P&ID symbols and instrument loop diagrams

In short

A Piping and Instrumentation Diagram (P&ID) is the master drawing of a process plant: every vessel, pump, line, valve, instrument and interlock shown schematically with standard symbols and tag numbers, but with no scale, no elevation and no piping route. An Instrument Loop Diagram (ILD) zooms into one single loop and shows its real wiring — field transmitter, junction box and terminal numbers, marshalling panel, I/O card, power supply and the DCS/PLC block — so a technician can commission or trouble-shoot it without reading the P&ID. The symbol language is ISA 5.1 (instrumentation symbols and identification) with ISA 5.4 for loop diagrams; Indian practice uses these alongside IS/ISO drawing conventions and client/EIL/PDIL standards. It is examined in Instrumentation "Plant practice / Process instrumentation" papers and in GATE-IN descriptive-style questions on tag identification, and it is the very first thing asked in interviews at L&T, Honeywell, Yokogawa, Emerson, ABB, Siemens, BPCL, IOCL, RIL, Tata Steel and EPC houses like EIL, Toyo and Technip.

Explained

The drawing family: where a P&ID sits

• BFD (block flow diagram): blocks for whole units with main streams — used in concept studies.
• PFD (process flow diagram): major equipment, main process lines, mass/energy balance, key control loops only; no spares, no small lines, no instrument detail.
• P&ID: every line with size-service-number, every valve, every instrument with its tag, interlocks, set points, insulation, slopes, line specs, utility tie-ins. It is the controlling document for HAZOP, procurement, erection and operating procedures.
• Downstream of the P&ID: instrument index, instrument specification sheets, hook-up (installation) drawings, loop diagrams (ILD), JB/cable schedules, cause-and-effect matrix, plot plan and isometrics.
• Revision discipline: P&IDs are issued "for review", "for HAZOP", "for construction", and finally as-built; a change needs a cloud + revision triangle and a revision note.

Instrument identification (tag numbers) — ISA 5.1

• A tag has two parts: functional identification (letters) and loop identification (number), e.g. FIC-101, often with an area/unit prefix (e.g. 21-FIC-101A).
• First letter = measured/initiating variable. A = analysis, C = conductivity (user-defined in some specs), D = density, E = voltage, F = flow, H = hand (manual), I = current, J = power, K = time/schedule, L = level, M = moisture/humidity, P = pressure (or vacuum), Q = quantity/totalised, R = radiation, S = speed/frequency, T = temperature, V = vibration, W = weight/force, Y = event/state, Z = position/dimension.
• Modifier after the first letter: D = differential, F = ratio, Q = integrating/totalising, S = safety, M = momentary. So PDT = pressure differential transmitter, FFIC = flow ratio indicating controller, PSV = pressure safety valve.
• Succeeding letters = function. I = indicate, R = record, C = control, T = transmit, E = primary element (sensor), S = switch, V = valve/damper, Y = relay/compute/convert, A = alarm, G = glass/gauge/sight, K = control station, Z = final actuator (safety).
• Suffixes: H = high, L = low, HH/LL = high-high/low-low (trip), so LAHH-205 is a level alarm high-high. Multiple identical items get A, B, C.
• Read examples: TE-301 thermocouple, TT-301 temperature transmitter, TIC-301 temperature indicating controller, TV-301 temperature control valve, TSHH-301 temperature switch high-high — all belong to loop 301.
• FE/FT/FQI: orifice plate (FE), DP transmitter (FT), flow totaliser indicator (FQI). LG = level gauge glass, PG/PI = pressure gauge.

