Flow Diagram Standards Fundamentals

1. Process Flow Diagrams (PFDs)

The Process Flow Diagram (PFD) is the primary engineering document that communicates the overall process design intent for a gas processing or pipeline facility. It establishes the heat and material balance, identifies major equipment items with their design conditions, and defines the process stream data that all subsequent engineering disciplines use as their design basis. The PFD is typically the first process document created during front-end engineering design (FEED) and remains the authoritative source for process conditions throughout the project lifecycle.

PFDs are governed by ISO 10628, which establishes the conventions for graphical symbols, stream identification, and the level of detail appropriate for process flow documentation. In the midstream and gas processing industries, PFDs also follow company-specific standards that supplement the ISO requirements with additional data presentation conventions tailored to hydrocarbon processing applications.

Scope and Purpose

The PFD serves several critical engineering functions that distinguish it from the more detailed P&ID:

• Process definition: The PFD defines the fundamental process scheme, showing the sequence of unit operations, the flow paths between major equipment, and the thermodynamic conditions at each point in the process. It answers the question "what does this facility do and how does it work?" at a conceptual level
• Heat and material balance: Every process stream on the PFD carries a stream number that references a detailed stream data table. This table provides flow rate (mass and volumetric), temperature, pressure, phase state, and full composition for each numbered stream. The heat and material balance is the quantitative backbone of the entire facility design
• Equipment design basis: Major equipment items shown on the PFD include design conditions (temperature, pressure, capacity) that define the performance requirements for detailed equipment specification. Each piece of equipment carries a tag number and a brief functional description
• Utility requirements: The PFD identifies heating medium, cooling medium, fuel gas, instrument air, and other utility requirements for the process, typically summarized in a utility summary table

What a PFD Includes

What a PFD Excludes

The PFD deliberately omits detail that would obscure the process intent. Items not shown on PFDs include:

• Individual isolation valves, drain valves, vent valves, and bypass valves
• Pipe sizes, line numbers, pipe specifications, and material designations
• Minor instrumentation (local gauges, test connections, sample points)
• Relief devices and their routing (shown on P&IDs)
• Start-up and shutdown piping arrangements
• Pipe fittings, reducers, flanges, and specialty items
• Detailed control system architecture and interlock logic

Stream Data Tables

The stream data table is an integral part of the PFD and provides the quantitative process data for every numbered stream. A properly constructed stream data table includes the following information for each stream:

Stream data tables are generated from process simulation software (such as commercial process simulators) and represent the converged heat and material balance for the facility design case. Multiple operating cases (design, normal, turndown, maximum) may each have their own stream data table set.

2. Piping and Instrumentation Diagrams (P&IDs)

The Piping and Instrumentation Diagram (P&ID) is the most detailed and most frequently referenced process engineering document in any gas processing or pipeline facility. While the PFD communicates the process concept, the P&ID defines exactly how the process is implemented—showing every pipe, valve, instrument, and equipment item required for construction, operation, maintenance, and safety. The P&ID is the primary reference document for operations, maintenance, safety reviews (HAZOP), and management of change (MOC) throughout the facility's operating life.

P&IDs are developed in accordance with ISA-5.1 (Instrumentation Symbols and Identification) for instrumentation representation and typically follow company-specific drafting standards for overall layout, symbology, and annotation conventions. In the midstream industry, API 14C (Recommended Practice for Analysis, Design, Installation, and Testing of Safety Systems for Offshore Production Facilities) provides additional guidance for safety system representation on P&IDs, and its conventions are widely adopted for onshore gas processing facilities as well.

Scope and Content

The P&ID includes every physical component in the piping and instrumentation system. The level of detail shown on a P&ID is sufficient for a qualified engineer or operator to understand exactly how the facility is built and how it operates:

• All equipment: Every vessel, column, heat exchanger, pump, compressor, tank, filter, and specialty item, shown with tag numbers, design conditions (pressure, temperature), and key dimensions or ratings
• All piping: Every process, utility, and safety-related pipe, identified by a line number that encodes diameter, service, sequence, piping specification, and insulation requirements
• All valves: Every gate valve, globe valve, ball valve, plug valve, check valve, control valve, relief valve, and specialty valve, each with a unique tag number
• All instrumentation: Every transmitter, indicator, controller, switch, analyzer, and safety device, identified using ISA-5.1 tag number conventions
• Control loops: Complete control loop definitions showing measurement devices, controllers, and final control elements with their connections and signal types
• Interlocks and safety systems: Emergency shutdown (ESD) systems, safety instrumented systems (SIS), and fire and gas detection systems with their trip logic

Line Numbering Conventions

Every pipe shown on a P&ID carries a unique line number that conveys essential information about the pipe in a standardized format. While exact formats vary by company, the most common convention in the midstream industry follows this structure:

For example, a line numbered 6"-PG-101-B1-H would indicate a 6-inch process gas line, sequence 101, piping specification B1 (e.g., carbon steel ANSI 150), with heat tracing (H). The specific codes used vary by company, but the underlying information conveyed is consistent across the industry:

Common Service Codes

Equipment Tag Numbering

Every piece of equipment on a P&ID carries a unique tag number that identifies its type and location within the facility. Equipment tags follow a structured format that enables any engineer or operator to identify the equipment type and the unit it belongs to at a glance:

The unit number (first digit or digits of the sequence) typically corresponds to the process area or unit operation. For example, 100-series numbers might designate the inlet separation area, 200-series the amine treating unit, 300-series the dehydration unit, and so on. This systematic numbering enables rapid identification of any equipment item's function and location within the facility.

