If you’ve ever walked into a control room in a Chinese chemical plant and tried to read the P&IDs as a Western-trained engineer, you know the moment of disorientation. The valves look wrong. The instrument bubbles use different letters. The line numbering follows a logic you weren’t taught.
It’s not that either system is incorrect. It’s that two parallel universes of symbology have evolved over decades, and your project’s success — from HAZOP reviews to commissioning — depends on picking one and sticking to it.
This article maps out the differences between ISA-5.1 (the dominant international standard) and GB/T 2625 (China’s national standard), so you can make the right call before your first P&ID hits the plotter.
Why This Matters More Than You Think
Before we dive into symbol-by-symbol comparisons, let’s be clear about the stakes. A P&ID is a legal document. It defines the process, the safeguarding, and the operating philosophy. If a symbol is ambiguous, the consequence can be:
A HAZOP node gets misidentified. During a 2019 petrochemical project in Jiangsu, a relief valve symbol that followed GB notation was misinterpreted by an international EPC team as a manual vent. The HAZOP action was closed based on the wrong assumption. It was caught during commissioning — but only because an operator noticed the discharge line routing looked odd.
A DCS programmer configures the wrong logic. ISA uses discrete letter codes for instrument functions. GB uses a different combination logic. If your I/O list follows one standard and your P&IDs follow another, someone is going to misread TAL (temperature alarm low) as something else entirely.
A contractor bids the wrong scope. One construction company I worked with lost ¥800,000 on a wastewater treatment project because they misread the instrument count on GB-standard P&IDs. They’d priced based on ISA conventions, where certain multi-function instruments are shown differently.
The bottom line: symbology isn’t cosmetic. It’s a communication protocol. And like any protocol, both ends need to speak the same language.
The Two Standards at a Glance
ISA-5.1 (ANSI/ISA-5.1-2009, reaffirmed 2019)
The Instrumentation, Systems, and Automation Society (ISA) published the first version of ISA-5.1 in 1949. It has been revised roughly every decade since, with the most recent reaffirmation in 2019.
ISA-5.1 is the de facto international standard. It is referenced by:
– ISO 14617 (Graphical symbols for diagrams)
– API RP 14C (Offshore production platform safety systems)
– PIP PNE00001 (Process Industry Practices)
– Virtually every international EPC contract specification
Its core philosophy: every instrument should be identifiable by a unique tag number, and the tag number should encode the measured variable, the instrument function, and the loop number. The tag TIC-1001 tells you everything: Temperature, Indicating, Controller, Loop 1001.
GB/T 2625-1981
China’s national standard GB/T 2625 (“Process measurement and control instrumentation — Symbolic representation for the function of instruments”) was originally published in 1981 and has seen minor updates through GB/T 2625-2004.
It draws heavily from early ISA conventions but diverges in significant ways:
– Letter code assignments differ. Some measured variable letters that ISA reserves for one purpose are used differently in GB.
– Instrument function representation uses a hierarchical approach. Instead of ISA’s flat “first letter = variable, subsequent letters = function” logic, GB sometimes applies a two-tier naming system.
– Graphical symbols for equipment differ. Valve bodies, actuator types, and line designations follow Chinese drawing conventions inherited from Soviet-era standards.
The key thing to understand: GB/T 2625 was designed for a domestic engineering ecosystem. Chinese design institutes, Chinese DCS vendors (HollySys, Supcon), and Chinese construction companies all speak GB. If your project is inside China and uses a Chinese EPC contractor, your P&IDs will almost certainly be in GB unless you explicitly contract otherwise.
Detailed Comparison: Instrument Tagging
This is where the differences hit you first. Let’s walk through the letter code tables.
