How to Read P&ID Instrument Symbols: ISA-5.1 vs JIS

A Japanese supplier sends you the P&IDs for a skid. Most of the tags look familiar: FIC-101, LT-201, PSV-301. Then you hit a bubble marked QI where you expected AI, a legend that says 計器室 and 現場 next to two circle styles, and signal lines with small slanted ticks instead of dashes. Is this ISA-5.1, a Japanese standard, or the supplier's own convention?

A piping and instrumentation diagram (P&ID), called 配管計装図 (haikan keiso-zu) in Japanese, uses symbols to show how a plant's equipment, piping, valves, instruments, and controls connect. It is not drawn to scale.

The short answer: tags on Japanese P&IDs follow the same basic grammar you already know. The first letter is what is measured, the letters after it are what the instrument does, and the number is the loop. The bar through a bubble shows where the operator sees it. The base is either ANSI/ISA-5.1 or the Japanese standard JIS Z 8204 (計装用記号, keiso-yo kigo, instrumentation symbols), and the two differ in a handful of letters, line types, and location symbols. In every case, the drawing's own legend sheet and the project standard override both.

This guide focuses on the instrument symbols: how to break down a tag, where JIS Z 8204 and ISA-5.1 differ, what the bubble lines and shapes mean, the Japanese words you will see in legends, and a short fictional P&ID to practice on. In short, a PFD shows the main equipment, flows, and operating conditions, and the P&ID builds on it to add all piping, valves, instruments, control loops, and standby equipment. For a full comparison and the order in which these drawings reach you, see PFD vs P&ID on Japanese projects.

How to Read an Instrument Tag: FIC-101

The circle on a P&ID (often called a bubble or balloon) holds an instrument tag. Its basic form is letters (the letter code) plus a number (the loop number). The first letter and the letters after it follow different rules, so read them separately.

First Letter: The Measured Variable

The first letter is the measured variable (JIS Z 8204 calls it the henryo kigo, variable letter). The four you see most often on plant drawings are F (flow), L (level), P (pressure), and T (temperature). Others in common use include S (speed) and H (hand, for manual operation). Analyzers are where the two standards split: ISA uses A, while JIS uses Q. More on that below.

A second letter can modify the first. The most common modifier is D for differential: PDI is a differential pressure indicator, such as the gauge that shows pressure drop across a filter. Do not read the P alone and assume a pressure gauge. Read the first letter and its modifier together as one variable.

Succeeding Letters: The Functions

The letters after the variable describe what the instrument does. The basic set is I (indicate), R (record), C (control), T (transmit), A (alarm), and S (switch). You will also see V (valve or other final control element), E (primary element, the sensor), and Y (relay, converter, or computing function).

Alarms and switches carry H (high), L (low), HH (high-high), or LL (low-low) to show which side they act on. LT is a level transmitter, TE is a temperature element such as a thermocouple or RTD, LAH is a high-level alarm, and PSLL is a pressure switch that acts on low-low pressure. On a P&ID, the letters tell you only which variable the instrument handles and what it does with it; the sensing principle and range are in other documents.

Number: The Loop

The number identifies a single measurement or control loop. JIS Z 8204 gives the example of loop 23, made up of an orifice, a transmitter, an indicating controller, and a control valve. As a rule, those instruments are tagged FE-23, FT-23, FIC-23, and FV-23. Collect every instrument with the same number, and you have the whole loop.

JIS Z 8204 numbers loops in series for each variable as the default (FIC-1, FR-2, FI-3 ... PIC-1, PI-2 ...). It also allows a single series across the plant or system regardless of variable. On real plants, numbers often carry an area or unit prefix, as in FIC-2105. The rule for that is written on the legend sheet.

ISA-5.1 vs. JIS Z 8204: Where the Letters Differ

Many Japanese drawings use ISA-5.1 conventions, especially on international projects and projects for foreign-owned companies. ISA-5.1 was revised in 2024 and renamed "Instrumentation and Control Symbols and Identification." JIS Z 8204 is still the 1983 edition. The Japanese Standards Association's list of standards under confirmation review, as of January 2024, lists the 1983 edition as a candidate for reaffirmation.