3. Equipment Symbols and Designations

Standardized equipment symbols provide a universal graphical language for representing process equipment on PFDs and P&IDs. These symbols are defined in ISO 10628 for process flow diagrams and in various company and industry standards for P&IDs. In the midstream and gas processing industries, equipment symbols must be immediately recognizable to engineers, operators, and maintenance personnel who may be working on facilities across multiple companies and geographic regions.

Vessel and Column Symbols

Process vessels are among the most commonly represented equipment items on flow diagrams. The symbol used conveys the vessel type and orientation:

Heat Exchanger Symbols

Heat exchangers are represented by symbols that indicate the exchanger type and flow configuration:

Rotating Equipment Symbols

Pumps, compressors, and other rotating equipment are represented by symbols that indicate the equipment type and driver:

Valve Symbols

Valves are one of the most numerous items on any P&ID. Each valve type has a distinct symbol that conveys its function, construction, and operating characteristics. The base symbol for all manual valves is two triangles meeting at a point (the bowtie shape), with modifications indicating the specific valve type:

API 14C Safety System Symbols

API Recommended Practice 14C (originally developed for offshore production facilities but widely adopted in onshore gas processing) defines specific symbols for safety devices and their functional relationships in the facility safety system:

• PSV (Pressure Safety Valve): Spring-loaded relief valve that opens at a set pressure to protect equipment from overpressure. Shown with the standard relief valve symbol and tagged with a PSV number
• PSH/PSL (Pressure Switch High/Low): Pressure-actuated switches that trigger shutdowns or alarms. Shown as instrument balloons with function letters per ISA-5.1
• LSH/LSL (Level Switch High/Low): Level-actuated switches for high-level shutdown or low-level pump protection
• SDV (Shutdown Valve): Automated isolation valve that closes on emergency shutdown signal. Shown with fail-safe position indicated (FC = fail closed)
• BDV (Blowdown Valve): Automated valve that opens to depressurize equipment to flare during emergency. Shown with fail-safe position (FO = fail open)
• FSV (Flow Safety Valve): Excess flow check valve that closes automatically on abnormal high flow, typically used on small-bore connections

4. Instrumentation Symbols (ISA S5.1)

ISA-5.1 (formerly ISA S5.1, titled "Instrumentation Symbols and Identification") is the governing standard for representing instrumentation on process diagrams throughout the oil and gas, petrochemical, and process industries. This standard defines a systematic method for identifying instruments and control devices by their measured variable and function, using a structured tag number system that is immediately interpretable by any engineer or technician familiar with the standard.

Instrument Tag Number Structure

Every instrument on a P&ID carries a tag number composed of letter codes and a loop number. The letter codes identify what the instrument measures and what it does with that measurement:

First Letter — Measured Variable

The first letter of the instrument tag identifies the process variable being measured or initiated:

Subsequent Letters — Instrument Function

The second and subsequent letters identify the functional role of the instrument device:

Common Instrument Tag Examples

Understanding how the letter codes combine to form complete instrument identification is essential for reading P&IDs. The following table shows the most frequently encountered instrument tags in gas processing and pipeline facilities:

Balloon Symbols — Device Location

ISA-5.1 uses different balloon (bubble) shapes to indicate where the instrument is physically located or accessed by the operator:

Control Valve Fail Positions

Control valves shown on P&IDs must indicate their fail-safe position—the position the valve assumes upon loss of signal or air supply. This information is critical for safety analysis and HAZOP studies:

• FC (Fail Closed): Valve closes on loss of signal. Used when the safe condition requires stopping flow (e.g., fuel gas to a fired heater, feed to a pressurized vessel)
• FO (Fail Open): Valve opens on loss of signal. Used when the safe condition requires maintaining flow (e.g., cooling water to a heat exchanger, vent path to flare)
• FL (Fail Last / Fail in Place): Valve holds its last position on loss of signal. Used when either fully open or fully closed creates a hazard, and the current operating position is safest

The fail position is shown on the P&ID as a notation adjacent to the control valve symbol (FC, FO, or FL) and is determined during the HAZOP or safety system design phase based on failure mode analysis for each specific application.

