to a single circle containing FT-501 with no horizontal line. This tells you a discrete Flow Transmitter is physically mounted in the field right at the orifice plate.

From FT-501, a dashed line (electrical signal, likely 4-20 mA) goes to a circle-within-a-square with a solid horizontal line containing FIC-501. This means the 4-20 mA signal travels to the main control room where a DCS acts as the Flow Indicating Controller.

From FIC-501, another dashed line runs to a discrete circle with no line containing FY-501 (an I/P transducer in the field). Finally, from FY-501, a line with double slashes (pneumatic signal, 3-15 psi) connects to the actuator of FV-501, a field-mounted flow control valve.

ISA-5.1 symbols let you interpret the functional intent shown on a P&ID, but physical locations, terminals, wiring, elevations, and installation details require the current loop, wiring, location, and equipment drawings.