Measured Variable (First Letter)
| Variable | ISA-5.1 | GB/T 2625 | Notes |
|———-|———|———–|——-|
| Temperature | T | T (温度) | Same — mercifully |
| Pressure | P | P (压力) | Same |
| Flow | F | F (流量) | Same |
| Level | L | L (物位) | Same, though GB extends L to cover solids level too |
| Analysis | A | A (分析) | Same |
| Speed/Frequency | S | S (速度/频率) | Same |
| Weight/Force | W | W (重量/力) | Same |
| Moisture/Humidity | M | M (湿度) — BUT also used for Motor in some Chinese plants | Watch this one. GB assigns M to moisture, but older Chinese drawings sometimes use M for motor-operated devices. ISA reserves M strictly for moisture. |
| Vibration | V | Z (振动) | Different letter. GB uses Z (zhèndòng), which can confuse ISA-trained engineers who expect Z for position. |
| Position | Z | W (位置) | Different letter. GB uses W for position (wèizhi), ISA uses Z. This is a common source of errors in multinational projects. |
Instrument Function (Subsequent Letters)
| Function | ISA-5.1 | GB/T 2625 | Notes |
|———-|———|———–|——-|
| Indicator | I | Z (指示) | Critical difference. ISA “I” means indicator. GB uses “Z” (zhǐshì). An ISA engineer seeing “TZ” on a GB drawing might think it’s a typo. |
| Recorder | R | J (记录) | GB uses J (jìlù). ISA reserves J for power or scan. |
| Controller | C | T (调节) | Major difference. GB uses T (tiáojié) for controller, not C. TIC in ISA = TIT in GB. If you don’t know this, you’ll spend hours looking for controllers that don’t exist. |
| Transmitter | T | B (变送) | GB uses B (biànsòng). So ISA “PT” = GB “PB”. Enough said. |
| Alarm | A | A or B (报警) | Mostly consistent, though some GB drawings use B for alarm (bàojǐng). |
| Switch | S | K (开关) | GB uses K (kāiguān). ISA “LSH” (level switch high) = GB “LKH” or similar. |
| Control Valve | CV | F (调节阀) | GB uses F (tiáojié fá) or sometimes V. |
| Solenoid | SV or SOV | DCF (电磁阀) | GB often uses pinyin-derived abbreviations. |
| Compute/Convert | Y or U | S (运算) | GB uses S (suàn). ISA reserves S for switch. |
Real-World Example
Let’s take a pressure control loop and tag it both ways:
ISA-5.1:
– PT-1001: Pressure Transmitter
– PIC-1001: Pressure Indicating Controller
– PY-1001: I/P Converter
– PV-1001: Pressure Control Valve
GB/T 2625:
– PB-1001: Pressure Transmitter (B = biànsòng)
– PIT-1001: Pressure Indicating Controller (T = tiáojié — wait, this conflicts with ISA where T = transmitter!)
If you’re reading a GB drawing as an ISA engineer, you’d read “PIT” as “Pressure Indicating Transmitter” — a nonsense combination. But in GB, it means “Pressure Indicating Controller.” This single mismatch has caused more confusion in Sino-foreign projects than any other symbology issue.
