P&ID total rules | easy understand about Process flow and instrument

P&ID rule
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I will explain about P&ID, which is the bible for mechanical and electrical engineers of chemical plants, especially about the entry rules.

The ability to “read“, “write“, and “explain” P&ID is essential for mechanical and electrical engineers.

The writing rules are flexible and there are many types, and you will be wondering which one to use.

So, I’ve narrowed it down to the ones I use the most.

If you can understand and put this into practice, you will be able to read and understand P&ID quite a bit.


P&ID stands for Piping & Instrumentation Flow Diagram.

Information on equipment, piping, control, etc. is comprehensively described.

It is an important document that contains important information on site .

P&IDs are used for many purposes.

Main uses of P&ID

  • special work on site
  • Confirmation of emergency situation
  • Consideration of facility expansion/renovation
  • technical consideration

I’ve been doing this job for over 10 years, but I’m worried that the “writing rules” aren’t enough.

Many people have a negative image, but let’s reverse the idea.

Since there are no rules, we can make our own rules.

Write freely!

Depending on the characteristics of the plant, it is possible to describe things to be careful about in terms of facilities.

  • Gasket paste range
  • Switching piping location
  • Blocking plate location
  • Equipment cleaning route
  • Piping used for each product

Various information can be included in the P&ID.

No two chemical plant production processes are the same and each has its own characteristics.

P&ID, which has no uniform rules and can be written freely, is more convenient in some respects.

Short flow and P&ID

The abbreviated flow of a chemical plant and P&ID have so much in common that it can be confusing for first-time users.

With other design documents, you can see that the contents are different just by looking at them.

However, there are many similarities between abbreviated flow and P&ID at first glance.

  • Fluid flow involved in the process
  • Equipment schematic
  • Device number/name
  • control instrument

Information around here is common.

It is as follows when described in the figure.

Common point

Up to this point, the outline flow and P&ID are common.

Short flow

An overview flow is a document that outlines the overall process.

Let’s take a look at the schematic flow below.

Short flow

This heat exchanges the raw material in T-001 with three heat exchangers X-001 to 003,

It is a virtual process of separating the liquid into T-002 and T-003.

A rough flow is a document that contains this on one to several sheets of paper.

It shows the positional relationship of 3 tanks and 3 heat exchangers as main equipment.

The “capacity” of the three tanks is also listed.


P&ID is a document that describes the details of an individual piece of equipment.

Take a look at the documentation below.


It is a document that describes one device on one sheet of paper.

In the example of the simplified flow above, you can write about 10 P&IDs.

Detailed information is provided for each piece of equipment.

Equipment information described in P&ID

  • instrument number
  • Equipment shape
  • internal structure
  • equipment accessories
  • Presence or absence of insulation
  • piping flow
  • Valves

Originally, the following information is also included, but this time it is omitted.

  • Equipment material
  • Insulation of equipment
  • Piping material
  • Fluid name of piping
  • Insulation of piping
  • Details of valves
  • fluid destination

I will describe the elements that actually make up the plant on each sheet.

This P&ID develops into construction materials such as piping diagrams for specific construction.

Rough flow → P&ID → Piping diagram

Construction design progresses in this way.

Batch and continuous P&ID

P&ID also differs between batch and continuous chemical processes.

Here are some typical examples of each.

Batch process P&ID

Introduce P&ID for batch process .

batch process

reactor unit

The batch process P&ID is in units of reactors.

One reactor per page.

Since 10 to 20 reactors are arranged, at least 20 pages are required.

P&IDs are usually listed so that they flow from left to right.

Temperature control with jacket

In a batch process , the reactor is usually jacketed and temperature controlled.

Even if some kind of reaction occurs in the reactor, there is no such thing as “keep warm” or “keep cold”.

The most common type is to warm up once, react, and then cool down after the reaction.

The order of heating → cooling . The reverse is rarely the case.

In the example above, S indicates steam and CW indicates cooling water.

The gas is cooled with a heat exchanger

The hot gas evaporated in the reactor is cooled in the heat exchanger through the gas line.

If the cooled gas is returned to the original reactor, it will be refluxed, and if it is transferred to another reactor, it will be distilled.

