10 Things Things You Need To Know About Pipe


By Tim Daly

If you’re going to be a pipefitter, there are some basic facts and figures that you need to know about pipes.

We’ll start with a little Geometry

 Outside Diameter (OD): The distance across the pipe measuring from the outside wall on one side of the pipe, and going across the center of the pipe to the outside wall on the other side.

Inside Diameter (ID): The distance across the pipe measuring from the inside wall on one side of the pipe, and going across the center of the pipe to the inside wall on the other side.

Circumference: The distance around the perimeter of the pipe.

Wall Thickness: How thick the wall of the pipe is.

Depending on the job you’re involved with, there may be more geometry involved regarding the size of your pipe. For instance, an engineer designing a boiler will need to know the heating area of a boiler tube, so he’ll need to use diameter and length to determine the internal surface area first.

Generally, high pressure systems or systems carrying potentially corrosive fluids or gases will use a heavier wall thickness. Special micrometers can be used to determine exact wall thickness.  If you need this knowledge, your employer may send you to a precision measuring class.

A handy tool to have if you’re in the maintenance business is an O.D. tape measure. Sometimes even a veteran can be wrong when trying to figure out the difference between 1 ¼” and 1 ½” pipe. An O.D. tape measure will help you be sure.

Pipes and Tubes

When we are talking about piping systems, it is important to know that pipe sizes are measured using the inside diameter.  IPS stands for International Pipe Size and NPT stands for Nominal Pipe Thread. For all intents and purposes, IPS and NPT represent the same measurement, except that NPT refers specifically to threaded pipe.

Tubing, however, is measured using the outside diameter.  This distinction most often becomes important when you are dealing with copper, because copper pipe and copper tubing are both abundant in the industry.  When you head out to the supply house or shop store, you need to know what you are asking for, and ensure that the clerk and you are both on the same page. If you’re an apprentice, you need to ensure that you and the mechanic are on the same page as well.

Metal pipe

There are a wide variety of materials used to make pipes. Generally speaking, if you need any real high end material, an engineer will provide the specific material, sizes, and wall thicknesses on a blueprint.

Again, if you’re in the maintenance business, you have to be able to recognize what kinds of pipes you are looking at when you’re going to fix a leak, or tap into a line to run a new branch. It should also be noted that sometimes older pipe is no longer up to code for new construction, so you may have to connect an older system to a newer system.

For instance, you may find an old domestic water line made out of galvanized steel pipe.  In such an instance you would probably be better served to replace the whole line with copper or perhaps PEX plastic pipe.  Also, if you’re going to connect dissimilar metals like black iron and copper, you may need to use aspecial coupling or union to help in the prevention of electrolysis. 

Plastic Pipe

Often when fitters are referring to plastic pipe they use the term PVC, and as often as not the pipe isn’t actually PVC. It could be ABS or CPVC. What you need to know most about plastic pipe is that when you are connecting it, you need to ensure that your fittings and your pipe are the same material, and that you are using compatible primer and glue/cement. 

For most installations it is usually desirable to use the purple primer, because it reminds you, and any future inspectors, that you did remember to prime it before adding the cement.

 Brazing & Soldering

These are the three methods of connecting pipes to each other, and also to fittings that are not considered mechanical (flange, union, or threads, etc). Of these, welding is the one that stands out because it is done on steel, steel derivatives like stainless steel or galvanized steel, or iron.

Brazing and soldering are often confused by people. Soldering is most often done on smaller or domestic piping systems like fresh water. It involves inserting the end of a copper pipe or tube into a copper fitting, heating it up with a torch, and flowing solder into it using the torch heat to pull (or direct) the flow of the solder into the fitting.

Brazing, on the other hand, in its most natural form uses a hotter torch (often combining oxygen and acetylene) to connect a pipe to a fitting. There is usually a brazing ring inserted into a groove in the fitting, that when heated, will flow into the joint, with the fitter using the torch heat to direct the flow of the ring material into the joint.

Usually a stick of brazing wire, called a brazing rod, is also heated and used to flow into the joint from the top to help make a good seal.  As stated above, many plumbers will refer to brazing as soldering, and many pipefitters will refer to soldering as brazing.  

Brazing rods can also be used to repair a split in copper pipe by heating the opened area and getting the rod to melt and flow over the opening. This is an art form and requires much skill to accomplish it.


Welding pipe takes different forms, depending on what material you’re welding, what sorts of pressures the pipe is going to have to withstand, what sort of fluid will be carried in the pipe, and in what setting the pipe will be located.

Butt welding takes place when you are joining two pipes in a run, or sometimes when you’re going to add a fitting like an elbow or tee. The key to butt welding is often in the bevels at the ends of the pipes and fittings.  There are “V” bevels and “J” bevels indicating the shape of the bevel’s slope.

When you are talking about critical systems like high pressure air, or nuclear systems for example, engineers have painstakingly determined what sort of bevel there should be, and also what angle the bevel should adhere to, and even how thick the very end can be.

