BFD vs PFD vs P&ID: Differences, Uses, and How They Relate

Engineers use different diagrams because no single drawing can answer every process, control, piping, maintenance, safety, and construction question clearly.

A Block Flow Diagram (BFD) tells the process story at the highest level. A Process Flow Diagram (PFD) develops that story into major equipment, process streams, operating conditions, and balance information. A Piping and Instrumentation Diagram (P&ID) shows the functional relationships among equipment, piping, valves, instruments, control loops, isolation features, and protective devices.

The three drawings are related, but they are not interchangeable.

Table
Drawing	Best short description	Typically used to understand
BFD	The process story	Major stages or blocks and principal flows
PFD	The process design basis	Major equipment, streams, operating conditions, stream-table data, and sometimes major control loops
P&ID	The detailed functional process configuration	Piping, valves, instruments, loops, isolation, drains, vents, bypasses, and protective devices per project standard

The common greenfield progression is BFD → PFD → P&ID, but that is not a mandatory sequence. Existing plants, modifications, packaged systems, and vendor documents may follow a different path.

This guide compares all three drawing types through one fictional heating-and-transfer process. It also provides a question-to-document decision table so that you can choose the correct drawing before relying on incomplete information.

What you will learn

By the end of this guide, you should be able to:

explain the difference between a BFD, PFD, and P&ID;
identify the questions each drawing can and cannot answer;
trace a process stage from a BFD into PFD equipment and then into P&ID details;
recognize where project conventions can change drawing content;
avoid treating a P&ID as an exact physical layout;
distinguish process-engineering block diagrams from control-system block diagrams;
select adjacent documents such as piping isometrics, line lists, loop diagrams, and cause-and-effect charts;
check whether a drawing is current and suitable for the intended task; and
read a simplified worked example without mistaking it for plant data or an approved design.

Educational disclaimer

This article, its diagrams, tables, calculations and examples are provided for education and general process-engineering orientation. They are not a design basis, operating procedure, isolation plan, construction document, relief-system design, control narrative, cause-and-effect chart or HAZOP record.

Do not use the simplified material to operate, modify, isolate, commission, construct or safeguard equipment. For real work, consult the latest approved controlled drawings, project specifications, calculations, datasheets, site procedures and applicable legal and technical requirements, with review by appropriately qualified personnel.

Illustrative operating conditions and calculated values are training inputs only. They do not establish equipment ratings, design limits, alarm or trip setpoints, valve fail actions, relief requirements or safe operating limits.

Terminology: BFD, PFD, P&ID, and PID

Block Flow Diagram

In this article, block diagram means a process-engineering Block Flow Diagram, abbreviated BFD. It represents major process stages as blocks connected by principal flow arrows.

The name needs clarification because “block diagram” is also used in several other disciplines:

A control engineer may use a block diagram to represent transfer functions, signal paths, summing points, and feedback.
An electrical engineer may use blocks to represent functional sections of a power or electronic system.
A software engineer may use blocks to represent modules, services, or data movement.
A process engineer uses a BFD to summarize process stages and material-flow relationships.

These diagram types may look superficially similar, but they answer different questions.

Process Flow Diagram

A Process Flow Diagram, abbreviated PFD, is a simplified process schematic. It typically shows major equipment, principal process streams, important operating conditions, and enough information to communicate the process design and basic control concept.

Other organizations may use related names, but the document title, legend, and project drawing procedure should govern interpretation.

Piping and Instrumentation Diagram

A Piping and Instrumentation Diagram, abbreviated P&ID, is a schematic that shows functional relationships among equipment, piping, valves, instruments, loops, interlocks, and related process features.

“Piping and Instrument Diagram” is also encountered as an expanded name. The abbreviation P&ID is preferable because it avoids confusion with a PID controller.

CALLOUT — PID controller versus P&ID A PID controller is a proportional-integral-derivative controller. A P&ID is a piping and instrumentation diagram. The terms describe different things. This article uses P&ID consistently for the drawing.

A P&ID is not a piping isometric, equipment layout, or scaled routing drawing. It shows functional relationships schematically and is normally not to scale.

Names do not replace document control

A familiar title does not guarantee familiar content. A company’s “PFD” may show selected control loops, while another company’s PFD may contain very little instrumentation. A BFD may be nearly conceptual, or it may carry summary flow and balance values.

Always check:

the drawing title;
drawing type;
legend and symbols;
notes;
status;
revision;
applicable project standard; and
whether the copy is controlled.

Why Engineering Uses Three Levels of Drawing

The purpose of using several drawing levels is not to repeat the same information three times. Each level removes details that would obstruct its main purpose and adds details needed for a particular set of decisions.

A BFD is useful when the reader needs to see the whole process without being distracted by equipment tags or valve arrangements. It can help answer:

What are the main processing stages?
What enters and leaves each major area?
How do the principal sections connect?
Where does a recycle or by-product return?

A PFD moves from process story to process definition. It can help answer:

What major equipment performs each stage?
Which major streams connect the equipment?
What are the normal operating temperatures, pressures, and flow rates?
What material or energy balance supports the process concept?
What is the basic control strategy?

A P&ID moves toward detailed functional definition. It can help answer:

Which valve isolates an item?
Where is a check valve installed functionally?
How is temperature measured and controlled?
Which instrument tags belong to a loop?
Where are required drains, vents, bypasses, and protective devices shown?
Which piping line identifier applies?

Trying to place all P&ID detail on a BFD would bury the process story. Trying to perform isolation planning from a PFD would rely on a drawing that normally omits many valves and connections. Trying to determine equipment coordinates from a P&ID would confuse functional representation with physical arrangement.

The drawings therefore form a connected information hierarchy, not a ranking of “bad, better, best.” More detail is not automatically more useful. The right drawing is the one suited to the question.

CALLOUT — General principle Use the least detailed drawing that completely and reliably answers the question. If the question involves operation, isolation, maintenance, safety, or field work, confirm the answer against the latest approved controlled documents and site procedures.

BFD, PFD and P&ID arranged from process overview to functional detail, with the progression identified as common rather than mandatory. — BFDs, PFDs and P&IDs commonly progress from process overview to increasing functional detail, but this sequence is not mandatory.

Block Flow Diagram Fundamentals

What a BFD shows

A BFD is normally the simplest of the three process diagrams. It uses labeled blocks and arrows to communicate the overall process sequence.

A block may represent:

one major equipment item;
a processing operation;
a group of related equipment;
a plant section;
a utility section; or
an entire facility.

The appropriate scope depends on the audience and the decision being made. A student exercise may use one block per operation. A large site overview may use one block for an entire production unit.

A BFD typically includes:

major process stages;
principal feeds;
principal products;
major by-products;
major recycle paths;
important inter-unit connections; and
arrows showing overall flow direction.

Left-to-right flow is commonly used where practical, but it is a convention rather than a physical-layout statement.

Common BFD conventions

A clear BFD normally uses:

simple rectangular blocks;
short, functional block names;
arrowheads that make flow direction unambiguous;
consistent terminology;
limited line crossings;
a title and boundary explaining the scope; and
enough labels to identify major feeds, products, and recycles.

Some BFDs carry summary values. These may include selected flow rates, compositions, production rates, temperatures, or balance information. Such values can support early studies, but their presence does not transform the drawing into a PFD.

The important distinction is purpose. A BFD communicates the process at the stage or area level. A PFD communicates major equipment and process-stream information in greater detail.

What a BFD normally leaves out

A BFD normally does not establish:

detailed equipment configuration;
equipment nozzles;
line numbers;
pipe sizes or classes;
individual manual valves;
detailed instruments;
control-loop tags;
drains and vents;
bypass arrangements;
relief-system design;
exact routing;
dimensions; or
physical coordinates.

Because a BFD is intentionally simplified, absence from a BFD does not mean that a feature does not exist. It means the feature is below the drawing’s normal level of detail.

BFD strengths

The BFD is especially useful for:

explaining an unfamiliar process quickly;
comparing alternative process routes;
establishing the scope of a unit or study;
introducing a process before presenting equipment details;
supporting early mass-balance thinking;
showing battery-limit or inter-area connections; and
giving non-specialists an understandable overview.

