P&ID (Piping & Instrument Drawings)

By Graphic Products Editorial Staff

P&ID drawings example

The numbering of equipment, piping and valves shown on P&IDs needs to be done in a logical way that allows for unlimited unanticipated additions to the system. In addition, the numbering system should provide useful information about the item being labeled. And the numbering system needs to be simple, and easy to understand and remember.

P&ID Criteria

These criteria eliminate numbering schemes in which numbers are assigned on-the-fly, meaning numbering things using whatever seems best at

Types of P&ID components to be numbered:

  • Mechanical equipment
  • Instruments
  • Piping and tubing
  • Valves
  • Actuators & controllers
  • Vessels & tanks

What type of information might be desired in a identification number? The following are some possibilities:

  • Equipment type code
  • Item Number (unique ID)
  • Identification of equipment function
  • Identification of the system
  • Location (plant, area number, train, system, stream, line, sub-line, etc.)
  • Item size
  • Sequence number
  • Contents (such as pipe contents)
  • Hazard information and warnings

Let's look at an approach to numbering equipment, instruments and components shown on the P&IDs that can be extended to to work with almost any situation.

An effective equipment ID number tells you about the object it identifies.  The information that is most useful includes its location, what it is, what it does, and specifically which one it is (if there are multiples of the same object).  An ID number might be formatted like:


The same numbering format should be used for all equipment that is alike.  However, different numbering formats might be appropriate for different types of equipment shown on the P&IDs.  For example, the numbering for instrumentation might be:


We'll look at possible numbering systems for both of these examples.

What Is The Location?

The first part of the ID number in both cases is used to identify the physical area where the equipment is located. This might be a process area, a building, or the area might be based on the function or system the equipment is used in.

Assign numbers to each area using a minimum of two or three digits. This allows for more detailed information and a greater number of locations. A two digit numbering system in a paper mill might look like:

00 – Wood chip handling
10 – Digesters
20 - Kilns
31 – #1 Recovery boiler building
32 - #2 Recovery boiler building
40 – Paper machines
50 – Power generation
60 - Converting
71 – Warehouse
72 – Administrative office

Notice several things about the numbering scheme chosen here. The first digit identifies the general area and the second digit identifies a specific part of that general area.  For example, every number in the 30's is in the recovery boiler area.  Number 31 is specifically the #1 recovery boiler building.  When equipment is so large that it has its own building, a numbering system that subdivides locations can be useful.

Also notice that even parts of the facility that may not be included on the P&IDs, such as the warehouse and offices, are included in the numbering scheme.  This should be a universal identification system, not one that is limited to the P&IDs.

P&ID Equipment Identification - What System?

The next part of the number identifies the system in which the equipment is usded. Each system should have a unique identifier. It can be a number or a text identifier. If text is used, keep it short, and use letter combinations that do not lead to confusion. Some examples in our paper mill example might include:

BLQ – Black liquor system
GLQ – Green liquor system
FOL – Fuel oil
NGA – Natural gas
CAR – Combustion air
HPS – High pressure steam
FWR – Feed water

What Type of Equipment?

Next identify the type of equipment. This, again, can be done with either a numerical code or text. Some examples include:

HV – hand valve 
PV – Power operated valve 
PU – pump 
HR – Heater 
CN - Condenser

At this point we might have an ID number that looks like:


This is a power operated valve in the #2 recovery boiler natural gas system. But, there are multiple burners and a number of power operated valves in the natural gas supply system. How can the specific valve be identified?

The Identifier Code

The specific valve is identified by adding a identifier number.  This is just a number assigned to each valve.  For example:


This would be the #8 valve in the natural gas system for the #2 recovery boiler. But, we may want to do a little more with this number.  For example, if there are six burners, we may also identify the burner number.  The following valve is the #2 valve used for the #4 burner:


As you can see it can be very easy to end up with long identification numbers.  As numbers get beyond ten or twelve digits they can become confusing and be more difficult to remember.  Don't go wild adding codes and identifiers to your ID numbers.  Include all of the essential information, and allow room for future expansion, but keep them as short as possible.

Pipe Markers

Numbering pipes may require a slightly different numbering scheme. However, the principle is the same. Include the information that is needed to quickly and easily identify each pipe.

The ANSI A13.1 code specifies that pipe markers must be placed such that the pipe can be identified from any location from which someone might be see the pipe. This includes labeling the pipe at bends and both sides of penetrations. ANSI A13.1 also requires the pipe contents be identified on the label using words that are easily understood. This is so that emergency responders, who will not be familiar with abbreviations you use, can easily identify the pipe contents. However, ANSI does not specify that an identification number be used. So you are free to devise a numbering system that is appropriate for your facility.

A pipe identification number might be:


Using the codes we have already established, a section of pipe might have the following identification:


This would be a natural gas supply line for the #2 recovery boiler, that is a two-inch pipe.  The sequence number is a number that is assigned to each section of pipe.

In some cases the pipe material is included in the identification number.  This would give a number such as:


The addition of “CS” indicates that this is a “carbon steel” pipe.  Other two-letter codes, such as “SS” for stainless steel, would identify other types of materials.

Instrument and Instrument Loop Numbers

Instrument numbering has some unique requirements.  A typical instrument number might include:


The AREA number is the same numbering system that has already been established.

The ASSOCIATED EQUIPMENT identifies the system or equipment the instrument is monitoring.  So if it is the #2 recovery boiler natural gas line pressure gauge,  the number might look like:


This identifies a pressure gauge (PG) in the #2 recovery boiler natural gas piping.  The pressure gauge is the #05 pressure gauge.
If there are redundant instruments, each has an alphabetical suffix added to the number.  In this case, if there are two natural gas pressure gauges, their numbers would be:

32-NAG-PG05A and  32-NAG-PG05B

P&ID Equipment Identification Standards

There is no right or wrong way to number the equipment, components and instruments shown on the P&IDs. What is important is that a numbering system be established and that it be used consistently. Once a numbering standard has been devised, use it throughout your facility and train everyone to understand what the numbering system, is communicating.

Making P&ID Equipment Identification Labels

When the time comes to make and apply the needed labels, be sure you have a DuraLabel label printer and tough-tested DuraLabel supplies. The most costly part of labeling is the labor cost to install the labels. That's one of the reasons you want labels that last – a long time. And that's what you get with DuraLabel.

DuraLabel vinyl labels are so tough that they are the only labels that come with a warranty on the life of the label after it is applied. No other brand has the tough-tested quality that allows them to provide a warranty like that!

Plus DuraLabel technical innovation has produced the DuraLabel TORO, the ONLY truly stand-alone four-inch label printer. The DuraLabel TORO is so complete that it even includes training built into the printer. Call 1-88-326-9244 today and ask for your free information package.

Read more on pipe marking & labeling in the Pipe Marking Resources Hub