Some of these welds will also include a ring of welding material that will sit between the beveled butts of the two surfaces, and such joints are usually held together with a cage that is constructed around the whole area of the joint, to hold everything in place until it can be tacked.

Generally speaking, only the most experienced welders and fitters will be in charge of making these joints up. If you’re an apprentice and have the opportunity to get involved in one or more of these types of butt welds, you should do so. You’ll be able to make a lot of money and friends if you can master this art. 

Socket welds involve welding a pipe inside of a fitting; usually a flange.  The pipe is bottomed out inside of the flange socket, tacked, and then welded where the neck of the flange meets the pipe; and also around the inside of the joint where the pipe bottoms out in the socket. 

Expansion and Contraction:

When you are working on steam pipes, expansion and contraction must always be on your mind.  When you remove the heat, the pipes are going to contract, and when you re-energize the line, it will expand again.

Engineers go to great lengths to account for expansion when they design steam systems.  They most often use expansion joints on piping runs in tight spaces. There are several different types of expansion joints in use, including the bellows type which have a corrugated belly that will move like an accordion, as the heat is added or subtracted.  There are also sliding expansion joints that use a piston type action, where one side slides into and out of the other side.

  Anchor points and rolling guides are usually installed as supports near expansion joints, to keep valves and flanges in the spots where they are needed. Pipefitters need to be aware of expansion and contraction when they are going to repair mechanical joints.

If enough anchor points are not installed before you break a flange on a joint that has only recently been de-energized, then there is a good chance the pipe will travel on you as it cools down. This travelling pipe will make it next to impossible to remake the connection again.  This is a very frustrating situation.

Another method of dealing with expansion and contraction in areas where the pipe is exposed, and has plenty of room, are expansion loops. These loops are more like a series of 90° bends than loops. They allow the pipe to move within the so-called loop when it heats and cools.

Like with expansion joints, it is important to know that your mating flanges are going to stay in the same spot when you disconnect them, so they can easily be made up again.

Threading Pipe

Threading Pipe is an art form, and takes practice. When you’re starting out, it seems like an easy enough procedure, especially if you’ve watched a pro who’s done it a lot. You just pick out the correct size thread cutter, install it into the threader, and feed in the pipe. But not so fast, Cowboy.

You need to ensure that all of the teeth are on the dye, and that the oiler is turned on and working. The fastest way to bring this project to a halt is to allow the pipe and cutter to heat up and expand, and then seize or break because you did not get a good flow of cutting oil on the surfaces.

Now that you have the dye inserted onto the threading machine, and the oil running, you need to bring the pipe up to the edge of the cutter and turn the machine on. You then need to ensure that you cut enough threads on the pipe to get a fitting on the end, with enough threads being inserted into the fitting for a tight seal that can withstand the pressure that will be exerted on it.

However, you also need to ensure that you don’t cut so many threads that the pipe can’t fit tight enough to get a good seal. Also, you have to stop the threader occasionally and back it up to remove the metal shavings that may be getting caught up in the threading die. These could get caught in your new threads and damage them. As I said before, it takes a lot of practice.

For a really fun time, try threading a piece of pipe remotely with a hand threader.  Finding a way to get the cutting oil into the pipe and dye area, as well as a way to contain the dripping oil, is fun in itself; add in the muscle it takes to turn the cutting head onto the pipe by hand, or the strength it takes to keep control of an electric hand threader, and it can really be a challenge.

Bending Pipe

Pipe bending, like certain kinds of pipe welding and grinding, is a real art form.  In most cases the actual bending is easy, as a large bending machine will do the work for you. The trick is in the measuring.  Even with a small tubing bender, if you don’t know how to mark the pipe out it will be nearly impossible to make the bend in the right spot, let alone at the right angle.

A pipefitter can only learn this art with a great deal of practice under the tutelage of somebody else already experienced in it.

Copper is a tough bend, and rigid copper pipe must be annealed before it can be bent.  Annealing is a heating process done with a torch to soften the copper up before bending. Failure to do so will have the copper pipe ripping during the bending process.

Flushing and Testing

After piping systems have been built or repaired, they are often put through one or more tests.  On high pressure systems, hydrostatic tests are often performed using a fluid medium like oil or water, depending on what the system will carry.

Hydrostatic tests of welded or brazed joints are generally performed at 150% of the system pressure. Fluid mediums are used because air and gasses tend to compress, and a failure could result in catastrophic injuries to the people involved.

Liquids tend to lose pressure much faster because they don’t compress like air. This is why it is imperative to get air out of the system through high point vents before you begin testing.  Often critical system components are removed and replaced with jumpers, because the mechanical parts could be damaged at the higher pressures. When this happens, mechanical joint tightness tests are performed at system pressure, sometime after the Hydrostatic test.

Quite often systems with brazed joints are flushed with hot water, after the brazing has been done and before the testing begins. This is because flaws in the brazing process could be evident when the pipes are heated up with the hot water.  The flushing also helps to remove any dirt or debris that may have fallen into the pipe during the brazing of the joints.

Leave a Reply