The ISA-5.1 details in this article follow the widely referenced 2009 edition. The two standards share the same basic logic, but a few letters and symbols mean different things. Check the legend first to see which standard the drawing leans on.

JIS Z 8204 also fixes the order of function letters: I, R, C, T, Q, S, Z, A. A flow indicating controller with an alarm is therefore FICA. If the indicator or the alarm does not need to be called out, the standard allows FCA or FIC.

For high and low alarms, JIS Z 8204 lets the drawing add H or L near the bubble as a specification symbol (shiyo kigo) instead of building it into the tag. On Japanese legends and alarm lists you may also see 上限 (jogen, high limit) and 下限 (kagen, low limit) for the same idea.

Bubble Lines and Shapes: Where It Is and What Runs It

The bubble tells you more than the tag. The bar through it shows where the instrument is, and the outline shows what kind of system handles the function. Together they tell you whether an operator can see the reading from the control room or has to walk out to the unit.

In today's chemical plants, many indicators and controllers are functions on a DCS screen rather than separate panel instruments. That is why P&IDs are full of "circle in a square with a single line" symbols.

JIS Z 8204 handles location a little differently. A bubble with no bar is in the field; a bubble with a horizontal bar is in the 計器室 (keiki-shitsu, instrument room), defined as the place where operators normally monitor and operate the plant. When the drawing needs to show a field panel (現場盤, genba-ban), JIS uses a separate symbol, and the panel's name can be written next to it. A dashed bar can mark an instrument mounted behind a panel. JIS also has its own symbols for a shared operator station (the standard still calls it a "CRT") and a multiloop controller.

Signal Lines: Dashes, Slashes, and Small Circles

Signal lines connect instruments and are drawn thinner than process piping. They are also where Japanese drawings can look the least familiar.

ISA-5.1-2009 also assigns separate line types to fieldbus communication (small diamonds) and to links between independent systems (filled dots). JIS Z 8204 defines an "internal link" (内部結合, naibu ketsugo): a connection made through a system's internal data path or software rather than external wiring.

The practical point: a dashed line on one drawing is not guaranteed to mean the same thing on another. On an ISA-style sheet it is usually an electric signal; on a JIS-style sheet it may be a substitute signal line drawn because the solid version could not be used. Confirm line types on the legend before you trace loops. For how Japanese panels receive 4–20 mA signals, see 4–20 mA vs 1–5 V on Japanese panels.

Valves on the P&ID: What to Check

Process piping is a thick solid line with arrows for flow direction. Each line carries a line number, usually built from the nominal size, fluid code, serial number, and piping class (a spec that bundles material and pressure rating). The format is set by each company or project.

Most valve symbols start from two triangles point to point, with marks or added parts that show the type. In Japan, piping drawing symbols are covered by JIS B 0011-1:1998, and ISO 10628-2:2012 covers graphical symbols for chemical plants. The fine distinctions between valve symbols vary by standard and company, so it pays more to know what each valve is for than to memorize shapes.

FO (fail open) and FC (fail closed) on a control valve show whether it opens or closes when its air supply or power is lost. JIS Z 8204 shows fail action with graphic symbols on the actuator, but where that is unavoidable it also allows the text marks FO and FC, plus FL (fail last, holding the last position). This is where you read whether the process moves to a safe state on a power or instrument-air failure.

Pumps drawn in pairs as A and B are often one running unit and one standby (予備機, yobiki). The isolation valves on each pump's suction and discharge, normally closed valves, and small drain and vent valves are easy to skip, but they are what maintenance staff look for when isolating a section of the system. For relief device details, see safety valve set pressure vs MAWP.

Japanese Legend Terms for Instruments

Legend sheets, called 凡例 (hanrei), and notes on Japanese P&IDs often mix English tags with Japanese labels. These are the instrument-related words you are most likely to meet. For drawing-level terms (drawing titles, line number, revision), see the table in PFD vs P&ID on Japanese projects.