Gases that are not completely cooled by the heat exchanger are transferred to another location for processing.

Most reactors are either under atmospheric pressure or under negative pressure, and there is almost no operation under pressure.

liquid is pumped

The liquid produced in the reaction process is pumped from the bottom of the reactor.

A bottoming nozzle is almost always installed in the reactor.

Batch processes typically have multiple pump destinations.

This is for liquid separation operation to separate the water layer and the oil layer generated by the reaction.

Continuous process P&ID

Introducing P&ID for continuous processes.

continuous process

many towers

There are many towers in continuous processes.

Unlike the batch process, there are few places to store the liquid in a large container.

This is because the liquid flows continuously.

Few temperature control switches

A continuous process does not have temperature control like a batch does.

If you warm it up, keep it warm, and if you cool it down, keep it cold.

The upper gas side is sometimes called a cooler, and the lower liquid side is called a reboiler.

From me who is a batch, both are heat exchangers.

There is no need to put steam and cooling water in the same container like batch,

The number of pipes tends to be small.

a lot of instruments

Continuous processes are instrument-heavy.

In the flow diagram, circled x is the instrument.

Various instruments such as thermometers, flowmeters, and pressure gauges are attached.

Continuous processes often use monitoring and adjusting instruments for stable operation.

It is now “automated”.

Unlike batch processing, there are few manual interventions, so it is advantageous in terms of cost.

However, it is difficult to switch products.

Transfer destination is fixed

A continuous plant usually has a fixed transfer destination.

We don’t switch the water layer and the oil layer by hand like in a batch.

This is the same as temperature control, in the direction of decreasing the number of pipes.

General writing rules for P&ID

P&ID rules vary by company and office, but there are some common rules.

Entry contents

A P&ID will generally include:

  • Equipment Number
  • operating conditions
  • Instrumentation equipment

The flow diagram is written as follows.

operating conditions

This form is said to be the standard.

Equipment Number

Equipment specifications must be included in the P&ID.

  • Equipment Number
  • Main specifications
  • Material
  • Supplementary specifications
  • operating conditions

These are shown on the drawing.

Equipment specification

P&IDs are generally written horizontally.

Equipment specifications are often listed in table format on the bottom left of this page.

The main specifications are generally as follows.

Heat exchangerm2

The unit varies depending on the scale of the factory.

There are various tank capacities such as kL, m3, L, and so on. Although kL and m3 are the same.

For pumps, m3/h, m3/min, L/min, kg/h, etc.

Within the range we are dealing with, we often use units that have a lower number of digits.

In a continuous system, the pressure is in MPa, but in a batch, it would be obvious to prefer kPa.

If it is a batch, it will not exceed 1.0 MPa, so if it is expressed in MPa, all pressures will be.

It becomes an expression starting with “0”. For example 0.100MPa, 0.450MPa, 0.700MPa.

If you use MPa, the digit will increase by the amount of “0”.

operating conditions

The operating conditions are as follows.

  • pressure
  • temperature
  • flow rate
  • composition

If the composition changes, it will be indicated on the drawing as follows.

operating conditions

However, this is limited to continuous processes.

Not listed for batch plants.

It is more correct to say that there is not enough space to write.

For a continuous plant, the operating conditions are mostly determined by one, and the conditions for start-up and slowdown may be added.

But batch plants have at least 4 to 5, and at most 20 operating conditions.

It is almost impossible to describe this in the form of a P&ID.

Instrumentation equipment

The following are listed as instrumentation equipment.

  • pressure gauge
  • thermometer
  • Flowmeter
  • Liquid level indicator
  • Weight scale
  • pH meter
  • density meter
  • conductivity meter
  • automatic valve

Instrumentation equipment is often combined with automatic valves.

In order to understand this master-slave relationship, it is often expressed with a different line type than the process.

For example:


What is linetype? If you are wondering, please see here.

flow direction

P&ID shows the flow of the process.

Write the flow from left to right.

It is not an expression that is conscious of left and right on site.

I am not conscious of left and right, but I am conscious of up and down.

In some cases, the floor height is forcibly written in the P&ID.

“Unreasonably” means that there is a large difference in the items written in the P&ID,

This is because it is difficult to classify floor heights perfectly cleanly. This is the part that drafters struggle with.

drawing connection

A drawing cannot be drawn on a sheet of paper.