Valve and Actuator Symbols
Valve Body Types
Both standards use similar basic shapes, but with important differences:
Gate Valve:
– ISA: A wedge shape (two angled lines meeting at the center) inside the valve body
– GB: Similar wedge shape, but often drawn with a horizontal line through the center (indicating the gate position)
Globe Valve:
– ISA: A circle bisected by a horizontal line, with an arrow indicating flow direction
– GB: Same basic shape, but the arrow convention differs — GB arrows point in the direction of increasing line number sequence rather than actual process flow in some design institutes
Butterfly Valve:
– ISA: Two opposing triangles meeting at the centerline
– GB: A diamond shape or a circle with a diagonal line through it, depending on the design institute’s internal standard
Check Valve:
– ISA: A circle with a right-angle line and a dot (swing check) or a spring symbol
– GB: Similar, but GB distinguishes between lift check, swing check, and tilting-disc check with more granular symbols
Actuator Types
| Actuator | ISA-5.1 | GB/T 2625 Common Practice |
|———-|———|—————————|
| Pneumatic diaphragm | Circle with Y-shaped yoke | Same, but sometimes simplified to a half-circle |
| Pneumatic piston | Circle with double lines | Square with diagonal hatching |
| Electric motor | Circle with “M” | Circle with “D” (diàn = electric) or “M” (motor) |
| Solenoid | Circle with “S” | Rectangle with “DCF” |
| Manual handwheel | Stem with a cross bar | Stem with a circle at the end |
| Hydraulic | Circle with “H” | Circle with “Y” (yèyā) |
Failure Position
Both standards use arrows or text to indicate valve failure position:
– FO (Fail Open) / FC (Fail Close) / FL (Fail Last) in ISA
– GB typically uses Chinese text: 气开 (air-to-open, fails closed), 气关 (air-to-close, fails open)
But note: some Chinese design institutes use a combination approach — an arrow on the actuator showing the spring direction, which is actually more intuitive once you learn it. An arrow pointing up = fails closed (spring pushes up, valve closes on air failure).
Line Designation and Numbering
ISA Convention
A typical ISA line number looks like:
10″-P-1001-A1A
Breaking it down:
– 10″: nominal pipe diameter
– P: service code (P = process, U = utility, C = chemical, etc.)
– 1001: sequential line number
– A1A: material/insulation specification code
GB Convention
A typical GB line number:
PL-1001-10-A1
Breaking it down:
– PL: service code (PL = process liquid, PG = process gas, CWS = cooling water supply, etc.)
– 1001: sequential line number
– 10: nominal diameter (DN, in mm — not inches!)
– A1: material class
Key differences:
1. GB uses metric diameters (DN) while ISA P&IDs typically use inches for pipe sizes
2. GB service codes are often two or three letters (pinyin-derived), while ISA tends toward single letters
3. GB line numbering often includes the unit/area number as a prefix, which ISA does less consistently
Service Code Comparison
| Service | ISA Common | GB Common |
|———|————|———–|
| Process fluid | P | PL (liquid) / PG (gas) |
| Cooling water | CW | CWS (supply) / CWR (return) |
| Instrument air | IA | IA |
| Nitrogen | N | N2 |
| Steam | S | LS (low pressure) / MS (medium) / HS (high) |
| Condensate | C or CD | SC |
| Fire water | FW | FW |
| Potable water | PW | DW (domestic water) |
| Wastewater | WW | WW |
Equipment Representation
Vessels and Tanks
Both standards represent vessels as simple geometric shapes, but there are subtle differences:
– ISA drawings tend to show vessels with rounded ends (dished heads), reflecting ASME vessel design conventions
– GB drawings often show vessels with flat ends or elliptical heads, reflecting GB 150 vessel design standards
– GB drawings for storage tanks often include a cone roof indicator for fixed-roof tanks, which is less common in ISA
Pumps
ISA shows pumps as a circle with the impeller line and discharge direction. GB is similar but adds more detail:
– GB pump symbols usually include the driver type (M for motor, T for turbine) adjacent to the pump symbol
– Spare pumps are explicitly marked in GB with a note character “（备）” meaning standby
– ISA uses a simpler “A/B” designation for duty/standby pairs
Heat Exchangers
This is one area where the differences are mostly cosmetic:
– ISA: Shell-and-tube exchangers shown as two concentric circles with process lines crossing
– GB: Same basic structure, but the tube-side and shell-side fluid identification follows Chinese fluid coding conventions
Practical Decision Framework: Which Standard for Your Project?
Now that you understand the differences, here’s the decision tree I use:
Use ISA-5.1 when:
1. Your EPC contractor is international. Fluor, Bechtel, Technip, Worley, Wood — they all default to ISA. Forcing them to use GB adds cost and schedule risk.
