PFD vs P&ID Explained with a Practical Level Control Automation Example

PFD and P&ID comparison with a practical level-control loop.

Quick Summary

A Process Flow Diagram, or PFD, explains the overall process and its major equipment. A Piping and Instrumentation Diagram, or P&ID, provides the detailed information required to understand piping, valves, instruments, control loops and process-safety functions.

Process engineers, production teams, maintenance personnel and automation engineers frequently use PFDs and P&IDs. Although both drawings represent the same process, they are prepared for different purposes and contain different levels of detail.

In this article, we will compare a PFD with a P&ID and then examine a practical automatic level-control loop for a storage tank.

1. What Is a Process Flow Diagram?

A Process Flow Diagram, or PFD, provides a high-level representation of a manufacturing process. It normally shows the major process equipment and the principal flow paths connecting that equipment.

A typical PFD may show:

• Reactors, tanks, columns and vessels
• Pumps, compressors and blowers
• Heat exchangers, condensers and reboilers
• Main process and utility streams
• Flow direction
• Important operating conditions
• Major process-control loops

A PFD is particularly useful when someone needs to understand how raw materials move through the plant, where major process transformations occur and how the principal equipment items interact.

Simplified PFD Representation

Feed → P-101 Pump → TK-101 Tank → Product

The PFD communicates the main process route without showing every valve, instrument or signal connection.

2. What Is a P&ID?

A Piping and Instrumentation Diagram, or P&ID, is a detailed schematic representation of the process equipment, piping, valves, instrumentation and control functions.

The P&ID helps engineering and plant teams understand how equipment is physically and functionally connected. It is commonly referenced during detailed engineering, installation, commissioning, operation, troubleshooting, maintenance and safety reviews.

A typical P&ID may include:

• Equipment identification numbers
• Process and utility piping
• Line numbers, sizes and specifications
• Manual and automated valves
• Transmitters, indicators, controllers and switches
• Instrument and control-loop tags
• Electrical, pneumatic and digital signal connections
• Vents, drains, bypasses and sample points
• Relief devices and selected protective functions
• Interlocks and shutdown references, where applicable

Important: A P&ID is a schematic drawing. It normally does not represent the exact physical location, piping length, elevation or scale of the installed equipment.

3. PFD vs P&ID: Main Differences

Comparison	PFD	P&ID
Primary purpose	Explain the overall process	Show detailed piping, instrumentation and control
Equipment	Major equipment	Major equipment with connected components
Piping	Main process streams	Detailed process and utility lines
Valves	Only important valves, if required	Manual, control and protective valves
Instrumentation	Limited to major controls	Detailed instruments and loop identification
Main users	Process engineering and project teams	Process, piping, automation, operation and maintenance teams

4. Practical Automation Example: Tank Level Control

Consider storage tank TK-101. Feed enters the tank through pump P-101, and product leaves the tank through the outlet line. The objective is to maintain the liquid level near a defined setpoint.

Level-Control Loop

LT
101 → LIC
101 → I/P → LV-101

How the loop works

1. LT-101, the level transmitter, measures the liquid level in TK-101.
2. The transmitter sends the measured process value to LIC-101.
3. LIC-101 compares the measured level with the operator-defined setpoint.
4. Based on the control error and its configured control action, the controller calculates an output.
5. If a pneumatic actuator is used, an I/P converter converts the electrical control signal into a pneumatic pressure signal.
6. LV-101 changes its opening and adjusts the tank outlet flow.
7. The resulting change in outlet flow brings the tank level back toward the setpoint.

5. Understanding the Instrument Tags

Instrument tags combine letters and numbers. The letters describe the measured variable and instrument function, while the number identifies the relevant loop or device according to the project tagging convention.

Tag	Meaning	Function
LT-101	Level Transmitter	Measures and transmits tank level
LIC-101	Level Indicating Controller	Displays level and controls the loop
LV-101	Level Control Valve	Manipulates outlet flow
P-101	Pump	Transfers liquid into the system
TK-101	Storage Tank	Stores or provides hold-up for liquid

6. Common P&ID Instrument Letters

Letter	Typical meaning	Example
F	Flow	FT, FI, FIC
L	Level	LT, LI, LIC
P	Pressure or vacuum	PT, PI, PIC
T	Temperature	TT, TI, TIC
I	Indication	LI, FI, PI
C	Control	LIC, FIC, PIC
T	Transmit, when used as a succeeding letter	LT, FT, PT

7. Control Philosophy Considerations

A P&ID shows the functional control arrangement, but the automation engineer must still define the detailed control philosophy. For the tank level-control loop, the design team should consider the following points:

• Should LV-101 fail open, fail closed or fail in its last position?
• What should happen during loss of electrical power or instrument air?
• Is high-level alarm LAH-101 required?
• Is an independent high-high level switch LSHH-101 required?
• Should high-high level stop P-101 or close an inlet valve?
• Is the controller action direct or reverse?
• What are the operating, alarm and trip setpoints?
• Will the loop operate in manual, automatic or cascade mode?
• How should bad transmitter signals be handled?
• Does the function belong in the PLC, DCS or safety system?

Safety note: The required valve failure position and shutdown action must be selected from the process hazard assessment and approved control philosophy. A generic drawing should never be used as the sole basis for a safety-critical design.

8. Frequent P&ID Mistakes

1. Using inconsistent equipment or instrument tags.
2. Failing to define signal-line types in the drawing legend.
3. Showing a control valve without its actuator failure position.
4. Leaving obsolete instruments on the drawing after plant modifications.
5. Not matching the P&ID with the PLC or DCS control narrative.
6. Confusing a process control action with an independent safety function.
7. Using symbols without checking the project-specific legend.
8. Failing to update the as-built drawing after commissioning or modification.

Conclusion

A PFD gives the big picture of a production process. A P&ID provides the detailed engineering information needed to understand how the piping, valves, instruments and control functions work together.

In the tank example, the PFD may show only P-101, TK-101 and the main process streams. The P&ID goes further by showing LT-101, LIC-101, LV-101, the signal relationships and the complete level-control function.

Engineers should always read a P&ID together with its drawing legend, equipment data sheets, instrument index, control narrative, cause-and-effect document and applicable project standards.

Final takeaway

PFD tells us what the process does.
P&ID tells us how the process equipment, piping and control system are connected.

Further Reading

• https://www.isa.org/standards-and-publications/isa-standards International Society of Automation Standards

Overview of Piping and Instrumentation Diagrams

Disclaimer: The symbols and control arrangement in this article are educational examples. Always follow the approved project legend, company engineering practices, applicable standards and process-safety requirements.