A look at water-soluble purge dam methods
July 11, 2006
Shielding gas is an often overlooked area of savings in tube and pipe welding. Creating a purge dam in one of several different ways can help you isolate the weld area and minimize the amount of gas and time needed to purge the weld zone properly.
|The first step in making your own water-|
soluble purge dam with water-soluble paper is to cut the paper in a diameter about 1.3 times the inner diameter of the pipe.
|Next, slit the edges and insert the paper disk into the pipe with the lip of the paper dam toward the weld area and then tape it into place.|
|The purge dam is ready; insert a purge gas needle into the root gap and introduce your purge gas.|
In gas tungsten arc welding (GTAW), the word consumable traditionally has referred to material that's consumed within the weld or in the process of welding. Wire, rod, tungsten electrodes, and shielding gas are consumables.
More recently welding supply companies and manufacturers have expanded the consumables category to include replacement parts such as nozzles, back caps, collets, collet bodies, and even gas lenses.
Companies often focus on minimizing the cost of individual replacement parts as a way to minimize the cost of the weld, but the fact is, welder downtime is more costly. To maximize welder uptime and keep costs lower, it's recommended that you buy quality replacement parts that last longer and keep your equipment—and your welders—at optimal levels of performance.
An often overlooked area of savings is in the use of shielding gas, which can be costly, especially in tube and pipe welding applications. It may be tempting to flood the entire piping system with shielding gas. While this may work fine in a small pipe system, it can be time-consuming and costly in the field, both in terms of shielding gas expense and crew waiting time.
Many accessories on the market are designed to create a purge dam to isolate the weld area and minimize the amount of gas needed, as well as the time it takes to purge the weld zone.
For a typical pipe run, a purge dam system allows you to use as little as 2 percent of the gas required and a fraction of the time required for a full system purge. This can lead to savings not only in gas consumption, but in crew time as well.
You can create a purge dam several different ways. Some welders have used everything from cardboard or plywood plugs to sponges.
Unfortunately, these ad hoc methods generally don't create a good weld environment, often leaving the welder with a lot of contamination.
Several components can be used to build an effective purge dam.
Purge Paper. Purge paper helps create a truly water-soluble purge dam that can be flushed out of the system with water or steam during hydrotesting or when the welding is complete and you're flushing and cleaning the line.
Manufacturers sell this paper as sheets, rolls, and pressure-sensitive tape with adhesive that dissolves with the paper. It's also available in precut, self-stick rounds to help save weld preparation time.
To form a purge dam, cut the paper in a diameter about 1.3 times the inner diameter of the pipe. Then slit the edges and insert the paper disk into the pipe with the lip of the paper dam toward the weld area. Then tape it into place with special soluble tape. Once the pieces to be welded are fitted into place and secured with weld tape, insert a purge gas needle into the root gap and introduce your purge gas. When the weld is complete, flush the line with water or steam to dissolve the paper dam.
Purge Film. In this method, as with purge paper, cut the film to fit your pipe, and use a special adhesive to attach the film and create a localized area for purging. The film is water-soluble and can be flushed away when welding is complete.
Like cellophane, purge film can be a bit unwieldy to work with, especially when welding pipes with a large ID. Film's flexible properties can help create a localized balloon behind the weld for your back shield needs.
In the past some were concerned that the purge film would become insoluble when heated to temperatures higher than 300 degrees F, and that purge paper, when installed incorrectly, also would become insoluble. Adherence to strict procedural guidelines is imperative to ensure not only that a impenetrable dam is created, but also that the film or paper dam will be completely soluble and leave no material behind once the weld is complete.
It is imperative to follow the manufacturer's instructions regarding construction and placement of the purge dam, and use only the materials recommended to ensure that nothing left in the system will contaminate the weld area. This is especially important when welding in a nuclear or food processing facility.
Purge Bladder. Purge bladders are tandem balloonlike devices connected by a spinal tube that carries gas to inflate them. The bladders are positioned on either side of the weld area and, once inflated, form a seal. Once the bladders are inflated, a release valve opens to flood the weld area between the two bladders with shielding gas. Purge bladders are available for many different pipe sizes and come in heat-resistant materials for preheated pipe work.
Once the weld is complete, the bladders can be deflated and extracted. This purge method may not be viable if the bladders need to be extracted through a long distance or through pipes with IDs smaller than the welded section.
Purge Plug. Pipe purging plugs are used on small-diameter pipe work and for pipe welding applications that include several branches and ends to seal. Plugs are particularly suitable for fabricating short lengths of tube and pipe assemblies in a shop.
The plugs have a hollow shaft with a sealed end cap and a connector for a gas inlet hose or vent hose.
Purge Baffle. Similar to purge bladders, double-ended purge baffles allow the weld zone to be isolated so that shielding gas can be applied locally. A gas diffuser produces a laminar flow of purge gas into the space between the baffles, which displaces the air in the space in between the baffles through ventilation holes.
Single-ended purge baffles allow tubes to be purged with an open end as long as the weld seam is the proper distance from the baffle. As with purge bladders, baffles must be extracted after welding. This may be difficult depending on the pipe length and the pipe system configuration.
Flange Purger. A flange purger uses an aluminum cone, which covers a range of tube sizes and is connected to a single-ended tube purge baffle with a gas diffuser.
Again, as with other mechanical purge devices, inserting and extracting the flange may not be feasible.
Once you've selected the proper purge system for your application, use an oxygen analyzer before you weld to test and monitor the amount of oxygen in the weld zone.
Especially with materials such as titanium and stainless steel, maintaining a completely oxygen-free weld area is imperative to creating a quality weld. Analyzers are available in hand-held models or as machines that can be integrated into an automated system.
Correctly installing a purge dam device will help save time as well as shielding gas, and help create a contaminant-free weld. Each method of localized purging has its advantages, and the method you choose will depend on your application.
Mechanical devices such as inflatable bladders, baffles, flanges, and plugs can be used repeatedly, which can make them a cost-effective option. However, also consider the exit strategy for these devices. With paper and film purge dams, remember to take into account your welding crew's skills and adherence to procedures.
|Tips for a Good Weld|
Always buy your gas from a trusted source.
Jennifer Simpson is marketing manager for Arc-Zone.com Inc., 2091 Las Palmas Drive, Suite C, Carlsbad, CA 92011-1551, 800-944-2243, fax 760-931-1504, firstname.lastname@example.org, www.arc-zone.com. Photos courtesy of Aquasol Corp., Amherst, N.Y.