Symbol shapes — balloons, lines and valves

• Instrument bubble (balloon) shapes tell you *where* the function lives:
  • plain circle = discrete/field-mounted instrument;
  • circle with a single horizontal line = mounted in a main control room panel, front of panel (accessible);
  • circle with a dashed line = control-room rear of panel / not accessible;
  • circle with a double line = field/local panel mounted;
  • square with an inscribed circle = shared display / shared control (DCS, operator accessible);
  • diamond inside a square = PLC / programmable logic control;
  • hexagon = computer function;
  • plain diamond/lozenge (or circle with a diamond) = shared logic/computing variants per the project legend.
• Signal line conventions (ISA 5.1):
  • plain continuous line = process/piping;
  • long-dashed line = electrical / electronic (4–20 mA) signal (some specs use a line with two short cross-hatches);
  • line with evenly spaced double hash marks // = pneumatic signal (0.2–1.0 kg/cm², i.e. 3–15 psi);
  • line with small circles -o-o- = data link / digital bus (fieldbus, HART digital, Modbus);
  • line with "X" cross marks = capillary tubing (filled/thermal system); line with "L" marks = hydraulic; line with a sine wave = guided electromagnetic/sonic.
• Valve body symbols: two opposed triangles = gate/generic; triangles with a curved body = globe; bowtie with a line across = butterfly (single line through circle); circle with a line = ball; "X" in a square = plug; a shape with a flap = check (non-return), with an arrow for flow direction.
• Actuators on top of a valve: semicircle/dome = diaphragm (pneumatic); rectangle = piston/cylinder; circle with "M" = motor operated (MOV); circle with "S" = solenoid; T-bar = handwheel; a spring symbol indicates spring-return.
• Fail action arrows: FC (fail closed), FO (fail open), FL (fail last/locked), FI (fail indeterminate) are written beside the valve; an arrow with the letters, or a down/up arrowhead on the actuator, states the air-failure position. Safety choice: fuel valve FC, cooling-water valve FO.
• Relief devices: PSV/PRV drawn as an angled body with a spring; rupture disc as a concave line in the line; conservation vent, flame arrestor and restriction orifice (RO) have their own legend symbols.
• Equipment: vessels and columns as outlines with trays/packing, shell-and-tube exchangers as a rectangle with tube passes, pumps as a circle with a tangential discharge, compressors, filters (Y-strainer as a Y), and tanks with roof type shown.
• Every project carries a Legend / Symbols and Abbreviations sheet — this governs over memory, because companies customise ISA.

Reading a loop on a P&ID, and the standard loop types

• Trace: primary element → transmitter → controller → I/P or positioner → final control element → process, then back to the measurement point.
• Simple feedback loop: LT-110 → LIC-110 → LV-110 on the outlet line of a tank.
• Cascade: master TIC output becomes the set point (SP) of a slave FIC; shown by a signal line into the slave bubble marked "SP" or "R" (remote set point).
• Ratio: FFIC with a ratio station keeping fuel in proportion to air.
• Split range: one controller output driving two valves over 4–12 mA and 12–20 mA; written as a note.
• Override/selector: a > (high select) or < (low select) inside a square/bubble; auto-selectors on compressor anti-surge.
• Interlock and trip: shown as a square with a diamond (PLC) or an "I" in a rectangle, cross-referenced to the cause-and-effect matrix; ESD and SIS loops are drawn separately or marked "S" (PSHH→ZV).
• Hand stations: HIC (manual loading station), HS (hand switch), HV (hand valve on the diagram when it has a tag).
• Cross-sheet continuation: an arrow/flag with "To P&ID 12-05, line 6"-P-1203 — follow the line number, not the picture.

Instrument Loop Diagram (ILD) — ISA 5.4

• Purpose: one drawing per loop showing every physical connection between field and control room, for installation, commissioning, loop checking and maintenance.
• Layout is in vertical zones, left to right or top to bottom: Field → Field junction box → multicore/trunk cable → Marshalling cabinet/TB → System cabinet I/O card → DCS/PLC database → Operator station.
• Contents: tag numbers; terminal block and terminal numbers at every break; cable number, pair number and core colours; loop power source (24 V DC, fused), fuse/MCB number; polarity (+/−); earthing and instrument earth bus; I/O card slot and channel address; signal range and calibrated span; action (direct/reverse); alarm/trip set points; air supply and I/P or positioner details; and the drawing's reference documents.
• Shows hardware, not process: no vessels, no line numbers.
• Related drawings: the hook-up drawing gives the mechanical installation bill of material (impulse tubing, manifold, root valve, mounting bracket, air filter regulator); the wiring/termination schedule is the table version of the ILD; the cable schedule lists route and length.
• Loop checking before start-up: inject a simulated 4, 8, 12, 16, 20 mA (or apply pressure to the transmitter), verify the DCS reading and the valve travel, and sign the loop folder — the ILD is the checklist.

