P&ID Preparation: What Every Process Engineer Needs to Know

Every process engineer works with P&IDs constantly. Far fewer actually prepare them correctly from scratch.

A P&ID is not just a drawing — it is the single document that every discipline on a project relies on: process, piping, instrumentation, electrical, safety, and operations. An error in a P&ID does not stay contained to one discipline. It propagates.

This article covers what a P&ID actually is, what it must contain, the standards that govern it, and where preparation most commonly goes wrong.

What Is a P&ID?

A Piping and Instrumentation Diagram (P&ID) is a detailed engineering drawing that shows the complete piping system of a process, including all equipment, piping, valves, instrumentation, and control elements — with enough detail to represent how the plant is actually built and operated.

A P&ID is not the same as a PFD. A Process Flow Diagram shows the process at a conceptual level — major equipment and key streams. A P&ID shows everything: every valve, every instrument, every line, every safety device, at the level of detail required for construction, commissioning, and operation.

What a Complete P&ID Must Contain

• All equipment — with correct equipment tag numbers matching the equipment list, drawn schematically to reflect actual nozzle configuration and relative elevation where relevant.
• All piping — every line, with line numbers matching the line list, including line size, pipe class/specification, and insulation type where applicable.
• All valves — manual valves, control valves, check valves, relief valves — each shown with the correct symbol for its type and function.
• All instrumentation — every transmitter, indicator, controller, and switch, tagged according to ISA instrument identification standards.
• Control loops — showing how instruments connect to control valves or other final control elements, including the signal type (pneumatic, electronic, digital/fieldbus).
• Safety systems — pressure relief devices, emergency shutdown valves, interlocks, and their trip logic references.
• Utility connections — steam, cooling water, nitrogen, instrument air connections to process equipment.

Line numbers, equipment tags, and instrument tags — every single one must be unique and must match exactly across the P&ID, the line list, the equipment list, and the instrument index. Inconsistency between these documents is one of the most common and most damaging errors in engineering packages.

The Standards That Govern P&ID Preparation

ISA 5.1 (Instrumentation Symbols and Identification) is the primary standard governing instrument symbols, tag numbering, and loop identification in the process industries. It defines the letter codes used in instrument tags — for example, “FT” for flow transmitter, “PIC” for pressure indicating controller, “LSH” for level switch high.

ISO 14617 and ISO 10628 provide the international standard equivalents for graphical symbols and flow diagrams for process plants, used more commonly outside North America.

Company-specific P&ID standards — most EPC companies and owner-operators maintain their own P&ID legend and standard symbols document, built on top of ISA 5.1 or ISO standards, with company-specific conventions for tag numbering, drawing borders, and revision control.

Understanding the applicable standard before starting P&ID preparation is not optional. Using inconsistent or incorrect symbology creates confusion across every discipline that reads the drawing — and in the worst case, creates ambiguity in safety-critical systems.

Instrument Tag Numbering: How It Works

ISA 5.1 instrument tags follow a structured format that communicates the instrument’s function at a glance:
[Loop Number]-[Function Letters]

For example: 101-FIC-201

• 101 — the unit or area number
• FIC — the function letters: F (flow), I (indicating), C (controller)
• 201 — the loop number, unique within that area

The function letters follow a defined logic:

• First letter = measured variable (F=flow, P=pressure, T=temperature, L=level, A=analysis)
• Succeeding letters = function (I=indicating, C=controller, T=transmitter, S=switch, A=alarm, V=valve)

Getting this tagging system right and applying it consistently across the entire P&ID set is fundamental — instrument technicians, control system engineers, and operators all rely on this tag structure to understand what each instrument does without needing to look up a separate reference every time.

P&ID Symbols: The Basics Every Engineer Should Know

• Equipment symbols — vessels are typically drawn as their approximate physical shape (vertical or horizontal cylinders with appropriate heads). Pumps, compressors, and heat exchangers have standardised schematic symbols specific to their type.
• Valve symbols — different symbols distinguish gate valves, globe valves, ball valves, butterfly valves, check valves, and control valves. The symbol also indicates whether the valve is manual or actuated, and the actuator type (pneumatic, electric, hydraulic) if actuated.
• Line symbols — different line weights and styles distinguish process piping, utility piping, instrument signal lines (pneumatic vs electronic vs data), and different line classes.
• Instrument bubbles — a circle represents a field-mounted instrument; a circle with a horizontal line represents a control room-mounted (board-mounted) instrument; a circle with a horizontal dashed line represents a DCS/computer function; a square with a circle represents a PLC-based function.
• Safety symbols — pressure relief valves, rupture discs, and safety instrumented system elements have specific standardised symbols that must be used consistently to avoid ambiguity in safety-critical documentation.