A BFD can also be the starting point for developing a PFD. This is a common approach in greenfield process development, not a universal rule. In an existing plant, engineers may begin with controlled PFDs and P&IDs and produce a new BFD as a communication aid. That is an occasional practical workflow, not evidence that all BFDs are created by working backward.

BFD limitations

A BFD should not be used to:

prepare an equipment-isolation plan;
identify every instrument in a loop;
determine pipe specifications;
locate a valve physically;
set operating limits;
size equipment or piping;
perform detailed construction;
define an interlock;
verify a relief system; or
replace approved procedures.

Its simplicity is its advantage and its limitation.

BFD versus control-system block diagram

BFD versus control-system block diagram
Feature	Process-engineering BFD	Control-system block diagram
Main subject	Material or process progression	Signal, mathematical, or functional control relationships
Typical blocks	Process stages, units, or plant areas	Controller, process transfer function, sensor, actuator
Typical arrows	Principal material or process flow	Signal direction or functional dependence
Main question	“What happens to the process, and in what broad sequence?”	“How does the control system respond or transmit information?”
Typical detail	Feeds, products, stages, major recycle	Setpoint, error, feedback, transfer functions, disturbances
Physical layout?	No	No
Interchangeable?	No	No

Plant-description diagrams such as BFDs, PFDs, and P&IDs must therefore be distinguished from transfer-function or control-signal block diagrams used in control education and analysis.

A process BFD follows material through sequential stages, while a feedback-control diagram returns a measured signal to the controller. — A process BFD organizes process stages; a feedback-control block diagram represents signal and response relationships.

Process Flow Diagram Fundamentals

What a PFD shows

A PFD is a simplified schematic showing basic equipment and stream flows. It develops the process stages identified on a BFD into major equipment and named or numbered streams.

A PFD typically shows:

major process equipment;
equipment tags and names;
principal process lines;
stream numbers;
flow direction;
major feeds and products;
selected utility connections;
normal operating temperatures and pressures;
normal or design-basis flow rates, as identified;
major stream composition or property information where relevant;
equipment duties;
material and energy balance information; and
instrumentation sufficient to illustrate the basic process-control concept.

Packaged systems may be represented as boxes when internal detail is not needed at PFD level. Operating temperatures and pressures may appear beside streams, inside equipment outlines, or in an associated stream table, depending on the drawing standard.

Major equipment and streams

The PFD converts a broad process stage into identifiable process objects.

For example, a BFD block called “Heating” may become:

a process heater or heat exchanger;
a heating-utility connection;
an inlet process stream;
an outlet process stream;
an equipment duty;
inlet and outlet temperatures and pressures; and
a major temperature-control concept.

The PFD should make the main process path understandable without requiring the reader to inspect every small branch or valve.

Stream tables

A stream table organizes the data associated with numbered streams. Depending on project practice, it may include:

stream number;
source and destination;
phase;
component or composition data;
mass flow;
molar flow;
volumetric flow;
temperature;
pressure;
density;
enthalpy; and
other properties relevant to the process.

Not every PFD has its stream table on the same sheet. Balance information may be on another sheet, in a continuation table, or in a controlled calculation package.

Stream numbers should remain consistent across related sheets and calculations. This allows a reader to move from a line on the PFD to its balance values without guessing.

Operating conditions are not automatically design limits

A temperature or pressure shown on a PFD must be interpreted according to its label and project convention.

Possible meanings include:

normal operating condition;
expected operating range;
selected design case;
equipment duty point;
stream-table basis; or
another defined calculation case.

A normal operating pressure is not automatically:

maximum allowable working pressure;
design pressure;
alarm setting;
trip setting;
relief set pressure; or
hydrotest pressure.

Similarly, a normal operating temperature is not automatically a design-temperature limit. Check equipment datasheets, line lists, process datasheets, relief files, operating procedures, and project notes before assigning a meaning the PFD does not state.

Material and energy balance information

A PFD often communicates the material and energy balance that supports process development. The information may include:

flow continuity;
feed and product rates;
component balances;
heating or cooling duties;
phase changes;
utility duties; and
physical or thermal conditions of major streams.

This makes the PFD a useful starting point for:

equipment lists;
equipment datasheets;
preliminary equipment sizing;
control decisions;
instrument datasheets;
P&ID development;
utility estimates; and
safety evaluations.

A PFD is an important design input, but it is not the only design basis. Calculations, specifications, datasheets, narratives, and project requirements remain necessary.

Does a PFD show control loops?

Sources and company practices differ. The most reliable general wording is:

A PFD typically shows the major process equipment, principal streams, operating conditions, and balance information. Depending on the project standard, it may also show major control loops or key control valves, but it normally omits the detailed instrumentation found on the P&ID.

A PFD may show:

a major temperature-control loop;
a key pressure-control valve;
a level-control relationship;
a seal leg;
an essential process-control connection; or
another control feature needed to explain the process concept.

It normally does not show every transmitter, switch, signal type, local indicator, alarm, interlock, or instrument connection.

What a PFD normally omits

Depending on the project standard, a PFD normally omits:

minor process lines;
startup lines;
detailed bypass lines;
most drains and vents;
most manual valves;
detailed isolation arrangements;
pipe classes;
detailed line identifiers;
instrument installation details;
complete interlock logic;
exact relief design;
supports;
physical routing;
dimensions; and
equipment coordinates.

Drives are not normally shown unless they are relevant to the process-control concept. Minor piping omitted from a PFD may still be essential on the P&ID and in the plant.

PFD strengths and limitations

A PFD is strong at connecting process calculations to equipment and streams. It is weaker at answering detailed configuration questions.

Use it to understand:

what major equipment does;
where principal streams go;
how normal process conditions change;
where heating or cooling occurs;
which major controls explain the process concept; and
whether the mass and energy balance is coherent.

Do not rely on it alone to determine:

every valve needed for isolation;
the complete instrument loop;
piping material;
detailed line routing;
construction dimensions;
exact alarm or trip implementation; or
whether a relief device is properly sized and located.

PFD anatomy connecting major equipment with numbered streams, process conditions, a stream-table reference and one major control loop. — A PFD emphasizes major equipment, numbered streams, key process conditions and selected major control functions.

Related reading: a practical level-control example on this blog, which uses a different tank-and-pump example.

Piping and Instrumentation Diagram Fundamentals

What a P&ID shows

A P&ID shows equipment, piping, and instrumentation schematically. It communicates functional relationships rather than exact physical arrangement.

A project P&ID typically includes, as applicable:

process equipment;
equipment tags;
process and utility piping;
line identifiers;
line-size and piping-class information per project standard;
manual valves;
control valves;
check valves;
isolation arrangements;
bypasses;
drains;
vents;
equipment connections;
principal instruments;
instrument tags;
measurement functions;
control loops;
signal relationships;
alarms;
interlocks;
shutdown functions;
protective devices;
off-page connectors;
references to other drawings; and
notes and legend references.

Process-safety guidance describes a P&ID as a schematic showing functional relationships among piping, equipment, instruments, loops, and interlocks. It may require multidisciplinary input because process intent, control implementation, piping design, operability, maintenance, and safety considerations all meet on the drawing.

Piping, valves, and isolation

At P&ID level, a single PFD line may expand into several functional features:

upstream isolation valve;
reducer or expander;
pump suction connection;
equipment nozzle;
discharge isolation valve;
check valve;
branch connection;
drain;
vent;
bypass;
sample point;
utility connection;
tie-in;
line break or class change; and
continuation to another sheet.

The drawing indicates that these features exist and how they relate functionally. It does not necessarily tell a field worker exactly where each valve is located or how much pipe lies between two items.

Isolation also requires more than spotting valve symbols. A safe isolation plan may need:

the latest approved P&ID;
site isolation procedures;
line walking;
equipment and valve identification in the field;
energy-source identification;
lockout/tagout requirements;
depressurization and draining provisions;
stored-energy assessment;
blinds or positive isolation requirements; and
authorization under the site’s safety-management system.

A training P&ID must never be used for actual isolation.