Standards, codes and Indian practice

• ISA 5.1 (Instrumentation symbols and identification) — the symbol/tag standard worldwide and in Indian EPC practice; ISA 5.4 loop diagrams; ISA 5.3/5.5 DCS and graphic symbols; ISA 20 spec forms; ISA 84 / IEC 61511 for safety instrumented systems with SIL, and IEC 61508 for device SIL capability.
• IEC 60617 / ISO 14617 give graphical symbols for diagrams; ISO 10628 covers process diagrams for the chemical industry. Indian Standards exist for engineering drawing practice (IS 696 family, superseded in parts by the SP 46 code of practice) and IS 962 as the code of practice for architectural and building drawings — exact current numbers and editions vary, so quote the project legend rather than a number you are unsure of.
• Indian statutory and regulatory links: PESO/CCOE approvals and IS/IEC 60079 for hazardous-area classification (Zone 0/1/2, Ex d / Ex ia) marked on the P&ID or instrument index; OISD standards (OISD-STD-132, 152 etc.) for oil and gas; Petroleum Rules 2002 and SMPV rules; CEA regulations and the Static and Mobile Pressure Vessels rules where relief devices are involved. Note that the four Labour Codes in force from 21 November 2025 (including the Occupational Safety, Health and Working Conditions Code) replaced the Factories Act, so plant safety documentation is now referenced to the OSH Code and its rules.
• Boiler-related instrumentation follows the Indian Boiler Regulations administered by state boiler inspectorates; relief-valve sizing to API 520/521 is standard in Indian refineries.
• Deliverable practice in India: EIL/PDIL/Tata Consulting style sheets, A1 size, title block with revision table, SmartPlant P&ID or AutoCAD/AVEVA as the drafting tool, and intelligent P&IDs linked to the instrument index database.

Advantages, limitations and common conventions

• Advantages: one unambiguous language across disciplines and vendors; makes HAZOP (node-by-node on the P&ID) possible; drives material take-off, loop count and I/O count; essential for operator training and permit-to-work isolation lists.
• Limitations: not to scale, no elevation (so a thermosiphon or a self-draining slope must be noted in words), no cable routing, no physical valve location — a P&ID can never replace an isometric or plot plan.
• Company-specific letters (e.g. using "C" for conductivity or "N" for a user choice) are legal only if defined on the legend sheet.
• Numbering practice: loops are numbered per unit (100 series, 200 series), duplicates get A/B, and spare I/O is reserved — typically 20% spare channels.

Key points

• P&ID - schematic of all equipment, piping, valves, instruments and interlocks; no scale, no elevation, no routing.
• PFD vs P&ID - PFD shows major equipment and balances; P&ID shows every line, valve and instrument tag.
• Tag structure - letters (function) + loop number; first letter = measured variable, succeeding letters = function.
• Modifiers - D differential, F ratio, Q totalising, S safety; PDT, FFIC, PSV follow from them.
• Suffixes - H, L, HH, LL for alarm and trip levels; LAHH = level alarm high-high (trip).
• Bubble shapes - plain circle field, single line main panel, dashed line rear of panel, double line local panel.
• Square-in-circle - shared display/shared control (DCS); diamond-in-square = PLC; hexagon = computer function.
• Signal lines - dashed = electronic 4–20 mA; double hash = pneumatic 0.2–1.0 kg/cm²; small circles = digital bus; "X" cross marks = capillary tubing.
• Fail action - FC, FO, FL, FI stated at the valve; chosen from process safety, not convenience.
• Final control element - usually a pneumatic diaphragm control valve with I/P converter and positioner.
• Cause-and-effect matrix - the table that defines every trip the P&ID interlock symbol refers to.
• ILD (ISA 5.4) - one loop, all wiring: field, JB terminals, cable/pair, marshalling TB, I/O card slot and channel, 24 V DC supply, DCS block.
• Hook-up drawing - mechanical installation detail and bill of material (impulse tube, manifold, root valve, AFR).
• Loop check - 4/8/12/16/20 mA injection and valve stroke verification, signed in the loop folder before commissioning.
• Legend sheet rules - project symbols and abbreviations override generic memory of ISA.
• Hazardous area - Zone and Ex marking (IS/IEC 60079) appear in the instrument index; PESO approval needed in India.
• SIS loops - ISA 84 / IEC 61511, SIL rated, drawn separate from the basic process control system (BPCS).
• Revision control - cloud + revision triangle; final issue is the as-built P&ID.

