Pipe is a hollow "tube" used for conveying products and pressure.
The products include fluids, gas, slurry, powders, pellets and more.
The pressure is hydraulic power. We usually designate the "tube" as pipe in the applicable line class but the definition includes any similar component designed as tubing, which is used for the same application.



One of the earliest methods of conveying fluids in the history of mankind was by pipe.
The earliest pipe on record was the use of bamboo for moving small quantities of water as a continues flow.
As man progressed, he began using hollow logs for his piping needs.
Probably the first recorded use of metal in piping systems was the use of lead or bronze during the "Bronze" age.

During the excavation at Pompeii, complete water distribution systems fabricated from lead have been uncovered.
These systems, include probably the first use of metal plug valves, are still workable.
Without piping our modern civilization and their attendant conveniences could not exist.

Today piping is used in almost every aspect of our lives.

Our drinking water is produced in plants full of piping and then comes to us through a vast network of pipes.
The waste from our homes and businesses flows away through another network of pipes and is then treated in a plant full of piping.
The fuel we use for travel or for heating was collected, processed and distributed using pipe.

No mater what you think about, power, food, paint, medicine, paper products, plastics, chemicals, and many more are all made in plants full of piping.

Our safety is also dependent on the piping in the fire water systems in our neighborhoods and buildings.

Materials of construction:

The various kinds of material from which pipe is, or can be, made is proved to be endless; among them are the more common carbon steel, along with chromes, stainless steel, iron, brass, copper, lead, aluminum, glass, rubber and various types of plastic material.

Over the years some of these materials have been combined to form lined pipe systems. These include carbon steel pipe lined with glass, carbon steel pipe that is lined with various plastics; carbon steel pipe lined with concrete.

Each one, plain or lined has certain advantages and disadvantages. Many things enter into making a choice of materials. Among the most important of these are commodity, pressure, temperature, size, ease of assembly availability and economics.

Pipe sizes:

Many years ago pipe was sized by its true inside diameter. i.e., a 1" pipe was actually 1" inside diameter.
However, as time went on and the methods of manufacturing were improved and made more standard, and because it became necessary to increase wall thickness to accommodate higher pressures and temperatures, it became necessary to size pipe by "nominal" size rather than actual size.
Because it was deemed too expensive to have a set of thread dies for each wall thickness in the smaller sizes, the outside diameter (O.D.) was held constant.
Thus wall thickness changes affect the internal diameter only and leave the O. D. constant for standardized fitting engagements.

Nominal size refers to the name by which we call a particular size pipe.
Nominal size and actual outside diameter of a pipe differs for size 12" and under.
For sizes 14" and larger the actual outside diameter and the nominal size are identical.

Pipe comes in a very wide range of sizes.
It is not uncommon to see piping as small as ½" or as large as 66".
Pipe mills can and will make almost any size for a price.
This does not always prove to be the economical choice because odd size fittings may not be available.
It is best to stick to the closest and most commercially available or common size to meet the need. 
The smaller common sizes in pipe include ½", ¾", 1", 2", 3", 4", 6", 8" 10" and 12".
The larger sizes, 14" and above increase in 2" increments.
The Nominal size pertains to calling the pipe size by name only.
The actual outside diameter or O. D. is different for the 12" and under sizes.


Nominal Size Actual O. D.

For all pipe sizes the inside diameter varies as the wall thickness increases thus the thicker the wall, the smaller the inside diameter.


Many years ago the only "weights" of pipe available were classed as standard weight, extra heavy and double extra heavy.
Within the last seventy-five years or so it became increasingly evident that this system was limited in scope and did not meet the needs of the growing state of the industry.
This was the direct result of the increasingly higher pressures and temperatures of the commodities being handled.
Consequently the use of schedule numbers came into being.
Today, both weight and schedule are the way of identifying the wall thickness.


Based on common practice pipe usually can be furnished in "single random" lengths, "double random" lengths, and under certain circumstances (pipeline work for example) in even longer lengths.
A single random will run from about 16' to 22' in length.
A double random will run from about 35' to 40' in length.
Pipe can be ordered to a specified fixed length but this will cost more.

