Process flow diagrams and P&IDs

Chemical Engineering (B.E./B.Tech) · Process control and design · free study notes for exams, placements and interviews

In short

A process flow diagram (PFD) shows the main equipment, process streams and operating conditions of a plant; a piping and instrumentation diagram (P&ID) shows every pipe, valve, instrument, interlock and utility connection needed to build and operate it. Together with the block flow diagram (BFD) they are the "language" of a chemical plant — design, procurement, construction, commissioning, operating procedures, HAZOP and audits all start from them. Examined in GATE (CH) design papers, in university Process Design / Plant Design courses, and asked in virtually every interview for process, piping, instrumentation and commissioning roles at EIL, L&T Hydrocarbon, Toyo, Technip, Jacobs, Tata Consulting Engineers, Reliance, IOCL, BPCL, GAIL, Aditya Birla, UPL and Deepak Nitrite. Symbols follow ISA 5.1 and ISO 10628 (adopted in Indian practice), with Indian drawing-sheet conventions in SP 46 / IS 10711 and company/OISD standards for oil and gas.

Explained

The drawing hierarchy: BFD → PFD → P&ID → isometrics

Block flow diagram (BFD): rectangles for whole process sections joined by arrows; shows feed, product and overall mass balance only. Used in feasibility reports and DPRs.
Process flow diagram (PFD): one drawing per process section showing all major equipment, equipment tags, interconnecting process lines, flow direction, and a material/stream balance table with flow, composition, temperature and pressure for each numbered stream. Control is shown only where it defines the process (main control loops).
Piping and instrumentation diagram (P&ID, also "process and instrumentation diagram"): not to scale, shows all equipment including spares, all piping with full line numbers, all valves, fittings, blinds, drains, vents, insulation, every instrument with its tag and loop, interlocks, relief devices, utility headers, and tie-ins to other drawings.
Utility flow diagram (UFD) / utility distribution diagram: steam, cooling water, instrument air, nitrogen, fuel gas headers with their pressures and users.
Downstream drawings: plot plan and equipment layout, piping GA, isometric drawings (3-D, to scale, with material take-off for fabrication), hook-up drawings, cable schedules, hazardous area classification drawings.
Order of issue: PFD at the licensor/basic-engineering stage → P&ID issued "for review" → HAZOP → "for construction" (AFC) → marked up during construction → as-built P&ID handed to operations. Revision control (Rev 0, A, B…) with a revision cloud and triangle on each change is mandatory.

What a PFD must carry — and what it must not

Must carry: all major equipment with tag and name; equipment data (duty, size, capacity) in a summary table; process lines with stream numbers in a flag/diamond; flow direction arrows; major control loops and major valves; heat duties of exchangers, fired heaters, reboilers; battery-limit conditions; stream table; recycles.
Must not carry: every isolation valve, drains, vents, instrument details, line sizes and specifications, spare pumps' piping, pipe class breaks — these belong on the P&ID.
Stream table is the examinable part: the row sums must satisfy the overall material balance; energy figures must satisfy the energy balance around each unit.
Typical title block contents: project name, client, drawing number, sheet number, revision, scale ("NONE"), drawn/checked/approved initials and dates — IS 962 is the code of practice for architectural and building drawings; for engineering drawings the sheet sizes, title block, line thickness and folding follow SP 46 and IS 10711 (ISO-based).

Equipment and line identification (tagging)

Equipment tag format commonly used in Indian EPC practice: <letter(s)>-<unit no><serial><suffix>, e.g. P-1501A/B = pump, unit 15, item 01, A and B (duty and standby). Common letters: C compressor, D drum/vessel, E heat exchanger, F furnace/fired heater, K machine, P pump, R reactor, T tower/column, TK tank, V vessel, X package.
Line number format: <size>"-<service code>-<unit>-<sequence>-<piping class>-<insulation code>, e.g. 6"-P-1501-015-A1A-H. Service codes: P process, S steam, C condensate, CWS/CWR cooling water supply/return, IA instrument air, PA plant air, N nitrogen, FG fuel gas, FO fuel oil, VT vent, DR drain, OWS oily water sewer.
Insulation codes: H hot/heat conservation, C cold conservation, P personnel protection, S steam traced, E electrically traced, F fireproofing.
Piping class fixes material, pressure–temperature rating, flange rating, corrosion allowance and branch table — it changes at a class break, always marked on the P&ID.
Tie-in point (circle/flag with a number) marks where new piping joins existing; continuation arrow names the P&ID number and co-ordinate it goes to.

