By Barbara K. Henon, Arc Machines Inc.
Editor's note: Pharmaceutical Online is pleased to present the final installment of a four-part article on orbital welding for bioprocess piping, authored by industry expert Barbara Henon of Arc Machines. This article was adapted from a talk given late last year by Dr. Henon at an ASME meeting.
Contents Passivation and Corrosion Resistance Quality Control Fabrication Technology SOPs Conclusions
Orbital welding has the additional advantage that there is less loss of corrosion resistance due to welding with orbital welding than with manual welding because of superior, more uniform control of the heat input. Critical pitting temperatures were found to be higher, indicative of better corrosion resistance, on orbital welds of AL-6XN superaustenitic stainless steel than comparable manual welds. In another study Cal-Chem Corp., Arc Machines Inc. and the University of Southern California Department of Materials Science and Engineering have shown a clear relationship between corrosion resistance and passivation of both welded and unwelded 316L stainless steel tubing. A total of 77 orbital welds purged at three different levels of purge gas purity were treated with several different mixed chelant formulations as well as nitric and phosphoric acids and subjected to potentiodynamic polarization tests for determination of pitting potentials.
Passivation with all of the passivating agents was clearly shown to increase the pitting potential indicating an increased resistance to corrosion. Two of the six mixed chelant formulations consistently showed higher pitting potentials than samples treated with nitric (according to ASTM A-380-94) or phosphoric acids while the other four chelant formulations were comparable to nitric and phosphoric acids. The pitting potentials of the orbitally welded samples were comparable to results obtained with unwelded tube samples (Lin, 1995) with the exception of mechanically polished tubing which was welded with argon gas containing more than 100 ppm of oxygen in the ID purge. These samples had active rather than passive polarization curves indicating a loss of corrosion resistance relative to the base metal.
The visual appearance of the welded samples showed a clear relationship between the amount of oxygen in the ID purge, the amount of heat tint on the HAZ, and the amount of visible corrosion after the potentiodynamic polarization testing. There was a clear correlation between the number of pits and the amount of surface etching in the weld HAZ. At the highest oxygen concentration passivation clearly reduced the amount of pitting and etching, but did not eliminate either the heat tint or the surface corrosion.
These results indicate that passivation improves the corrosion resistance of unwelded as well as welded tube samples. Orbital welding with standard or higher purity purge gas appeared to have little or no detrimental effects on corrosion resistance, while welding with poor quality purge gas may result in a loss of corrosion that cannot be restored by passivation.
Test coupons are used to establish acceptable orbital weld quality standards before the start of a job and at specified intervals during an installation. Photo courtesy of Sandia National Laboratories.
Test coupons are used to establish acceptable orbital weld quality standards before the start of a job and at specified intervals during an installation. Photo courtesy of Sandia National Laboratories.
As engineers, managers, and end users, you need to know enough not to take welding quality for granted. With either manual or orbital welding, there is no assurance that all the welds in a piping system will meet the desired specification unless there is good onsite quality control. Quality control is the responsibility of the facility owner. Personnel representing the owner or third party QC is generally preferred to QC by the contractor for obvious reasons. End users must be aware of potential welding problems and of the possibility of defects and be knowledgeable enough to demand excellent weld quality.
We have often seen written welding specifications that are not possible to achieve with the materials provided. For example, the specification of a very narrow weld bead width or a total lack of concavity on low sulfur materials. To avoid conflict once the job is under way, criteria for weld acceptance should be established by the contractor and the owners' representative before the start of the job. Test coupons on the actual materials to be used on the particular application should be welded, and bend and tensile testing done according to ASME Section IX. During the qualification process and at least once a day during an installation, test coupons should be made and presented to the quality control personnel.
The initial test coupon used during weld procedure qualification should be used as a standard against which the daily test coupons are judged to assure that all visual criteria specified are satisfied. All test coupons should be kept for future reference. In addition to the daily test coupons, test coupons may be required for a change in conditions such as a change in material heats, move to a new location, or change in gas supply, etc. The importance of quality control in avoiding welding defects can not be overstated.
While some welding defects are obvious from viewing the outside of the weld others, perhaps more critical, can only be seen from the inside of the weld. It is recommended that a minimum of 20% of the welds be selected at random for borescopic inspection of the ID. This must be done on a routine basis throughout the duration of an installation to insure that all the welds are completely penetrated, well purged, and that there are no discernable voids or crevices.
Borescopes used to inspect the ID of welds during installation, such as this sterile fill line at the Pfizer Animal Health facility. Photo courtesy of Rand & Son mechanical contractor.
Borescopes used to inspect the ID of welds during installation, such as this sterile fill line at the Pfizer Animal Health facility. Photo courtesy of Rand & Son mechanical contractor.
Modern borescopes, sometimes referred to as video endoscopes, have flexible probes which are inserted into the pipe to view the welds which may be located 20 ft. from the end of the tube or on a branch or bend in the tubing. The built-in electronic imaging devices project an image of the weld to a video monitor. Usually a spacer of plastic or other soft material is used so the tip of the borescope can be rotated around the weld joint without scratching the polished interior of the tubing. The use of a borescope makes it possible for inspectors to see the entire inner surface of a weld joint and to detect any welding defect that could be seen by eye. In viewing the weld ID with a borescope, it is sometimes difficult to tell whether a weld bead is concave or convex. Some borescopes have the technology to measure the depth or height of the inner weld bead.