Instruments and loops

Instrument tags make process functions traceable. The exact letters, symbols, bubble styles, line types, and numbering structure are governed by the project legend and identification standard.

A loop may show:

a process variable measured in the equipment or line;
a sensing or transmitting function;
a signal sent to a controller;
a controller comparing the measurement with a target;
an output signal sent to a final control element; and
a valve or other final element changing the process.

The P&ID communicates the functional loop, but detailed implementation may require:

an instrument index;
instrument datasheets;
a loop diagram;
wiring or termination drawings;
logic diagrams;
control-system configuration;
a cause-and-effect chart;
a shutdown narrative; or
a Safety Instrumented Function (SIF) lifecycle package where applicable.

A line joining instrument functions on a P&ID does not automatically specify cable routing, protocol, terminal numbers, logic solver configuration, or software code.

Alarms, interlocks, and protective functions

P&IDs may show alarms, interlocks, shutdown functions, and protective devices per project standard. Their presence does not eliminate the need to consult supporting documents.

For example, a high-pressure alarm symbol may establish that a pressure-related alarm is intended, but it may not show:

alarm setpoint;
alarm priority;
delay;
voting;
operator response;
suppression rules;
reset method; or
relationship to an automated trip.

Similarly, an interlock reference may require a cause-and-effect chart or logic narrative to understand the complete action.

Relief-device representation requires particular care. A symbol does not by itself establish:

the protected scenario;
required relief rate;
set pressure;
accumulation basis;
inlet or outlet pressure-drop acceptability;
discharge destination;
material compatibility;
relief-device type; or
compliance with applicable codes.

Those questions belong in the relief design basis, calculation, datasheet, and applicable engineering review.

The P&ID is not an exact physical layout

A P&ID is a schematic of functional relationships. It is not to scale.

The following may not correspond to physical reality:

the distance between two symbols;
the orientation of equipment;
the number of pipe bends;
the location of a valve on the page;
the elevation of a vent;
the physical sequence of nearby components when drafting conventions simplify the line;
the geographic position of equipment; and
the length of piping.

Use a plot plan or equipment layout for overall physical location. Use piping layouts, a three-dimensional model, spool drawings, and piping isometrics for routing and fabrication detail.

Legends and project conventions

The first P&ID in a drawing set should carry or reference a legend according to the cited process-safety guidance. The legend explains symbols and line conventions used by the project.

A legend may define:

equipment symbols;
valve symbols;
instrument symbols;
signal line types;
line-break notation;
off-page connectors;
package boundaries;
control-system locations;
normally open or normally closed notation;
revision marks;
abbreviations; and
special project symbols.

Each symbol should be traceable to the applicable legend. Do not assume that a symbol learned from one company has exactly the same meaning in another drawing set.

P&IDs and construction

P&IDs are vital to people constructing and commissioning a process because they define many required functional relationships. However, a P&ID is not a standalone construction package.

Construction may also require:

approved equipment drawings;
piping plans;
piping isometrics;
support details;
civil and structural drawings;
electrical drawings;
instrument hook-ups;
wiring and termination drawings;
specifications;
line lists;
equipment and instrument datasheets;
vendor drawings;
testing requirements; and
approved field-change documentation.

The correct conclusion is not “P&IDs cannot support construction.” It is that a P&ID is an essential input but does not provide every physical and fabrication detail needed for construction.

Configuration control

P&IDs are frequently used for configuration control because they record functional process relationships across disciplines. That makes drawing status critical.

A printed or downloaded copy can become outdated. Before using one, verify:

drawing number;
revision;
status;
approval;
issue purpose;
outstanding markups;
whether changes are incorporated;
whether vendor documents are current; and
whether the document-control system lists a later revision.

P&ID anatomy showing functional piping, valves, vents, drains, instruments, a control loop and drawing references rather than physical layout. — A P&ID records piping and control functionality, references and connections; it is not an exact physical layout.

BFD, PFD, and P&ID Side by Side

BFD, PFD, and P&ID Side by Side
Drawing	Primary question	Typically includes	Normally does not establish
BFD	What is the overall process story?	Major stages or blocks, principal feeds and products, major flow paths, optional summary values	Equipment-level configuration, detailed controls, valves, line classes, isolation, routing, or dimensions
PFD	How does the major process equipment transform the principal streams?	Major equipment, stream numbers, normal operating conditions, balance information, duties, and sometimes major controls or key control valves	Most manual valves, detailed instrumentation, complete isolation, detailed line specifications, exact routing, or construction dimensions
P&ID	How are the equipment, piping, valves, instruments, and protection functions connected?	Equipment, piping, valves, isolation, bypasses, drains, vents, instruments, loops, interlocks, protective devices, and line identifiers per project standard	Exact coordinates, scaled distances, exact pipe routing, support design, fabrication dimensions, or complete control-system implementation

A practical mental model

Think of the three drawings as three zoom levels:

BFD: the chapters of the process story;
PFD: the main characters, conditions, and calculations in each chapter;
P&ID: the functional connections needed to implement and manage the process.

The analogy has limits. A P&ID is not simply a magnified PFD. It contains a different class of information, especially valves, instruments, isolation features, and line designations. Likewise, a PFD is not just a BFD with equipment pictures. It connects process calculations to tagged equipment and streams.

Detail does not equal physical accuracy

Moving from BFD to PFD to P&ID increases functional and identification detail. It does not move toward a scaled physical map.

Even a highly detailed P&ID remains schematic. For dimensions and routing, the reader must move sideways to layout and piping documents rather than “down” to another process-flow drawing.

Heating expands from one process block to E-230 and then to functional isolation and temperature control, while candidate and unresolved items remain visibly qualified. — The Heating function remains traceable from BFD to PFD and P&ID detail; candidate, illustrative and TBD items are not final plant data.

Traceability Between the Drawings

Traceability allows a reader to follow the same process intent through increasingly detailed documents without losing identity or design basis.

Four key identification systems

1. BFD stage names

A BFD may identify a stage such as:

feed storage;
transfer;
heating;
separation; or
product storage.

These names organize the process story but may not provide unique equipment identity.

2. Equipment and stream tags on the PFD

The PFD introduces tags such as:

V-210 for a vessel;
P-220 for a pump; and
E-230 for a heat exchanger.

It also introduces stream numbers such as S-01 and S-02. Stream numbers should remain consistent across related sheets and calculations per the cited engineering drawing guidance.

3. Line and instrument identifiers on the P&ID

The P&ID adds traceable identifiers for:

piping lines;
instruments;
control loops;
valves where tagged;
off-page connections;
equipment nozzles per project practice; and
protective devices.

A PFD stream number and P&ID line number are not necessarily the same identifier. One PFD stream can expand into multiple P&ID lines, especially across branches, specification breaks, equipment boundaries, or phase changes.

4. Registers and supporting records

Tags connect drawings to controlled records such as:

equipment lists;
equipment datasheets;
line lists;
instrument indexes;
instrument datasheets;
loop diagrams;
valve lists;
relief-device registers;
cause-and-effect charts;
logic narratives;
calculations; and
revision records.

Traceability map

Traceability map
Process level	Example identity	Typical linked records
BFD stage	“Heating”	Process description, study basis, overall balance
PFD equipment and stream	E-230; S-02; S-03	Equipment list, exchanger datasheet, stream table, heat balance
P&ID tags and lines	TT-231; TIC-231; TCV-231; project line IDs	Line list, instrument index, valve datasheet, loop diagram, control narrative
Supporting document control	Drawing number, revision, note, legend reference	Datasheet revisions, calculations, change records, approvals, revision history

Reading down and reading up

Reading down means moving from broad intent to implementation detail:

Identify the BFD stage.
Find the equipment and streams that perform that stage on the PFD.
Find the tagged equipment on the P&ID.
Follow the connected lines, valves, instruments, and references.
Open supporting lists, datasheets, loop documents, and calculations.

Reading up means testing whether detailed features still support process intent:

Start with a valve, instrument, or line on the P&ID.
Identify the connected equipment and function.
Locate the related stream or equipment on the PFD.
Confirm the operating case and process objective.
Relate it to the overall BFD stage.