Methods of manufacture:

Pipe is made two ways. It is made by taking a flat plate, called a skelp, and rolling it into a tube shape and then welding the two edges together to form a tube. This pipe is commonly called "welded pipe" or ERW pipe. The other way is to take a solid bar or billet and pierce a hole through the length. This pipe is commonly called seamless pipe.

Determining wall thickness:

The wall thickness for pipe is generally covered in the piping material specifications by calling out the Schedule Number for a large majority of sizes.
However, as pressure and temperature increase, and sometimes the corrosion allowance, it becomes necessary to calculate the required wall thickness for a specific case.
Please note that generally as the specifications change into higher-pressure classes, wall thickness calculations must be made for smaller size pipe.
Wall thicknesses are by strict adherence to the rules set forth in the code for Pressure Piping.
For more detailed information on specific pipe sizes and it's various wall thicknesses, schedules and pipe weights see the "tools," "piping", "pipe chart" on this website


In steel pipe, the word "grade" designates divisions within different types based on carbon content or mechanical properties (tensile and yield strengths).
The tensile strength is the ultimate amount of stretching the steel can bear without breaking.
The yield strength is the maximum amount of stretching steel can bear before it becomes permanently deformed or before it loses its ability to return to its original shape.
Grade A steel pipe has lower tensile and yield strengths than Grade B steel pipe.
This is because it has a lower carbon content.
Grade A in more ductile and is better for cold bending and close coiling applications.
Grade B steel pipe is better for applications where pressure, structural strength and collapse are factors.
It is also easier to machine because of its higher carbon content.
It is generally accepted that Grade B welds as well as Grade A.


Steel pipe can generally be specified with a specific end preparation at the time of purchase.
Three end preps are standard.
  • There is plain end (PE). This would be the choice for small sizes where socket welded fittings will be used to join pipe to pipe or pipe to fittings. This is also the default end prep if no end prep is specified.
  • There is threaded end (TE). This would be the choice for small sizes where the pipe to pipe or pipe to fitting assembly is to be threaded.
  • There is also bevel end (BE). This would be the choice for most all 3" and larger steel pipe (or other metallic pipe) where "butt welding will be used to join pipe to pipe or pipe to fittings.


The information given above is what you should know about pipe.
There are also some things that you should understand about pipe.
There is a big difference between what you know about a subject and what you understand about that subject. 
With pipe, most novice designers think that all they have to do is "draw" or "place" the pipe symbol (on that pipe support beam symbol) in whatever CAD system they are currently using and they are done.
They do not understand what that pipe symbol really means.
That pipe is (or represents) what will be almost a living thing and as such it will have a growing problem.
It will be installed at a certain ambient temperature and then on start-up it will operate at a totally different temperature.
That difference between the installation temperature and the operating temperature will cause the pipe to expand or contract.
No matter what the designed tries to do he or she cannot stop this action.
This expansion (or contraction) will cause stress, strain and force in both the piping system and the pipe support system. 
This pipe will also have a weight problem.
The pipe it self has a certain weight.
The pipe next to it may be the same size but it may not weight the same.
This pipe may be both high pressure and high temperature.
This means that the wall schedule may be much thicker therefore it will weigh more.
Let's say we do have two lines side by side.
They are both 14", one (Line A) is a low temperature, low pressure cooling water line and the other (Line B) is a high pressure, high temperature hydrocarbon process line.
The span for both lines is 25'.


Line A Line B
Pipe weight/foot 54.6 189.1
Water weight/foot 59.7 42.6
Insulation weight/foot 0 15
Total weight of span 2857 lbs. 6170 lbs.
This does not include any forces that may be imposed by the total piping configuration on this specific pipe support.
However, it does indicate that there must be some close coordination with the structural department so they do not assume that all 14" lines are equal. 
As for the piping designer, does this line need extra space for movement?
Do either or both of these lines need a pipe guide at this specific pipe support?
Does either of these lines need anchors at this specific pipe support?
If an anchor is required will the anchor forces on each side of the support be the same or will the anchor farces be unbalanced?
Both cases must be brought to the attention of the structural group. 
With the hot line there is normally an insulation shoe required which is added material and which changes the dimensional reference point for the centerline of this line and can cause design errors if not understood and allowed for.

For additional information about fittings see the "Codes / Standards" section of this website. 

About the Author


{cb: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 This email address is being protected from spambots. You need JavaScript enabled to view it..}

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