Reading Order: Six Steps

A P&ID carries so much information that tracing symbols from one corner loses the big picture. Guides from engineering firms and training sources point to the same approach: work from the main flow down to the details. The six steps below are our summary of that approach.

At step 4, treat each loop as three parts: what is measured (element and transmitter), where the decision is made (controller), and what is moved (final element). The measurement point and the valve it drives are often far apart on the sheet, for example level measured on a tank and the valve on its outlet, or temperature measured downstream and the valve on the steam line. The shared loop number is what ties them together.

An interlock at step 5 stops a pump or closes a valve automatically when its conditions are met. On an ISA-style drawing it appears as a diamond with an "I" inside, the interlock logic function. The detailed conditions are usually defined in a separate interlock diagram or logic diagram, and a PLC may be the device that runs the logic. See interlock circuit diagrams on Japanese drawings.

Worked Example: A Simple Fictional P&ID

Here is a short practice run. Liquid is pumped out of a feed tank, heated with steam in a heat exchanger, and sent to the next step. This is a fictional example made for explanation, not a real plant.

Start with the flow. The pump P-101A draws liquid from tank TK-101 and sends it through the flow control valve FV-102 on its discharge. The liquid is heated in heat exchanger E-101 and goes on to the next step. Steam enters the exchanger from the top, and condensate leaves from the bottom.

Next, read the control loops. There are three:

• Loop 101: Level transmitter LT-101 measures the tank level and shows it on indicator LI-101 in the DCS. In this example the loop only monitors; it does not control.
• Loop 102: Flow indicating controller FIC-102 in the DCS receives the signal from flow transmitter FT-102 and adjusts control valve FV-102 to hold the flow steady.
• Loop 103: TIC-103 receives the signal from temperature transmitter TT-103 on the exchanger outlet and moves control valve TV-103 on the steam side to hold the outlet temperature.

Loop 103 is the typical case where the temperature is measured on the outlet piping but the valve sits on the steam line. From their positions alone the link is hard to see, but following "103" shows they are one loop.

PI-104 on the pump discharge has no bar, so it is a local pressure gauge read in the field. Last, check the safety devices. PSV-101 on top of the tank opens when tank pressure exceeds its set point and discharges to the destination shown on the drawing. The LT-101 signal also goes to an interlock (the diamond with an I), which shows the intent to stop the pump on low-low level and prevent it from running dry. On a real project, the exact trip conditions are in the interlock diagram or another separate document.

Checklist: Reviewing Instrument Symbols on a Japanese P&ID

• The legend sheet (hanrei) is included, and it names the base standard (ISA-5.1 and its edition, JIS Z 8204, or a company standard)
• Every letter used in the tags appears in the legend, including Q, Z, and any user-defined letters
• The loop numbering rule (per variable or plant-wide, area prefix) is stated
• Bubble bars match the real location: field, control room (keiki-shitsu), local panel (genba-ban), or behind the panel
• The legend explains the outlines used for DCS, SIS, PLC, and computer functions
• Signal line types (electric, pneumatic, software link) are defined, and solid and dashed versions are not mixed
• For each control loop, the measurement, controller, and final element can be traced through the loop number, where applicable
• Control valve fail positions (FO, FC, or FL) are shown and agree with the control specification
• Alarm and trip points (H, L, HH, LL, or jogen/kagen) are marked, with set values in the instrument list
• Every PSV shows the equipment it protects and its discharge destination
• Interlocks shown on the P&ID point to an interlock diagram or logic diagram
• Revision clouds, revision triangles, and HOLD marks have been checked against the latest issue

Questions to Ask the Japanese Side

Short, specific questions get faster answers. You can paste these into an email or a comment sheet and adjust the tags:

• Which symbol standard do these P&IDs follow: ISA-5.1 (which edition), JIS Z 8204, or your company standard? Could you send the legend sheet in English or bilingual form?
• On sheet [number], what does the letter Q (or Z) mean in tag [tag]? Is it quality/analysis or quantity/totalizing?
• Does the horizontal bar on [tag] mean the DCS in the central control room or a local panel? If a local panel, which one?
• What does the dashed line between [tag] and [tag] represent: an electric signal or another signal type?
• What is the fail position of [valve tag] on loss of instrument air or power, and where is it defined?
• Where are the set values for [alarm/trip tag] listed: the instrument list, the alarm list, or the interlock diagram?
• Which interlock diagram defines the conditions for interlock [number]?
• Has the field piping been checked against this P&ID revision after the last modification?

