ISA 5.1 defines how instrumentation is identified and drawn. Besides the tag letter code, it sets how loops are numbered (parallel or serial), how repeated instruments are distinguished with suffixes, how accessories are treated and which rules apply to symbols and signal lines on a diagram.
What ISA 5.1 sets
ISA 5.1 “Instrumentation Symbols and Identification” has a specific purpose: to establish a uniform way of designating the instruments and instrumentation systems used for measurement and control. To do so it defines a system of symbols and an identification code. Inductive, capacitive and photoelectric proximity sensors, with PNP or NPN output, are explained in inductive, capacitive and photoelectric sensors.
It applies in the chemical, petroleum, power generation, air conditioning, metal refining and many other process industries. Process equipment symbols are not part of the standard; they appear only to show how instrument symbols are applied.
How the tag is read (the variable and function letters) is explained in P&ID Diagrams and Instrument Identification. This guide covers what comes next: loop numbering, special cases and drawing rules.
How a loop is identified
A loop identification consists of a first letter and a number. All instruments in the same loop carry the same number and, in parallel numbering, the same first letter. Each loop has a unique identification. An instrument shared by two or more loops carries the identification of the loop considered predominant.
The standard allows two ways of numbering:
| Method | How it works | Example |
|---|---|---|
| Parallel | Each first letter starts its own sequence | TIC-100, FRC-100, LIC-100, AI-100 |
| Serial | A single sequence for the project or for large sections, regardless of the letter | TIC-100, FRC-101, LIC-102, AI-103 |
The sequence can start at 1 or at any convenient number, such as 001, 301 or 1201. The number may include coded information, but the standard recommends keeping it simple.
Repeated instruments, accessories and multiple functions
There are three cases where the basic tag is not enough.
Several instruments with the same function in a loop. A suffix is added to the loop number, in upper-case letters (FV-2A, FV-2B, FV-2C). On a multipoint temperature recorder that prints numbers to identify each point, the primary elements can be numbered TE-25-1, TE-25-2, TE-25-3. If it is more logical, a pair of flow transmitters can be designated FT-2 and FT-3 instead of FT-2A and FT-2B.
One instrument with several functions. It is designated with all of them: FR-2/PR-4 for a flow recorder with a pressure pen, PR-7/8 for a two-pen pressure recorder. When there are no separate devices, the slash is not needed, as in TAHL-21.
Accessories. Accessories that are not shown on the diagram but need designation, such as purge meters, air sets or seal pots, are tagged according to their function and with the same loop as the instrument they serve. For example, the orifice flange associated with FE-7 is tagged FX-7, and a thermowell for TI-9 is tagged TW-9. They may also use the same tag as the instrument with an explanatory word, such as PI-8 PURGE.
Rules for drawing symbols
The standard’s general rules for diagrams are these:
- The bubble may label distinctive symbols, such as a control valve. In that case, the line joining the bubble to the symbol is drawn close to it, without touching. In all other cases, the bubble represents the instrument itself.
- A symbol whose relationship to the rest of the loop is evident does not need its own tag. An orifice flange or a control valve that is part of a larger system can be shown without a number. The primary element symbol is optional when it is connected to another instrument.
- A short note can be added next to the symbol or line to clarify the function. For example, 3-9 psig and 9-15 psig next to the signals of two split-range valves, or 3-15 psig and 15-3 psig in a blending or diverting operation.
- Symbols can be drawn in any orientation and signal lines can enter or leave at any angle. Function designators and tags are always horizontal.
- Direction arrows are added to signal lines when they clarify the direction of information. In complex diagrams they help understanding.
- The electric or pneumatic power supply of the instrument is not expected to be drawn, unless it is essential to understand its operation.
- A single signal line is enough to represent the connection between two instruments on a flow diagram, even if physically there is more than one.
- The sequence of instruments on the diagram reflects the functional logic or the flow of information, which does not always match the physical order of the connections.
- Instrument lines are drawn thin relative to the process piping lines.
Signal lines: user options
The line section of the standard offers alternatives “at the user’s choice”. For the electric signal, the standard offers the dashed-line symbol or the triple cross-hatch one. It recommends choosing one and applying it consistently. To distinguish analog from binary (on/off) signals, the standard offers an optional symbol with a reverse slash. If the project does not require that distinction, it can be omitted. The recommendation is to keep the same convention across all documents in a set.
Where diagrams go wrong
The most common errors are of consistency, not of symbol:
- Mixing parallel and serial numbering in the same project.
- Repeating a tag in two different loops by copying a diagram.
- Using two electric line styles in the same set of drawings.
- Taking symbols from another discipline without a note explaining the change in meaning. The standard warns that this is critical when a misinterpretation could cause harm to people or equipment.
From the tag to the signal in SCADA
Every instrument on the diagram ends up as a signal in the control system. We integrate those signals with the control system and SCADA of your plant or substation, with a signal list consistent with the project tags. See SCADA integration and gateways and write to us through contact. Engineering is done remotely, worldwide; field work is done in Colombia, or elsewhere on request.
Related guides
- P&ID Diagrams and Instrument Identification
- Control Loop: PV, SP and MV
- Documenting an Automation Project
- 2-Wire and 4-Wire Transmitters
- Instrument Calibration: Concepts
- Industrial Electrical Diagrams: IEC and NEMA Symbols
Referencias
- ANSI/ISA-5.1, Instrumentation Symbols and Identification.
- IEC 60617, Graphical symbols for diagrams.
Frequently asked questions
What is the difference between parallel and serial loop numbering?
In parallel numbering each first letter starts its own sequence, for example TIC-100, FRC-100, LIC-100. In serial numbering a single sequence is used for the project regardless of the letter, for example TIC-100, FRC-101, LIC-102. The standard allows both; what matters is to use only one throughout the project.
How is an instrument that performs two functions identified?
It is designated with all of its functions. The standard gives as an example a flow recorder with a pressure pen, tagged FR-2/PR-4, and a common high and low temperature alarm window, tagged TAHL-21.
Does the electrical supply of each instrument have to be drawn on the diagram?
No. The standard does not expect electric or pneumatic power supplies to be shown, unless they are essential to understand the operation of the instrument or loop.