Instrument symbols — ISA 5.1 (ANSI/ISA-5.1-2009, reaffirmed later)

Bubble (circle) with a tag: first letter = measured variable, succeeding letters = function. Variables: F flow, L level, P pressure, T temperature, A analysis, D density, S speed, W weight, Q quantity, H hand (manual), V vibration, Z position. Functions: I indicate, R record, C control, T transmit, A alarm, S switch/safety, V valve, Y relay/compute/convert, E primary element (sensor), G glass/gauge.
So FRC-101 = flow recording controller, LAH-205 = level alarm high, PSV-310 = pressure safety valve, TE/TT-402 = thermocouple and its transmitter, FY-101 = I/P converter in the flow loop.
Bubble graphics (location): plain circle = field mounted; circle with a single horizontal line = control room / primary location, accessible to operator; circle with a dashed line = behind-panel / inaccessible; circle with a double line = secondary or auxiliary location; square-with-circle = shared display shared control (DCS); diamond-in-square = PLC/programmable logic; hexagon = computer function. Field-mounted DCS items use the same symbols at the field level.
Signal lines: plain line = piping; dashed = electrical; dashed with double cross-hatch marks = pneumatic (crossed slashes); line with small circles = data link (software/system bus); line with hollow circles = internal system link; capillary = —x—x—; hydraulic —L—L—.
Interlock / logic: interlock shown as a bubble with I or a logic box (e.g. UZ, SIS) and a cause-and-effect matrix sheet. Safety instrumented functions are tagged separately and reference IEC 61511 / IS compliance.
Final control element: valve body symbol plus actuator — diaphragm (dome), piston, motor (M), solenoid (S); fail position written as FC (fail closed), FO (fail open), FL (fail last/locked), FI (fail indeterminate).

Piping, valve and specialty symbols

Valves: gate (plain bow-tie), globe (bow-tie with solid circle), ball (bow-tie with open circle), butterfly, plug, check/NRV (bow-tie with an arrow or flap), diaphragm, needle, three-way (tee shape), four-way.
Control valve = bow-tie with actuator; bypass arrangement = control valve plus upstream/downstream block valves and a globe bypass (a classic P&ID reading question).
Relief devices: PSV/PRV (spring symbol on a valve), rupture disc (dashed arc), vacuum breaker, flame arrester, pressure vacuum relief valve on tanks (OISD-STD-118 practice for tank farms).
Specialties: strainer (Y, T, basket, temporary cone strainer shown with a flag), steam trap, sight glass, spectacle blind (two circles joined — one open, one solid), spacer and blind, restriction orifice (RO), silencer, expansion joint, flexible hose, reducer, orifice flange, sample point.
Equipment is drawn as a schematic outline (column with trays/packing, shell-and-tube exchanger, air-fin cooler, centrifugal pump as a circle with a tangential outlet, PD pump, vessel with nozzles) with nozzle sizes, elevations where critical (NPSH, gravity flow), internals and manholes.
Elevations and slopes are noted where they matter: minimum liquid seal legs, pump suction free of pockets, lines sloped to drain, NO POCKET, FREE DRAINING, FIELD ROUTE, SLOPE 1:100.