5. Drawing Management and Revision Control

Drawing management and revision control are critical quality assurance functions that ensure process diagrams accurately reflect the as-built and current operating condition of a facility throughout its lifecycle. In the midstream industry, where facilities may operate for 30+ years with continuous modifications, the integrity of the P&ID set is fundamental to safe operations, effective maintenance, and regulatory compliance. Management of Change (MOC) procedures directly depend on accurate, up-to-date process diagrams as the baseline for evaluating proposed modifications.

Revision Control Process

Every change to a P&ID or PFD follows a structured revision control process that maintains an auditable history of all modifications:

Revision Block Information

Every revision to a drawing is documented in the revision block (typically located in the lower right corner of the drawing), which records:

• Revision number: Sequential numbering (Rev 0, Rev 1, Rev 2...) or letter-based (Rev A, Rev B...) for preliminary issues
• Date: Date the revision was issued
• Description of change: Brief narrative describing what was changed and why
• Prepared by / Checked by / Approved by: Names and signatures (or electronic equivalents) of the individuals responsible for the change
• MOC reference: Management of Change document number authorizing the modification (for operating facilities)

Changes on the drawing itself are indicated by revision clouds (irregular closed curves drawn around the modified area) and revision triangles containing the revision number, allowing anyone reviewing the drawing to quickly identify what changed in each revision.

Red-Line Markup Practices

Red-line markups are the standard method for documenting field changes and construction deviations that need to be incorporated into the master drawings. Proper red-line practices are essential for maintaining drawing accuracy:

• All markups must be made in red ink on official printed copies of the current revision
• Each markup must include the date, initials of the person making the change, and a brief description
• Additions are drawn in red using standard symbols; deletions are crossed out with a single red line (never erased or obscured)
• Red-line drawings must be submitted to the engineering document control group for incorporation into the master within a defined timeframe (typically 30–90 days for operating facilities)
• The original red-line markup is retained as a permanent record even after incorporation into the master drawing

Management of Change (MOC) Requirements

Any modification to a facility that alters the process, equipment, instrumentation, or safety systems documented on P&IDs must go through a formal Management of Change process. MOC requirements for drawing updates include:

• Engineering review: All proposed changes must be reviewed by a qualified engineer to assess their impact on process safety, operability, and regulatory compliance
• HAZOP or safety review: Significant changes require a formal hazard analysis (HAZOP, What-If, or equivalent) with the results documented and action items tracked to completion
• Drawing update: P&IDs must be updated before or concurrent with the physical modification. Operating a facility with drawings that do not reflect the current configuration is a serious safety and regulatory violation
• Training: Operations and maintenance personnel must be trained on the changes reflected in the updated drawings before the modification is placed in service
• Pre-startup safety review (PSSR): Before operating with the modification, a formal PSSR verifies that all MOC requirements have been met, including drawing updates, training, and inspection

Drawing Registers and Document Control

A drawing register (or drawing index) is the master list that catalogs every drawing associated with a facility. The register tracks:

Electronic Drawing Systems

Modern facility design and operations increasingly rely on electronic drawing systems that provide capabilities beyond traditional 2D CAD drafting:

• CAD-based P&IDs: Standard 2D CAD drawings (AutoCAD, MicroStation) remain the most common format for P&IDs in the midstream industry. They provide drafting efficiency but limited data integration
• Smart P&IDs: Intelligent P&ID software (such as smart plant tools) creates drawings where every symbol is a database object with associated attributes (tag number, specifications, design data). Smart P&IDs enable automated data extraction, consistency checking, and integration with other engineering databases
• 3D model integration: P&ID data can be linked to 3D piping models, enabling automatic generation of equipment lists, line lists, and instrument schedules from the combined model-drawing dataset
• Electronic document management: Enterprise document management systems (EDMS) control access, revision tracking, approval workflows, and distribution of all engineering documents including P&IDs. These systems maintain a complete audit trail of all document changes

Facility Documentation Packages

Flow diagrams exist within a broader documentation package that evolves through the project lifecycle. The key project phases and their documentation deliverables include:

Maintaining accurate and current flow diagrams throughout all project phases is not merely a best practice—it is a regulatory requirement under OSHA Process Safety Management (29 CFR 1910.119) and EPA Risk Management Program (40 CFR 68) for facilities handling regulated quantities of hazardous materials. These regulations explicitly require that process safety information, including P&IDs, be current and accessible to operating personnel at all times.

References

1. ISA-5.1 — Instrumentation Symbols and Identification
2. API Recommended Practice 14C — Analysis, Design, Installation, and Testing of Safety Systems for Offshore Production Facilities
3. ISO 10628 — Diagrams for the Chemical and Petrochemical Industry
4. ASME Y32.11 — Graphical Symbols for Process Flow Diagrams
5. OSHA 29 CFR 1910.119 — Process Safety Management of Highly Hazardous Chemicals
6. API 520 — Sizing, Selection, and Installation of Pressure-Relieving Devices (Part I and II)