Basic rules of P&ID | Expression method and instrument symbols

P&ID instrument
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I will explain the instrument symbol of P&ID.

This is very important content, including information on instrumentation control.

Process engineers are responsible for deciding the operation method, so as an instrumentation engineer, I have a passive attitude.

Mechanical and electrical engineers tend to think that instrumentation equipment belongs to instrumentation engineers, and even more shy away from it.

Even mechanical and electrical engineers will have an advantage if they know about control, so it would be a shame to avoid it.

variable character symbol

It is common to write instrument symbols on the P&ID.

Instrument symbols are defined as variables in JIS Z 8204 instrument symbols.

We will introduce how to use instrumentation symbols in chemical plants.

JIS Z 8204 Instrument symbol

First, let’s check the JIS standard.

Instrumentation symbols consist of three characters: variable symbol, variable modification symbol, and function symbol.

variable symbol

Regarding the variable symbol, which is the first character, we have picked up the ones used in chemical plants from the JIS standard.

Fflow rateFlow
LLiquid surfaceLevel

You will notice that most of the letters are linked to English.

The exceptions are Q: conductivity and U: others.

Q is defined as conductivity in the category of quality, and U is in the category of large number of variables.

U is often used for on-off automatic valves.

Just by looking at these variable symbols on the P&ID, you can tell at a glance which instrumentation device it is.

variable modifier symbol

Introducing the second character, the variable modifier symbol.

Jautomatic scanningJIdou?

This is rare and there are many exceptions.

I is not defined as the second character in JIS.

I don’t really understand why J is automatic scanning.

Q is an integral, but wouldn’t it have been better to say Integral?

I have some doubts, but in any case, it’s enough to understand that “that’s the way it is”.

Almost all instruments use I or Q.

This is an image where I is an instantaneous value and Q is an integrated value. This is enough.

Function symbol

Introducing the third character, the function symbol.

Zsafety emergency

Like the second character variable modifier symbol, I and Q appear.

There are some other characters.

To summarize these,

  • The variable symbol for the first character is quite strict.
  • The second and third characters are relatively flexible.

You will see that.

Combination example

Let’s look at some examples of combinations of variable symbols.



PIA means an instrument that indicates pressure (P) (I) and issues an alarm (A).

Whether or not to give an A depends on the philosophy of the factory.

It may be okay to just write it as PI.

This is because adding a pressure indicator means that the amount of pressure will affect the process.

That means it would be better to issue a warning.

Of course, DCS allows you to set alarms for HH, H, L, and LL.

flow rate


FQC is a meter that adjusts (C) the integration (Q) of the flow rate (F).

It’s a wonderful flow meter that adjusts the flow rate and also integrates it.

If you have done this up to this point, the batch system is almost perfect.

Used for dripping, etc.


FIQ is a common method of integrated flow rate.

Use this when you do not need to adjust the flow rate but want to know the total value.

In batch mode, it is often used to feed a fixed amount of water for each batch, such as when preparing solvent or water at the start of operation.

A flow meter is not often used when a predetermined amount of process liquid has been introduced and reacted and is sent to another location.

This is because the amount has already been determined.

This type of integrating flow meter is useful in the stage before the amount is determined.


FIC adjusts the instantaneous flow rate.

It is recognized that it ranks lower than the cumulative flow rate.

It is used when you want to control the flow rate to a certain degree, but not to the point of dripping.

For example, this applies to steam flow rate adjustment.



The overwhelming majority of thermometers are TIA.

Just indicate the temperature.

Almost the same as a pressure gauge.

Set the upper and lower temperature limits to the DCS alarm.


Some thermometers have an adjustment function as a TIC.

A typical example is when distilled with steam.

Cascade control using FIC and TIC is also often used.

For continuous plants, TIC is common for temperature.

Liquid surface


The majority of the liquid surface is LI.

If it approaches the upper limit, an interlock will be applied to stop the vehicle from driving.

The upper and lower limits are somewhat complicated in batch because they are set for each process.

  • The liquid level must be zero at the start of the process.
  • After filling the tank with liquid, the liquid level must not be below a certain level when adding the next liquid.
  • At the end of the batch, the liquid level must be below a certain value.

Judgment conditions change for each process.

This is a very important function for safe driving.


There are rare cases where the liquid level is controlled by LIC.

This is when controlling the liquid level in the dripping tank in addition to FQC using the flow meter when sending dripping liquid.

Use it as a sub-judgment condition, not the main judgment condition.



WIC is an adjustment using weight.

Used in a drip reaction using a load cell to control the drop amount according to the load cell reading.

When the liquid level is high, a large amount of liquid will flow if the valve opening is high, so reduce the valve opening.

When the liquid level gets low, the head gets lower and it becomes difficult for the liquid to flow, so open the valve to secure the flow rate.

I would like to adjust the dripping amount to a constant level, but the problem is that the dripping amount changes depending on the liquid level.

Controlling with FQC may work better in some cases, but WIC has a higher probability.


UJV is an image of an on-off valve.

It combines many other U’s, automatic scanning J’s, and valve V’s.

There seem to be cases where X and V are used.

HJV is also sometimes used, but this is an image of an on-off valve that supplies air by manually opening and closing.

Opening and closing is not operated from the DCS, but is operated on-site.

The driving force for opening and closing the valve is air, not humans.

This is a case where you would like to use an automatic valve, such as a large valve, but there is no need to import it into the DCS because manual work is required.

This is unique to batches that involve a lot of manual work.


Related article

Related information


We explained the expression rules for P&ID instrumentation symbols.

There may be some differences depending on the company, but I think the general symbol rules for flow rate, temperature, liquid level, pressure, etc. are the same.

This content alone provides general information about the control, so if you don’t draw it carefully in the P&ID, you won’t be able to create a successful DCS sequence.

I want to understand the rules before driving.

Please feel free to post any concerns, questions, or concerns you may have regarding the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

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