How a P&ID is read and used

Start at the feed battery limit, follow the main process line through each equipment item, noting line numbers and class breaks.
At each control loop identify the measured variable, the controller, the final element and its fail position; check the loop makes physical sense (level controlled by outflow, pressure by vent, etc.).
Check protection: every vessel that can be overpressured has a relief path; pumps have minimum-flow protection; isolation and double-block-and-bleed where required for maintenance.
Check operability: drains at low points, vents at high points, spectacle blinds for positive isolation, sample connections, startup bypasses, spare pump changeover.
HAZOP takes the P&ID node by node (a line or an equipment item) and applies guide words to parameters — so an incomplete P&ID makes a HAZOP invalid.
The P&ID is the source for operating manuals, SOPs, pre-commissioning punch lists, line checking, hydro-test packages, loop checking, MOC (management of change) records and statutory submissions.

Standards, codes and Indian statutory practice

ISA 5.1 — instrumentation symbols and identification (the global reference); ISA 5.2 binary logic, ISA 5.3/5.5 for display and graphic symbols.
ISO 10628 — diagrams for the chemical and petrochemical industry (BFD/PFD/P&ID content and symbols); ISO 14617 graphical symbols for diagrams.
IS 962 — code of practice for architectural and building drawings/civil interface; Indian drawing-office conventions (sheet sizes A0–A4, title block, line types) follow IS/ISO practice. Equipment design codes referenced on the drawings: IS 2825 (unfired pressure vessels, now largely superseded in practice by ASME Section VIII Div. 1 for export work), ASME B31.3 for process piping, API 650/620 and OISD-STD-118 for storage tanks, TEMA for exchangers.
Statutory: SMPV(U) Rules and the Gas Cylinders Rules under the Explosives Act for pressurised storage; Factories-type safety duties now sit under the Occupational Safety, Health and Working Conditions Code, 2020, in force from 21 November 2025 (it replaced the Factories Act, 1948); MSIHC Rules and the Chemical Accidents (EPPR) Rules under the Environment (Protection) Act for major accident hazard installations — all of which require approved drawings, safety reports and on-site emergency plans built on the P&ID. PESO (Petroleum and Explosives Safety Organisation) approves layouts and drawings for petroleum, LPG and explosives installations; CCOE licences reference the plot plan and P&ID.
Hazardous area classification drawings follow IS/IEC 60079-10-1 (zones 0/1/2 for gases) and drive instrument and motor selection (Ex d, Ex e, Ex ia); the P&ID notes the area class for electrical items.
Exact clause numbers and editions of Indian Standards change; quote the current edition from the drawing's reference list rather than from memory.

Key points

BFD — section-level blocks and overall balance; no equipment detail.
PFD — major equipment, numbered streams, stream table with flow/composition/T/P, major loops, heat duties.
P&ID — every pipe, valve, instrument, interlock, utility and relief device; not to scale; the construction and HAZOP document.
Not to scale — P&IDs and PFDs show topology, not geometry; geometry lives in the plot plan and isometrics.
Equipment tag — letter(s) for type + unit + serial + A/B for duty/standby (P-1501A/B).
Line number — size, service, unit, sequence, piping class, insulation code, all on one tag.
Piping class break — point where material/rating changes; always marked.
ISA first letter = measured variable; succeeding letters = function (FRC, LAH, PSV, TE, FY).
Bubble line — none = field, single line = control room, square+circle = DCS, diamond+square = PLC.
Signal types — solid piping, dashed electrical, cross-hatched pneumatic, circled line = data link.
Fail position — FC/FO/FL chosen by safety logic: fuel valves fail closed, cooling-water valves fail open.
Spectacle blind / double block and bleed — positive isolation for maintenance, shown on the P&ID.
Relief path — every pressure-containing item needs an identified and sized relief or safe vent.
Revision control — clouds, revision triangles, Rev 0/A/B, AFC issue, as-built handover.
Tie-in point and continuation arrow — how a P&ID links to other sheets and existing plant.
Legend/symbol sheet — the first P&ID sheets define all symbols, abbreviations and line codes used in the project.
HAZOP node — a line or item on the P&ID; guide words applied to it. An out-of-date P&ID invalidates the study.
As-built — the only legally and operationally trustworthy version; MOC keeps it current.