Where Practice Varies

The drawing's legend and the project standard come before any standard you know. Symbols, abbreviations, and numbering often follow company rules, and drawings that mix ISA-5.1 and JIS Z 8204 conventions are not unusual. If a symbol looks unfamiliar, ask instead of guessing.

A P&ID does not always match the field. The risk assessment guideline from the High Pressure Gas Safety Institute of Japan (KHK) treats the P&ID as the basic subject of a HAZOP study and lists checking the field for differences between the P&ID and the actual piping as part of the preparation. If a drawing was not updated after a modification, staff can misread which valves to close when isolating equipment. Isolation for maintenance is decided through the site's approved procedures and permit-to-work system, not from the drawing alone.

P&IDs are revised many times as design progresses, and after construction they are kept as as-built drawings (完成図, kansei-zu). Look for the revision number in the title block, revision clouds and triangles around changes, and "HOLD" marks on items not yet fixed.

Finally, a P&ID does not hold everything. Measurement ranges and set values are in the instrument list, valve specifications in the valve list, interlock conditions in interlock diagrams, and safety instrumented function requirements in the safety requirements specification. The symbol figures in this article are simplified, and the process in Figure 8 is fictional; neither is a design to build from.

FAQ

What Does FIC Mean on a P&ID?

FIC is a flow indicating controller. F is the measured variable (flow), I is indicate, and C is control. The number after it, as in FIC-101, is the loop number shared with the flow element, transmitter, and control valve in the same loop.

What Does the Line Through an Instrument Bubble Mean?

It shows where the instrument is accessed. In ISA-5.1, no line means field-mounted, a single line means a primary location such as the central control room, a double line means an auxiliary location such as a local panel, and a dashed line means behind the panel. In JIS Z 8204, a line means the instrument room (keiki-shitsu).

Do Japanese P&IDs Use ISA-5.1 or JIS Z 8204?

Either, and sometimes both. International projects often use ISA-5.1, while some domestic drawings follow JIS Z 8204 or a company standard built on it. The basic tag logic is the same, but letters such as A, Q, and Z and the line types can differ. Read the legend sheet for each project.

Does the P&ID Show Instrument Ranges and Set Points?

Usually not. Most P&IDs show tag numbers and how the instruments connect. Ranges and set values are in the instrument list, valve details in the valve list, and interlock conditions in the interlock diagram.

How Is a P&ID Different from an Isometric Drawing?

A P&ID shows connections and control and is not drawn to scale. Actual pipe lengths, bends, and equipment positions are shown in layout drawings and isometric drawings.

Sources

• JIS Z 8204:1983 Instrumentation symbols (Japanese, text reproduction): letter codes, bubbles, signal lines, loop numbers, FO/FC/FL
• Japanese Standards Association: JIS confirmation review list, as of January 25, 2024 (Japanese)
• ISA: Widely Used Engineering Symbols and Drawings Standard ISA-5.1 Updated (October 2024)
• Formación Industrial: Instrument mounting location on a P&ID according to ISA 5.1
• InstruNexus: ISA 5.1 Instrumentation Symbols and Identifications, Detailed Analysis
• ISO 10628-1:2014 Diagrams for the chemical and petrochemical industry, Part 1 (preview)
• JIS B 0011-1:1998 Technical drawings, simplified representation of pipelines, Part 1 (Japanese, text reproduction)
• High Pressure Gas Safety Institute of Japan (KHK): Risk Assessment Guideline, Ver. 2 (February 2016, Japanese)
• Engrity: How to Read P&ID Drawings, A Beginner's Guide
• Creately: How to Read a Piping and Instrumentation Diagram