PROCESSING & PROCESSORS The use of water soluble film When stainless steel pipes and tubes, hereafter refer- some made of other materi- als such as chrome moly carbon steel, copper nickel, the material near the weld zone to prevent it from re- acting with the hot metal and causing coking, porosi- Mostly, argon is used for welding. In some countries gen for some of this work, versally applied for purging combine with hot titanium to cause undesirable nitri- To obtain a quality purge customary to fill the pipe as- sembly with gas and keep thought to be a suitable flow rate for an estimated period In today's climate of ever improving quality control procedures with traceability, such hit and miss methods to simply pour gas into a pipe assembly in the hope that a good purge will be advantage of this is that it will cost the operator more money than is necessary, meter and longer length oxygen causes lengthy waiting periods by costly specialists. Furthermore there is no guarantee that such voluminous quantities purging periods will actually When confronted with Solid Disc Purge Dam Assemblies large volumes to purge, the specialist will attempt to damming the pipe in some Many specialists still use poor quality materials to dam pipes. Cloth, wooden discs, foam bungs are all common. Such materials ge- nerally have a high vapour taminating vapours into the space being purged. Clearly products which are made products available include diagram 1 above), solid rubber disc purge dam as- above) and water soluble materials for users to manu- facture their own dams (see Bladders and rubber disc assemblies require an open STAINLESS STEEL FOCUS

PROCESSING & PROCESSORS Diagram 3: A water soluble purge dam arrangement Diagram 4: Argweld water soluble film purging kit weld is finished. Dams made from water soluble materials can be used for open assem- blies and closing welds. Water soluble materials are available in both plastic and paper forms. The paper product may leave relatively large fibres after dissolving and they can cause filters to become blocked. Further- more paper is becoming the water soluble plastic A British made water soluble plastic is available dams for the purging of pipe welds and this is made from a PVOH (polyvinyl...

PROCESSING & PROCESSORS holes will be made as appro- priate to feed argon gas into release air and excess argon. Because argon is heavier than air it is best to arrange purge cavity at the bottom at a flow rate of 10 litres a minute or less, to avoid tur- argon mixing with the air. The gas exit hole should be to ensure that the whole cavity is filled with argon prevent the molten weld root from being blown out- that the oxygen level has been reduced to an accept- able content, usually 1% or less for stainless steel connect a purge monitor to the exhaust area. This will ensure that time and gas...

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