Formulas and examples

Overall material balance per stream table: Σ(mass in) = Σ(mass out) + accumulation; at steady state accumulation = 0. Units: kg/h or t/h.
Component balance: Σ(Fᵢ xᵢ,in) + generation − consumption = Σ(Fᵢ xᵢ,out) (+ accumulation); F = mass flow (kg/h), x = mass fraction. For a non-reacting unit, generation = consumption = 0, so in = out.
Energy balance across a unit: Q = ṁ cp ΔT (sensible) or Q = ṁ λ (latent); Q in kW, ṁ in kg/s, cp in kJ/kg·K, λ in kJ/kg.
Exchanger duty on a PFD: Q = U A ΔTlm; U in W/m²·K, A in m², ΔTlm = log-mean temperature difference (K).
Line size from flow: D = √(4 Q̇ / (π v)); Q̇ = volumetric flow (m³/s), v = design velocity (m/s), D = internal diameter (m).
Orifice/flow element: Q̇ ∝ √(Δp) — so a square-root extractor (FY) is needed between a differential-pressure transmitter and the controller unless the transmitter does it.
Control valve sizing (liquid, non-choked): Q = Cv √(Δp/SG), Q in US gpm, Δp in psi, SG = specific gravity; Δp allocated to the valve is typically 20–30% of system dynamic loss.
Pump NPSH available: NPSHa = (P_suction_abs − P_vapour)/(ρg) + static head − friction head (m); must exceed NPSHr with a margin (commonly ≥ 1 m or per API 610).
PSV relief area (gas, critical flow, API 520): A = W √(T Z) / (C K_d P₁ K_b K_c √M); A in mm², W in kg/h, T in K, P₁ absolute relieving pressure.

Example 1 — closing a PFD stream table. A distillation column feed is 10,000 kg/h containing 40% benzene, 60% toluene (mass). The distillate is 98% benzene and recovers 95% of the benzene in the feed. Complete the stream table.

Benzene in feed = 0.40 × 10,000 = 4,000 kg/h; toluene = 6,000 kg/h.
Benzene in distillate = 0.95 × 4,000 = 3,800 kg/h.
Distillate total D = 3,800/0.98 = 3,877.55 kg/h → toluene in distillate = 3,877.55 − 3,800 = 77.55 kg/h.
Bottoms B = 10,000 − 3,877.55 = 6,122.45 kg/h; benzene = 4,000 − 3,800 = 200 kg/h; toluene = 6,000 − 77.55 = 5,922.45 kg/h.
Bottoms benzene purity = 200/6,122.45 = 3.27%.

Stream	1 Feed	2 Distillate	3 Bottoms
Total, kg/h	10,000	3,877.55	6,122.45
Benzene, kg/h	4,000	3,800	200
Toluene, kg/h	6,000	77.55	5,922.45
Benzene, mass %	40.0	98.0	3.27

Example 2 — exchanger duty and area shown on a PFD. 25,000 kg/h of process liquid (cp = 2.1 kJ/kg·K) is cooled from 120 °C to 60 °C by cooling water entering at 32 °C and leaving at 42 °C, counter-current. U = 600 W/m²·K.

ṁ = 25,000/3,600 = 6.944 kg/s → Q = 6.944 × 2.1 × (120 − 60) = 875.0 kW.
Cooling water: ṁw = 875.0/(4.18 × 10) = 20.93 kg/s = 75,360 kg/h ≈ 75.4 m³/h.
ΔT₁ = 120 − 42 = 78 K; ΔT₂ = 60 − 32 = 28 K; ΔTlm = (78 − 28)/ln(78/28) = 50/1.0245 = 48.80 K.
A = Q/(U ΔTlm) = 875,000/(600 × 48.80) = 29.9 m².
The PFD shows "E-1502, Duty 875 kW, Area 30 m²"; the P&ID then adds TI on both outlets, a TRC on the process outlet throttling the CWR valve, CWS/CWR line numbers, vents, drains and a bypass.