Reading up is valuable during reviews. A detail may be technically drawable but unnecessary, inconsistent, or contrary to the process objective.

Legends, notes, and references

Traceability is not limited to tags. It also depends on:

symbol legends;
general notes;
line-type conventions;
equipment designation rules;
off-page connector references;
package-boundary definitions;
drawing indexes;
revision clouds;
change descriptions; and
document-status fields.

Equipment designations should match across equipment lists, datasheets, PFDs, and P&IDs. A mismatch can indicate an incomplete change, drafting error, duplicate identifier, or outdated document.

Revision control

Before relying on traceability, confirm that the documents belong to a compatible revision set. A current P&ID linked to a superseded PFD can create false confidence.

Ask:

Are all referenced documents approved for the intended use?
Does the drawing index identify the current revision?
Have marked-up field changes been incorporated?
Are off-page references valid?
Does the equipment list use the same tags?
Are calculations based on the current process case?
Has the change passed the required review and approval gates?

CALLOUT — A tag is a link, not proof Matching tags help locate related information. They do not prove that every document is current, internally consistent, or approved. Revision and status checks remain necessary.

Tag relationships connect process stages and drawing identifiers to controlled equipment, line, instrument, loop, legend and revision records. — Consistent tags connect process stages and drawings to controlled equipment, line, instrument, loop, legend and revision records.

Same-Process Walkthrough: Training Liquid Heating and Transfer System

Example basis and limitations

ILLUSTRATIVE EXAMPLE — NOT PLANT DATA

A fictional, nonreactive, single-phase, water-like liquid flows at 2.00 kg/s through P-220 Transfer Pump and E-230 Process Heater/Heat Exchanger. The liquid enters the system at 20 °C and is heated to 45 °C before reaching V-240 Product Tank.

The assumed heat capacity is 4.00 kJ/(kg·K), and the assumed density is 1000 kg/m³. These are fictional calculation assumptions, not property claims for a real fluid.

The example is designed only to show how the same process appears on a BFD, PFD, and P&ID. It does not size the pump, exchanger, piping, control valve, or relief system.

8.1 BFD view: four process stages

At BFD level, the process can be reduced to four blocks:

Feed storage  →  Liquid transfer  →  Heating  →  Product storage
    B1                  B2              B3

The BFD answers the main process-story question:

A liquid is stored, transferred, heated, and stored again.

It does not need to show V-210, P-220, E-230, or V-240 unless the project chooses to identify equipment at this level. Optional labels B1, B2, and B3 can help reference study boundaries, although the four textual stages are sufficient for this example.

Some BFDs could also carry the 2.00 kg/s process-flow value and the inlet and outlet temperatures as summary information. That would be acceptable if it supports the BFD’s purpose and remains readable. Those values would still require a stated basis and revision control.

Four ordered process stages carry the fictional liquid from feed storage through transfer and heating to product storage. — Training-system BFD: feed storage, liquid transfer, heating and product storage.

8.2 PFD view: equipment, streams, conditions, and control concept

At PFD level, the four stages become tagged equipment:

V-210 Feed Tank
P-220 Transfer Pump
E-230 Process Heater/Heat Exchanger
V-240 Product Tank

The principal process sequence is:

V-210 ──S-01──> P-220 ──S-02──> E-230 ──S-03──> V-240
                                      ↑
                               S-04 heating utility

A major temperature-control concept is shown as:

E-230 outlet temperature → TT-231 → TIC-231 → TCV-231
                                                |
                                      Heating-utility flow

This PFD representation communicates:

the major equipment performing each stage;
the direction of the liquid flow;
the principal stream numbers;
the heating-utility connection;
the inlet and outlet process conditions; and
the major temperature-control concept.

It does not yet attempt to define every isolation valve, check valve, drain, vent, instrument detail, or piping identifier.

Illustrative stream table

Selected example equipment and streams define the heating-transfer process, while all S-04 conditions and relief-protection decisions remain unresolved. — The illustrative PFD links V-210, P-220, E-230 and V-240 to defined process streams; every S-04 condition and the relief-protection evaluation remain TBD.

Illustrative stream table
Stream	From → To	Material/phase	Mass flow	Temperature	Pressure
S-01	V-210 → P-220 suction	Fictional single-phase liquid	2.00 kg/s	20 °C	100 kPa(a)
S-02	P-220 discharge → E-230	Fictional single-phase liquid	2.00 kg/s	20 °C	300 kPa(a)
S-03	E-230 outlet → V-240	Fictional single-phase liquid	2.00 kg/s	45 °C	280 kPa(a)
S-04	Heating utility → E-230	Utility, not specified	TBD	TBD	TBD

All pressures remain in kPa(a), meaning kilopascals absolute. They are not converted to gauge pressure.

The S-04 utility flow is deliberately left TBD. The process-side heat duty can be estimated from the illustrative inputs, but utility mass flow cannot be derived without a defined utility, inlet and outlet state, heat losses, exchanger basis, and other design information.

8.3 Calculations linked to the PFD

Temperature rise

Δ T = 45 - 20 = 25 K

A temperature difference of 25 °C is numerically equal to 25 K, but kelvin is used in the heat-duty equation.

Illustrative heat duty

Q = ṁc_pΔ T

Q = 2.00 kg/s × 4.00 kJ/(kg·K) × 25 K

Q = 200 kW

Unit check:

kg/s × kJ/kg·K × K = kJ/s = kW

The result is CALCULATED from illustrative inputs. It is not a real exchanger duty.

Ignoring heat losses:

|Q_utility| ≈ 200 kW

This does not determine utility mass flow.

Actual volumetric flow

Using the fictional density assumption:

V̇ = (2.00 kg/s) / (1000 kg/m³) = 0.00200 m³/s

Using the exact conversion 3600 s/h:

0.00200 m³/s × 3600 s/h = 7.20 m³/h

This is actual volumetric flow, not standard volumetric flow.

Pump differential pressure

Δ P_pump = 300 - 100 = 200 kPa

This subtraction uses two absolute pressures. The numerical difference is 200 kPa.

Heater process-side stream-pressure difference

Δ P_{E-230, streams} = 300 - 280 = 20 kPa

The 20 kPa result is the difference between the illustrative PFD stream pressures. It is not a validated exchanger hydraulic-design basis.

Optional hydraulic power

P_hydraulic = Δ P V̇

P_hydraulic = 200 kPa × 0.00200 m³/s = 0.400 kW

This is ideal hydraulic power based on the illustrative pressure difference and volumetric flow. It is not pump shaft power or motor power. Determining those values would require efficiency and equipment-selection information.

Process-side mass balance

2.00 - 2.00 = 0.00 kg/s

The process-side mass balance closes for the steady illustrative case. The heating-utility circuit is separate.

Process-side mass balance
Calculated item	Result	Limitation
Temperature rise	25 K	Based on 20 °C inlet and 45 °C outlet
Process heat duty	200 kW	Uses fictional c<sub>p</sub> = 4.00 kJ/(kg·K); ignores losses
Actual volumetric flow	0.00200 m³/s = 7.20 m³/h	Uses fictional density of 1000 kg/m³
Pump differential pressure	200 kPa	Difference between illustrative stream pressures
Heater stream-pressure difference	20 kPa	Not a validated hydraulic-design basis
Ideal hydraulic power	0.400 kW	Not shaft or motor power
Process mass-balance difference	0.00 kg/s	Steady illustrative process side only
Approximate utility duty magnitude	approximately 200 kW	Utility mass flow remains TBD

8.4 P&ID view: functional configuration

At P&ID level, each simple PFD connection is expanded into functional piping, valve, instrumentation, and protection information.

The educational P&ID concept includes:

V-210 connection;
suction isolation;
P-220;
discharge isolation;
discharge check valve;
PI-221, a pump discharge pressure indicator only;
E-230 process-side isolation;
a candidate process bypass;
TT-231, TIC-231, and TCV-231;
signal relationships among TT-231, TIC-231, and TCV-231;
heating-utility isolation;
a candidate utility bypass only if the control and operating philosophy permits it;
process-required low-point drains;
process-required high-point vents;
connection to V-240;
candidate PAH-221;
a relief-protection evaluation note (device need, type, location and tag TBD);
project line identifiers; and
legend references.

