P&ID drawing and ISA instrumentation symbols

Why it matters

The piping and instrumentation diagram (P&ID) is the master document of a process plant. Instrument engineers design from it, contractors build from it, operators and safety reviewers (HAZOP) work from it, and maintenance staff use it to find the right transmitter at 3 a.m. Being able to read and draw ISA-5.1 symbols and tags fluently is expected from your first day in a plant job.

Key ideas

PFD versus P&ID. A process flow diagram (PFD) shows major equipment, main streams and operating conditions (flows, temperatures, pressures, heat duties). A P&ID adds every pipe with size and specification, every valve, every instrument, control loop, interlock, vent, drain and relief device, but usually no operating data. Electrical power wiring and physical layout are not shown on a P&ID.

Instrument tag (ISA-5.1). A tag = identification letters + loop number, e.g. TIC-101.

• The first letter is the measured or initiating variable: A analysis, F flow, L level, P pressure, T temperature, S speed, W weight/force, Z position, E voltage, I current, J power, H hand (manual).
• A modifier may follow the first letter: D differential (PD = differential pressure), F ratio (FF = flow ratio), Q totalise (FQ = flow totaliser), S safety (PSV = pressure safety valve).
• Succeeding letters give the functions, in the order readout/passive then output: I indicate, R record, E primary element, T transmit, C control, V valve or final element, A alarm, S switch, Y relay/compute/convert, and modifiers H high and L low (LSHH = level switch high-high, PAL = pressure alarm low).
• The loop number is shared by every instrument in the loop: FE-101 (orifice), FT-101 (transmitter), FIC-101 (controller), FY-101 (I/P), FV-101 (valve).

Balloons (instrument symbols).

• Circle — a discrete (stand-alone) instrument.
• Circle inside a square — shared display/shared control, i.e. a DCS function.
• Hexagon — a computer function; diamond inside a square — a PLC function.
• Location: no line across the symbol = field-mounted; a single solid horizontal line = central control room, accessible to the operator; a double line = local (auxiliary) panel; a dashed line = behind the panel or not normally accessible.

Signal and connection lines.

• Process piping: thick solid line. Instrument connection to the process / instrument supply: thin solid line.
• Electric signal: dashed line.
• Pneumatic signal: line with double diagonal hash marks (// //).
• Capillary tube (filled system): line with crosses (X).
• Hydraulic signal: line with "L" marks.
• Software or data link (DCS/fieldbus internal link): line with small circles.
• Guided electromagnetic or sonic signal: line with sine-wave symbols.

Valves and actuators. Body symbols distinguish globe, gate, ball, butterfly, three-way and angle valves. A dome on the stem is a spring-diaphragm actuator, a box with "M" is a motor actuator, a rectangle with "S" a solenoid. Failure action is written near the valve: FO (fail open), FC (fail closed), FL (fail locked/last position). Relief valves (PSV) and rupture discs have their own symbols.

Equipment and other items. Pumps, compressors, vessels, columns and exchangers carry equipment tags (P-201, V-101, E-301). Interlocks are shown as a diamond with "I" and a number, and safety-instrumented functions are often flagged separately. Line numbers carry size, fluid code, material class and insulation.

Reading a loop. Follow it from the primary element through the transmitter, along the signal line to the controller balloon, then to the converter and the final element. Check the location of each balloon, the signal type of each line and the valve's fail position. Mistakes in any of these change how the plant behaves.

Formulas

I = 4 + 16·(x − x_min)/(x_max − x_min) and inversely x = x_min + (x_max − x_min)·(I − 4)/16

• I: transmitter output (mA); x: measured value in engineering units; x_min, x_max: calibrated range. Used when a P&ID loop is read together with its instrument data sheet.

Q = Q_max·√((I − 4)/16)

• Flow from a differential-pressure transmitter whose output is linear in ΔP (no square-root extraction); Q_max: flow at 20 mA. When the P&ID shows an FY "√" function block, the extraction is done there and the result is linear in flow.

P = 20 + 80·(u/100)

• Pneumatic signal (kPa) from an I/P converter for controller output u (%).

Worked examples

Example 1 (standard) — a flow loop. A P&ID shows FE-101 (orifice) → FT-101 (DP transmitter, field) → FIC-101 (circle in square with a solid line) → FY-101 → FV-101 (FC). The flow range is 0–50 m³/h at 20 mA. The transmitter output reads 12 mA. Interpret the loop and find the flow (a) if FT-101 has square-root extraction, (b) if it outputs a signal linear in ΔP and the extraction is missing.

1. FIC-101 in a circle-in-square with a solid line is a DCS flow indicating controller in the control room; FY-101 is the I/P converter; FV-101 is a fail-closed control valve.
2. Signal fraction = (12 − 4)/16 = 0.5.
3. (a) Linear in flow: Q = 0.5 × 50 = 25 m³/h.
4. (b) Linear in ΔP: Q = 50 × √0.5 = 50 × 0.7071 = 35.4 m³/h. Misreading this is a classic commissioning error.

Example 2 (GATE level) — a temperature loop. TT-301 (range 0–200 °C) sends a 4–20 mA signal (dashed line) to TIC-301 (shared display). TIC-301 sends its output to TY-301, and a line with double hash marks goes from TY-301 to TV-301, marked FC. TT-301 reads 14 mA and the controller output is 60 %. Find the temperature, the pneumatic signal and the valve position, and identify the signal types.

1. Temperature = 0 + 200 × (14 − 4)/16 = 200 × 0.625 = 125 °C.
2. TY-301 is an I/P converter: the dashed input is electric and the hashed output is pneumatic.
3. P = 20 + 80 × 0.60 = 68 kPa.
4. FC means air-to-open, so with a linear calibration the valve is 60 % open. On loss of air it closes.

Common mistakes

• Reading the second letter as the measured variable: in PDT the D modifies P (differential pressure); in TIC the I and C are functions.
• Assuming a solid line across a balloon means "field mounted" — it means control room; no line means field.
• Mixing up electric (dashed) and pneumatic (double-hash) signal lines.
• Giving different loop numbers to instruments in the same loop, or the same number to unrelated loops.
• Forgetting the square-root relationship in DP flow loops.
• Leaving out the valve's failure position; reviewers need it for HAZOP.

For GATE IN

Questions are mostly identification: decode tags such as LSHH, PDIC, FQI, TIC; recognise balloon types and their locations; identify signal-line types; and pick the correct P&ID for a described control scheme (cascade, ratio, feedforward). Combine this with 4–20 mA conversions and DP-flow square-root numericals.

Quick check

1. What do the letters in PDIC stand for?
2. What does a circle inside a square with a solid horizontal line represent?
3. Which line symbol represents a pneumatic signal?
4. A level transmitter with range 0–4 m reads 8 mA. What is the level?
5. What does LSHH mean?

Answers: 1. Pressure differential indicating controller. 2. A shared display/control (DCS) function accessible to the operator in the control room. 3. A line with double diagonal hash marks. 4. 1 m. 5. Level switch high-high (usually a trip).