Practical Applications of Orbital Tube and Pipe Welding
Practical Applications of Orbital Tube and Pipe Welding
Orbital welding was developed in the 1960s for welding in the aerospace industry when stronger, more reliable joining techniques were needed for hydraulic lines in rocket-powered aircraft. The failure of joints in the hydraulic lines when the fittings used to join them came apart from high vibrations and stresses of the rocket engines while flying at or near the speed of sound inspired the development of orbital welding technology.
One of the first aircraft on which the new technology was applied was the B-70. Orbital welding provided improved weld joint soundness, while maintaining good ductility and mechanical properties.
Since then, orbital welding has found niches in a variety of diverse industries, not only because of the improved reliability and quality of the weld joints, but for reasons of improved productivity, weld bead smoothness, improved corrosion resistance, and practicality when compared to manual welding or other joining methods.
Orbital welding employs the gas tungsten arc welding (GTAW) process, in which the tungsten electrode is not consumed. An inert gas, typically argon, is used to shield the electrode, the arc, and molten metal. To start the weld, an arc is struck and maintained between the electrode and the work. The electrode rotates or travels around the weld joint to complete the weld while the tube or pipe remains stationary.
The GTAW process, commonly referred to as TIG, or tungsten inert gas, can be done as a fusion or autogenous weld, in which the ends of the tube or pipe are melted and fused together without the addition of filler, or with filler in the form of wire or metal inserts added to the weld. Orbital fusion welding is generally limited to wall thicknesses of 0.154 inch or less, while orbital welding using filler can be done successfully on thicknesses of up to 6 inches using special narrow-gap equipment.
The equipment for orbital welding consists of a power supply, a weld head, and various cables and accessories needed for the particular application. Most modern power supplies are accurate and repeatable and use microprocessors to control the weld parameters.
The ability to deliver exactly the same values for welding current, pulse times, and travel speed for fusion welding, as well as the precise control of wire feed speed, torch oscillation across the weld joint, and control of the arc gap for wire feed applications, make it possible to achieve consistency from weld joint to weld joint.
The repeatability of process is particularly important for materials such as duplex stainless steel alloys, for which precise control of heat input is essential to maintain the favorable mechanical properties of the alloy.
Boiler tube and furnace tube installation and repair are major applications of orbital pipe welding. This type of application has become practical in recent years because of the availability of smaller, less expensive pipe welding power supplies, some with the ability to operate the full-function weld heads, which are capable of wire feed, torch oscillation, and automatic control of the arc gap.
Some weld heads can be mounted between rows of tubing, which would be difficult or impossible for a manual welder to access with his torch, or would require mirrors to view the weld joint. Inaccessibility can result in poor weld quality of manual welds and a high reject rate, which translates into time required for rework and a loss of productivity.
A comparison between orbital and manual welding technology was provided by Carolina Power & Light Company. CP&L used both manual and orbital welding on its superheater and reheater tubes at its Roxboro Electric Generating Plant. An outage was scheduled for 12 weeks to repair 1,500 tubes, which was originally planned to be done manually.
About four weeks into the outage, the contractor experienced a 16 percent reject rate on the manual welds and a shortage of labor to complete the project on schedule. The plant contracted with another company to weld the superheater header tubes automatically. The reheater tubes were completed manually by the original contractor.
Both jobs had to be 100 percent completed during the outage. Since the superheater and reheater jobs were similar in scope, it presented an opportunity for a realistic, on-site comparison of orbital and manual welding. The results are shown in Figure 1.
Other power plants have successfully used orbital pipe welding for boiler tube repair and replacement. In addition, these companies have successfully done orbital welds on the bimetallic welds between stainless steel and chrome-moly steel on the superheater headers, as well as welds at numerous other locations on the boilers.
Nuclear Piping Systems
The nuclear industry was one of the early users of orbital pipe welding because of the need for high-quality welds that would resist intergranular stress corrosion cracking (IGSCC) that frequently occurred in nuclear piping systems.
Since new construction in the nuclear industry has been limited in the U.S. in recent years, most of the orbital welding jobs have been to repair or replace piping systems damaged by IGSCC, which occurs under conditions of stresses on the welds in severe service environments. Special weld heads have been designed for repairing valves by cladding the inside diameter (ID) to reinforce the structurally weakened valve.
Vision systems were an early development to allow welding operators to view welds in radioactive areas from a remote location. This limited the exposure to radiation to the time required to set up the orbital weld head on the pipe.
Earlier fiber-optic vision systems, in which the individual fibers tended to break over time, thus losing resolution, have been replaced with direct-view cameras mounted at the torch, which allow the welding operator to view the leading and trailing edge of the weld in progress. Corrections can be made in cross-seam adjustment (centering), wire feed speed, or welding currents.
Food and Dairy Industries
The food and dairy industries use large amounts of stainless steel tubing. It is important to note that among the various food-related applications, piping carrying meat products has been orbitally welded, breweries were among the first users of orbital welding, and some wineries have recently welded their piping orbitally.