A compact educational representation is:

V-210
  |
[Process suction isolation]
  |
P-220
  |
[Discharge isolation]--[Check valve]--PI-221----E-230----> V-240
                                                   |
                                            TT-231 measurement
                                                   :
                                                TIC-231
                                                   :
Heating utility --[Isolation]--TCV-231-------------+

Candidate features requiring design review:

- Process bypass around E-230
- Utility bypass, only if the approved philosophy permits it
- PAH-221 high-pressure alarm
- Relief-protection evaluation required — device need, type, location and tag TBD
- Required low-point drains and high-point vents
- Final line numbers, sizes, classes, and valve details

Dotted or dashed signal notation is intentionally not standardized in this text drawing. The controlled project legend must define signal types and instrument locations.

Functional piping and control connect V-210 through P-220 and E-230 to V-240, with bypasses, drains, vents and PAH-221 marked candidate and relief protection left for evaluation. — The educational P&ID-style view distinguishes selected example functions from candidate features and unresolved relief-protection decisions.

8.5 Reading the example from left to right

V-210 Feed Tank and pump suction

The PFD shows V-210 feeding P-220 through S-01. The P&ID must develop the functional suction arrangement, including required connections and isolation per project standard.

The P&ID alone does not prove that the suction design is hydraulically acceptable. A pump review may also need:

minimum liquid level;
static head;
pipe losses;
fluid vapor pressure;
temperature cases;
available and required net positive suction head;
nozzle sizes;
suction-line geometry;
startup conditions; and
vendor requirements.

No suction sizing or cavitation conclusion is made in this example.

P-220 discharge

S-02 begins at the illustrative pump-discharge condition of 300 kPa(a). At P&ID level, the discharge path includes a check valve and isolation functionality.

PI-221 is defined only as a pump discharge pressure indicator. It is not automatically:

a transmitter;
an alarm source;
a shutdown input;
a control input; or
a safety instrument.

A candidate PAH-221 is listed for review. Its need, measurement source, setpoint, alarm priority, operator response, delay, and relationship to other protective functions remain undetermined.

E-230 process side

The PFD shows the process stream entering E-230 at 20 °C and 300 kPa(a) and leaving at 45 °C and 280 kPa(a).

The P&ID develops:

process-side connections;
isolation;
temperature measurement;
candidate bypass functionality;
required drains and vents; and
connections to the downstream product tank.

A process bypass should not be added merely because bypasses are common. Review must determine:

why bypassing is required;
whether bypassing defeats heating or protection;
how the bypass is controlled or secured;
whether operating procedures permit its use;
whether it creates an unintended flow path; and
whether it affects startup, shutdown, maintenance, or cleaning.

Temperature-control loop

The educational control concept is:

TT-231 measures E-230 outlet temperature.
TIC-231 receives the temperature measurement and performs the indicating/controller function.
TCV-231 changes heating-utility flow.
Changing utility flow changes heat transfer.
The process outlet temperature responds.

At PFD level, this major loop may be shown because it explains how the process reaches 45 °C. At P&ID level, the loop receives tags, functional signal connections, and relevant piping details.

The example does not define:

controller tuning;
sensor technology;
sensor insertion details;
signal protocol;
control-system platform;
valve characteristic;
valve size;
actuator type;
positioner;
shutoff class;
leakage acceptance;
alarm settings; or
trip logic.

Those details belong in additional design documents and reviews.

8.6 TCV-231 fail-action review

A preliminary educational proposal is for TCV-231 to fail closed, removing heat input on loss of actuator energy. This is not a final design decision and should not be depicted as one on the educational drawing.

The review must consider:

freezing if heat stops;
solidification;
viscosity increase;
plugging;
any exothermic behavior;
minimum-temperature process needs;
heating-utility pressure and temperature;
control-valve leakage and shutoff capability;
stored thermal energy;
startup;
shutdown;
loss of process flow;
blocked process flow;
loss of instrument power;
whether closing the utility valve creates another hazard;
whether another final element is more suitable;
whether an independent trip is needed;
alarm and operator-response requirements;
interlock requirements; and
whether Safety Instrumented Function analysis is required.

“Steam valve fails closed” or “heating valve fails closed” must not be treated as a universal rule. Fail action follows the defined safe-state analysis for the actual process and utility.

8.7 Relief-protection evaluation and overpressure review

Relief-protection evaluation required — device need, type, location and tag TBD. No relief device or device tag is selected in this example. The need, device type, location, sizing basis, set pressure, discharge destination, and code basis are undetermined.

The overpressure review should evaluate at least:

pump deadhead or blocked discharge;
blocked-in thermal expansion;
heating utility available while process flow is stopped;
external fire where applicable;
utility-to-process leakage or exchanger tube rupture;
credible simultaneous or dependent scenarios;
relief destination;
backpressure;
isolation philosophy;
applicable codes;
equipment and piping design limits; and
whether pressure relief, thermal relief, alarm, trip, design pressure, or another safeguard is appropriate.

No claim is made that thermal relief is always required between isolation valves. No final relief-device position is established. No device is sized.

8.8 Drains, vents, and bypasses

A P&ID may show process-required low-point drains and high-point vents. Their need and position depend on:

actual piping geometry;
drainage and venting objectives;
commissioning;
hydrotest and dewatering requirements;
maintenance;
flushing;
process hazards;
trapped-liquid risk;
emissions or containment requirements; and
destination systems.

Because a P&ID is not a physical layout, the “high” or “low” designation cannot be verified from page position. Layout and piping design must establish the actual elevation.

A utility bypass around TCV-231 should appear only if the approved control and operating philosophy permits it. An uncontrolled bypass could defeat temperature control or a protective action.

8.9 What each example drawing proves—and does not prove

8.9 What each example drawing proves—and does not prove
Drawing	What the example communicates	What it does not prove
BFD	The liquid moves through storage, transfer, heating, and product storage	Equipment selection, conditions, controls, isolation, routing
PFD	V-210, P-220, E-230, V-240; streams S-01 to S-04; illustrative conditions; 200 kW duty; major temperature-control concept	Detailed valve arrangement, line class, alarm implementation, relief adequacy, exact routing
P&ID concept	Functional piping, isolation, check valve, PI-221, TT/TIC/TCV loop, candidate drains, vents, bypasses, alarm, and a relief-protection evaluation note	Final valve fail action, relief design, field location, dimensions, construction completeness, safe isolation plan

8.10 How to trace the example

Start with the BFD stage Heating.

On the PFD, identify E-230.
Identify inlet stream S-02 and outlet stream S-03.
Read the stream-table conditions.
Locate the TT-231 → TIC-231 → TCV-231 control concept.
Move to the P&ID and find E-230.
Follow the process line through the required isolation features.
Follow TT-231’s signal to TIC-231.
Follow TIC-231’s output to TCV-231.
Check the heating-utility isolation and any permitted bypass.
Locate candidate protection references, notes, and off-page connections.
Open the line list, instrument index, loop diagram, valve datasheet, equipment datasheet, cause-and-effect chart, and relief-design file as applicable.
Confirm current revisions before drawing a conclusion.

Include/Exclude Matrix

The following matrix summarizes typical practice. It is not a substitute for the project drawing standard.

