Effective weld purging is only achieved by making sure that oxygen is displaced from the purge zone prior to and during welding. Any residual oxygen can cause significant loss of corrosion resistance and a reduction in joint strength. It is therefore essential to seal the pipe on either side of the joint and maintain this seal throughout the process. The residual level of oxygen in the purge zone needs to be consistent with the welding procedure, so continuously monitoring the purging zone to ensure compliance is crucial.
No. 1: Choose a dependable sealing material — The cheapest solution is seldom the best, so examine all of the options available. Don’t be tempted to use sealing discs made from polystyrene foam, wood, and cardboard. At best, these substances leak or emit contaminants, and at worst, they catch fire during the hot weld cycle.
No. 2: Use a complete purging system — Don’t try to economize by making use of whatever happens to be around at the time. Separate seals for the pipe and rubber tubing for the inert gas all held together with bits of tape seldom succeed. Invariably, this is time-consuming and can only be used once.
Find systems using inflatable dams. Commercial equipment is now available in which gas flow, pressure, and purge gas quality are all pre-set. Complete monitoring instrumentation can be incorporated to ensure a high level of quality control. These systems have been designed for multiple use and are rugged enough to cope with site conditions whilst still significantly reducing overall purging and welding time.
No. 3: Establish what level of oxygen in the purge gas is acceptable — There is plenty of published information available that establishes what the maximum oxygen content needs to be to prevent loss of mechanical and physical properties in the weld. This depends on the material being welded, but, generally, some stainless steel welding requires a low level whereas most carbon steels are much less sensitive.
No. 4: Take care with gas flow — Whatever system is selected, ensure the inert gas enters slowly. Argon is heavier than air, so introduce it slowly at the bottom of the weld purge space and discharge from the highest point. Helium is lighter than air and needs to be inserted at the top of a cavity and removed at the base.
No. 5: Don’t rush into the welding sequence — Wait until all the air has been displaced before welding. For many metals, this means ensuring a residual oxygen level below 100 ppm.
This is a traditional practice and one followed by major fabricators, this is based on pure guesswork. If the oxygen content is too high, or varies during welding, oxidation occurs and this often means rejection with the expense of re-machining and, therefore, production delays. It can also lead to loss of corrosion resistance in stainless steels.
No. 6: Use a purge gas oxygen monitor — Don’t assume that allowing plenty of time for purging will remove all the oxygen. If there are leaks in the system, turbulence or simply poor quality purge gas could cause oxygen levels to exceed levels necessary to prevent contamination. The best solution is to use a device capable of accurate and reliable measurement of oxygen level.
No. 7: Read published information about purging — There’s plenty of it available. Learn from the experience of others rather than using trial-and-error methods that could cost time and necessitate re-welding.
Information courtesy of Huntingdon Fusion Technologies (HFT). For more information, visit www.huntingdonfusion.com.
Publication date: 10/2/2017