These industries share, with other high-purity industries, the need for piping systems without crevices which can entrap contaminants and that can be operated and maintained in a clean condition without degradation by corrosion.
Most hygienic piping systems are designed to be cleaned or sterilized in-place (CIP/SIP) without being disassembled. For CIP to be effective, the inner surface of the piping, including the welds, must be smooth and free of crevices and cracks that could entrap product and provide a colonization site for bacteria. Without the orbital welding of the various fitting, piping runs, valve assemblies, and similar joints, CIP would not be practical.
Although the dairy and food-related industries have been slower to accept orbital welding as the standard joining method because of the misconception that they do not need the quality or that it would be more expensive, many of the major players in this industry have begun to specify orbital welding in their plants. With more and more of the major companies specifying orbital welding for food and dairy applications, its use may become industry-wide within the next five to 10 years.
Other applications of orbital pipe welding include the welding of furnace tubes, in which access is limited and special materials that are difficult to weld by hand are used.
Similarly, boiler tube repair in the pulp and paper industry has been successful on welds that would have been difficult to access by hand or because the working environment, such as the high temperatures inside an economizer drum, would not have been conducive to the production of quality manual welds.
Orbital pipe welding has also been applied successfully in a number of offshore projects using corroslon-resistant materials such as titanium, copper-nickel, and duplex alloys, which are difficult to weld by hand and for which manual welders are in short supply.
Semiconductor manufacturers are the largest industrial users of orbital fusion welding equipment since the mid 1980s. Semiconductor manufacturers have performed millions of orbital (fusion) tube welds on stainless steel process gas lines, mainly in 1/4-, 3/8-, and 1/2-inch diameters with wall thicknesses of 0.030, 0.035, and 0.049 inch, respectively.
Manufacturers of semiconductor chips have miles of stainless steel tubing running under the fabrication areas. A typical process gas line, which brings a process gas from its location on the gas pad to the point of use, has about 300 orbital welds.
This industry demands that the inner surface of the tubing transporting the process gas be extremely smooth so that moisture or particulates cannot accumulate and cause contamination. In this industry, contamination has a harmful effect on process yield.
The ID of the tubing is electropolished to achieve an average surface roughness of 10 Ra microinches or less. Ra is a measurement of surface roughness used to describe the surface finish of the tubing ID. It means average roughness and is determined with a profilometer, which measures peak-to-valley distances and averages these measurements over a length known as cutoff. The higher the number, the rougher the surface. Since the welds are an integral part of the piping system, it is important for the weld ID to be as smooth as possible.
Orbital welding makes a crevice-free and smoother weld than can typically be accomplished manually, but the alloy composition also has an effect on smoothness. The industry has kept the sulfur content of stainless steel low to achieve a better surface finish and a smoother weld bead. Stainless steel manufacturers and suppliers have experimented with the base composition of stainless steel to achieve the smoothest welds possible.
Semiconductor processing requires the use of hazardous pyrophoric, toxic, and corrosive gases which must be doubly contained. In other words, the line carrying the gas must be enclosed within a second outer line. In case of a leak in the inner line, which could otherwise be catastrophic, a leak into the outer line would instantly be detected by a monitoring system to alert maintenance personnel.
Biopharmaceutical piping systems, including water-for-injection (WFI), deionized water (DI), and process lines, are typically installed by mechanical contractors using orbital fusion tube welding systems.
To be successful, orbital welding requires highly proficient personnel. A minimum of about a week is required for operator training. At this point, the welders should be capable of operating the power supply, which controls the travel speed of the torch, welding current, arc voltage, wire feed speed, and, when required, oscillation of the torch across the weld joint.
They must also be proficient at mounting the weld head on the pipe, setting the correct torch angles and wire feed adjustments, and, for remote welding operations, making these adjustments while watching the weld on a video monitor. Given more time and hands-on experience, welders become more proficient without additional formal training.
If the operators are required to program the power supply and to troubleshoot weld procedures, a longer training period is needed. Experience with manual welding and/or the ability to "read" a weld puddle is a big advantage in learning to operate an orbital pipe welding system.
Tube welder training is available at more than 50 local unions of the United Association throughout the U.S. and Canada.
For high-purity welding, welders must learn cleanroom procedures and high-purity purging techniques. Welding gas, typically argon, must be purified to the low parts-per-million or even the low parts-per-billion levels and maintained at this level in the tube ID during welding to prevent discoloration. An oxygen analyzer is often used to ensure that the purge gas leaving the tube ID is of the same purity as the source gas.
Successful orbital welding demands good cutting and facing equipment to properly prepare the tube ends for welding. Tube end preparation must be done in such a way as to prevent particulates from entering the tube ID. This is usually accomplished by purging the tube ID during the facing operation. Finally, a reliable quality control program is required to ensure that weld quality specifications are met.
Orbital tube and pipe welding is an established method of joining tube and pipe in a variety of industries. An orbital welding supplier can help manufacturers determine possible applications not discussed in this article.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.