About Backing Strips

I have seen magnetic clamps to push the strips against the wall on the back side, for easy removal afterwards.

What is the purpose of backing strip on support pad (full encirclement pad) for pipe line? To have the header move freely due for thermal expansion or contraction? Is that so?

The purpose of a backing strip is to provide a 100% penetration weld where only one side is accessible. It is usually welded continuously with a fillet weld to one piece, covering the seam and welded from the other side. Thick plates may require multiple passes with back gouging between passes. Peening of the weld is not permitted on critical welds to vessels, pipes, tanks, etc that are subject to high pressure. Welding a submarine hull is a good example. Although both sides of a submarine's hull are accessible for welding, there are sometimes small areas where it is impossible to get into and there, a backing strip is used.

In addition to the prescribed good answerers, one of the most important use of backing strips is in fabrication of cross-country pipelines, where no way to allow formation of protrudes arising from the field weld of circumferential root pass, which will obstruct or retard both the flow of fluid and also running of pigs used for flushing, cleaning, sealing or inline inspection of the pipeline. .

Backing Rings and consumable inserts as per ASME B31.3.

If you make a sample and pull a tensile test, this will explain to you as to whether you need one or not.

PEbob

The backing strips are not only for a flat looking weld on the inside. They also come preformed in different "bulge" (strip is hollow) shapes. Just as additional info.

Thank you for the answers, please see if I am understanding this correctly, so we will usually use backing strips at places where we don't have access to the under side of the weld (e.g: a very small diameter pipe), and we will use this because there are no room for us to do back grinding, and therefore, a backing strip is used to ensure that there are no excessive weld on the under side of the weld.

I can see how I can use this for butt welds (it will just be a strip of cylinder), but how about if I am going to do a longitudinal weld? Then how must the backing strip look in order to fit the profile of the cylinder circumference?

And how will you take out the backing strip if you don't have access to the space in the first place?

Thanks!

Welding technique is vital when welding from one side, and full penetration is required. By choosing different electrodes, one may achieve full penetration welds from one side only. A case in point is from the Pipeline industry in which the use E6010 electrodes for the root pass in a "downhill" mode and full penetration is achieved, as verified by X-Ray. This is a technique used in many pipe applications and should be useful for longitudinal welding, as well. The caveat is that the system for which the part to which the longitudinal weld is made and the applicable governing code do not prohibit the use of the E-6010 electrode. And, depending on the part size and practicality and so forth, GMAW for the root pass is also a method for making the root pass to achieve a full penetration joint from one side only. Back-gouging is thus eliminated.

Why don't you explain your problem a little more detailed? What you call small others will call big e.g.

I have welded 1/4 O.D. Stainless 316L 200 Bar pipe, but definitely not with a welding rod. What you call cylinder, others call tank. If you cannot remove or apply backing strips, you'd better shouldn't.

The strips are mostly used for bigger projects. For smaller projects you should think of acetylene welding, TIG or microplasma. Depending on what, where and how to weld. Techniques are somewhat different.

But with a separate heat supply torch, you have more control of the weld for small pieces.

The use of backing strips depends on your preference or prescriptions, the speed you want to put in the process, and also the skill of the welder. Sometimes you'll start welding from the inside bottom, through a small gap, build up as long it works and end up at the outside top of the pipe, for circumference welds. I Have seen welding with extensions to the rod of 3 feet for big diameter pipe too. Sometimes 3 rods weld together.