Purging Methods for Weld Purity

purging methods purity weld

By Dr Michael Fletcher of Delta Consultants 

Design, fabrication, and maintenance practices that can meet exacting purity requirements of ASME 31.3 are crucial to the food, pharmaceutical, and semiconductor industries.

Demands for improvements in piping fabrication quality have risen exponentially in recent years.

Fig. 1 — Schematic section through tube joint illustrating gas seals on each side of the weld. The identifiers show access routes for inert gas supply/exhaust.The latest version the ASME 31.3 Process Piping code Ref. 1) is a formal recognition of this emerging requirement that has been stimulated mainly by the bioprocessing sector, but also by associated industries such as pharmaceuticals, semiconductors, and food production.

An essential element in pipework is production of welded joints. The stringent inspection procedures imposed ASME 31.3 apply as much to welded joints as they do to all the other fabrication processes involved.

The article noted in Ref. 2 highlighted more general aspects of the significance the latest edition of the code as it applies to the manufacture of high-quality pipework.

Producers of welding accessories designed specifically to meet the requirements have been quick to meet new challenges ASME B31.3 impose.

The Problem

Figure 1. Pipestoppers Nylon PlugsOne of the fundamental requirements imposed during the welding of pipes is to prevent oxidation of the weld during the first pass. The welding torch provides inert gas coverage of the top of the fusion zone, but unless precautions are made there is no coverage of the weld root. The problem has always been recognized and over the years a variety of solutions have evolved, some eccentric, others practical but largely ineffective.

While minimum standards are set for fusion welding, the application to the pharmaceutical, semiconductor, and food production sectors demands particularly high standards of cleanliness. Hygienic purity is the driving force for joints destined for use in pharmaceutical and food production. Elimination of particulate contamination is the crucial requirement in semiconductor manufacture.

Figure 3. Inflatable StoppersWith some semiconductor manufacturers producing chips with dimensions at the 32-nanometer (nm) level, and research going on at the 15-nm level, it is easy to see why the design, fabrication, and maintenance practices required to ensure exacting purity requirements of their process fluid distribution systems are of paramount importance.

In the food processing industries, statutory legislation and a plethora of litigation suits have forced plant manufacturers to introduce quality control levels previously considered unnecessary. Contamination introduced during fabrication is now unacceptable.

The Solution

Figure 4. Quick Purge Pip eWeld Purge SystemSatisfying the high level of protection lies in selection of specialized “purging” equipment.

Inserting low-quality paper or other barrier material on either side of the joint and filling the space between them with inert gas may be good enough for low-level requirements but is unsuitable to meet the requirements ASME B31.3 imposes.

Significant progress was made in purging equipment in the 1980s when welding accessory manufacturers developed expanding plugs and inflatable stoppers (Figs. 1–3). These devices gave assurance of effective sealing with the pipe wall and prevented leakage of inert gas from the weld zone, thus precluding backflow of oxidizing gases from the atmosphere.

Figure 5. Hot Purge PreHeated Pipework Purge SystemsGenerally effective, these developments still allowed considerable scope for innovative improvements. Devices such as shown in Fig. 4 integrate a pair of inflatable bladders and provides pressure- controlled gas ports for both inflating and inert gas supplies.

Other systems have been designed to satisfy the requirements of pre- and postweld heating through the use of thermally resistant materials — Fig. 5. Some are produced with no metallic materials in the weld vicinity so that postweld nondestructive examination can be undertaken with the purge system still in place.
There have been significant developments recently in gas-monitoring instruments. These incude devices such as monitors (Fig. 6) designed specifically for measurement of low oxygen levels in purge gases during welding.

The challenges of the ASME Process Piping code have provided a stimulus for further developments, and advanced versions of commercial purge systems are becoming available. Some of these employ high-stability engineering polymers to cover all exposed metal components so that the risk of transfer of metallic materials onto pipe surfaces is minimized. Many are able to provide fully automatic control over inert gas flow and pressure. The welding supply industry is responding proactively to the demands imposed by the latest edition of ASME 31.3.

1. ASME B31.3, Process Piping, Chapter X, High Purity Piping. 2010. New York, N.Y.: American Society of Mechanical Engineers.
2. Huitt, W. M., Henon, B. K., and Molina III, V. B. 2011. New piping code for high-purity processes. Chemical Engineering, July

purging methods for welding

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