Example 3 — line number and size for the same duty. Cooling water 75.4 m³/h at design velocity 2.0 m/s.

Q̇ = 75.4/3,600 = 0.02094 m³/s → D = √(4 × 0.02094/(π × 2.0)) = √(0.013333) = 0.1155 m = 115.5 mm.
Next standard size up: NPS 6 (DN 150) carbon steel; with ID ≈ 154 mm, actual velocity = 0.02094/(π/4 × 0.154²) = 1.12 m/s — acceptable (CW design range roughly 1–3 m/s). NPS 5 (ID ≈ 128 mm) gives 1.63 m/s and is also valid where the class allows 5".
Line tag: 6"-CWS-1501-021-A1A-N (N = no insulation) with the return as 6"-CWR-1501-022-A1A-N.

Example 4 — reading a control loop and choosing fail position. A fired heater outlet temperature is controlled by the fuel gas valve.

Loop: TE-2201 (thermocouple) → TT-2201 (transmitter, electrical dashed line) → TIC-2201 (DCS, square-with-circle) → TY-2201 (I/P converter) → TCV-2201 (pneumatic diaphragm control valve, pneumatic signal).
Fail position: FC — loss of air must shut fuel off. A low-flow trip FSLL-2202 and high-temperature trip TSHH-2201 act through an interlock I-22 on a shut-off valve XV-2201 (solenoid-operated, fail closed), recorded in the cause-and-effect matrix.
Compare: the cooling-water valve on E-1502 is FO, because loss of air must leave cooling in service.

Example 5 — control valve Cv from the stream data. Water 75.4 m³/h (= 332 US gpm, since 1 m³/h = 4.403 gpm → 75.4 × 4.403 = 332.0), SG = 1.0, valve Δp = 1.0 bar = 14.5 psi.

Cv = Q √(SG/Δp) = 332.0 × √(1/14.5) = 332.0/3.808 = 87.2.
Select a valve with rated Cv around 120–145 so the design point sits near 60–70% travel; the P&ID notes "CV-1502, 4" body in 6" line, Cv 130, FO, with 6" block valves and a 4" globe bypass".

Feature	BFD	PFD	P&ID
Purpose	Concept, feasibility	Process definition, heat & mass balance	Construction, operation, HAZOP
Equipment shown	Blocks only	All major items	All items including spares, internals
Piping	Arrows	Main process lines	All lines with full line numbers
Valves	None	Major/control valves only	Every valve, blind, drain, vent
Instruments	None	Key loops	All instruments, alarms, interlocks
Stream data	Overall balance	Full stream table	None (refer PFD)
Scale	None	None	None (not to scale)
Primary users	Management, DPR	Process engineers, licensors	Piping, instrumentation, construction, operations

Where it is used

Refineries and petrochemicals: IOCL, BPCL, HPCL, Reliance, HMEL, GAIL — P&IDs are the controlled documents for every turnaround, tie-in and MOC; OISD audits check as-built accuracy.
EPC and consultancy: EIL, L&T Hydrocarbon, Toyo India, Technip Energies, Jacobs, Tata Consulting Engineers, Thyssenkrupp — graduate process engineers spend their first years developing PFDs, P&IDs, line lists and stream tables in AutoCAD P&ID, SmartPlant/SPPID or Plant 3D.
Bulk chemicals, agrochemicals and pharma: UPL, Deepak Nitrite, Aarti, SRF, Divi's, Sun Pharma — P&IDs plus URS/DQ-IQ-OQ documents form part of GMP qualification and schedule-M compliance.
Fertiliser, cement, power and water: ammonia-urea plants, CPPs, ZLD and ETP packages, municipal water schemes — UFDs and P&IDs define utility headers and interlocks.
Job roles: process design engineer, piping engineer, instrumentation/C&I engineer, commissioning engineer, plant operations and HAZOP facilitator; software skills in SmartPlant P&ID, AutoCAD Plant 3D, Aspen Plus/HYSYS (for the stream table behind the PFD).
Statutory and insurance work: PESO/CCOE approvals, MSIHC safety reports, consent applications to State Pollution Control Boards and insurer risk surveys all require current P&IDs and plot plans.