Formulas and examples

Example 1 — decode a tag list. Identify each: PDIT-402, LAHH-115, FFIC-220, TSV-307B, AIT-508.

• PDIT-402 — P pressure, D differential, I indicate, T transmit → differential-pressure indicating transmitter, loop 402 (typical across a filter).
• LAHH-115 — level alarm high-high, loop 115 → trip-level alarm, normally wired to the SIS.
• FFIC-220 — flow ratio indicating controller, loop 220 (air/fuel ratio control).
• TSV-307B — temperature safety valve, loop 307, second of two identical items.
• AIT-508 — analysis indicating transmitter (e.g. pH or O₂), loop 508.

Example 2 — level transmitter output. A DP level transmitter is calibrated 0–2,500 mmWC for a tank of 0–5 m water level. Find the current at 3.2 m and the level at 14.4 mA.

• Span fraction at 3.2 m = 3.2/5 = 0.64. I = 4 + 16 × 0.64 = 4 + 10.24 = 14.24 mA.
• At 14.4 mA: fraction = (14.4 − 4)/16 = 10.4/16 = 0.65 → level = 0.65 × 5 = 3.25 m (and DP = 0.65 × 2,500 = 1,625 mmWC).

Example 3 — orifice flow loop (square-root law). FE-101/FT-101 is sized for 200 m³/h at 2,500 mmWC DP. The field reading is 12.0 mA, with no square-root extraction in the transmitter.

• ΔP fraction = (12.0 − 4)/16 = 0.5 → ΔP = 0.5 × 2,500 = 1,250 mmWC.
• Q = 200 × √0.5 = 200 × 0.7071 = 141.4 m³/h — not 100 m³/h. (If the DCS applies √ extraction, 12 mA would be shown as 50% flow only when the transmitter itself outputs √ΔP.)

Example 4 — loop power and burden. A 24 V DC two-wire loop has a transmitter needing minimum 11 V, 250 Ω DCS input resistor, a 100 Ω indicator and 0.6 km of cable at 0.08 Ω/m per core (two cores).

• Cable resistance = 2 × 600 × 0.08 = 96 Ω. Total load = 250 + 100 + 96 = 446 Ω.
• Voltage dropped at 20 mA = 0.020 × 446 = 8.92 V → voltage left at the transmitter = 24 − 8.92 = 15.08 V > 11 V, so the loop works.
• Maximum allowable load = (24 − 11)/0.020 = 650 Ω; margin = 650 − 446 = 204 Ω.

Example 5 — I/P output and split range. A controller output is 60%. Find the I/P air signal, and the position of two split-range valves.

• Current = 4 + 16 × 0.60 = 13.6 mA; air = 0.2 + 0.8 × 0.60 = 0.68 kg/cm² (≈ 3 + 12 × 0.6 = 10.2 psi).
• Split range: valve A (4–12 mA) is at 13.6 mA → fully open (100%); valve B (12–20 mA) = (13.6 − 12)/8 = 0.20 → 20% open.

Where it is used

• Refineries, petrochemicals and fertiliser plants (IOCL, BPCL, HPCL, RIL, GAIL, RCF, IFFCO) — P&ID-driven HAZOP, SIL study, loop folders and as-built handover.
• EPC and design houses (EIL, PDIL, L&T Hydrocarbon, Toyo, Technip, Tata Consulting Engineers) — graduate jobs as instrument design engineers producing P&IDs, instrument index, hook-ups and ILDs in SmartPlant/AVEVA.
• Automation vendors and system integrators (Honeywell, Yokogawa, Emerson, ABB, Siemens, Schneider) — I/O counting from P&IDs, DCS database build from ILDs, FAT/SAT loop checking.
• Power, steel and cement plants (NTPC, Tata Steel, JSW, UltraTech) — boiler drum level three-element control, interlock schemes, cause-and-effect matrices referenced on the P&ID.
• Pharma, food and water treatment — P&IDs with sanitary symbols, CIP loops, and GMP documentation; ETP/STP instrumentation under state pollution control board monitoring requirements.
• Maintenance and commissioning roles — instrument technicians and shift engineers isolate, calibrate and trouble-shoot using the P&ID for process isolation and the ILD for terminal-level fault finding.