Include/Exclude Matrix
Information	BFD	PFD	P&ID	Qualification
Major process stages	Include	Typically	Typically	PFD/P&ID show these through equipment and connectivity rather than abstract blocks
Principal feeds and products	Include	Include	Include	P&ID may use off-page connectors
Major equipment	Sometimes	Include	Include	A BFD block may represent one item or an entire area
Minor equipment	Exclude	Normally exclude	Typically include if functionally relevant	“Minor” depends on process significance
Stream numbers	Sometimes	Include	Sometimes referenced	P&ID normally relies on line identifiers rather than only PFD stream numbers
Stream table	Sometimes summary only	Typically	Normally exclude	It may be on another PFD sheet or calculation
Operating temperature and pressure	Sometimes summary only	Typically	Sometimes	P&ID may show selected values or limits per project standard
Material and energy balance	Sometimes summary only	Typically	Normally exclude	P&ID is not normally the balance document
Major controls	Normally exclude	Sometimes/typically where needed	Include	PFD variability depends on project standard
Detailed instruments and loops	Exclude	Normally exclude	Include	Supporting loop and logic documents remain necessary
Manual valves	Exclude	Normally exclude	Typically include	Exact scope follows project standard
Isolation and bypass	Exclude	Normally exclude	Include as required	Suitability for actual isolation still requires procedure and site verification
Drains and vents	Exclude	Normally exclude	Include as required	Determined by process, operability, and piping design
Relief devices	Exclude	Sometimes major concept	Include as applicable	Symbol does not replace relief calculation
Pipe size, class, and line ID	Exclude	Normally exclude	Typically include	Exact encoding follows project standard and line list
Physical coordinates	Exclude	Exclude	Exclude	Use plot plan or equipment layout
Exact routing and dimensions	Exclude	Exclude	Exclude	Use piping layout, model, and piping isometric
Construction dimensions	Exclude	Exclude	Exclude	Use approved fabrication and construction drawings

How to use the matrix

Treat “Include,” “Exclude,” “Typically,” “Normally,” and “Sometimes” as disciplined qualifiers:

Include means the information is central to the usual purpose.
Exclude means the drawing is normally the wrong source.
Typically means commonly included, subject to the project standard.
Normally acknowledges legitimate exceptions.
Sometimes signals substantial variation among organizations and projects.

If the matrix conflicts with a controlled project standard, follow the controlled standard.

Who Creates, Reviews, and Uses Each Drawing

Drawing responsibility varies by organization. The following roles are typical, not universal.

Who Creates, Reviews, and Uses Each Drawing
Role	Typical contribution to BFD	Typical contribution to PFD	Typical contribution to P&ID
Process engineer	Defines process stages, boundaries, feeds, products, and major recycles	Develops process basis, major equipment, streams, balances, duties, and basic control concept	Commonly determines process content; defines process intent, operating requirements, and functional piping needs
Instrumentation/control engineer	Limited input where control architecture affects the concept	Reviews or develops major control concepts	Develops instrument functions, loop structure, control-valve requirements, signals, alarms, interlocks, and references
Piping/mechanical engineer	Reviews broad equipment or area implications	Reviews equipment and major piping feasibility	Develops piping details, classes, valves, mechanical connections, and constructability interfaces
Process-safety reviewer	Identifies high-level hazard boundaries and major inventories	Reviews process conditions, balance cases, safeguards, and safety-evaluation inputs	Reviews isolation, protection, relief interfaces, interlocks, drains, vents, bypasses, and hazard-study actions
Operator	Uses the overall process story for orientation	Uses major flows, equipment duties, and conditions	Uses current controlled P&IDs for system understanding with approved procedures and field verification
Maintenance technician	Uses the overview for system context	Uses equipment and process context	Uses current controlled P&IDs to understand functional connections, while relying on procedures and field verification for work
Student/trainee	Learns process sequence and boundaries	Learns equipment, streams, balances, and major control concepts	Learns symbols, loops, valves, isolation concepts, and document relationships without using training drawings for plant work

Process engineer

The process engineer commonly establishes the BFD and PFD basis. Hanford guidance states that individual P&ID content is generally determined by the process engineer. That statement is organization-specific authoritative guidance, not a universal assignment rule.

In practice, P&ID development requires multidisciplinary input. The process engineer may own process intent while other disciplines define details within their authority.

Instrumentation and control engineer

The instrumentation and control engineer helps turn a PFD control concept into implementable functions. Responsibilities may include:

measurement selection;
instrument tagging;
control-loop structure;
control-valve requirements;
signal and system interfaces;
alarm and interlock definition;
instrument datasheets;
loop diagrams; and
cause-and-effect or logic documentation.

The P&ID remains schematic. Detailed implementation usually continues in supporting documents.

Piping and mechanical engineer

This role checks whether process intent can be translated into appropriate equipment and piping arrangements. Contributions may include:

piping-class application;
valve type and mechanical requirements;
equipment connections;
maintainability;
tie-ins;
piping flexibility interfaces;
drainage and venting feasibility; and
layout coordination.

Physical routing is developed in layouts, models, and piping isometrics, not established by the P&ID’s page geometry.

Process-safety reviewer

A process-safety reviewer examines whether the documents support hazard identification and risk control. Typical review questions include:

Are credible pressure and temperature deviations represented?
Are protection functions traceable?
Can hazardous material be isolated, drained, vented, or contained?
Are bypasses controlled?
Are relief references linked to an approved basis?
Are interlocks explained in supporting logic documents?
Have modifications been incorporated through Management of Change?

A P&ID is often an important hazard-review input, but no claim should be made that a particular P&ID format is universally mandated for HAZOP.

Operators and maintenance technicians

Operators may use all three levels:

BFD for plant-wide orientation;
PFD for process behavior and normal conditions;
P&ID for detailed functional understanding.

Maintenance technicians commonly use the P&ID to understand equipment connections and instrumentation. For actual work, the drawing must be combined with approved work instructions, isolation procedures, permits, and field verification.

Students and trainees

A strong learning sequence is:

explain the process in words;
sketch a BFD;
identify major equipment on a PFD;
read stream conditions;
identify the major control concept;
trace the same equipment on a P&ID;
interpret valves and instrument tags using the legend; and
identify which questions require adjacent documents.

The goal is not symbol memorization alone. It is learning to ask what each document can legitimately prove.

Drawing Use Across the Project Lifecycle

The drawing types often develop in increasing detail, but stages overlap and documents are revised iteratively.

Drawing Use Across the Project Lifecycle
Lifecycle stage	BFD	PFD	P&ID
Concept/feasibility	Commonly used to compare process routes and boundaries	May begin for selected concepts	Normally limited or absent
Process development	Refined as the process story changes	Developed with major equipment, streams, conditions, and balances	May begin for critical systems
Front-End Engineering Design (FEED)/basic design	Used for scope communication	Major design document	Developed to define functional configuration
Detailed engineering	Usually stable but may be updated	Updated with approved process changes	Expanded and coordinated across disciplines
Construction/commissioning	Used mainly for orientation	Used for process context and commissioning basis	Essential functional input, but not a standalone construction package
Operations/maintenance	Useful for overview and training	Useful for process conditions and troubleshooting context	Used for detailed system understanding under document control
Modification/Management of Change	Updated if overall process story changes	Updated if equipment, streams, conditions, balances, or control concept change	Updated for affected piping, instrumentation, controls, and protection
Decommissioning	Useful for defining process areas and boundaries	Helps identify equipment and inventories	Helps identify functional connections, isolation, drains, vents, and interfaces

No rigid sequence

BFD → PFD → P&ID is a common greenfield progression. It is not mandatory.

Variations include:

an existing facility beginning a modification from current P&IDs;
a vendor package supplying a detailed internal P&ID before the owner’s overall PFD is finalized;
a brownfield team preparing a simplified BFD for communication after reviewing existing detailed drawings;
safety findings causing revisions to both the PFD control concept and P&ID;
field discoveries requiring controlled updates; and
packaged systems represented as boxes on a PFD but detailed on vendor documents.

The documents should converge toward a consistent approved configuration even when their development sequence differs.

Choosing the Right Drawing for the Question

Use the question—not the desired answer—to choose the document.