Interview questions

Q: What is the single biggest difference between a PFD and a P&ID? A: A PFD defines the process — major equipment, streams and a stream table with flows, compositions and conditions; a P&ID defines the hardware — every line, valve, instrument, interlock and utility needed to build and run it, with no stream data.
Q: Are P&IDs drawn to scale? A: No. They show connectivity and function only; geometry, elevations and routing come from the plot plan, piping GA and isometric drawings.
Q: Decode LIC-201 and PSV-310. A: LIC-201 is a level indicating controller on loop 201; PSV-310 is a pressure safety (relief) valve, tag 310.
Q: What does a square with a circle inside mean on a P&ID? A: A shared-display, shared-control function — typically a DCS-based instrument accessible to the operator; a diamond inside a square means a PLC/programmable logic function.
Q: How do you choose whether a control valve fails open or closed? A: By the safer state on loss of motive power: fuel, feed and steam-admission valves usually fail closed; cooling water, quench and reflux valves usually fail open; valves whose movement either way is hazardous are fail-last.
Q: What information is packed into a line number? A: Nominal size, service code, unit number, sequence number, piping class (material and rating) and insulation code — enough to specify the pipe without reading another document.
Q: What is a piping class break and why mark it on a P&ID? A: The point where material, rating or corrosion allowance changes — it governs procurement, welding, testing boundaries and must not be crossed by the wrong spec.
Q: What is a spectacle blind and when is it used? A: A single plate with one open and one solid half that can be swung between flanges for positive isolation during maintenance; valves alone are not accepted as positive isolation.
Q: Why is a square-root extractor shown in some flow loops? A: An orifice meter output varies as the square root of flow, so the differential-pressure signal needs square-root extraction (in the transmitter or as an FY block) before it is used as a linear flow signal.
Q: What is a tie-in point, and why is it numbered? A: The interface between new and existing piping; numbering lets each tie-in be scheduled, isolated, tested and signed off during shutdown, with a register tracking it.
Q: Which document does a HAZOP work from, and what is a node? A: The latest approved P&ID; a node is a defined section — usually a line or an equipment item — to which guide words are applied against parameters such as flow, pressure and temperature.
Q: Why is the "as-built" P&ID so important? A: It is the only version that matches the plant; operating procedures, isolation plans, relief sizing and HAZOP all become unsafe if field changes are not back-marked through management of change.
Q: On a PFD, how do you check that the stream table is right? A: Close the overall and component mass balances around every unit and the whole plant, and verify each exchanger or heater duty with an energy balance.

Common mistakes

Putting instruments, drains, vents and every isolation valve on a PFD, or expecting stream flows and compositions on a P&ID.
Reading a P&ID as a layout — assuming equipment is physically where it is drawn, or inferring elevations and pipe lengths from it.
Reversing ISA letters: treating the first letter as the function instead of the measured variable (writing CF instead of FC for a flow controller, or confusing FC the tag with FC the fail-closed note).
Confusing signal-line types — drawing an electrical signal as pneumatic, or missing that a solid line means piping, not a signal.
Forgetting the relief path: a vessel, a blocked pump discharge or a thermally expanding blocked-in line left without a relief or safe vent.
Omitting the insulation and piping-class fields from a line number, or inventing a tag that does not match the project legend sheet.
Working from a superseded revision instead of the AFC/as-built sheet, and not clouding changes with a revision triangle.
Quoting a withdrawn Indian Standard or an old statute — Factories Act duties now sit under the OSH Code, 2020 in force from 21 November 2025, and IS numbers and editions must be taken from the project reference list.