There are always a few welds in a pharmaceutical installation that are inaccessible to visual inspection of the ID. ASME BPE-97 recommends that owners and contractors agree upon a pro-cedure for blind welds. In this situation, test coupons made just before and just after the blind weld may substitute for ID inspection with a borescope.
Orbital welding technology provides the potential for achieving an excellent piping installation of the highest quality, but it is not a panacea. Mechanical contractors have developed standard operating procedures (SOPs) for materials handling and orbital welding which have resulted in a steady reduction in the number of rejected orbital welds over the years. Kinetic Systems kept records from installations at 36 BioPharmaceutical manufacturing and filling facilities tracked between 1991 and 1994. They reported an average of 2,900 welds per project and inspected over 100,000 orbital tube welds. The average weld rejection rate during this time was 0.74% with a mean of 0.45%. They reported a continual year-by-year drop in reject rate from a high of 1.83% in 1991 to 0.20% in 1994. They said that their SOPs were critical to keeping the rejection rate low. In addition, the size of the project was found to affect the weld rejection rate with a higher percentage of rejects at the largest job size (10,000 - 30,000 welds). The reject rate was somewhat higher than average for the smallest jobs (< 500 welds) as well.
SOPs for orbital welding typically include:Methods of inspection and handling of materials including cleaningTracking of material heats - especially avoiding sulfur mismatchesControl of proper tungsten type, dimensions and installation proceduresPrecision end-preparationAccurate fit-up and alignment of components prior to weldingPurging tube ID during manual tackingControl of purge gas purity and flow ratesUse of trained personnelMaintaining a welding log
Inspection of incoming materials and proper materials handling are essential SOPs. Inspectors must make sure that the materials meet the required specifications and that all heat numbers are recorded. All new heats of material should be tested for weldability and approved by QC before installing them. When possible, all tubing and fittings should be obtained from a single source.
Separate tools suitable for each type of material should be provided. For example, no tools of carbon steel or tools used on carbon steel should ever be used on stainless steel because such a tool could provide a carbon source that might result in carbide precipitation during welding.
A welding log listing each weld by operator and welding machine, location and time of day should be maintained throughout the project. The heat-of-material and lot numbers should also be recorded for each weld.
Alignment tool, made by Binsky & Snyder Mechanical Contractors, to hold tubes in position for orbital welding.
Alignment tool, made by Binsky & Snyder Mechanical Contractors, to hold tubes in position for orbital welding.
Mechanical contractors are continuously seeking more effective ways of fabricating. Alignment and purging are areas where SOPs can typically be improved. Tools, such as the purge mandrel tool, can be used to align tubes and fittings without the need for pretacking, while confining the ID purge to the area close to the weld. Some contractors have designed their own alignment tools such as the one shown above from Binsky & Snyder Mechanical Contractors.
Improvements in purging have been brought about by the availability of highly purified argon and by improved analytical methods of determining gas purity such as oxygen and moisture analyzers. Cleaner materials with better surface finishes and more widespread use of electropolished 316L stainless steel tubing have contributed to overall improvements in hygienic piping systems over the past decade.
Use of qualified personnel. Successful installation of biopharmaceutical equipment and piping systems requires a dedicated crew of trained welding personnel. However, the availability of trained personnel to operate orbital welding equipment is limited, as is the availability of skilled manual welders, both in the United States and Europe, and in some countries qualified welders able to quickly accept specialized training for orbital welding are difficult, if not impossible, to find. Although welding operators can be trained to operate fusion welding equipment in two days, it takes longer to become well experienced in its use. Welding operators must be able to identify bad welds and make adjustments in the welding parameters.
It takes some experience for operators to be able to develop weld parameters on their own. Purchasers of welding equipment will, usually, train a crew of operators; when the trained personnel leave for other job opportunities, newly-hired operators do not receive the same training as the original group and the situation deteriorates over time.
The labor pool of skilled manual welders is diminishing, and to overcome this problem, many labor unions in US have purchased equipment and conduct training classes in orbital welding for their apprentices and journeymen. Nevertheless, in many areas, it is still difficult to get a crew of fully trained welding operators. This means that the contractors must train their own people and the self-training that is typically provided is not always adequate, since there is usually a lot of pressure to begin the project immediately. The most successful manufacturers of orbital welding equipment will provide operator training for welders anywhere in the world. Users of such equipment should be aware of this service and are encouraged to avail themselves of it.
Orbital welding, together with developments in fabrication technology and SOPs, and improved materials, has advanced the state-of-the-art of biopharmaceutical process piping. Orbital welding is no longer a novelty, but has become the standard technology for joining piping systems in industries where cleanliness and purity are of concern. This is certainly true of bioprocess installations. Orbital welding makes it possible to routinely produce high-quality welds that afford better corrosion resistance than similar welds done manually, without sacrificing productivity, and with reduced man-hours.
Probably the most important recommendation for engineers or contractors planning an installation in which orbital welding will be used is to plan far enough in advance to allow for proper training, materials handling, weld procedure development, quality control, and all the other details that must be considered for a successful job.
To see the previous installments of this article, follow these links:
I. Considerations for Orbital Welding in BioProcess Piping Applications II. Considerations for Orbital Welding in BioProcess Piping Applications III. Considerations for Orbital Welding in BioProcess Piping Applications
For more information: Barbara Henon, manager, Technical Publications, Arc Machines Inc., 10280 Glenoaks Blvd., Pacoima, CA 91331. Telephone: 818-896-9556. Fax: 818-890-3724.
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