Choosing the Right Drawing for the Question
Question	Start with	Then confirm with
What are the main process stages?	BFD	Process description or PFD
What major equipment performs the process?	PFD	Equipment list and datasheets
Where do the major streams go?	PFD	Stream table and connected PFD sheets
What are the operating conditions?	PFD plus stream table	Process basis, datasheets, calculations, and operating procedures
Which valve isolates this equipment?	Latest approved P&ID	Approved isolation procedure and site verification; never use an educational drawing
How is a variable measured and controlled?	P&ID	Loop diagram, control narrative, logic documents, and instrument datasheets
How is an instrument implemented in detail?	P&ID plus instrument index	Instrument datasheet, loop diagram, hook-up, wiring, and control-system documentation
Where is equipment physically located?	Plot plan or equipment layout	Site/model verification
What is the exact pipe route?	Piping layout or model	Piping isometric and field verification
What pipe material or specification applies?	P&ID line designation	Controlled line list and piping-class specification
What is the relief scenario and sizing basis?	Relief reference on relevant documents	Approved relief calculation and design-basis file
What does an interlock do?	P&ID	Cause-and-effect chart, logic narrative, and approved control-system documentation

Adjacent documents and what they add

Plot plan

A plot plan shows major physical locations and site relationships. It is the appropriate starting point for questions about where equipment or buildings are located.

Equipment or piping layout

A layout shows physical arrangement more accurately than a P&ID. It supports accessibility, maintenance, routing, and coordination reviews.

Piping isometric

A piping isometric shows a pipe run in an isometric format and may include dimensions, fittings, welds, supports or support references, material information, and fabrication details per project practice.

Equipment datasheet

A datasheet records equipment design and performance requirements. It may contain design pressure, design temperature, capacity, materials, nozzle information, duty, and other specifications.

Line list

A line list connects line identifiers to information such as:

service;
size;
piping class;
design pressure;
design temperature;
insulation;
tracing; and
testing requirements,

depending on project practice.

Instrument index and datasheet

The instrument index provides a controlled list of tagged instruments. Datasheets define service and technical requirements.

Loop diagram

A loop diagram provides implementation detail that a P&ID normally does not, such as terminal and connection relationships per project standard.

Cause-and-effect chart or logic narrative

These documents explain the actions associated with alarms, trips, permissives, and interlocks. The P&ID may show the relevant function but not the full logic.

Reader checklist: before you rely on a drawing

[ ] Is this the correct drawing type for my question?
[ ] Is the drawing number correct?
[ ] Is the revision current?
[ ] Is the status approved for the intended use?
[ ] Is the copy controlled?
[ ] Have I read the title, scope, notes, and legend?
[ ] Are off-page references complete?
[ ] Do equipment and instrument tags match supporting registers?
[ ] Are pressure references clearly absolute, gauge, or otherwise defined?
[ ] Are values normal operating conditions, design cases, or limits?
[ ] Am I treating a schematic as if it were a physical layout?
[ ] Does the task require a line list, datasheet, piping isometric, loop diagram, or logic document?
[ ] Are there outstanding field markups or Management of Change actions?
[ ] Does the task require an approved procedure or permit?
[ ] Has the actual field condition been verified where required?

Engineering questions branch to the controlled drawing, register, calculation or design-basis document intended to answer them. — Start with the question you need answered, then consult the controlled document designed to hold that information.

Text equivalent: choosing the document for the question

Use a BFD for major process stages and principal material-flow direction.
Use a PFD for major equipment, principal streams, process conditions, balance information and selected major control concepts.
Use a P&ID for functional piping, valves, instruments, loops, isolation features and protective-device relationships.
Use a plot plan or equipment layout for equipment location and overall physical arrangement.
Use a piping layout for routing and spatial coordination.
Use a piping isometric for a detailed fabricated piping view, dimensions and construction information within its approved scope.
Use an equipment datasheet for equipment-specific process and mechanical requirements.
Use a line list for controlled line attributes and references.
Use a loop diagram for detailed instrument-loop wiring, termination and connection information within the project convention.
Use a cause-and-effect chart for defined initiating events and resulting protective actions.
Use the relief design-basis file for relief scenarios, assumptions, calculations and sizing basis.

Always confirm the latest approved revision, applicable legend, project convention and document-control status before relying on any document.

Common Mistakes and Misconceptions

1. Treating the three drawings as interchangeable

A BFD cannot answer detailed valve questions. A P&ID should not be used as a process balance table. Match the question to the document.

2. Assuming every project uses identical content

Some PFDs show major control loops and key valves; others show very little instrumentation. Read the project standard and legend.

3. Calling a P&ID an exact physical layout

A P&ID is schematic and normally not to scale. Use layouts, models, and piping isometrics for routing and dimensions.

4. Reading page direction as elevation

A line drawn higher on a P&ID is not necessarily physically higher. Actual high-point vents and low-point drains require piping-layout knowledge.

5. Treating a BFD block as one equipment item

A block can represent one item, a unit operation, a plant area, or an entire facility.

6. Assuming a BFD never contains values

Some BFDs carry summary values or simplified balance information. The defining feature is the stage-level process view.

7. Assuming every PFD stream table is on the same sheet

The table may be on a continuation sheet or in another controlled calculation document.

8. Treating stream-table values as live plant data

A PFD normally describes a defined design or operating basis. It is not a live historian display unless explicitly integrated and identified as such.

9. Confusing normal operating data with design limits

Normal pressure is not automatically design pressure, alarm pressure, trip pressure, or relief set pressure.

10. Ignoring absolute versus gauge pressure

The worked example uses kPa(a). Never silently treat absolute pressure as gauge pressure.

11. Assuming one PFD stream equals one P&ID line

A principal PFD stream can become several P&ID lines because of branches, class breaks, equipment connections, or other detailed divisions.

12. Assuming an omitted item does not exist

A PFD may omit valves, drains, vents, and minor lines that are present and essential on the P&ID.

13. Treating every instrument bubble as a complete implementation

The P&ID normally does not provide every wiring, terminal, configuration, protocol, tuning, and software detail.

14. Inferring an alarm setpoint from normal pressure

An alarm tag does not establish its setpoint or operator response. Consult the approved alarm and control documentation.

15. Assuming a relief symbol proves adequate protection

Relief adequacy requires a documented scenario, design basis, sizing calculation, set pressure, installation review, and discharge-path review.

16. Applying a universal control-valve fail position

Fail-open or fail-closed decisions depend on the process hazard and operating analysis. TCV-231’s proposed fail-closed action remains an illustrative review item.

17. Adding bypasses without considering defeat of control or protection

A bypass can undermine control, isolation, or a safeguard. Its purpose, normal state, authorization, and use must be defined.

18. Using a P&ID alone as a construction package

A P&ID is vital to construction but does not contain every physical, fabrication, support, electrical, or installation detail.

19. Using an uncontrolled print

Printed copies can become outdated. Confirm the current revision in the document-control system.

20. Confusing process and control block diagrams

A process BFD communicates major processing stages and material flow. A control block diagram communicates signal or transfer-function relationships.

21. Assuming the drawing sequence is mandatory

BFD → PFD → P&ID is common for greenfield development, but brownfield work, modifications, and vendor packages may develop differently.

Troubleshooting a drawing inconsistency

If two documents appear to disagree:

Stop relying on the disputed value or connection.
Record the drawing numbers and revisions.
Check drawing status and revision history.
Read the applicable notes and legend.
Check off-page references.
Compare the equipment list, line list, instrument index, and datasheets.
Determine whether an approved Management of Change is in progress.
Ask the responsible engineering discipline or document-control function.
Resolve the discrepancy through the project’s controlled process.
Do not “correct” a controlled drawing informally.

Standards and Company Conventions

Standards provide common frameworks, but project-specific conventions still matter. This article does not reproduce standards text or symbol tables.

ISO 10628-1:2014

ISO 10628-1:2014, “Diagrams for the chemical and petrochemical industry — Part 1: Specification of diagrams,” addresses classification, content, representation, and drafting rules for flow diagrams. ISO states that it does not apply to electrical engineering diagrams. It was confirmed current in the ISO catalogue’s 2026 review.

ISO 10628-2:2012

ISO 10628-2:2012, “Diagrams for the chemical and petrochemical industry — Part 2: Graphical symbols,” covers graphical symbols and is listed as valid in the ISO catalogue, which states that it was last reviewed and confirmed in 2024.

ANSI/ISA-5.1-2024

ANSI/ISA-5.1-2024, “Instrumentation and Control — Symbols and Identification,” addresses uniform instrumentation and control symbols and identification codes. It recognizes alternative organizational methods.

Use wording such as ISA-style or per project standard rather than assuming every project implements one convention identically.

