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This article is intended to answer some of the questions that come up about the subject of Line Numbers and Line Lists for a typical process plant project.

Some of the questions that come up about Line Numbering include: What is it? Why is it? What does it mean? Who does it? Another question is that is often asked “is there a common pipe line numbering system? Do engineering companies and clients have different line numbering systems? Another question is: Does ASME B31.3 or CSA (Canadian STDs), have a standard for line numbering?

These are all good questions and it is important for every piper to know the answers to each one. I could be wrong but I think it is safe to say there is no “common pipe line numbering system” in use in the process plant engineering and design world. There are just too many companies and people and clients and project types to be able to achieve such commonality. I don’t remember seeing anything in ASME B31.3 (or any other Code), but then again it is not the purpose of the B31 Code to dictate a line numbering system.

To properly discuss this subject we need to understand the purpose of the “Line Number.” We also need to acknowledge that some companies use the term “Line Designation.” Whether your company uses the term “Line number” or the term “Line designation” the purpose is still the same. It is the name for the line, it is for positive identification. It is the same as for you your name is the way to positively identify you. The pipe line identification is necessary through out the project and is used for many purposes by many different people or groups. The line numbering is an important aspect of the piping on a project and should only be created and controlled by the piping group for the project, specifically by the piping material engineer for that project.

As with any of the other “gray hair” pipers, I too have seen many different line numbering systems in my working past. Of all of them I have determined that while there are some similarities there is no totally common system.

Here are some details that I have been able to determine for myself. There are two basic areas that tend to govern line numbering. These are the line number method and the line number elements.

* Methods - There are two basic line numbering “methods.” The first method is based on the Purpose or Function of the line. The second method is based on the line “Size.”
* Elements - There are many potential line identification elements that may be included in a line numbering system. The elements that are most commonly included in a line numbering scheme are: Plant Location, Line Identity, Line Class (or Line Specification), Size, Insulation Type (when required) and Heat Tracing Type (when required).


The “purpose or functional” method of line numbering is where the line is best defined as a single line having an origin and a terminus consistent with a purpose or function. An example of this might be a pump suction line. It might come from a Storage Tank, a Tower Bottom or an Accumulator. It travels some distance and then splits and connects to the two pumps designated for that one service or function. This is one line, it serves one purpose or function therefore it has only one line number. Another example would be the pump discharge that leaves both of these pumps and join to form the single pipe line that runs to the next piece of equipment. This line also has one purpose or function therefore it has only one number.

The “line size” method is where the line number changes whenever and wherever the line size changes. When compared to the first (purpose or function) method, the pump suction line might have five line numbers instead of one. The pump discharge might also have five or more depending on what happens at the destination end of the line.

One or the other of these two methods becomes the “Rule” for a company, client or project. As we all know, for every rule you will always be able to find an exception. One exception that is common to both of these “Rules” is the rule of the change of line class (or line specification). The change in line class is most often caused by a change in pressure or material. A line might be the same size from one point to another but includes a pressure control valve that lets the pressure down to a level that allows a lower flange rating and or another material therefore a different line class. There are other line class change initiators but I think you get the idea. Any change in a lines conditions or material causes a mandatory change in the line number no mater which line numbering method is being used.


The elements of a line number might be arranged and look something like this:

10-1021-CA1A-12”- IH -ST


10 = the plant Unit or Area (mandatory)

1021 = the numeric line number (mandatory)

CA1A = the line class (mandatory)

12” = the line size (mandatory)

IH = Hot Insulation (only when required) (1)

ST = Steam Tracing (only when required) (2)

(1) Other types of insulation include IC = Cold Insulation, IS = Safety Insulation, IA = Acoustic (or Sound) Insulation, etc.

(2) Other types of tracing include: ET = Electric Tracing, CWT = Chill Water Tracing, TT = Thermon (Hot Oil) Tracing, etc.

* In this case the “10” refers to a physical plant area as defined by the project work breakdown structure document. Other Areas might be 20, 30, 40 or 11, 12, 13, etc.
* The “1021” represents the twenty-first line in this area. All line numbers on this project will be four place numbers starting with 1000 in each area.
* The “CA1A” represents the project piping material line class code for: 300# (C), Carbon Steel (A), 1/32” corrosion allowance (1) and a specific gasket type/material (A).

I have no doubt that there are other items of information that could be or are added in some form or another based on the preference of a specific company of client. My credo is “Keep It Simple.” The manner of arranging the Elements to form a line number is something that is also dictated by the Company and or in some cases the Client. The sequence that is shown above is in the order of information priority that I am used to.

The line numbering activity should only be done when the P&ID’s are well along in the development process. The more complete the P&ID’s are the less hours will be wasted and the better the results of the line numbering effort. The degree of P&ID completeness prevents recycle when new equipment or systems are added or when equipment or systems are deleted.

As the Piping Material Engineer (or designate) performs the line numbering of the P&ID he or she also initiates the first draft of the Line List (or Line Designation Table) data base. The resultant document from this data base will be extensive for a whole project but will be broken down by plant area.

In addition to the line identification elements (indicated above), the Line List (data base and document) will or should also include important information for the design process and construction process such as the following:

* The line commodity
* The phase (liquid or gas)
* The origin of the line
* The destination of the line
* The line pressure, both the normal operating pressure and the maximum operation (or Design) pressure
* The line temperature, both the normal operating temperature and the maximum operation (or Design) temperature
* An indicator code for Stress Analysis requirements
* PWHT requirements
* The insulation thickness (optional)
* The line schedule (optional)

The line list once initiated will have many additions, deletions and be used by lots of people throughout the project both in the design office and in the field.

One of the first and most important activities where the line list is used is when the definitive labor hour estimate is generated. The number of lines on a project has a direct bearing on the labor hours required. This is true for the piping design activities in the home office, the pipe fabrication shop and for the installation contractor in the field. So it is very important to get it right.


About the Author


James O. Pennock has more than forty-five years in the process plant design profession. He has been involved in both home office and job site assignments on refinery, chemical, petrochemical, power and other projects. His experience ranges from entry level designer to engineering manager. Much of this was with Fluor. He is also the author of the book "Piping Engineering Leadership for Process Plant Projects." He is now retired, living in Florida, USA and does only occasional consulting work.

Mr. Pennock can be contacted via E-Mail at

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