Interview questions

• Q: What is the difference between a PFD and a P&ID? A: A PFD shows major equipment, main streams and balances with only key control loops; a P&ID shows every line, valve, instrument tag, interlock and set point, and is the controlling document for construction and HAZOP.
• Q: What does FIC-101 mean, and what are its related tags? A: Flow indicating controller, loop 101; the same loop usually has FE-101 (primary element), FT-101 (transmitter) and FV-101 (control valve).
• Q: How do you tell from a bubble where an instrument is mounted? A: A plain circle is field mounted, a single solid line means front of main control panel, a dashed line means rear of panel (not accessible), and a double line means a local field panel.
• Q: What symbol represents a DCS function and what represents a PLC? A: A square with an inscribed circle is shared display/shared control (DCS); a diamond inside a square is a PLC or programmable logic function.
• Q: How are pneumatic and electronic signals distinguished on a P&ID? A: Pneumatic lines carry double hash marks across them (0.2–1.0 kg/cm²); electronic 4–20 mA signals are dashed lines; digital bus links are drawn with small circles along the line.
• Q: What does "FC" next to a control valve mean, and how is it chosen? A: Fail closed — the valve goes shut on loss of air; the choice follows safety, so a fuel or feed valve is FC while a cooling-water or quench valve is FO.
• Q: What extra information does a loop diagram carry that a P&ID does not? A: Terminal numbers at the field JB, marshalling TB and I/O card, cable and pair numbers, polarity, loop power supply and fuse, I/O address, calibrated range, action and set points.
• Q: Which standards govern instrument symbols and loop diagrams? A: ISA 5.1 for symbols and identification, ISA 5.4 for instrument loop diagrams, with IEC 60617/ISO 14617 for general graphical symbols and ISA 84/IEC 61511 for safety instrumented systems.
• Q: A transmitter without square-root extraction reads 12 mA on an orifice loop ranged 0–200 m³/h. What is the flow? A: 141.4 m³/h, because 12 mA is 50% of the DP span and flow varies as the square root of DP.
• Q: What is a cause-and-effect matrix and how does it relate to the P&ID? A: A table listing each initiating input (e.g. PSHH-301) against every action it triggers; the P&ID interlock symbol refers to it, and the SIS logic is configured from it.
• Q: What is a hook-up drawing? A: A standard installation detail for an instrument showing impulse tubing, root valve, manifold, mounting and the air supply with its bill of material — it does not show wiring.
• Q: How do you carry out a loop check at site? A: Isolate the loop, inject 4, 8, 12, 16 and 20 mA (or apply process input at the transmitter), verify the DCS reading, alarms and valve stroke against the ILD, then sign the loop folder.
• Q: If the project legend defines a symbol differently from ISA 5.1, which one do you follow? A: The project legend sheet — it is contractually binding and ISA permits user-defined letters and symbols provided they are defined there.

Common mistakes

• Treating a P&ID as a scaled layout drawing — it has no scale, elevation or piping route, so never estimate pipe length or valve location from it.
• Misreading the first letter and the modifier: PDT (differential pressure transmitter) confused with PT, or PSV (safety valve) with PS (pressure switch).
• Forgetting the suffix meaning, so LAHH is drawn as an ordinary alarm instead of a trip in the SIS.
• Mixing bubble conventions — using a plain circle for a DCS point, or forgetting that a dashed-line bubble is rear-of-panel and not operator accessible.
• Using 100 m³/h for 12 mA on an orifice flow loop — the square-root relation makes it 70.7% of span.
• Giving the same loop number to unrelated instruments, or omitting A/B suffixes on duplicate items, which breaks the instrument index and I/O count.
• Showing wiring or terminal numbers on a P&ID, or process vessels on a loop diagram — each drawing has its own scope.
• Choosing a valve's fail action from convenience rather than a safety analysis, and not stating FC/FO on the drawing at all.

Related topics

Control valves and final control elements · Flow, level, pressure and temperature measurement · DCS, PLC and SCADA architecture · PID controller tuning and cascade control · Safety instrumented systems, SIL and HAZOP · Hazardous area classification and intrinsic safety