2. Your licensor requires it. If you’re licensing a process from UOP, Lummus, Axens, or Shell, their process design packages come in ISA. Retagging everything to GB is a massive effort with zero value-add.
3. Your DCS is from a Western vendor. Honeywell Experion, Emerson DeltaV, Yokogawa Centum, Siemens PCS 7 — the tag naming conventions in these systems are built around ISA logic. Using GB tags creates a impedance mismatch between the P&IDs and the DCS database.
4. The plant will be operated by an international team. If your operators have experience in multiple countries, ISA is the common denominator.
Use GB/T 2625 when:
1. Your entire supply chain is Chinese. If the design institute (设计院), the EPC contractor, the DCS vendor (中控、和利时), and the operators are all Chinese, using GB is faster and cheaper. Everyone already thinks in GB.
2. The project is inside China and won’t have international partners. Chinese regulatory submissions and safety reviews expect GB-standard drawings. Using ISA can cause delays during the safety permit process (安全设施设计审查).
3. The owner’s existing plants use GB. Consistency across sites matters more than which standard is technically better. If a company already has five plants documented in GB, switching to ISA for the sixth plant creates a maintenance headache.
The Hybrid Approach (Recommended for Sino-Foreign JVs)
The most successful multinational projects I’ve seen use a dual-standard approach:
1. Process design basis: ISA-5.1 for all P&IDs, with a GB cross-reference table in the legend
2. Instrument index: Maintained in both ISA and GB tag formats, with a mapping column
3. DCS configuration: Uses ISA tags internally, with GB aliases configured for Chinese operator displays
4. Legend sheet: Shows both ISA and GB symbols side-by-side for every instrument type used in the project
5. Line list: Includes both inch and DN sizes, both ISA and GB service codes
This costs about 15-20% more during detailed engineering but saves far more during construction and commissioning, when you’re managing a mixed team of Chinese and international engineers.
I’ve seen the alternative — a pure ISA project in China where the construction team couldn’t read the drawings efficiently — and it cost the project three months of schedule slippage due to RFIs alone.
The Human Factor: Training Your Team
Standards are only as good as the people reading them. If you’re running a hybrid project, invest in:
A symbology training session (4-8 hours) for all engineers. Walk through the tag comparison table above. Show real P&IDs from both standards. Have people identify instruments on drawings from the “other” standard. It’s boring but essential.
A one-page cheat sheet posted in the engineering office. List the 10 most confusing symbol differences between ISA and GB. Laminate it. Put it on the wall next to the plotter.
A tag mapping tool. This doesn’t need to be fancy — an Excel spreadsheet that converts ISA tags to GB tags and vice versa, with a lookup table. Share it with the entire project team.
A Note on Smart P&IDs
If you’re using SmartPlant P&ID, AVEVA P&ID, or AutoCAD Plant 3D, you have an advantage: these tools can enforce symbology standards through their symbol libraries and templates. You can set up rules so that:
– Only approved ISA symbols can be placed on ISA sheets
– GB sheets pull from GB symbol libraries
– Tag formats are validated against the selected standard
But be warned: setting up dual-standard templates in these tools is non-trivial. You’ll need a dedicated administrator who understands BOTH standards, not just a CAD operator who knows where to click. I’ve seen companies spend 200+ hours getting this right. Budget for it.
Summary
The choice between ISA-5.1 and GB/T 2625 is not about which standard is “better.” It’s about which standard your team can execute consistently. The worst outcome isn’t picking the wrong standard — it’s picking one and then having half your team silently default to the other.
My rule of thumb: if the project crosses a border, use ISA as the master and provide GB as a translation layer. If the project lives entirely inside China, use GB and provide ISA translations only for licensor packages and imported equipment.
Either way, document your choice in the project P&ID legend, train everyone on it, and enforce it in every review.
And if you see “PIT” on a drawing and you’re not sure whether it’s a controller or a transmitter — stop and ask. A five-minute conversation during design beats a five-day rework during commissioning.
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