Welding sections of pipe is challenging for many reasons, not least of which is the limited access available to interior pipe weld joints for cleaning. In field welding, especially, it is often physically impossible to access welds inside of joined pipe, and that means that joint cleanup efforts will be limited to chemical methods. However, there is another way to go about ensuring clean, strong joints, and that is by preventing the need to clean pipe joints altogether. Below is why and how to use paper as a gas containment barrier when purging the pipe's interior of combustive gases:
What is purging?
Simply put, purging is the removal of oxygen and other gases that combine in a chemical reaction with hot metals; this reaction creates oxidation which can weaken a pipe joint and serve as a future catalyst for rust and corrosion.
The preferred method of purging is to create a sealed chamber around the area to be welded, and introduce a chemically-inert gas to force out the oxygen. Argon, the same gas used in incandescent light bulbs, is most commonly used since it is plentiful and inexpensive.
Why use paper as a barrier?
The purging chamber can be formed by several means, and pipeline welders have used inflatable bladders and a variety of other mechanical devices to create sealed-off areas. The problem experienced by welders in using these devices in field applications is that accessibility is highly limited; pipe sections are often joined rapidly and in adverse circumstances that make the use of bulky or slow equipment undesirable.
That is why paper is a great alternative. Paper is lightweight, portable and inexpensive, and just as important in some applications, purge paper leaves no residue. Here is how you can make paper purge barriers when welding pipe:
How to create paper purge barriers
Obtain purge paper and purge tape – while technically any paper or tape can be used to build an effective purge barrier, many pipeline applications require that there be no residue remaining once the pipe is in-use. Specially-designed purge paper and purge tape is available from a variety of manufacturers, and both are dissolved by water. Ordinary paper or other materials such as cardboard will often fail to completely dissolve in the presence of water; such materials can remain inside pipelines and clog equipment or contaminate products.
Mark the paper – when creating a barrier with purge paper, it is important that it be cut to an appropriate size and in a perfectly round shape. Paper that is poorly-cut will not provide an adequate seal inside the pipe. While you can successfully mark paper circles by using a compass, it's simpler and just as effective to trace the inside of a small piece of pipe, or fold the paper over the end of the pipe. The folding will create an indentation that shows the pipe's diameter.
Cut the circle – once you have marked the paper, cut around the diameter of the circle, but allow for an edge that increases the size of the circle by one-third. For example, if your pipe's marked diameter is 12 inches, then you will need to cut a circle that is 16 inches in diameter.
Cut folding slits – after cutting the circle, you need to make slits around the edge of the circle that allow the paper to be folded and inserted into the pipe. Each slit should be made perpendicular from the edge of the circle and extend inward toward the edge of the pipe's marked circle that you either drew or indented in step 2. Cut the slits within one-fourth of an inch to the marked circle, but be careful not to extend past it.
Fit and tape the circle – once you have cut the circle with slits, insert the circle into the pipe section to an appropriate distance away from the pipe joint. The exact distance away from the location to be welded depends upon several variables, and you should consult with your project engineer or supervisor for specifics. However, in general, you don't want the paper to be too close to the weld so that it doesn't suffer heat damage and allow oxygen to enter.
Attach the circle to the inside of the pipe by folding over the tabs and taping them down. Inspect your work to be sure that you have sealed all areas and that there are no loose tabs. You should now be ready to introduce the purging gas and continue with the weld.
After finishing the weld, you can leave the paper and tape in place until it is ready to be flushed and dissolved. Follow the recommendations of the paper and tape manufacturer or a company like Suburban Welding & Steel LLC to be sure that you have eliminated all traces of the materials; excess paper that is not properly dissolved can cause problems once the pipeline is operational.