IEC 62424:2016

IEC 62424:2016 Edition 2.0, dated 2016-07-15, is titled “Representation of process control engineering — Requests in P&I diagrams and data exchange between P&ID tools and PCE-CAE tools.”

Its scope concerns representation of process-control engineering requests and data exchange between P&ID tools and Process Control Engineering/Computer-Aided Engineering tools.

Historical ASME reference

ASME Y32.11-1961 (R1998) is a historical reference only. It is no longer an ASME-approved/ANSI standard and should not be presented as current.

United States OSHA context

For United States facilities and only for processes covered by OSHA’s Process Safety Management standard, 29 CFR 1910.119 includes relevant process-safety-information requirements.

The cited provisions identify:

under 29 CFR 1910.119(d)(2)(i)(A), a block flow diagram or simplified process flow diagram as process technology information;
under 29 CFR 1910.119(d)(3)(i)(B), piping and instrument diagrams as process equipment information;
under 29 CFR 1910.119(d)(3)(i)(G), material and energy balances for processes built after May 26, 1992;
relief-system design and design basis;
under 29 CFR 1910.119(l)(1), written Management of Change procedures except for replacement in kind; and
under 29 CFR 1910.119(l)(4), updating affected process safety information.

OSHA is United States-specific and scope-limited. Do not generalize these requirements to every facility worldwide. Do not claim that OSHA mandates a specific HAZOP drawing format.

Project rules remain essential

Standards do not eliminate the need for:

a project legend;
tagging procedures;
line-numbering rules;
drawing templates;
document status definitions;
revision procedures;
engineering approval workflows; and
discipline responsibilities.

When a general textbook convention conflicts with the controlled project standard, the controlled project standard governs the project.

Standards, regulations, and supporting references

These links point to catalogue pages, regulations or guidance for scope and status checks. This article does not reproduce standards text or symbol tables. Access date for all links: 2026-10-06.

Frequently Asked Questions

What is the main difference between a BFD, PFD, and P&ID?

A BFD explains the process at the stage or block level. A PFD shows major equipment, principal streams, operating conditions, balance information, and sometimes major control loops or key control valves. A P&ID shows detailed functional relationships among equipment, piping, valves, instruments, loops, isolation features, and protective devices per project standard.

Which drawing should I read first?

For an unfamiliar process, normally start with the BFD to understand the overall story, then read the PFD for equipment and process conditions, and finally use the P&ID for functional piping and instrumentation detail. This sequence is useful for learning but is not mandatory for every project or task.

Does a PFD show valves and instruments?

A PFD may show major control loops, key control valves, seal legs, or other controls needed to explain the basic process-control concept. It normally omits most manual valves and the detailed instrumentation found on the P&ID. Exact content follows the project standard.

Does a P&ID show process conditions?

A P&ID may show selected operating values, limits, setpoints, or notes per project standard, but it is normally not the main material-and-energy-balance document. Use the PFD, stream table, process basis, datasheets, and calculations to establish process conditions and their meaning.

Is a P&ID drawn to scale?

No. A P&ID is a schematic of functional relationships and is normally not to scale. It does not establish exact equipment coordinates, pipe length, routing, elevation, or construction dimensions. Use plot plans, equipment layouts, piping models, piping layouts, and piping isometrics for physical information.

How is a process BFD different from a control block diagram?

A process BFD uses blocks and arrows to show major process stages and principal material flows. A control block diagram shows signal paths, feedback, transfer functions, controllers, and dynamic relationships. They may have similar visual forms, but they describe different systems and are not interchangeable.

Can a PFD or P&ID be used for construction?

A PFD provides process context but normally lacks construction detail. A P&ID is vital to construction because it defines functional piping and instrumentation relationships, but it is not a standalone construction package. Construction also requires approved layouts, piping isometrics, supports, specifications, datasheets, electrical and instrument drawings, vendor documents, and other project deliverables.

Who uses each drawing?

Process engineers, students, managers, and reviewers often use BFDs for process overview. Process engineers, equipment specialists, control engineers, and safety reviewers use PFDs for major equipment, conditions, balances, and process-control concepts. Process, piping, instrumentation, safety, operations, maintenance, construction, and commissioning personnel use P&IDs for detailed functional understanding, subject to role, project practice, document control, and approved procedures.

Practice, Planned Resources, and Wrap-Up

Practice exercises

The following exercises use only the fictional Training Liquid Heating and Transfer System.

Exercise 1: Choose the drawing

Which drawing should you consult first for each question?

a. What are the four major process stages? b. What is the illustrative E-230 inlet temperature? c. Where is PI-221 connected functionally? d. What is the exact physical route of the pump-discharge pipe? e. What document should contain the relief-sizing basis?

Exercise 2: Trace a loop

Starting at the controlled variable, place these tags in functional order:

TCV-231
TIC-231
TT-231
E-230 outlet temperature

Exercise 3: Check the energy calculation

Calculate the duty using:

mass flow = 2.00 kg/s;
fictional heat capacity = 4.00 kJ/(kg·K);
inlet temperature = 20 °C; and
outlet temperature = 45 °C.

State the result and one limitation.

Exercise 4: Interpret pressure

Calculate:

a. P-220 differential pressure from S-01 and S-02; b. E-230 process-side stream-pressure difference from S-02 and S-03.

Explain why the second result is not automatically a validated hydraulic-design basis.

Exercise 5: Identify unresolved design items

Name at least four items that remain unresolved for the relief-protection evaluation or TCV-231.

Exercise 6: Find the misconception

A trainee says, “The bypass line is drawn above E-230, so it must be physically higher than the exchanger.” What is wrong with that conclusion?

Answer key

Exercise 1

a. BFD. b. PFD and stream table: 20 °C. c. Latest approved P&ID, followed by supporting instrument documents where needed. d. Piping layout/model and piping isometric, not the P&ID. e. Approved relief calculation or relief design-basis file.

Exercise 2

E-230 outlet temperature → TT-231 → TIC-231 → TCV-231 → heating-utility flow response.

Exercise 3

Δ T = 45 - 20 = 25 K

Q = 2.00 × 4.00 × 25 = 200 kW

Limitation: the heat capacity is fictional and the calculation ignores heat losses. It does not size E-230 or establish utility mass flow.

Exercise 4

a. 300 - 100 = 200 kPa. b. 300 - 280 = 20 kPa.

The 20 kPa value comes only from illustrative stream pressures. Hydraulic design requires verified cases, line and exchanger geometry, physical properties, fouling assumptions, fittings, control-valve behavior, and other design inputs.

Exercise 5

Acceptable answers include:

whether a relief device is required;
device type;
location;
sizing;
set pressure;
discharge destination;
credible overpressure scenarios;
applicable code basis;
final TCV-231 fail action;
utility pressure and temperature;
valve leakage or shutoff;
startup and shutdown behavior;
response to loss of flow;
need for an alarm, trip, interlock, or SIF.

Exercise 6

A P&ID is schematic and normally not to scale. Page position does not establish physical elevation. Use piping-layout information, the model, piping isometric, and field verification.

Potential future resources

The following are proposed companion resources, not claims about currently available downloads:

proposed BFD/PFD/P&ID comparison checklist;
proposed blank question-to-document worksheet;
proposed tag-traceability practice sheet;
proposed stream-table calculation template;
proposed beginner P&ID legend-reading exercise; and
proposed controlled-drawing verification checklist.

No affiliate relationship, endorsement, download availability, performance result, or commercial benefit is claimed.

Conclusion

The practical difference between BFD, PFD, and P&ID is the question each drawing is designed to answer.

Use a BFD to understand the process story.
Use a PFD to understand major equipment, principal streams, operating conditions, balance information, and the basic control concept.
Use a P&ID to understand detailed functional relationships among piping, valves, instruments, loops, isolation, and protection features.

Then move to adjacent controlled documents whenever the question involves physical location, exact routing, detailed instrument implementation, relief sizing, interlock behavior, construction, operation, maintenance, or isolation.

The most important drawing-reading habit is not memorizing every symbol. It is asking:

What does this drawing establish, what does it leave unresolved, and what approved document must I check next?