How P&IDs Are Developed Through a Project

P&ID development is not a one-time activity — it evolves through defined stages as the project matures:

• Preliminary / Process P&IDs (BEP stage) — developed during Basic Engineering, showing major equipment, main process lines, primary control loops, and conceptual safety systems. These establish the process philosophy but are not yet complete enough for construction.
• Issued for HAZOP — the P&IDs are frozen at a defined revision specifically for the HAZOP study. This is a critical checkpoint: the HAZOP team systematically reviews every line and node on the P&ID for deviation scenarios, and the output directly drives P&ID revisions afterward.
• Issued for Design (IFD) — after HAZOP actions are incorporated, P&IDs are issued to all engineering disciplines (piping, instrumentation, electrical, civil) as the basis for detailed design work.
• Issued for Construction (IFC) — the final, fully detailed P&ID revision, incorporating all discipline review comments, used for actual field construction.
• As-Built — updated after construction and commissioning to reflect any field changes made during installation and startup, becoming the permanent operating reference document.

Every revision must be tracked, and every discipline working from a P&ID must be working from the current revision. Using an outdated P&ID revision during construction is a direct cause of costly field rework.

Preparing a P&ID: The Practical Workflow

1. Start from the PFD and heat and material balance. The P&ID must be consistent with the process design established in the PFD — equipment, major streams, and design conditions all originate there.
2. Add all equipment with correct tag numbers, matching the equipment list exactly.
3. Route all process piping, assigning line numbers consistent with the line list, and specifying line size and pipe class per the piping specification.
4. Add all valves required for operation, isolation, maintenance, and safety — including block valves for maintenance isolation, drain and vent valves, and bypass valves where operationally required.
5. Add instrumentation and control loops, working closely with the instrumentation engineer to define the control philosophy — what is measured, what is controlled, and how the control loop is configured.
6. Add safety systems — relief devices sized per the relief system design, emergency shutdown valves, and interlock references, coordinated with the process safety engineer and the HAZOP/SIL study outcomes.
7. Cross-check against every related document — equipment list, line list, instrument index, cause and effect diagram — before issuing for review.

This cross-checking step is where many errors are caught — or missed. A P&ID that has not been systematically checked against these supporting documents will contain inconsistencies that surface later, usually during construction or commissioning, when they are far more expensive to fix.

Common Mistakes in P&ID Preparation

• Inconsistent tag numbering between the P&ID and the equipment list, line list, or instrument index — this is the single most common error, and it creates confusion across every discipline using the documents.
• Missing isolation valves for maintenance — failing to include block valves that allow equipment to be isolated for maintenance without shutting down the entire unit, forcing unnecessary downtime later.
• Incomplete or missing safety system representation — relief devices, interlocks, or emergency shutdown logic that is not fully or correctly shown, creating ambiguity in safety-critical systems.
• Not updating P&IDs after HAZOP — HAZOP studies generate action items, many of which require P&ID changes (additional instrumentation, interlocks, or relief devices). Failing to formally close out and incorporate these actions into the P&ID revision is a serious gap.
• Drawing symbols inconsistent with the project’s legend — using non-standard or ambiguous symbols that different disciplines interpret differently.
• Not reflecting actual constructed conditions in as-built P&IDs — field changes made during construction or commissioning that are never fed back into the P&ID, leaving the operating plant with inaccurate reference documentation — a serious problem for future modifications, troubleshooting, or safety studies.

Frequently Asked Questions

What does a P&ID include?

A complete P&ID includes all equipment with correct tag numbers, all piping with line numbers and specifications, all valves (manual, control, relief, and safety), all instrumentation and control loops with ISA-compliant tags, safety systems including relief devices and interlocks, and utility connections to process equipment. Every element must be consistent with the equipment list, line list, and instrument index.

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

A Process Flow Diagram (PFD) shows the process at a conceptual level — major equipment and key process streams with basic flow, temperature, and pressure data. A P&ID shows the complete, detailed piping system including every valve, instrument, and safety device, at a level of detail sufficient for construction. The PFD is prepared earlier in the project; the P&ID is developed and refined through multiple stages up to construction.

What standard governs P&ID symbols and instrument tagging?

ISA 5.1 (Instrumentation Symbols and Identification) is the primary standard used in process industries for instrument symbols, tag numbering format, and loop identification. ISO 10628 and ISO 14617 provide equivalent international standards. Most companies also maintain their own project-specific P&ID legend built on top of these base standards.

When are P&IDs frozen for HAZOP?

P&IDs are typically frozen at a defined revision specifically for the HAZOP study, since the HAZOP team systematically reviews the drawings node by node for deviation scenarios. After the HAZOP is complete, the resulting action items are incorporated into the next P&ID revision before the drawings are issued for detailed design.

What is an as-built P&ID and why does it matter?

An as-built P&ID is the final revision updated after construction and commissioning to reflect any field changes made during installation and startup. It matters because it becomes the permanent reference document used for plant operation, future modifications, troubleshooting, and safety studies — an inaccurate as-built P&ID undermines every activity that relies on it afterward.