This means that the drawings must be connected.

For example:


Page.1 is written on the bottom right.

“T-002 Page.2” is written on the right side.

This means connecting to T-002 on Page.2.


This is Page.2.

There is an expression “P-001 Page.1” on the upper left.

This means that you will be connected from “P-001” on “Page.1”.

Since it is a connection of two drawings, I will express it so that it can be referenced from each of the two drawings.

P&ID Detail Rules

I will explain the detailed rules of P&ID.

This is a “local rule” that varies considerably depending on the company or place of business.

However, the absence of this sub-rule would result in a human-dependent P&ID.

This means that rules can change even within the same office.

Absolutely hard to see.

Let me introduce what should be defined as a detailed rule and a specific example.

Please refer to here for general P&ID entry rules.

Utilities and processes on the same page

Write utility and process piping on the same page.

Putting utility plumbing on the process page means:


Conversely, if you don’t write utility piping, it will look like this.

no utility

This is fine for a schematic flow, but it is unacceptable when it comes to detailed drawings at the P&ID level.

This is because utility piping also includes controls and operations and is directly linked to the process.

Utility piping, such as heat exchangers, that are clearly directly connected to the process are essential.

In some cases, you may not write something that is not directly related to the process, such as pump or agitator seal water, but is necessary for the operation of the equipment.

In such a case, create a utility flow diagram separately so that the utility system can be managed.

write a spec break

A spec break is a boundary between specs.

For example:

spec break

This means that the pipe material is separated before and after the valve.

The valve is made of SUS304 and the piping flange that connects to the valve is SGP.

In addition to the material, heat retention is written in the same way.

Indicate the slope of the pipe

There are rules that dictate the slope of the pipe.

For example:


From left to right, they have the following meanings:

  • do not reverse
  • tilt 1/10
  • tilt 45 degrees

Reverse gradient and 1/10 are limited to metal piping.

45 degrees is applicable to not only metal piping but also glass lining piping and fluorine resin lining piping.

  • It is OK if the reverse slope is not extremely sloped by applying a level
  • If you have a 1/10 slope, clearly indicate the start and end heights of the sloped pipe on the piping diagram
  • If you want a 45 degree tilt, add a 45 degree elbow

Almost any material can be tilted at 45 degrees, but a subtle tilt can only be done with metal piping.

You should have thought like this.

It is not impossible to make an elbow with a special angle even with glass lined piping.

On the other hand, the initial cost is higher, the delivery time is longer, and the maintenance is inferior.

write the required dimensions

Write the required dimensions on the P&ID.

For example:

write the dimensions

It indicates the height of a structure called an inverted U seal.

The height itself doesn’t matter much, but if it’s too low, you won’t be able to drive.

For that reason, please secure 300 mm as a constant height.

This is the example.

In addition to inverted U seals, there are many other examples where the piping height must be specified.

There are many in the reflux line.

write keep warm

P&ID equipment writes thermal insulation.

For example:

write keep warm

Appropriate hatching is put in the place where the heat insulation of the equipment is attached, and the thickness is described.

In this example, the shell of the heat exchanger is insulated and the channel cover is not insulated.

In batch-type chemical plants, there are many condensers that condense inside the pipes, so there are many cases where heat insulation is not attached to the shell but on the channel cover side.

The classification of heat and cold insulation, which cannot be specified only by thickness, will vary greatly depending on the company.

The need for heat and cold insulation is unique to batch-type chemical plants.

Heating/cooling may be necessary in some cases if heating/cooling is performed during reaction with a stirrer.


There aren’t many books on process flow.

I think that there are many cases where rules are established in the company, but if there are no rules, let’s gradually create them by referring to this article and the following books.

created by Rinker
¥3,850 (2024/04/19 02:23:45時点 Amazon調べ-詳細)

Related article

Related information


I explained the basic rules of P&ID for batch chemical plants.

Abbreviated flow and P&ID, batch and continuous, reactor units, symbols, line specifications, variable character symbols.

There is a degree of freedom and there are many types, but the items that can be used are limited, so let’s master them little by little.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

*I will read all the comments and answer them seriously.